Microhydration of PAH
+
cations: evolution of
hydration network in naphthalene
+
-(H
2
O)
n
clusters
(n#5)†
Kuntal Chatterjee and Otto Dopfer *
The interaction of polycyclic aromatic hydrocarbon molecules with water (H
2
O¼W) is of fundamental
importance in chemistry and biology. Herein, size-selected microhydrated naphthalene cation
nanoclusters, Np
+
-W
n
(n#5), are characterized by infrared photodissociation (IRPD) spectroscopy in the
C–H and O–H stretch range to follow the stepwise evolution of the hydration network around this
prototypical PAH
+
cation. The IRPD spectra are highly sensitive to the hydration structure and are
analyzed by dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ) to
determine the predominant structural isomers. For n¼1, W forms a bifurcated CH/O ionic hydrogen
bond (H-bond) to two acidic CH protons of the bicyclic ring. For n$2, the formation of H-bonded
solvent networks dominates over interior ion solvation, because of strong cooperativity in the former
case. For n$3, cyclic W
n
solvent structures are attached to the CH protons of Np
+
. However, while for
n¼3 the W
3
ring binds in the CH/O plane to Np
+
, for n$4 the cyclic W
n
clusters are additionally
stabilized by stacking interactions, leading to sandwich-type configurations. No intracluster proton
transfer from Np
+
to the W
n
solvent is observed in the studied size range (n#5), because of the high
proton affinity of the naphthyl radical compared to W
n
. This is different from microhydrated benzene
+
clusters, (Bz-W
n
)
+
, for which proton transfer is energetically favorable for n$4 due to the much lower
proton affinity of the phenyl radical. Hence, because of the presence of polycyclic rings, the interaction
of PAH
+
cations with W is qualitatively different from that of monocyclic Bz
+
with respect to interaction
strength, structure of the hydration shell, and chemical reactivity. These differences are rationalized and
quantified by quantum chemical analysis using the natural bond orbital (NBO) and noncovalent
interaction (NCI) approaches.
1. Introduction
The interactions and chemical reactions of aromatic molecules
and their cations with surrounding solvent molecules are of
fundamental importance for many phenomena in chemistry
and biology.
1–4
In particular, the interaction of benzene and
polycyclic aromatic hydrocarbon molecules (PAH) and their
cations (PAH
+
) with water (H
2
O¼W) plays a crucial role in the
areas of combustion,
5
organic chemistry,
6–14
astrochemistry,
15–26
and biomolecular recognition.
3,4,27–29
For example, intermolec-
ular interactions involving aromatic hydrocarbons and water
are the driving force for many biochemical processes, such as
the conformation and folding of proteins, base pair stacking in
DNA, drug design, macromolecular assemblies, and biological
membranes. The relative strengths of the various interaction
types (e.g.,OH/p,CH/O, cation/p)
4,28,30–32
strongly depend
on the charge state of the PAH. In the context of astrochemistry,
PAH molecules, their radical cations (PAH
+
), and their proton-
ated species (H
+
PAH) are suggested to be carriers of the
unidentied infrared emission bands as well as the diffuse
interstellar bands.
33–38
Concerning ions of the most simple PAH
(naphthalene, C
10
H
8
, Np), the postulation
39
and tentative
identication of the Np
+
cation as a DIB carrier
40,41
provided
a great stimulus for the plethora of laboratory spectroscopy of
both Np
+
(ref. 39 and 42–51) and H
+
Np
52–58
in the gas phase and
isolated in rare gas matrices in both the infrared and optical
range of the electromagnetic spectrum. In addition, the spec-
troscopy and photochemistry of Np and Np
+
deposited in ice or
on ice grains as well as in aqueous solution have been
studied.
6,18,19,23,59,60
For a deeper understanding of the interaction between water
and PAH molecules in various charge states at the molecular
level, the accurate knowledge of the involved interaction
potential is required. To this end, the combination of spec-
troscopy of molecular clusters isolated in the gas phase with
Institut f¨
ur Optik und Atomare Physik, Technische Universit¨
at Berlin, Hardenbergstr.
36, 10623 Berlin, Germany. E-mail: dopfer@physik.tu-berlin.de; Tel: +49 30 31423018
†Electronic supplementary information (ESI) available: Detailed NBO and NCI
analyses, structures and IR spectra of less stable Np
+
-W
n
isomers and W
n
clusters, Cartesian coordinates of all optimized structures. See DOI:
10.1039/c7sc05124g
Cite this: Chem. Sci.,2018,9,2301
Received 1st December 2017
Accepted 24th January 2018
DOI: 10.1039/c7sc05124g
rsc.li/chemical-science
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quantum chemical calculations provides the most direct access
to this potential. Herein, we combine infrared photodissocia-
tion (IRPD) spectroscopy with dispersion-corrected density
functional theory (DFT) calculations to probe the initial
microhydration steps in size-selected cold Np
+
-W
n
clusters
produced in a supersonic plasma expansion using electron
ionization. This approach has recently been applied in our
laboratory to a variety of microhydrated aromatic ions.
61–72
Signicantly, the employed cluster ion source generates
predominantly the most stable isomer of a given cluster ion,
because of the sequential cluster growth realized in the high-
pressure region in the supersonic expansion.
73,74
Surprisingly, despite their importance only very scarce
information is available for the spectroscopy of PAH
()
-W
n
clusters, and this is even true for the most simple Np
()
-W
n
clusters. For example, no spectral data have been reported yet
for neutral Np-W
n
clusters, and thus all information about their
structure and interaction potential has to barely rely on rather
limited computational studies available only for n#2.
75
The
latter predict pH-bonded structures, in which the W ligands
bind via OH/pstacking to the aromatic p-electron system of
Np either as single W ligands or as a H-bonded W
2
dimer. The
situation is more favorable for Np
-W
n
anion clusters, which
were characterized by photoelectron
76–78
and IR
79
spectroscopy
up to the size range n¼6. While Np
is unstable with respect to
electron detachment because of the negative electron affinity of
Np, microhydrated Np
-W
n
clusters with n$1 are stable and
can be produced in supersonic expansions. Computational
analysis of the IR spectra of Np
-W
n
reveals that also the anion
clusters prefer pH-bonded structures, in which W (n¼1), a H-
bonded W
2
dimer (n¼2), or cyclic W
n
clusters (n¼3 and 4)
bind to the pcloud of Np
via (multiple) OH/pinteractions.
79
The rst and only information about Np
+
-W
n
cation clusters
prior to our work came from a very recent mass spectrometric
study,
14
in which Np
+
-W
n
clusters up to n¼6 were generated by
injecting Np
+
cations produced by electron ionization into
a dricell of an ion mobility mass spectrometer containing
water vapor. For the n¼1 dimer, the signal was strong enough
to measure clustering equilibria, which yield a binding enthalpy
of DH¼7.8 1 kcal mol
1
(2730 350 cm
1
) for Np
+
-W. Out
of the four isomers calculated for Np
+
-W, the three lowest-
energy structures have a bifurcated CH/O H-bond, while the
p-bonded structure is least stable (Fig. 1). Although the calcu-
lated binding energy of the global minimum (7.7 kcal mol
1
)
agrees with the measured enthalpy, the other two H-bonded
isomers are also quite low in energy (6.8 and 6.5 kcal mol
1
)
and thus no reliable conclusion about the cluster structures can
be drawn from the mass spectra.
14
For n> 1, only very weak mass
peaks were reported, and the most stable computed structures
determined for n¼2–6 have a linear H-bonded W
n
chain
attached to Np
+
.
14
In a recent spectroscopic study,
72
we analysed the IRPD
spectrum of Np
+
-W recorded in the C–H and O–H stretch (n
CH/
OH
) range by dispersion-corrected DFT calculations (B3LYP-D3/
aug-cc-pVTZ) to obtain the rst spectroscopic information
about the interaction of a PAH
+
cation with W. This spectral
range is highly sensitive to the details of the H-bonded
structure. The analysis of the observed IR spectrum conrms
that the bifurcated CH/O binding motif involving two CH
groups of two different rings (denoted Np
+
-W(18) in Fig. 1)
corresponds indeed to the global minimum of Np
+
-W predicted
by the calculations, and no other isomer is identied in the cold
molecular beam. In addition to the reliable structure determi-
nation, frequency-dependent photofragmentation branching
ratios monitored for IRPD of cold Np
+
-W-Ar clusters yield
a spectroscopic determination of the Np
+
-W binding energy (D
0
¼2800 300 cm
1
), in excellent agreement with the calculated
value (D
0
¼2773 cm
1
) and the enthalpy derived from mass
spectrometry (DH¼2730 350 cm
1
).
14
Detailed analysis
using the natural bond orbital (NBO) approach elucidates the
binding mechanisms and interaction strengths of the various
possible bifurcated CH/O H-bonding motifs,
72
as well as
differences in structure and binding energy between the most
stable Np
+
-W isomer and that of the monohydrated benzene
cation (Bz
+
-W).
Herein, we extend our combined IRPD and DFT approach to
larger Np
+
-W
n
clusters up to n#5 to reliably determine the
structure of the microhydration network formed around this
most simple PAH
+
cation. Surprisingly, our results yield cyclic
solvent structures, in disagreement with the recent computa-
tional study predicting linear W
n
structures.
14
Comparison
between neutral,
72,75
anionic,
76–79
and cationic
14,72
polyhydrated
Np
()
-W
n
clusters elucidates the drastic effects of the charge
state on the hydration network and the interaction strength.
Comparison of Np
+
-W
n
with the related and well-studied (Bz-
W
n
)
+
clusters
10,12,61,62,73,80–88
reveals the qualitative difference in
structure and chemical reactivity of both cluster systems. The
Np
+
-W interaction is quite different from that of Bz
+
-W with
respect to structure, interaction strength, and binding mecha-
nism.
14,72
In addition, the proton affinity of the naphthyl radical
is much larger than that of the phenyl radical. Hence, the
structure of the hydration shell and also the chemical reactivity
with respect to intracluster proton transfer are expected to be
very different for (Np-W
n
)
+
and (Bz-W
n
)
+
.
2. Experimental and computational
techniques
2.1 Experimental methods
IRPD spectra of mass-selected Np
+
-W
n
clusters with n#5 are
recorded in a tandem quadrupole mass spectrometer coupled
to an electron-ionization cluster ion source described in detail
elsewhere.
73,89
Briey, Np
+
-W
n
clusters are produced in a pulsed
supersonic plasma expansion by electron and/or chemical
ionization of Np and subsequent clustering reactions in the
high-pressure regime of the expansion.
72
The expanding gas
mixture is generated by passing Ar carrier gas (8–10 bar)
through a reservoir containing solid Np (Sigma-Aldrich, >99%,
heated to 70 C). To produce hydrated clusters, 10 ml of distilled
water are added to the gas line before the sample reservoir. The
desired Np
+
-W
n
parent clusters are mass-selected in the rst
quadrupole mass lter and irradiated in an adjacent octopole
ion guide with a tunable IR laser pulse (n
IR
) of an optical
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parametric oscillator (IR-OPO), which is pumped by a nano-
second Q-switched Nd:YAG laser. The IR-OPO laser pulses are
characterized by an energy of 2–5 mJ in the C–H/O–H stretch
range, a repetition rate of 10 Hz, and a bandwidth of 1 cm
1
.
Calibration of n
IR
to better than 1 cm
1
is accomplished by
a wavemeter. Resonant vibrational excitation upon single-
photon absorption leads to evaporation of a single W ligand
corresponding to following photodissociation reaction:
Np
+
-W
n
+n
IR
/Np
+
-W
n1
+W
No other fragmentation channel is observed under the
employed experimental conditions. The generated Np
+
-W
n1
fragment ions are mass-selected by the second quadrupole
mass lter and monitored by a Daly detector as a function of n
IR
to extract the IRPD spectrum of the Np
+
-W
n
parent cluster. All
IRPD spectra are linearly normalized for laser intensity varia-
tions monitored with a pyroelectric detector. Several spectra in
the 2950–3800 cm
1
range are composed of separate scans
recorded in the O–H and C–H stretch ranges, respectively. They
are connected such that the intensities of overlapping bands are
adjusted. The peak width in the IRPD spectra mainly arises
from unresolved rotational structure, sequence hot bands
involving inter- and intramolecular modes, lifetime broad-
ening, and possible overlap of transitions of different isomers.
To separate the laser-induced Np
+
-W
n1
fragments from those
generated by metastable decay in the octopole region, the ion
source is triggered at twice the laser frequency, and signals from
alternating triggers are subtracted. The composition of the
mass-selected Np
+
-W
n
clusters is conrmed by collision-
Fig. 1 Structures of W, Np
+
(with numbering of carbon atoms), and the (18), (12), (23), and (p) isomers of Np
+
-W calculated at the B3LYP-D3/aug-
cc-pVTZ level. Binding energies (D
0
) and bond lengths are given in cm
1
and ˚
A, respectively. Numbers in parenthesis correspond to relative
energies and free energies (E
0
,G).
