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2020, 22,28123
Microhydration of substituted diamondoid radical
cations of biological relevance: infrared spectra of
amantadine
+
-(H
2
O)
n= 1–3
clusters†
Martin Andreas Robert George, Friedrich Buttenberg, Marko Fo
¨rstel and
Otto Dopfer *
Hydration of biomolecules and pharmaceutical compounds has a strong impact on their structure,
reactivity, and function. Herein, we explore the microhydration structure around the radical cation of the
widespread pharmaceutical drug amantadine (C
16
H
15
NH
2
, Ama) by infrared photodissociation (IRPD)
spectroscopy of mass-selected Ama
+
W
n=1–3
clusters (W = H
2
O) recorded in the NH, CH, and OH stretch
range of the cation ground electronic state. Analysis of the size-dependent frequency shifts by dispersion-
corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about
the acidity of the protons of the NH
2
group of Ama
+
and the structure and strength of the NHOand
OHO hydrogen bonds (H-bonds) of the hydration network. The preferred sequential cluster growth
begins with hydration of the two acidic NH protons of the NH
2
group (n= 1–2) and continues with