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induced dissociation experiments, facilitated by lling the
octopole ion guide with N
2
collision gas (10
5
mbar).
2.2 Computational techniques
Quantum chemical calculations are carried out for Np
+
-W
n
at
the B3LYP-D3 level to determine their structural, vibrational,
and energetic properties.
90
Initial extensive manual screening of
the potential energy surface is conducted using the cc-pVTZ
basis set and nal optimization is performed with the larger
aug-cc-pVTZ basis set. The dispersion-corrected B3LYP-D3
functional describes the electrostatic, induction, and disper-
sion forces of aromatic clusters rather well, and reproduces
their experimental IR spectra and binding energies to satisfac-
tory accuracy using the aug-cc-pVTZ basis set.
67,68,71,91
This is
particularly true for the properties of Np, Np
+
, W, and the Np
+
-W
interaction, as was demonstrated in our recent report.
72
In
addition, spin contamination of the radical cation clusters is
negligible at this computational level.
72
All coordinates are
relaxed in the search for stationary points, and their nature as
minima or transition states is veried by harmonic frequency
analysis. Harmonic intramolecular vibrational frequencies are
linearly scaled by the factor 0.96266, derived from tting
calculated harmonic O–H and C–H stretch frequencies of W and
Np to experimental values.
72
Harmonic scaled IR stick spectra
are convoluted with a Gaussian line shape (fwhm ¼10 cm
1
) for
facile comparison to the experimental IRPD spectra. All re-
ported binding energies (D
0
), relative energies (E
0
), and relative
free energies (G) are corrected for harmonic vibrational zero-
point energies. Energies are not corrected for basis set super-
position error, because it was found to be very small (<1%) for
Np
+
-W and related microhydrated aromatic cations at this
computational level.
71
Natural bond orbital (NBO) analysis is
employed to evaluate charge distribution and charge transfer in
Np
+
-W
n
, as well as second-order perturbation energies (E
(2)
)of
donor–acceptor orbital interactions involved in the H-
bonds.
92,93
Interaction types and strengths are determined using
the noncovalent interaction (NCI) approach.
94,95
To this end, the
reduced gradient of the electron density, s(r)|grad(r)|/r
4/3
,is
evaluated as a function of the electron density roriented by the
sign of the second eigenvalue l
2
of the Hessian, r*¼rsign(l
2
).
The r*values (given in a.u.) provide a measure of the strengths
of the intermolecular bonds.
3. Results and discussion
Fig. 2 compares the IRPD spectra of Np
+
-W
n
in the 2950–
3800 cm
1
spectral range recorded in the Np
+
-W
n1
fragment
channel. This range covers the aromatic C–H modes of Np
+
(n
CH
) and the O–H stretch modes of the W ligands (n
OH
), and
thus provides a sensitive probe of the evolution of the structure
of the H-bonded hydration network around Np
+
in its
2
A
u
electronic ground state. The positions and widths of the tran-
sitions observed are listed in Table 1, along with their suggested
vibrational and isomer assignments. In general, the bands
labeled A–C occurring in the 3600–3800 cm
1
range are attrib-
uted to free O–H stretch modes of the W ligands, whereas
transitions D in the 3200–3600 cm
1
range arise from H-bonded
O–H stretch vibrations of the W
n
hydration network. Finally,
band E near 3080 cm
1
is assigned to aromatic C–H stretch
modes of Np
+
. In the following, we discuss the structural
evolution of the hydration network of Np
+
-W
n
by comparing the
measured IRPD spectra to computed linear IR absorption
spectra of the most stable isomers as a function of the cluster
size (n). Clearly, the IRPD spectra of Np
+
-W
n
in Fig. 2 show
a large variation in appearance as the number of W molecules in
the cluster increases, thereby illustrating the qualitative
changes in the hydration network structure induced by
sequential addition of W ligands in this size regime.
3.1 Np
+
and W monomers
The geometric, vibrational, and electronic structures of W, Np,
and Np
+
obtained at the B3LYP-D3/aug-cc-pVTZ level were dis-
cussed in detail in our earlier report on Np
+
-W.
72
Briey, the
O–H bond parameters of W in its
1
A
1
ground state (R
OH
¼0.9617
˚
A, n
1
¼3656 cm
1
,n
3
¼3755 cm
1
) agree well with the experi-
mental data (0.9578 ˚
A, 3657 and 3756 cm
1
).
96,97
The optimized
planar structures of both Np and Np
+
have D
2h
symmetry. The
calculated geometry and vibrational frequencies of neutral Np
in its
1
A
g
electronic ground state are in excellent agreement with
Fig. 2 IRPD spectra of Np
+
-W
n
(n¼1–5) recorded in the Np
+
-W
n1
fragment channel in the O–H and C–H stretch range. The positions,
widths, and vibrational and isomer assignments of the transitions
observed (A–E) are listed in Table 1. The dashed lines are included to
guide the eye for illustrating relative positions of the free O–H stretch
bands (A–C).
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the experimental data.
98–100
The
2
A
u
ground electronic state of
Np
+
is derived by removal of a bonding p(a
u
) electron from the
highest occupied molecular orbital of Np, which is delocalized
over the aromatic ring.
48,50,51,72
The NBO analysis reveals that the
excess positive charge is mostly distributed over the peripheral
hydrogen atoms of the bicyclic aromatic ring.
72
In contrast to
large C–C bond length changes, ionization induces only minor
contractions of the C–H bonds (#1m
˚
A). Importantly, the IR
activities of all C–H stretch fundamentals is very weak (<2 km
mol
1
),
72
and hence n
CH
fundamentals of Np
+
have never been
detected, neither in the gas phase
72
nor in rare gas matrices.
43,101
3.2 Np
+
-W
The possible structural isomers of Np
+
-W and the assignment of
its IRPD spectrum have been discussed in detail in the previous
communication.
72
They shall be briey summarized herein for
completeness, because they provide the basis for the discussion
of the larger clusters. The four nonequivalent minima on the
Np
+
-W potential
14,72
are shown in Fig. 1, and their predicted IR
spectra are compared in Fig. 3 to the measured IRPD spectrum.
For comparison, also the spectra calculated for bare Np
+
and W
are shown (although at a different intensity scale). The attrac-
tion in all four Np
+
-W isomers is largely given by the electro-
static charge–dipole interaction between the positive charge on
Table 1 Positions, widths (FWHM in parentheses), and suggested
vibrational and isomer assignments of the transitions observed in the
IRPD spectra of Np
+
-W
n
compared to frequencies of most stable
isomers calculated at the B3LYP-D3/aug-cc-pVTZ level. For
comparison, available data for W
n
with n#2 are also listed
Cluster Exp (cm
1
) Vibration Calc
a
(cm
1
) Isomer
W 3657
b
n
1
3656 (5, a
1
)
3756
b
n
3
3755 (63, b
2
)
W
2
3601
b
n
b
3540 (341, a0)
3654
b
n
1
3650 (9, a0)
3735
b
n
f
3727 (87, a0)
3746
b
n
3
3745 (84, a00)
Np
+
-W E 3072 (13) n
CH
3062 (59, a
1
)Np
+
-W(18)
B 3635 (10) n
1
3641 (34, a
1
)Np
+
-W(18)
A 3722 (9) n
3
3729 (95, b
1
)Np
+
-W(18)
Np
+
-W
2
E 3068 (10) n
CH
3051 (143, a0)Np
+
-W
2
(18)
n
CH
3063 (108, b
3u
)Np
+
-W
2
(1845)
D 3496 (33) n
b
3434 (696, a0)Np
+
-W
2
(18)
B 3646 (13) n
1
3649 (22, a0)Np
+
-W
2
(18)
n
1
3642 (65, a
g
)Np
+
-W
2
(1845)
C 3696 (14) n
f
3708 (84, a0)Np
+
-W
2
(18)
A2 3728 (10) n
3
3731 (186, b
1u
)Np
+
-W
2
(1845)
A1 3740 (9) n
3
3741 (99, a00)Np
+
-W
2
(18)
Np
+
-W
3
E 3065 (24) n
CH
3123 (18) Np
+
-W
3
(c1)
n
CH
3079 (113) Np
+
-W
3
(c2)
D3 3248 (broad) 2b
OH
D2 3402 (broad) n
b
3415 (162) Np
+
-W
3
(c1)
n
b
3417 (85) Np
+
-W
3
(c2)
D1 3507 (broad) n
b
3485 (313) Np
+
-W
3
(c1)
3528 (197)
n
b
3472 (380) Np
+
-W
3
(c2)
3502 (335)
C 3703 (28) n
f
3713 (79) Np
+
-W
3
(c1)
3711 (137)
3708 (120)
n
f
3717 (68) Np
+
-W
3
(c2)
3715 (143)
3714 (126)
Np
+
-W
4
E 3082 (4) n
CH
3087 (4) Np
+
-W
4
(c)
D2 3210 (broad) n
b
3240 (56) Np
+
-W
4
(c)
2b
OH
D1 3433 (broad) n
b
3319 (752) Np
+
-W
4
(c)
3332 (778)
3375 (170)
C 3703 (16) n
f
3710 (94) Np
+
-W
4
(c)
3708 (122)
3708 (153)
3706 (18)
Np
+
-W
5
D2 3230 (49) n
b
3180 (182) Np
+
-W
5
(c1)
2b
OH
D1 3365 (broad) n
b
3256 (1327) Np
+
-W
5
(c1)
3281 (816)
3317 (429)
3356 (134)
C 3700 (13) n
f
3712 (117) Np
+
-W
5
(c1)
3710 (146)
3708 (84)
3707 (74)
3706 (29)
a
IR intensity (in km mol
1
) and vibrational symmetry are listed in
parentheses. For the n
CH
modes, only the by far most intense
calculated vibration is listed.
b
Ref. 97, 107, 127 and 128.
Fig. 3 Comparison of experimental IRPD spectrum of Np
+
-W to linear
IR absorption spectra of all four nonequivalent isomers calculated at
the B3LYP-D3/aug-cc-pVTZ level (Fig. 1, Table 1). For comparison, also
the spectra calculated for Np
+
and W are shown (at a different intensity
scale).
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Np
+
and the dipole moment of W. As a consequence, the dipole
of W points away from the Np
+
cation in all isomers. The Np
+
-
W(18) global minimum with C
2v
symmetry features a symmetric
bifurcated CH/O ionic H-bond, in which two neighboring CH
groups of the two different aromatic rings act as proton donors
for two intermolecular H-bonds to the two in-plane lone pairs
of W. Its calculated dissociation energy of D
0
¼2773 cm
1
is in
good agreement with the values extracted from IRPD spectra of
Np
+
-W-Ar recorded in various fragment channels (D
0
¼2800
300 cm
1
),
72
and mass spectrometric experiments (DH¼2730
350 cm
1
).
14
The other two planar Np
+
-W(12) and Np
+
-W(23)
isomers with C
s
and C
2v
symmetry have a similar bifurcated
CH/O H-bond motif, with the main difference that both CH
proton donor groups belong to the same aromatic ring. As
a result, the CH/O H-bonds deviate substantially more from
linearity than those in the (18) isomer. Therefore, they are
signicantly weaker and longer, leading to correspondingly
smaller binding energies of D
0
¼2458 and 2340 cm
1
, respec-
tively. This view is fully supported by the NBO analysis of the
donor–acceptor energies of the orbital interactions involved in
the CH/O H-bond motif.
72
The three in-plane Np
+
-W isomers
are separated by calculated barriers of V
b
¼99–485 cm
1
. The
fourth Np
+
-W(p) isomer with a p-bonded structure and D
0
¼
1947 cm
1
is less stable than the in-plane isomers.
The consequences of the formation of the Np
+
-W dimer on
the appearance of its IR spectrum in the O–H and C–H stretch
range are visualized in Fig. 3 for the four ligand binding sites.
The large disparity in the ionization energies of Np and W (IE ¼
65 687 and 101 787 cm
1
)
51,102
implies that the positive charge
in (Np–W)
+
mainly remains on the Np moiety, justifying the
notation of Np
+
-W. Nonetheless, formation of the Np
+
-W dimer
is accompanied by modest charge transfer from Np
+
to the W
ligand, which scales with the interaction strength. For example,
Dq¼12, 7, and 8 me for the (18), (12), and (23) isomers. As
a result of this charge transfer, the O–H bonds of W become
weaker, leading to minor red shis in the symmetric and anti-
symmetric O–H stretch modes from the frequencies of bare W,
n
1/3
¼3656/3755 cm
1
. For the (18) global minimum, the effects
are largest, with Dr
OH
¼1.7 m˚
A and Dn
1/3
¼15/26 cm
1
.In
addition to the Dn
1/3
shis, the relative IR intensity of n
1
is
strongly enhanced compared to that of bare W, which is typical
for cation-W dimers.
61,73,80
Concerning the aromatic C–H bonds
of Np
+
, the largest impact is observed for the CH proton donor
groups. The elongation of the C–H bonds upon formation of the
CH/O ionic H-bond is rather minor, and the corresponding
red shiin n
CH
is small. For example, for the (18) global
minimum, the values are Dr
CH
¼0.8 m˚
A and Dn
CH
¼5cm
1
.
However, the IR intensity enhancement is substantial with up to
two orders of magnitude. As a consequence, the nearly IR
inactive n
CH
modes of bare Np
+
(I
CH
#2 km mol
1
) become
visible in the Np
+
-W spectrum of the in-plane isomers, and
reach intensities comparable to the n
OH
fundamentals (e.g.,I
CH
¼59 km mol
1
for (18)). Of course, for Np
+
-W(p) there is
essentially no effect on the C–H bond properties upon
monohydration.
The comparison of the IRPD spectrum measured for Np
+
-W
with the linear IR spectra calculated for the four Np
+
-W isomers
suggests an immediate assignment of the experimental spec-
trum to the most stable (18) isomer, because of the good
agreement with respect to both the positions and relative
intensities of the transitions.
72
The bands A, B, and E observed
at 3722, 3635, and 3072 cm
1
are attributed to n
3
,n
1
, and n
CH
of
(18) calculated at 3729, 3641, and 3062 cm
1
, with systematic
deviations of 7–10 cm
1
, respectively. The single intense n
CH
normal coordinate of (18) corresponds mostly to a symmetric
elongation of the C1–H and C8–H bonds, which is in phase with
a minor elongation of the opposite C4–H and C5–H bonds. The
detection of three single narrow transitions in the IRPD spectra
of Np
+
-W (and also the colder Np
+
-W-Ar cluster not shown here)
indicates that the experimental spectrum is mainly produced by
the (18) isomer.
72
The predicted IR spectra of the other three
isomer have n
OH
and n
CH
frequencies, which are clearly different
from those of (18) when considering the achieved spectral
resolution. Thus, if any of the less stable local minima would be
present in the expansion, the spectral features measured in the
n
OH
and n
CH
range should exhibit splittings and/or shoulders, in
disagreement with the experimental observation. Hence, the
population of the (12), (23), and (p) isomers of Np
+
-W is below
the detection limit. This result is in line with the relative (free)
energies of the four isomers (Fig. 1).
3.3 Np
+
-W
2
Fig. 4 summarizes the low-energy Np
+
-W
2
structures considered
in this work, along with the geometry of bare W
2
.Np
+
-W
2
clusters with p-bonded W ligands are no longer discussed here,
because of their low binding energies. There are two principal
ways to add a second W ligand to the H-bonded Np
+
-W dimer to
form Np
+
-W
2
. In the rst category, which corresponds to the
formation of a H-bonded solvent network, an H-bonded W
2
dimer is attached to the Np
+
cation via an in-plane bifurcated
CH/O ionic H-bond. Depending on the three nonequivalent
binding sites, these are denoted Np
+
-W
2
(18), Np
+
-W
2
(12), and
Np
+
-W
2
(23). Their total binding energies of D
0
¼5529, 5019,
and 4851 cm
1
follow the same trend as found for the Np
+
-W
dimers (Fig. 1), and their relative free energies (G) are similar to
the relative energies (E
0
). The formation of the H-bonded
solvent network is strongly cooperative because of the nonad-
ditive induction forces, as will be illustrated in some detail
quantitatively for the (18) isomers of n¼1 and n¼2. Attach-
ment of the second W ligand strongly strengthens the ionic
CH/O bond, which contracts from 2.327 ˚
A(n¼1) to 2.242 ˚
A(n
¼2). This result is in line with the larger proton affinity of W
2
as
compared to W (PA ¼808 and 691 kJ mol
1
).
103,104
As a conse-
quence of the stronger CH/O bond, the C–H proton donor
bonds are elongated from 1.0828 to 1.0836 ˚
A, and the corre-
sponding C–H stretch frequency decreases from 3062 to
3051 cm
1
along with an enhancement of the IR intensity from
59 to 143 km mol
1
. On the other hand, the H-bond of the W
2
dimer becomes much stronger by the presence of the Np
+
cation. The nearby positive charge polarizes the rst W ligand
and the additional induced dipole strengthens the W–WH-
bond. The isolated W
2
dimer has a calculated dissociation
energy of D
0
¼1108 cm
1
, in good agreement with the
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experimental value of 1105 10 cm
1
,
105
illustrating that the
chosen B3LYP-D3/aug-cc-pVTZ level reliably describes the W–W
interaction. The W–WOH/O H-bond contracts from 1.947 ˚
Ain
free W
2
to 1.826 ˚
AinNp
+
-W
2
(18). The total binding energy of D
0
¼5529 cm
1
for Np
+
-W
2
(18) exceeds the sum of the dissociation
energies of W
2
(1108 cm
1
) and the Np
+
-W(18) dimer
(2773 cm
1
) by 42%, providing a quantitative energetic measure
for the strong cooperativity. The formation of the W
2
dimer in
Np
+
-W
2
has unique signatures in the n
OH
range of the IR spec-
trum (Fig. 5). The localized bound O–H stretch of the proton
donor is largely red shied to n
b
¼3434 cm
1
with high IR
intensity (696 km mol
1
), because of the elongated O–H bond
(0.9759 ˚
A), while the uncoupled free O–H stretch of the dangling
OH group (0.9617 ˚
A) of this W molecule occurs at n
f
¼
3708 cm
1
. The n
b
mode in Np
+
-W
2
(18) is shied to lower
frequency by 106 cm
1
compared to bare W
2
(3540 cm
1
), as
a result of the stronger OH/O H-bond. The coupled n
1
and n
3
frequencies of the proton acceptor OH groups (0.9627 ˚
A) are
predicted at 3649 and 3741 cm
1
,i.e. they occur 8 and 11 cm
1
blueshied from those of the n¼1 cluster, because the W
molecule is further away from the Np
+
charge and thus their
O–H bonds are shorter than in n¼1 (0.9634 ˚
A). Further
measures of the degree of noncooperativity provided by the
NBO and NCI analyses are detailed in ESI (Fig. S1–S3, Table
S1†). Both the NBO and NCI indicators suggest that the neutral
OH/O H-bond in Np
+
-W
2
(18) is stronger than the CH/O ionic
H-bond! Similar cooperative effects as found for the (18) struc-
ture of Np
+
-W
2
are also observed for the (12) and (23) isomers,
although both the OH/O and CH/O H-bonds are weaker in
these less stable local minima as compared to the (18) isomer.
In the second category of Np
+
-W
2
structures, which corre-
sponds to interior ion solvation, both W ligands bind separately
to the Np
+
cation via two individual CH/O ionic H-bonds. The
three most stable examples of this category are Np
+
-W
2
(18/45),
Np
+
-W
2
(18/12), and Np
+
-W
2
(12/23), with binding energies of
D
0
¼5395, 4817, and 4512 cm
1
, respectively (Fig. 4). Other
possible isomers such as (18/23), (18/34), etc. are less stable. In
contrast to clusters with an H-bonded solvent network, the
nonadditive polarization forces are slightly noncooperative for
interior ion solvation, mainly because of enhanced charge
delocalization. This effect shall be illustrated in some detail
quantitatively for the (18) and (18/45) isomers of n¼1 and n¼
Fig. 4 Optimized structures of W
2
and most stable isomers of Np
+
-W
2
calculated at the B3LYP-D3/aug-cc-pVTZ level. The (18), (12), and (23)
isomers correspond to formation of a H-bonded solvent network, while the (18/45), (18/12), and (12/23) isomers are examples of interior ion
solvation. Binding energies (D
0
) and bond lengths are given in cm
1
and ˚
A, respectively. Numbers in parenthesis correspond to relative energies
and free energies (E
0
,G).
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2, respectively, which are both characterized by bifurcated
CH/O ionic H-bonds to the CaH groups of Np
+
. The (18/45)
isomer with D
2h
symmetry has two equivalent CH/OH-
bonds with R¼2.341 ˚
A and D
0
¼2698 cm
1
, while the
corresponding bond in the n¼1 cluster (C
2v
) is slightly stronger
(R¼2.327 ˚
A, D
0
¼2773 cm
1
). Energetically, the non-
cooperativity thus is evaluated as 3%. As a consequence of the
slightly weaker CH/O H-bond, the C–H proton donor bonds
contract from 1.0828 ˚
A(n¼1) to 1.0826 ˚
A(n¼2), and the
corresponding (averaged) C–H stretch frequency increases from
3064 to 3070 cm
1
. In addition, the O–H bonds of the
individual W ligands are less affected by the interaction with
Np
+
in n¼2 as compared to n¼1(Dr
OH
¼1.5 vs. 1.7 m˚
A, Dn
1/3
¼
14/24 vs. 15/26 cm
1
). Similar noncooperative effects are
observed for other isomers with interior ion solvation.
According to the calculations, the most stable Np
+
-W
2
isomer
is the (18) isomer with W
2
attached to C1H and C8H. The next
stable isomer is (18/45) with E
0
¼134 cm
1
and G¼521 cm
1
,
i.e. the formation of an H-bonded network is favored over
interior ion solvation (Fig. 4). The other considered isomers of
both categories are substantially higher in energy, and from
experience with the analysis of the n¼1 spectrum, they are not
expected to substantially contribute to the experimental IRPD
spectrum of Np
+
-W
2
. To this end, the IR spectra calculated for
the (18) and (18/45) isomers are compared in Fig. 5 to the
measured IRPD spectrum. The latter exhibits six transitions at
3740 (A1), 3728 (A2), 3696 (C) 3646 (B), 3496 (D), and 3068
(E) cm
1
. The bands D and C are newly observed in the n¼2
spectrum (Fig. 2) and are a clear signature of a Np
+
-W
2
isomer
with a H-bonded W
2
dimer. They correspond to the bound and
free O–H stretch modes of the proton donor W molecule (n
b/f
)in
the W
2
dimer. As a consequence, transitions D and C at 3496
and 3696 cm
1
are assigned to n
b
and n
f
of the (18) isomer, in
good agreement with the calculated frequencies of 3434 and
3708 cm
1
. The remaining free O–H stretch modes of the
terminal W acceptor molecule are predicted at n
1
¼3649 and n
3
¼3741 cm
1
, and thus are assigned to transitions B and A1
observed at 3646 and 3740 cm
1
, respectively. Transition E at
3068 cm
1
is then attributed to n
CH
of the (18) isomer predicted
at 3051 cm
1
. The experimental red shiof n
CH
upon attach-
ment of the second W ligand (4cm
1
) is somewhat smaller
than the predicted shi(11 cm
1
). In addition, the enhanced
relative intensity of n
CH
compared to the free n
OH
bands
observed experimentally is nicely reproduced by the spectrum
calculated for (18). While for n¼1, n
CH
is less intense than n
OH
,
the opposite is true for n¼2. Overall, most of the major tran-
sitions of the IRPD spectrum of Np
+
-W
2
agree well with those
predicted for the (18) global minimum with respect to position
and relative intensity. The other less stable Np
+
-W
2
local
minima with a W
2
structure, (12) and (23), have a qualitatively
similar IR spectrum (Fig. S4 in ESI†) but may be excluded for
several reasons. First, the calculated relative energies are rather
high (E
0
> 500 cm
1
), and thus their presence may be excluded
for thermodynamic reasons. For the n¼1 cluster, all local
minima with E
0
> 300 cm
1
are below the detection limit. In
addition, since the OH/O H-bonds are weaker in the (12) and
(23) isomers as compared to the (18) isomer (R¼1.841 and
1.846 vs. 1.862 ˚
A), their bound O–H stretch frequencies occur at
signicantly higher energies (n
b
¼3456 and 3462 vs.
3434 cm
1
). However, band Dassigned to n
b
occurs as a single
peak and does not show any splitting. The asymmetric blue-
shaded band contour of band D with a sharp rise on the red side
and a long monotonic decreasing tail on the blue side is typical
for excitation of proton donor stretch vibrations
73
and not an
indication for the presence of further isomers. Similarly, band E
assigned to n
CH
does not exhibit any splitting or shoulder. While
for the (12) isomer the n
CH
intensity is predicted to be much
weaker than for (18) (I
CH
¼23 vs. 143 km mol
1
) and thus
difficult to detect, the most intense n
CH
fundamental of (23) is
predicted to occur with substantial intensity at 3094 cm
1
(52
km mol
1
). This frequency is far from the measured n
CH
band E
at 3068 cm
1
, and thus the population of (23) is clearly below
the detection limit.
Although at rst glance the IRPD spectrum of Np
+
-W
2
can
mostly be assigned to the dominating (18) global minimum,
closer inspection reveals subtle hints for the minor presence of
a second isomer, which belongs to the class of interior ion
solvation. The clearest signature is transition A2 at 3728 cm
1
,
which occurs close to band A of Np
+
-W at 3722 cm
1
. This band
thus is attributed to the free n
3
mode of a W ligand directly
Fig. 5 Comparison of experimental IRPD spectrum of Np
+
-W
2
to
linear IR absorption spectra of the most stable (18) and (18/45) isomers
of Np
+
-W
2
calculated at the B3LYP-D3/aug-cc-pVTZ level (Fig. 4,
Table 1). For comparison, also the spectrum calculated for bare W
2
is
shown (at a different intensity scale). Comparison of the IRPD spec-
trum to linear IR spectra of less stable isomers is available in Fig. S4 in
ESI.†
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attached to Np
+
. The small observed blue shiof 6 cm
1
from n
¼1ton¼2 is consistent with the noncooperative effect when
attaching the second W ligand. Hence, band A2 of Np
+
-W
2
is
assigned to n
3
of the by far most stable (18/45) isomer with
interior ion solvation predicted at 3731 cm
1
. The other tran-
sitions calculated for this isomer (n
3
¼3642 cm
1
,n
CH
¼
3063 cm
1
) overlap with bands B and E at 3646 and 3068 cm
1
mainly assigned to the (18) global minimum. This overlap may
explain the somewhat higher relative intensity of band B in the
IRPD spectrum, which cannot be explained by the calculated
spectrum of (18) alone. Comparing the ratio of the integrated
band intensities of bands A1 and A2 (>3 : 1) with the IR cross
sections calculated for n
3
of the (18) and (18/45) isomers (1 : 2),
we estimate the maximum abundance of the (18/45) local
minimum to be below 20%. This result is consistent with its
relative (free) energy of E
0
¼134 (521) cm
1
. An alternative
explanation for band A2 may be unresolved structure of
(hindered) internal rotation of the terminal W ligand of the (18)
isomer of Np
+
-W
2
. Although we cannot exclude this interpreta-
tion, we favor currently the scenario with the coexistence of the
two isomers. Other isomers with interior ion solvation may
safely be excluded for thermodynamic reasons (E
0
> 700 cm
1
).
In addition, their spectra predicted in the n
CH
range do not
match well with band E, because of their weaker CH/OH-
bonds (Fig. S4 in ESI†).
3.4 Np
+
-W
3
Interestingly, the IRPD spectrum of Np
+
-W
3
in Fig. 2 is domi-
nated in the free O–H stretch range by band C at n
f
¼3703 cm
1
.
Unlike the spectra for the n¼1 and n¼2 clusters, the n¼3
spectrum does not show anymore pronounced peaks A and B
assigned to coupled n
3
and n
1
transitions of W ligands acting
barely as proton acceptors in H-bonding. While there is den-
itively no signal around 3650 cm
1
(B, n
1
), some weak signal
intensity in the blue shoulder of band C near 3730 cm
1
may be
attributed to n
3
transitions (A). On the other hand, there are
intense and broad transitions D1 and D2 peaking at 3507 and
3402 cm
1
, which clearly come from H-bonded O–H stretch
modes. These observations are taken as evidence that the
predominant Np
+
-W
3
structures contributing to the IRPD
spectrum have only W ligands, which act as proton donor in H-
bonds, either as single donor or double donor. In such struc-
tures, a cyclic H-bonded W
3
trimer is attached to the Np
+
cation.
The still high intensity of transition E at 3065 cm
1
indicates
that at least in some of these Np
+
-W
3
clusters, the cyclic W
3
unit
is connected to the Np
+
cation via CH/O ionic H-bonding.
The calculations yield a large number of stable structures for
Np
+
-W
3
. Similar to the case of Np
+
-W
2
, the possible Np
+
-W
3
isomers may be classied into those with interior ion solvation
and those forming a single hydration network. The latter may
further be divided into those having a linear, branched, or cyclic
W
3
unit. Several of the low-energy structures representing
prototypes for these classes of Np
+
-W
3
clusters are shown in
Fig. 6 and S5 in ESI.†Amongst all optimized structures, Np
+
-
W
3
(c1) with D
0
¼8355 cm
1
is the most stable isomer (E
0
¼0).
In this structure, a cyclic neutral distorted H-bonded W
3
trimer
is attached to the Np
+
cation via two nearly linear CH/O ionic
H-bonds involving the C1H and C8H proton donor groups. For
comparison, the structure and IR spectrum calculated for bare
W
3
are shown in Fig. S6 and S7 in ESI.†Its dissociation energy
for W loss (D
0
¼2809 cm
1
) is in good agreement with the
experimental value of 2650 150 cm
1
.
106
The three intermo-
lecular OH/O bonds in Np
+
-W
3
(c1) are rather different (R¼
1.880, 1.904, 2.059 ˚
A), giving rise to three isolated bound O–H
stretch oscillators (n
b
¼3415, 3485, 3528 cm
1
, Fig. 7) due to
rather different O–H bond lengths (0.9762, 0.9733, 0.9712 ˚
A). In
contrast, the free O–H bonds have essentially the same length
(0.9619 0.0001 ˚
A), producing three free O–H stretch bands
with similar frequency (3708–3713 cm
1
). A similar cyclic
structure Np
+
-W
3
(c2) shown in Fig. 6 with D
0
¼7846 cm
1
is
slightly higher in energy (E
0
¼509 cm
1
). In the most stable
linear (18) isomer with D
0
¼8140 cm
1
(E
0
¼215 cm
1
), a linear
W
3
chain is attached to the Np
+
cation via a bifurcated CH/O
ionic H-bond between the acidic C1H and C8H protons and the
rst W ligand of the chain (Fig. S5 in ESI†). Such linear chains
may also start between the less acidic C1H and C2H protons or
the C2H and C3H protons, yielding the less stable (12) and (23)
isomers with D
0
¼7524 and 7262 cm
1
(E
0
¼831 and
1093 cm
1
). Np
+
-W
3
clusters with a branched W
3
network are
less stable than those with a linear W
3
chain (e.g.,D
0
¼7787
versus 8140 cm
1
for attachment of W
3
at the C8H and C1H
protons). Amongst all the isomers with interior ion hydration,
Fig. 6 Optimized structures of the two most stable cyclic Np
+
-W
3
isomers obtained at the B3LYP-D3/aug-cc-pVTZ level. Binding energies (D
0
)
and bond lengths are given in cm
1
and ˚
A, respectively. Numbers in parenthesis correspond to relative energies and free energies (E
0
,G).
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the most stable (18/45) isomer with D
0
¼8059 cm
1
(E
0
¼
296 cm
1
) binds a W
2
unit via bifurcated CH/O H-bonds to
C1H and C8H and a single W ligand to the C4H and C5H
protons. As expected, the most stable Np
+
-W
3
isomer, in which
all three W ligands bind separately to Np
+
via individual ionic
CH/O H-bonds, is the (18/45/12) isomer with D
0
¼7300 cm
1
(E
0
¼1055 cm
1
). When considering entropic effects at room
temperature, the linear and branched structures as well as
structures with interior ion solvation become more favorable
than cyclic structures because the latter ones are more rigid (see
Gvalues in Fig. 6 and S5 in ESI†).
The IRPD spectrum of Np
+
-W
3
is compared in Fig. 7 to the IR
spectra calculated for the two considered cyclic isomers (c1) and
(c2), while comparison to IR spectra of the other isomers is
available in Fig. S8 in ESI.†As mentioned above, the lack of
bands A and B in the Np
+
-W
3
spectrum provides a strong indi-
cation that the experimental spectrum is largely dominated by
cyclic isomers, and indeed it can fully be rationalized by the two
lowest-energy cyclic isomers (Fig. 7). The resulting assignments
are summarized in Table 1. While the (c1) global minimum can
account well for free and bound O–H stretch bands (C, D1, D2),
it fails to reproduce the intense n
CH
band E. In contrast, the (c2)
isomer can explain the latter transition and, in addition, also
the O–H stretch part of its spectrum is compatible with the
experimental one. The minor signal in the blue wing of band C
near 3730 cm
1
may provide weak evidence for the presence of
the most stable linear (18) isomer (Fig. S8 in ESI†), which for
energetic reasons may also have a signicant population.
Because its terminal W is far away from the Np
+
charge, its n
1/3
bands have low IR activity, and thus may not readily be detected
here. The broad transition D3 near 3250 cm
1
cannot be
rationalized by any of the calculated Np
+
-W
3
isomers. This band
thus is tentatively attributed to the rst overtone of the bending
vibrations of the W ligands (2b
OH
), which is expected in this
spectral range and not included in the harmonic calculations.
Clearly, the IR spectrum predicted for cyclic W
3
(Fig. 7) is quite
different from that of Np
+
-W
3
(c1). The perturbation by the
nearby Np
+
cation arises from both charge-induced and steric
effects.
3.5 Np
+
-W
4
The evolution of the IRPD spectra of Np
+
-W
n
in Fig. 2 yields the
following major conclusions for the n¼4 cluster. Bands A and B
assigned to n
3
and n
1
modes of W ligands not acting as H-bond
donors, which are strong for n#2 and very weak and/or absent
for n¼3, denitively disappear for n¼4. This result indicates
that the n¼4 clusters are dominated by isomers with a cyclic
W
4
unit. Second, compared to the spectra of the n¼1–3 clus-
ters, also the relative intensity of band E attributed to n
CH
of Np
+
becomes very weak relative to the n
OH
transitions for n¼4. This
observation indicates that the population of Np
+
-W
4
isomers
with CH/O ionic H-bonds of Np
+
to the W
4
unit is small, and
the cyclic W
4
ring may preferentially bind above or below the
Np
+
ring. The O–H stretch spectral range is now completely
dominated by the sharp band C and a broader band D1 char-
acteristic for n
f
and n
b
of single-donor W ligands.
Out of the many stable structures computed for Np
+
-W
4
, the
cyclic Np
+
-W
4
(c) isomer shown in Fig. 8 is the most stable one,
with a total binding energy of D
0
¼12 557 cm
1
. In this struc-
ture, a distorted neutral H-bonded W
4
unit is located above the
aromatic Np
+
ring, with a strongly nonplanar bifurcated CH/O
H-bond motif to one W molecule of the W
4
cycle. As a result, the
C1H/O and C8H/O H-bonds are rather weak (2.685 and 2.491
˚
A) compared to the planar bifurcated CH/O H-bonds found for
n#3. Like for n¼3, also for n¼4 all free O–H bonds of the
cyclic W
4
ring point away from the Np
+
cation to maximize the
electrostatic charge–dipole attraction. For comparison, also
a low-energy linear Np
+
-W
4
(l) isomer is shown in Fig. 8 (D
0
¼
11 103 cm
1
), although its relative energy is already rather high
compared to Np
+
-W
4
(c), E
0
¼1454 cm
1
. Note that this linear
isomer differs from that reported in ref. 14 such that the W
ligands are closer to the Np
+
cation. The global minimum re-
ported in ref. 14 could not be optimized at the current theo-
retical level.
The IRPD spectrum of Np
+
-W
4
is compared in Fig. 9 to the IR
spectra calculated for the cyclic and linear isomers shown in
Fig. 8. In general, the overall appearance of the Np
+
-W
4
(c)
spectrum agrees well with the measured spectrum, in particular
in view of the fact that the computations somewhat
overestimate the red shiof the bound O–H stretch bands
Fig. 7 Comparison of experimental IRPD spectrum of Np
+
-W
3
to
linear IR absorption spectra of the most stable cyclic isomers of Np
+
-
W
3
calculated at the B3LYP-D3/aug-cc-pVTZ level (Fig. 6, Table 1). For
comparison, also the spectrum calculated for bare W
3
is shown (at
adifferent intensity scale).
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(see Fig. 5 for n¼2). This is true for the overall appearance, the
positions and relative intensities of the transitions, and the
simplicity of the spectrum. Hence, bands C and D1 at 3703 and
3433 cm
1
in the IRPD spectrum are assigned to n
f
and n
b
modes of Np
+
-W
4
(c), calculated in the narrow 3706–3710 cm
1
range for n
f
and more widely spread out at 3240, 3319, 3332, and
3375 cm
1
for n
b
(Table 1). The transition D2 at 3210 cm
1
is
again tentatively attributed to 2b
OH
, and may also cover the
lowest-frequency n
b
mode (3240 cm
1
). Consistent with the
experimental spectrum, the n
CH
intensity of this isomer is very
small (4 km mol
1
). In fact, the visible band E at 3082 cm
1
is
probably not (only) produced by the Np
+
-W
4
(c) global minimum
but may be an indicator for the presence of less stable cyclic
and/or linear/branched Np
+
-W
4
isomers with stronger CH/O
contacts. One such example may be the computed Np
+
-W
4
(l)
isomer. On the other hand, this isomer has the n
1
and n
3
tran-
sitions typical of the terminal W ligand, which are not detected
in the experimental spectrum, although their IR activity is
predicted to be higher than that of n
CH
. Interestingly, the
spectrum calculated for cyclic W
4
(Fig. S6 in ESI†) is not too
different from that of Np
+
-W
4
(c1) (Fig. 9). The strongly IR active
ring stretch mode of W
4
is measured at n
b
¼3416 cm
1
,
107
i.e.
somewhat lower than the maximum of band D1 of Np
+
-W
4
at
3433 cm
1
. In addition, it is substantially higher than the pre-
dicted frequency of n
b
¼3297 cm
1
, conrming that the DFT
calculations overestimate the red shiof the bound O–H stretch
frequencies.
3.6 Np
+
-W
5
The following conclusions are evident from the comparison of
the IRPD spectrum of Np
+
-W
5
with those of the smaller hydrates
in Fig. 2. First, band C near 3700 cm
1
gains further in relative
intensity, because the number of single-donor W ligands in the
clusters increases and their n
f
bands occur all at the same
position. In addition, its width becomes narrower, because the
binding energy for W elimination becomes smaller as the
cluster size increases. Second, like for the other n$3 clusters,
bands A and B are absent, indicating that clusters with cyclic W
5
units dominate the cluster population. Third, band E disap-
pears completely for n¼5, revealing that the observed cluster
structures have no (or only a weak) CH/O ionic H-bond. Thus,
the trend observed already for n¼4 continues for n¼5, and the
spectrum points toward Np
+
-W
5
clusters with cyclic W
5
rings
above the Np
+
plane. Fourth, the transition D1 associated for n
b
Fig. 8 Optimized structures of the most stable Np
+
-W
4
isomers with a cyclic (c) and linear (l) W
4
solvent cluster obtained at the B3LYP-D3/aug-
cc-pVTZ level. Binding energies (D
0
) and bond lengths are given in cm
1
and ˚
A, respectively. Numbers in parenthesis correspond to relative
energies and free energies (E
0
,G).
Fig. 9 Comparison of experimental IRPD spectrum of Np
+
-W
4
to
linear IR absorption spectra of the most stable cyclic and linear isomers
of Np
+
-W
4
calculated at the B3LYP-D3/aug-cc-pVTZ level (Fig. 8,
Table 1). For comparison, also the spectrum calculated for bare W
4
is
shown (at a different intensity scale).
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transitions becomes broader and shied to lower frequency,
indicating that the OH/O H-bonds of the W
5
solvent network
become stronger. Finally, similar to band C, the band D2
assigned to the 2b
OH
overtone increases in relative intensity,
because the number of W ligands in the cluster becomes larger
and possibly the anharmonic interaction (Fermi resonances)
with the nearby n
b
fundamentals becomes stronger. Part of the
D2 intensity may also arise from n
b
transitions.
The most stable structure out of the plethora of minima
found for Np
+
-W
5
is the Np
+
-W
5
(c1) isomer shown in Fig. 10,
with a total binding energy of D
0
¼15 927 cm
1
. In this struc-
ture, a distorted cyclic neutral W
5
pentamer is connected to the
most acidic C1H and C8H protons by a strongly nonplanar
bifurcated CH/O ionic H-bond to a single W molecule. This
structure is similar to the most stable Np
+
-W
4
(c) structure of the
n¼4 cluster, with the simple insertion of one W ligand into the
cyclic water cluster ring. All W molecules act as single-donor
single-acceptor, except for the W molecule connecting the W
5
cycle to Np
+
. For comparison, a related cyclic Np
+
-W
5
(c2) isomer
is also shown in Fig. 10, in which one W ligand is a double
acceptor in the cyclic W
5
pentamer. This less stable structure is
substantially higher in energy (E
0
¼2718 cm
1
), illustrating that
W
5
rings with only single-donor single-acceptor molecules are
energetically very favorable. Other linear or branched structures
or cyclic structures with smaller W
n<5
rings (ring-tail structures)
are not considered in detail because they are less stable and,
similar to Np
+
-W
4
, do not t the experimental spectrum in the
free O–H stretch range. Similar to the n¼4 cluster, the global
minimum reported in ref. 14 with a linear W
5
chain pointing
away from Np
+
could not be found at the current theoretical
level.
In Fig. 11 the IRPD spectrum measured for Np
+
-W
5
is
compared to the IR spectra calculated for the (c1) and (c2)
isomers of Np
+
-W
5
and bare W
5
. The spectrum of the most
stable Np
+
-W
5
(c1) isomer agrees well with the measured spec-
trum, when we recall that the calculations underestimate the
frequencies of the bound O–H stretch bands (n
b
). The resulting
assignments are listed in Table 1. All n
f
modes of Np
+
-W
5
(c1) are
predicted in the narrow range of 3709 3cm
1
, in good
agreement with band C at 3700 cm
1
(which has a width of
13 cm
1
). The highly red shied n
b
modes assigned to the
OH/O H-bonds at 3180, 3256, 3281, 3317, and 3356 cm
1
correlate with the broad bands D2 and D1 peaking at 3230 and
3365 cm
1
, respectively. As for the other cluster sizes, band D2
may also contain contributions from 2b
OH
overtone transitions.
Although the Np
+
-W
5
(c2) isomer is quite high in energy, clusters
of this type may account for the signal observed in the
Fig. 10 Optimized structures of the two most stable cyclic Np
+
-W
5
isomers obtained at the B3LYP-D3/aug-cc-pVTZ level. Binding energies (D
0
)
and bond lengths are given in cm
1
and ˚
A, respectively. Numbers in parenthesis correspond to relative energies and free energies (E
0
,G). For
comparison, also the spectrum calculated for bare W
5
is shown (at a different intensity scale).
Fig. 11 Comparison of experimental IRPD spectrum of Np
+
-W
5
to
linear IR absorption spectra of the cyclic Np
+
-W
5
isomers calculated at
the B3LYP-D3/aug-cc-pVTZ level (Fig. 10, Table 1).
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3400–3600 cm
1
range because of their weaker OH/O bonds
and thus higher n
b
frequencies. Comparison of the IR spectra
calculated for Np
+
-W
5
(c1) and bare W
5
reveals that the presence
of Np
+
causes a smaller perturbation than for the n¼4 case.
Again, the strongly IR active n
b
modes of W
5
calculated at
3248 cm
1
(averaged value) are around 100 cm
1
lower than the
measured band (3360 cm
1
),
107
conrming that the calculations
overestimate the red shis of the bound O–H stretch frequen-
cies of the cyclic W
n
ring.
4. Further discussion
4.1 Cluster growth
The analysis of the IRPD spectra utilizing the DFT calculations
provides a detailed and consistent picture of the preferential
sequential cluster growth in microhydrated Np
+
-W
n
clusters.
The observed IRPD spectra can readily be explained by the most
stable clusters predicted by the calculations. For the most
probable cluster growth, we obtain the following scenario. As
discussed in detail in our previous report,
72
the population of
the Np
+
-W dimer (n¼1) is dominated by the Np
+
-W(18) isomer,
in which W binds in a planar bifurcated CH/O H-bond to the
most acidic C1H and C8H protons of Np
+
. For n¼2, the
formation of a H-bonded solvent network is more favorable
than interior ion solvation, even though the cation–dipole
interaction in Np
+
-W (D
0
¼2800 300 cm
1
)
72
is nearly three
times stronger than the neutral W–W H-bond bond (D
0
¼1105
10 cm
1
).
105
This at rst glance surprising result is attributed
to the massive effects of the nonadditive three-body induction
forces. For the formation of the H-bonded solvent network,
these are highly cooperative, while for interior ion solvation
they are slightly noncooperative. For Np
+
-W
2
, these can be
quantied as +42% for the H-bonded Np
+
-W
2
(18) global
minimum and 3% for the Np
+
-W
2
(18/45) local minimum,
respectively. For Np
+
-W
2
, the cooperative effects involved in H-
bonded networks eventually override the large difference
between the Np
+
-W and W–W binding energies. Thus, the
detected Np
+
-W
2
clusters are mostly attributed to the Np
+
-
W
2
(18) global minimum, in which a W
2
dimer is attached to the
acidic C1H and C8H protons via a planar bifurcated CH/O
ionic H-bond (D
0
¼5529 cm
1
). A minor part of the population
(<20%) is tentatively assigned to the less stable Np
+
-W
2
(18/45)
isomer, in which two individual W ligands separately bind to
Np
+
via equivalent planar bifurcated CH/O H-bonds (D
0
¼
5395 cm
1
). For n$3, isomers with interior ion solvation are
not observed anymore, and only the more stable Np
+
-W
n
structures with cyclic H-bonded W
n
rings are detected. However,
while for n¼3 these rings bind in the Np
+
plane to the acidic
C–H protons of Np
+
via two linear CH/O H-bonds, for n¼4
and 5 the cyclic W
n
rings are located above the aromatic plane
and are connected to Np
+
via strongly nonplanar bifurcated
CH/O ionic H-bonds. In such sandwich-type structures, the W
ligands are closer to the positive charge of the Np
+
cation, and
thus can maximize the strong electrostatic charge–dipole and
inductive charge-induced dipole interactions. The gain in elec-
trostatic and induction energy is larger than the energy penalty
arising from the deviation from linearity and planarity of the
CH/O ionic H-bonding. Some additional stabilization may
also come from increased dispersion between the cyclic W
n
ring
and the p-electron system of the aromatic Np
+
ring in the
sandwich-type conguration. Thus, the Np
+
-W
n
structures with
n¼4 and 5 strongly benet from the cooperative induction
forces arising from the cyclic H-bonded ring, and the sandwich
conguration maximizes both dispersion and electrostatic
forces. In addition, the W
n
rings in Np
+
-W
n
are all more or less
distorted structures derived from those of isolated neutral cyclic
W
n
rings (Fig. S6 in ESI†). Although all of them have only single-
donor single-acceptor solvent molecules in the most stable Np
+
-
W
n
isomers, the free dangling OH groups are all pointing away
from the Np
+
cation to optimize the cation–dipole attraction.
When considering the predicted Np
+
-W
n
global minima, the
incremental dissociation energies, DD
0
(n)¼D
0
(n)D
0
(n1),
are 2773, 2756, 2826, 4202, and 3370 cm
1
for n¼1–5,
respectively. Although there is no monotonic trend in the whole
considered size range, DD
0
increases for small nand peaks at n
¼4, illustrating the strong cooperativity effects for the hydra-
tion network above the Np
+
cation. The magnitude of these
incremental binding energies is also in line with the experi-
mental observation of single W loss under the single-photon
IRPD conditions employed in the present work (with hn
IR
#
4000 cm
1
). The structure of the cyclic neutral W
n
clusters are
distorted by the presence of the Np
+
cation, and their IR spec-
troscopic signatures change as well, although their coarse
geometries remain unaffected (Fig. S6 and S7 in ESI†). Also,
their total and incremental binding energies, which vary as
DD
0
(n)¼1108, 2810, 3539, and 2521 cm
1
for n¼2–5,
respectively, are smaller than those of the corresponding Np
+
-
W
n
clusters, illustrating the cooperative effect of the nearby
positive charge on the cyclic H-bonded network. As expected,
the averaged OH/O bond length in bare W
n
clusters decreases
monotonically as 1.947 > 1.903 > 1.764 > 1.733 ˚
A for n¼2–5.
Interestingly, the corresponding variation in Np
+
-W
n
is strongly
nonmonotonic (1.826 < 1.972 > 1.789 > 1.753 ˚
A for n¼2–5). In
addition, for n$3 the H-bond lengths in Np
+
-W
n
is even larger
than in bare W
n
although the incremental binding energies are
smaller. This effect is attributed to the steric effects of the Np
+
cation on the structure of the H-bonded W
n
network. Interest-
ingly, also the interaction of the W
n
cluster with the CH groups
of Np
+
changes nonmonotonically in the most stable Np
+
-W
n
clusters, with averaged CH/O bond lengths of 2.327 > 2.242 <
2.467 < 2.588 > 2.571 ˚
A. The drop from n¼1ton¼2 is due to
cooperativity, while the increase for n$3 arises from the fact
that the CH/O bond becomes weaker because of the formation
of the sandwich structures with the W
n
rings lying above the Np
+
plane. The destabilization of the CH/O interaction with
increasing cluster size is clearly visible in the decreasing
intensity of n
CH
in the IR spectra (band E in Fig. 2). Interestingly,
the evolution of the microhydration network around Np
+
is
similar to that inferred for the heterocyclic aromatic imidazo-
lium ion, in which the two acidic NH groups of the monocyclic
ve-membered ring are protected by substitution with bulky
alkyl groups.
108
Finally, the solvation of the Np
+
cation by
nonpolar ligands (such as Ar) is preferentially via p-stacking,
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because for these clusters dispersion and polarization forces
dominate the attraction.
48,49,72,109
Signicantly, the cluster growth scenario derived herein
from IRPD spectroscopy and DFT calculations is very different
from the one reported previously from mass spectrometry and
computations.
14
In the latter study, Np
+
-W
n
clusters are gener-
ated by electron ionization of Np and injecting these ions into
a dricell lled with He buffer gas and water vapor with partial
cooling down to 249 K. In the resulting mass spectra, Np
+
-W
n
clusters can be detected up to n¼6, although for cluster sizes
with n$2 the ion signal is barely above the noise level. This is
in marked contrast to our much more efficient electron-impact
supersonic plasma cluster ion source, in which via the same
sequential cluster growth mechanism much higher yields of
Np
+
-W
n
clusters can be generated under cold molecular beam
conditions and at concentrations sufficient to record IRPD
action spectra. The low Np
+
-W
n
yield in the previous study
14
allowed to perform cluster aggregation equilibria to be
measured only for n¼1, and the obtained binding enthalpy of
DH¼2730 350 cm
1
is in good agreement with our recent
spectroscopic value of D
0
¼2800 300 cm
1
.
72
In addition to
mass spectra, the previous study also reported results of DFT
calculations at the B3LYP and M06-2X levels for Np
+
-W
n
clusters
up to n¼6.
14
For n¼1 and 2, the reported minima
14
are in good
agreement with our structures. However, for n$3 there is
a qualitative discrepancy between the previous study
14
and the
most stable structures derived herein from both IR spectroscopy
and DFT computations. No cyclic structures were reported (and
possibly also not considered) previously,
14
and it was claimed
that in the most stable Np
+
-W
n
clusters, a linear H-bonded W
n
chain is attached to Np
+
via CH/O bonding. This is in contrast
to our computations, which clearly show that cyclic structures
are by far more stable than linear ones for n$3, a conclusion
strongly supported by our IRPD spectra. If linear H-bonded
chains were attached to Np
+
at the C1H and C8H protons,
transitions A, B, and E should be detected in the Np
+
-W
n
spectra
with substantial intensity (in disagreement with the experi-
mental observation). Since our electron-impact cluster ion
source is known to produce the most stable isomers of a given
cluster size,
73,74
we are condent that the cyclic structures are
indeed the global minima for Np
+
-W
n
with n$3. The failure of
nding the global minima in the previous study
14
illustrates the
rather limitated capabilities of mass spectrometry and incom-
plete structural survey by computations for nding the most
stable cluster structures. At the same time, it highlights the high
sensitivity of vibrational spectroscopy in the X–H stretch range
for the determination of the H-bonded solvation networks in
polyhydrated clusters.
The ionization energy of Np (IE ¼8.144 eV)
51
is by far lower
than that of W
n
clusters (>10 eV for n#20).
110,111
Consequently,
the excess positive charge is mostly localized on the aromatic
molecule even for large n, in line with the notation Np
+
-W
n
.
There is some partial charge transfer from Np
+
to the W
n
network, which for the most stable Np
+
-W
n
isomers amounts to
Dq¼12, 14, 22, 28, and 32 me for n¼1–5 according to the NBO
analysis (Fig. S3 in ESI†). Interestingly, the magnitude of the
charge transfer for the cyclic solvent structures is slightly larger
than that predicted for linear chain structures (e.g.,Dq¼23 me
for n¼6).
14
Similarly, the proton affinity of the naphthyl radical
(PA ¼234.5 kcal mol
1
)
11
is by far larger than those of W
n
clusters in this size range (PA ¼167–216 kcal mol
1
for n¼1–
6).
112
Thus, no proton transfer to the solvent cluster occurs in
(Np-W
n
)
+
, because C
10
H
8+
-W
n
clusters are much more stable
than C
10
H
7
–H
+
W
n
. Test calculations for (Np-W
n
)
+
clusters with
a protonated H
+
W
n
solvent cluster with n¼4 and n¼5 reveal
that they are more than 1 eV less stable than corresponding Np
+
-
W
4
clusters (Fig. S9 in ESI†).
It is instructive to compare the properties of Np
+
-W
n
clusters
characterized herein with those of neutral
72,75
and anionic
clusters
76–79
to extract the considerable impact of the charge on
the hydration network of PAHs. So far, no experimental data are
available for neutral Np-W
n
clusters, and thus all information
for this complex has to rely on computations carried out for n#
2.
72,75
According to the calculations, the most stable Np–W and
Np–W
2
clusters have pH-bonded structures, in which either
a single W ligand or a H-bonded W
2
forms two intermolecular
OH/pH-bonds with the aromatic pelectron system of Np. In
such pH-bonds, the OH protons can favourably interact with
the negative pelectron cloud.
72,75,113
Because of the lack of the
positive charge, the interaction is much weaker in neutral Np-
W
n
than in the Np
+
-W
n
cation. Ionization of the n¼1 and n¼2
clusters thus causes a drastic change in the potential energy
surface with respect to both the structure and interaction
strength of the aromatic molecule with W
n
from the OH/pH-
bonded neutral clusters to CH/O H-bonded cation clusters.
Such ionization-induced changes in the hydration motif give
rise to solvent rearrangement reactions occurring on the pico-
second timescale.
87,114
These may be probed in the future for Np-
W
n
clusters by time-resolved pump-probe IR spectroscopy,
114,115
a technique recently applied to monitor solvent rearrangement
reactions in related aromatic clusters in real time.
116–120
Although Np
is an unstable anion because of its negative
electron affinity, stable microhydrated Np
-W
n
clusters can be
produced in supersonic expansions and have been character-
ized by photoelectron (n#8)
76–78
and IR (n#6)
79
spectroscopy.
The computational analysis of these IR spectra
79
yields multiple
pH-bonded structures for Np
-W
n
for n#4, in which
H-bonded W
n
clusters are attached to a single side of Np. For
n¼3 and 4, cyclic solvent structures with single-donor single-
acceptor W molecules are most favourable. However, in contrast
to the cation clusters, in Np
-W
n
the H atoms not involved in
the OH/O H-bonded W
n
network point toward the Np
anion
to form OH/pH-bonds. For Np
+
-W
n
cations, this conguration
is repulsive and the free OH groups point away from Np
+
. In all
three charge states, the formation of W
n
networks strongly
benet from cooperativity.
4.2 Comparison to (Bz-W
n
)
+
clusters
Comparison between Np
+
-W
n
and (Bz-W
n
)
+
clusters reveals the
effects of the second aromatic ring present in Np on the inter-
action between PAH
+
cations and W. The geometric and elec-
tronic structure of (Bz-W
n
)
+
clusters are well characterized by IR
(n#23)
61,62,73,80–83
and electronic (n#4)
84
spectroscopy,
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photoionization spectroscopy (n¼1),
85
mass spectrometry (n#
40),
10,12,86
and quantum chemistry.
87,88
A detailed comparison
between the Np
+
-W and Bz
+
-W dimers (n¼1) has been pre-
sented in our previous report.
72
Briey, the structures and H-
bonding motifs in Bz
+
-W and Np
+
-W are principally different.
While in the Np
+
-W(18) global minimum, the W ligands forms
a bifurcated CH/O H-bond to two adjacent C
a
H groups of two
different rings, in the most stable Bz
+
-W(12) structure W is
attached to two CH groups obviously from the same ring.
Because in Bz
+
the excess charge is delocalized over only a single
ring, the electrostatic interaction in Bz
+
-W is stronger than in
Np
+
-W. This view is consistent with the NBO
72
and NCI (Table S1
in ESI†) analyses of the charge distribution in both ions, which
show that the CH protons in Np
+
carry a lower positive partial
charge. This electrostatic effect is partly compensated for by the
more linear H-bonds in Np
+
-W(18) enabled by simultaneous
binding of W to the two fused rings. As a net result, the disso-
ciation energy of Bz
+
-W (D
0
¼3290 120 cm
1
)
83
inferred from
spectroscopic measurements is still substantially larger than
that of Np
+
-W (D
0
¼2800 300 cm
1
),
72
in good agreement with
the values calculated at the B3LYP-D3/aug-cc-pVTZ level (D
0
¼
3209 and 2773 cm
1
) and binding enthalpies extracted from
mass spectrometry (DH¼3150 350 and 2730
350 cm
1
).
12,14
For both aromatic monohydrates, there is a large
geometry change from the neutral cluster (pH-bonded)
113,121–126
to the cation cluster (CH/O H-bonded).
61,73,80,87
Similar to Np
+
-W
2
, also for the Bz
+
-W
2
trimer the isomer with
aW
2
dimer attached to Bz
+
is found to be more stable than
isomers with individual W ligands and both isomers are
observed in the IR spectra.
62,81
Previous computations and IR
spectra indicate that the low-energy isomers of Bz
+
-W
3
have
linear and branched W
3
networks attached to Bz
+
, while cyclic
isomers have not been identied as minima on the poten-
tial.
13,62,81
This is in contrast to Np
+
-W
3
, for which our calcula-
tions indicate that cyclic structures are not only stable but also
yield the global minimum. This difference is rationalized by the
stronger Bz
+
-W bond with larger electrostatic interactions,
which are considerably stronger than the W–W bonds and thus
favor branched and linear structures via directional charge–
dipole forces. In Np
+
-W
n
, the Np
+
-W bond is weaker and less
dominant. Therefore, in Np
+
-W
3
the W–W H-bond interactions
become more competitive and can maximize the number of H-
bonds by forming a cyclic ring.
Starting from n$4, in (Bz-W
n
)
+
clusters one proton from Bz
+
is transferred to the W
n
solvent cluster, i.e. the ground state
structure has the form C
6
H
5
–H
+
W
n
, in which a phenyl radical
binds to the surface of a protonated water cluster. This intra-
cluster ion–molecule reaction is inferred from IR
62,81,82
and
electronic
84
spectroscopy and is consistent with mass spec-
trometry
10,12,13
and calculations.
13,81,88
The proton affinity of W
n
clusters increases with cluster size n(PA ¼691, 808, 862, 900,
904, 908 kJ mol
1
for n¼1–6),
10,103,112
and starting from n¼4it
becomes larger than the one of the phenyl radical
(884 kJ mol
1
).
103
Thus, for (Bz-W
n
)
+
with n$4, the proton-
transferred structure C
6
H
5
–H
+
W
n
is thermodynamically more
stable than the Bz
+
-W
n
form. In contrast, the proton affinity of
the naphthyl radical is rather high (981 kJ mol
1
),
11
so that
proton transfer from Np
+
to W
n
in (Np-W
n
)
+
is thermodynami-
cally not feasible in the cluster size range studied here.
5. Concluding remarks
In summary, microhydrated clusters of Np
+
are characterized by
IR spectroscopy and dispersion-corrected DFT calculations to
probe the stepwise evolution of the structure of the micro-
hydration network around this prototypical PAH
+
cation.
Signicantly, the presented data provide the rst spectroscopic
impression of the Np
+
-W
n
(and thus the PAH
+
-W
n
) interaction
and the stepwise hydration process at the molecular level. The
IR spectra recorded in the C–H and O–H stretch range are highly
sensitive to the solvation network and change substantially with
cluster size. Hence, their computational analysis provides
unprecedented insight into the preferential cluster growth.
Notably, the cold Np
+
-W
n
clusters are produced in a supersonic
plasma beam expansion. Thus, the predominant population
corresponds to the global minima on the respective potential
energy surfaces, which is supported by the analysis of the IR
spectra. The salient results can be summarized as follows.
(1) The most stable Np
+
-W dimer (n¼1) is stabilized by
a strong, planar, and symmetric bifurcated CH/O ionic H-
bond, in which the two lone pairs of W bind to two CH
protons belonging to different rings of the bicyclic Np
+
ion. The
CH/O H-bond in Np
+
-W differs qualitatively in both structure
and interaction type from the corresponding CH/O H-bond in
Bz
+
-W. The H-bond in Bz
+
-W is stronger than in Np
+
-W (3290
120 versus 2800 300 cm
1
), because the positive charge
density is more localized in the monocyclic Bz
+
cation leading to
larger charge–dipole attraction. However, because the bifur-
cated H-bonds in Np
+
-W deviate much less from linearity,
enabled by the simultaneous interaction with two fused
aromatic rings, the molecular orbital interaction stabilizing the
H-bond is stronger than in Bz
+
-W. These qualitative differences
in bonding between Bz
+
-W and Np
+
-W will also hold for larger
PAH
+
-W clusters, because they arise from the presence of fused
rings in Np
+
and other PAH
+
ions. The quantum chemical NBO
and NCI analyses quantitatively explain these differences.
(2) For Np
+
-W
n
clusters with n$2, two types of clusters can
compete, namely those with interior ion solvation and those
with a H-bonded hydration network. The former are slightly less
stabilized by small noncooperative nonadditive induction
forces due to charge delocalization. In contrast, the latter
strongly benet from cooperative nonadditive polarization
forces due to enhanced H-bond strengths in the multiple H-
bonded network. Consequently, Np
+
-W
2
clusters (n¼2) with
a H-bonded W
2
dimer attached to the Np
+
cation via a bifur-
cated CH/O H-bond dominate the population (>80%), because
they are more stable than structures in which two individual W
ligands bind separately to Np
+
. The degree of cooperativity and
noncooperativity can be quantied by considering the binding
energies, the vibrational frequency shis upon H-bonding, and
the NCI and NBO analyses (charge transfer and molecular
orbital interactions). Concerning the binding energy, the
cooperativity is quantied as +42% for the most stable Np
+
-W
2
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isomer with a W
2
dimer, while the noncooperativity is 3% for
the most stable Np
+
-W
2
isomer with two single W ligands.
(3) Np
+
-W
n
clusters with n$3 prefer a cyclic W
n
network
anchored to the Np
+
cation via CH/O ionic H-bonds. In all
these Np
+
-W
n
clusters, the cyclic W
n
network is composed of
single-donor single-acceptor W ligands. In the most stable Np
+
-
W
3
cluster, cyclic W
3
is attached to Np
+
in the aromatic plane via
two single linear CH/O H-bonds, while in Np
+
-W
n
with n¼4
and 5 the cyclic W
n
clusters bind to Np
+
via a single nonplanar
bifurcated CH/O H-bond to form sandwich-like structures.
This subtle change in structural motifs arises from the opti-
mization of the sum of various competing contributions, such
as steric hindrance (repulsion), strengths and number of the
different H-bonds (OH/O, CH/O), charge–dipole (electro-
statics) and charge-induced dipole (polarization) attractions,
and dispersion forces. In the larger n¼4 and 5 sandwich
structures, electrostatic, polarization, and dispersion forces as
well as the OH/O H-bonds of the W
n
subclusters are enhanced,
while the CH/O H-bond is less favored but also less important
for the total interaction energy. For the smaller n¼3 cluster, the
CH/O H-bonds still provide an important contribution.
(4) The presence of the Np
+
cation in Np
+
-W
n
has an
important impact on the structure and IR spectrum of the W
n
unit, although the coarse structure of W
n
remains unaffected
(e.g., monocyclic for n¼3–5). In particular, the free OH groups
of the single-donor single-acceptor cyclic network in Np
+
-W
n
with n¼3–5 all point away from the positive charge.
(5) The cyclic Np
+
-W
n
clusters (n¼3–5) obtained here by IR
spectroscopy and DFT calculations deviate substantially from
the ones with linear W
n
chains predicted recently by mass
spectrometry and DFT calculations,
14
indicating that the former
combined approach is much more sensitive in predicting reli-
able cluster structures.
(6) Comparison of cationic Np
+
-W
n
with neutral Np-W
n
and
anionic Np
-W
n
illustrates the important impact of the charge
state on the PAH
()
-W
n
interaction. While anionic and neutral
Np
()
-W
n
clusters benet from (multiple) OH/pH-bonds and
thus have H-bonded W
n
clusters attached to the aromatic p-
electron system, these congurations are repulsive for cationic
Np
+
-W
n
, because the cation–dipole forces turn the free OH
groups of W
n
away from Np
+
.
(7) Comparison of (Np-W
n
)
+
with (Bz-W
n
)
+
reveals the drastic
differences in intermolecular interaction, microhydration, and
chemical reactivity upon attachment of the second ring. First,
for n#2, the H-bonds in Bz
+
-W
n
are stronger than in Np
+
-W
n
,
because the charge is more localized in the former ion, leading
to increasing electrostatic and polarization forces. Second, for
the same reason, Bz
+
-W
3
prefers a linear/branched W
3
structure
over a cyclic W
3
ring observed in Np
+
-W
3
. Third, (Bz-W
n
)
+
clus-
ters with n$4 exhibit proton transfer to solvent, because the
proton affinity of W
n$4
exceeds the one of the phenyl radical. No
such intracluster proton transfer is observed for (Np-W
n
)
+
because of the substantially higher proton affinity of the
naphthyl radical. The same will be true for (PAH-W
n
)
+
clusters
with larger PAH molecules because the proton affinity of their
radicals will be even higher.
In future work, this combined spectroscopic and computa-
tional strategy may be employed to extend these cluster studies
to larger degree of hydration, larger PAH
+
cations, different
solvents (polar/nonpolar, protic/nonprotic), and protonated
PAH. Exploration of larger (PAH-W
n
)
+
cations eventually
converges to the limit of PAH
+
cations embedded in ice or
located on ice surfaces, which are particularly relevant for
astrochemical applications. Because the charge remains on Np
+
even in large polyhydrated clusters, the highly reactive Np
+
radical cation generated in ice grains or on their surfaces by
radiation or particle impact is readily available for ion–molecule
reactions involving other organic molecules deposited in and/or
on the grains.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by Deutsche Forschungsgemeinscha
(DFG, DO 729/3).
References
1 E. A. Meyer, R. K. Castellano and F. Diederich, Angew.
Chem., Int. Ed., 2003, 42, 1210–1250.
2 L. M. Salonen, M. Ellermann and F. Diederich, Angew.
Chem., Int. Ed., 2011, 50, 4808–4842.
3 J. P. Schermann, Spectroscopy and Modelling of Biomolecular
Building Blocks, Elsevier, Amsterdam, 2008.
4 P. Hobza and K. M¨
uller-Dethlefs, Non-covalent Interactions,
The Royal Society of Chemistry, Cambridge, 2009.
5 P. Weilm¨
unster, A. Keller and K. H. Homann, Combust.
Flame, 1999, 116,62–83.
6 S. Steenken, C. J. Warren and B. C. Gilbert, J. Chem. Soc.,
Perkin Trans. 2, 1990, 335–342.
7 M. Mons, I. Dimicoli and F. Piuzzi, Int. Rev. Phys. Chem.,
2002, 21, 101–135.
8 B. Brutschy, Chem. Rev., 1992, 92, 1567–1587.
9 H. Mohan and J. P. Mittal, J. Phys. Chem. A, 1999, 103, 379–
383.
10 A. Courty, M. Mons, J. Le Calve, F. Piuzzi and I. Dimicoli, J.
Phys. Chem. A, 1997, 101, 1445–1450.
11 W. Y. Feng and C. Lifshitz, Int. J. Mass Spectrom. Ion Process.,
1996, 152, 157–168.
12 Y. Ibrahim, E. Alsharaeh, K. Dias, M. Meot-Ner and M. S. El-
Shall, J. Am. Chem. Soc., 2004, 126, 12766–12767.
13 Y. M. Ibrahim, M. Meot-Ner, E. H. Alshraeh, M. S. El-Shall
and S. Scheiner, J. Am. Chem. Soc., 2005, 127, 7053–7064.
14 I. K. Attah, S. P. Platt, M. Meot-Ner, M. S. El-Shall, S. G. Aziz
and A. O. Alyoubi, Chem. Phys. Lett., 2014, 613,45–53.
15 L. J. Allamandola, M. P. Bernstein, S. A. Sandford and
R. L. Walker, Space Sci. Rev., 1999, 90, 219–232.
16 T. Henning and F. Salama, Science, 1998, 282, 2204–2210.
17 K. Sellgren, Spectrochim. Acta, Part A, 2001, 57, 627–642.
18 M. S. Gudipati, J. Phys. Chem. A, 2004, 108, 4412–4419.
2316 |Chem. Sci.,2018,9, 2301–2318 This journal is © The Royal Society of Chemistry 2018
Chemical Science Edge Article
Open Access Article. Published on 24 January 2018. Downloaded on 4/7/2021 12:27:25 PM.
This article is licensed under a
Creative Commons Attribution-NonCommercial 3.0 Unported Licence.
View Article Online
19 M. P. Bernstein, S. A. Sandford, A. L. Mattioda and
L. J. Allamandola, Astrophys. J., 2007, 664, 1264–1272.
20 M. P. Bernstein, S. A. Sandford, L. J. Allamandola,
J. S. Gillette, S. J. Clemett and R. N. Zare, Science, 1999,
283, 1135–1138.
21 S. G. Murthy and J. A. Louis, Astrophys. J., Lett., 2003, 596,
L195–L198.
22 A. M. Cook, A. Ricca, A. L. Mattioda, J. Bouwman, J. Roser,
H. Linnartz, J. Bregman and L. J. Allamandola, Astrophys. J.,
2015, 799, 14.
23 J. Krausko, J. K'Ekuboni Malongwe, G. Bicanova, P. Klan,
D. Nachtigallova and D. Heger, J. Phys. Chem. A, 2015,
119, 8565–8578.
24 A. Simon, J. A. Noble, G. Rouaut, A. Moudens, C. Aupetit,
C. Iner and J. Mascetti, Phys. Chem. Chem. Phys., 2017,
19, 8516–8529.
25 J. A. Noble, C. Jouvet, C. Aupetit, A. Moudens and
J. Mascetti, Astron. Astrophys., 2017, 599, A124.
26 A. L. F. de Barros, A. L. Mattioda, A. Ricca, G. A. Cruz-Diaz
and L. J. Allamandola, Astrophys. J., 2017, 848, 112.
27 P. Ball, Chem. Rev., 2008, 108,74–108.
28 G. A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological
Systems, Springer, Heidelberg, 1991.
29 D. P. Zhong, S. K. Pal and A. H. Zewail, Chem. Phys. Lett.,
2011, 503,1–11.
30 P. Hobza and R. Zahradnik, Intermolecular Complexes: The
Role of van der Waals Systems in Physical Chemistry and in
the Biodisciplines, Elsevier, Amsterdam, 1988.
31 P. Hobza and Z. Havlas, Chem. Rev., 2000, 100, 4253–4264.
32 D. A. Dougherty, Science, 1996, 271, 163–168.
33 L. J. Allamandola, A. G. G. M. Tielens and J. R. Barker,
Astrophys. J., 1985, 290, L25–L28.
34 A. Leger and L. d'Hendecourt, Astron. Astrophys., 1985, 146,
81–85.
35 A. Leger and J. L. Puget, Astron. Astrophys., 1984, 137,L5–L8.
36 G. H. Herbig, Annu. Rev. Astron. Astrophys., 1995, 33,19–73.
37 A. G. G. M. Tielens, Annu. Rev. Astron. Astrophys., 2008, 46,
289–337.
38 T. Snow, L. V. Page, Y. Keheyan and V. M. Bierbaum, Nature,
1998, 391, 259–260.
39 F. Salama and L. J. Allamandola, Astrophys. J., 1992, 395,
301–306.
40 S. Iglesias-Groth, A. Manchado, D. A. Garc´
ıa-Hern´
andez,
J. I. G. Hern´
andez and D. L. Lambert, Astrophys. J., Lett.,
2008, 685, L55–L58.
41 T. P. Snow, Astrophys. J., 1992, 401, 775–777.
42 F. Salama and L. J. Allamandola, J. Chem. Phys., 1991, 94,
6964–6977.
43 D. M. Hudgins, S. A. Sandford and L. J. Allamandola, J. Phys.
Chem., 1994, 98, 4243–4253.
44 D. Romanini, L. Biennier, F. Salama, A. Kachanov,
L. J. Allamandola and F. Stoeckel, Chem. Phys. Lett., 1999,
303, 165–170.
45 L. Biennier, F. Salama, L. J. Allamandola and J. J. Scherer, J.
Chem. Phys., 2003, 118, 7863–7872.
46 L. Biennier, F. Salama, M. Gupta and A. O'Keefe, Chem.
Phys. Lett., 2004, 387, 287–294.
47 O. Sukhorukov, A. Staicu, E. Diegel, G. Rouill´
e, T. Henning
and F. Huisken, Chem. Phys. Lett., 2004, 386, 259–264.
48 H. Piest, G. von Helden and G. Meijer, Astrophys. J., Lett.,
1999, 520, L75–L78.
49 T. Pino, N. Boudin and P. Brechignac, J. Chem. Phys., 1999,
111, 7337–7347.
50 J. Oomens, A. J. A. van Roij, G. Meijer and G. von Helden,
Astrophys. J., 2000, 542, 404–410.
51 M. C. R. Cockett, H. Ozeki, K. Okuyama and K. Kimura, J.
Chem. Phys., 1993, 98, 7763–7772.
52 U. J. Lorenz, N. Solca, J. Lemaire, P. Maitre and O. Dopfer,
Angew. Chem., Int. Ed., 2007, 46, 6714–6716.
53 H. Knorke, J. Langer, J. Oomens and O. Dopfer, Astrophys. J.,
Lett., 2009, 706, L66–L70.
54 A. M. Ricks, G. E. Douberly and M. A. Duncan, Astrophys. J.,
2009, 702, 301–306.
55 I. Alata, R. Omidyan, M. Broquier, C. Dedonder, O. Dopfer
and C. Jouvet, Phys. Chem. Chem. Phys., 2010, 12, 14456–
14458.
56 I. Alata, M. Broquier, C. Dedonder-Lardeux, C. Jouvet,
M. Kim, W. Y. Sohn, S. Kim, H. Kang, M. Sch¨
utz, A. Patzer
and O. Dopfer, J. Chem. Phys., 2011, 134, 074307.
57 A. Patzer, M. Sch¨
utz, C. Jouvet and O. Dopfer, J. Phys. Chem.
A, 2013, 117, 9785–9793.
58 O. Krechkivska, Y. Liu, K. L. K. Lee, K. Nauta, S. H. Kable
and T. W. Schmidt, J. Phys. Chem. Lett., 2013, 4, 3728–3732.
59 M. P. Bernstein, J. P. Dworkin, S. A. Sandford and
L. J. Allamandola, Meteorit. Planet. Sci., 2001, 36, 351–358.
60 M. P. Bernstein, S. A. Sandford and L. J. Allamandola,
Astrophys. J., Lett., 1996, 472, L127–L130.
61 N. Solc`
a and O. Dopfer, Chem. Phys. Lett., 2001, 347,59–64.
62 N. Solc`
a and O. Dopfer, J. Phys. Chem. A, 2003, 107, 4046–
4055.
63 M. Schmies, M. Miyazaki, M. Fujii and O. Dopfer, J. Chem.
Phys., 2014, 141, 214301.
64 J. Klyne, M. Schmies, M. Fujii and O. Dopfer, J. Phys. Chem.
B, 2015, 119, 1388–1406.
65 J. Klyne, M. Schmies, M. Miyazaki, M. Fujii and O. Dopfer,
Phys. Chem. Chem. Phys., 2018, DOI: 10.1039/
c1037cp04659f, in press.
66 H. S. Andrei, N. Solca and O. Dopfer, J. Phys. Chem. A, 2005,
109, 3598–3607.
67 M. Sch¨
utz, Y. Matsumoto, A. Bouchet, M. ¨
Ozt¨
urk and
O. Dopfer, Phys. Chem. Chem. Phys., 2017, 19, 3970–3986.
68 A. Bouchet, M. Sch¨
utz and O. Dopfer, ChemPhysChem, 2016,
17, 232–243.
69 O. Dopfer, A. Patzer, S. Chakraborty, I. Alata, R. Omidyan,
M. Broquier, C. Dedonder and C. Jouvet, J. Chem. Phys.,
2014, 140, 124314.
70 M. Sch¨
utz, K. Sakota, R. Moritz, M. Schmies, T. Ikeda,
H. Sekiya and O. Dopfer, J. Phys. Chem. A, 2015, 119,
10035–10051.
71 J. Klyne, M. Miyazaki, M. Fujii and O. Dopfer, Phys. Chem.
Chem. Phys., 2018, DOI: 10.1039/c1037cp06132c, in press.
72 K. Chatterjee and O. Dopfer, Phys. Chem. Chem. Phys., 2017,
19, 32262–32271.
73 O. Dopfer, Z. Phys. Chem., 2005, 219, 125–168.
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74 N. Solc`
aandO.Dopfer,J. Phys. Chem. A, 2001, 105, 5637–5645.
75 E. M. Cabaleiro-Lago, J. Rodr´
ıguez-Otero and ´
A. Pe˜
na-
Gallego, J. Phys. Chem. A, 2008, 112, 6344–6350.
76 J. Schiedt, W. J. Knott, K. Le Barbu, E. W. Schlag and
R. Weinkauf, J. Chem. Phys., 2000, 113, 9470–9478.
77 S. A. Lyapustina, S. Xu, J. N. Nilles and K. H. Bowen Jr, J.
Chem. Phys., 2000, 112, 6643–6648.
78 S. H. Lee, J. H. Kim, I. Chu and J. K. Song, Phys. Chem. Chem.
Phys., 2009, 11, 9468–9473.
79 B. J. Knurr, C. L. Adams and J. M. Weber, J. Chem. Phys.,
2012, 137, 104303.
80 M. Miyazaki, A. Fujii, T. Ebata and N. Mikami, Chem. Phys.
Lett., 2001, 349, 431–436.
81 M. Miyazaki, A. Fujii, T. Ebata and N. Mikami, Phys. Chem.
Chem. Phys., 2003, 5, 1137–1148.
82 M. Miyazaki, A. Fujii, T. Ebata and N. Mikami, J. Phys. Chem.
A, 2004, 108, 10656–10660.
83 M. Miyazaki, A. Fujii and N. Mikami, J. Phys. Chem. A, 2004,
108, 8269–8272.
84 M. Miyazaki, A. Fujii, T. Ebata and N. Mikami, Chem. Phys.
Lett., 2004, 399, 412–416.
85 B.-M. Cheng, J. R. Grover and E. A. Walters, Chem. Phys.
Lett., 1995, 232, 364–369.
86 Y. Ibrahim, E. Alsharaeh, M. Rusyniak, S. Watson,
M. M. N. Mautner and M. S. El-Shall, Chem. Phys. Lett.,
2003, 380,21–28.
87 H. Tachikawa and M. Igarashi, J. Phys. Chem. A, 1998, 102,
8648–8656.
88 M. Shimizu, E. Yamashita, M. Mitani and Y. Yoshioka,
Chem. Phys. Lett., 2006, 432,22–26.
89 O. Dopfer, Int. Rev. Phys. Chem., 2003, 22, 437–495.
90 M. J. Frisch, et al.,GAUSSIAN09, D.01, Gaussian, Inc.,
Wallingford, CT, 2009.
91 J. Klyne and O. Dopfer, J. Mol. Spectrosc., 2017, 337,124–136.
92 E. D. Glendening, J. K. Badenhoop, A. E. Reed,
J. E. Carpenter, J. A. Bohmann, C. M. Morales,
C. R. Landis and F. Weinhold, NBO 6.0, Theoretical
Chemistry, University of Wisconsin, Madison, 2013.
93 A. E. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988,
88, 899–926.
94 E. R. Johnson, S. Keinan, P. Mori-Sanchez, J. Contreras-
Garcia, A. J. Cohen and W. T. Yang, J. Am. Chem. Soc.,
2010, 132, 6498–6506.
95 J. Contreras-Garcia, E. R. Johnson, S. Keinan, R. Chaudret,
J. P. Piquemal, D. N. Beratan and W. T. Yang, J. Chem. Theor.
Comput., 2011, 7, 625–632.
96 A. G. Csaszar, G. Czako, T. Furtenbacher, J. Tennyson,
V. Szalay, S. V. Shirin, N. F. Zobov and O. L. Polyansky, J.
Chem. Phys., 2005, 122, 214305.
97 G. Herzberg, Molecular Spectra and Molecular Structure. II.
Infrared and Raman Spectra of Polyatomic Molecules,
Krieger Publishing Company, Malabar, Florida, 1991.
98 S. N. Ketkar and M. Fink, J. Mol. Struct., 1981, 77, 139–147.
99 V. I. Ponomarev, O. S. Filipenko and L. O. Atovmyan,
Kristallograya, 1976, 21, 392–394.
100 K. B. Hewett, M. H. Shen, C. L. Brummel and L. A. Philips, J.
Chem. Phys., 1994, 100, 4077–4086.
101 J. Szczepanski, D. Roser, W. Personette, M. Eyring,
R. Pellow and M. Vala, J. Phys. Chem., 1992, 96, 7876–7881.
102 A. W. Potts and W. C. Price, Proc. R. Soc. London, Ser. A,
1972, 326, 181–197.
103 P. J. Linstrom and W. G. Mallard, NIST Chemistry WebBook,
NIST Standards and Technology, Gaithersburg MD, 20899,
2017, http://webbook.nist.gov.
104 D. J. Goebbert and P. G. Wenthold, Eur. J. Mass Spectrom.,
2004, 10, 837–845.
105 B. E. Rocher-Casterline, L. C. Ch'ng, A. K. Mollner and
H. Reisler, J. Chem. Phys., 2011, 134, 211101.
106 L. C. Ch'ng, A. K. Samanta, Y. Wang, J. M. Bowman and
H. Reisler, J. Phys. Chem. A, 2013, 117, 7207–7216.
107 F. Huisken, M. Kaloudis and A. Kulcke, J. Chem. Phys., 1996,
104,17–25.
108 J. M. Voss, B. M. Marsh, J. Zhou and E. Garand, Phys. Chem.
Chem. Phys., 2016, 18, 18905–18913.
109 T. Vondr´
ak, S. Sato and K. Kimura, Chem. Phys. Lett., 1996,
261, 481–485.
110 S. Tomoda and K. Kimura, Chem. Phys. Lett., 1983, 102,
560–564.
111 L. Belau, K. R. Wilson, S. R. Leone and M. Ahmed, J. Phys.
Chem. A, 2007, 111, 10075–10083.
112 R. Knochenmuss, O. Cheshnovsky and S. Leutwyler, Chem.
Phys. Lett., 1988, 144, 317–323.
113 S. Suzuki, P. G. Green, R. E. Bumgarner, S. Dasgupta,
W.A.GoddardIIIandG.A.Blake,Science, 1992, 257, 942–945.
114 O. Dopfer and M. Fujii, Chem. Rev., 2016, 116, 5432–5463.
115 M. Fujii and O. Dopfer, Int. Rev. Phys. Chem., 2012, 31, 131–
173.
116 K. Tanabe, M. Miyazaki, M. Schmies, A. Patzer, M. Sch¨
utz,
H. Sekiya, M. Sakai, O. Dopfer and M. Fujii, Angew.
Chem., Int. Ed., 2012, 51, 6604–6607.
117 M. Wohlgemuth, M. Miyazaki, M. Weiler, M. Sakai,
O. Dopfer, M. Fujii and R. Mitric, Angew. Chem., Int. Ed.,
2014, 53, 14601–14604.
118 M. Miyazaki, T. Nakamura, M. Wohlgemuth, R. Mitric,
O. Dopfer and M. Fujii, Phys. Chem. Chem. Phys., 2015,
17, 29969–29977.
119 M. Wohlgemuth, M. Miyazaki, K. Tsukada, M. Weiler,
O. Dopfer, M. Fujii and R. Mitric, Phys. Chem. Chem.
Phys., 2017, 19, 22564–22572.
120 M. Miyazaki, A. Naito, T. Ikeda, J. Klyne, K. Sakota,
H. Sekiya, O. Dopfer and M. Fujii, Phys. Chem. Chem.
Phys.,2018, DOI: 10.1039/c1037cp06127g, in press.
121 H. S. Gutowski, T. Emilsson and E. Arunan, J. Chem. Phys.,
1993, 99, 4883–4893.
122 R. N. Pribble and T. S. Zwier, Science, 1994, 265,75–79.
123 T. S. Zwier, Annu. Rev. Phys. Chem., 1996, 47, 205–241.
124 D. Feller, J. Phys. Chem. A, 1999, 103, 7558–7561.
125 B. Brutschy, Chem. Rev., 2000, 100, 3891–3920.
126 K. S. Kim, P. Tarakeshwar and J. Y. Lee, Chem. Rev., 2000,
100, 4145–4185.
127 Z. S. Huang and R. E. Miller, J. Chem. Phys., 1989, 91, 6613–
6631.
128 K. Kuyanov-Prozument, M. Y. Choi and A. F. Vilesov, J.
Chem. Phys., 2010, 132, 014304.
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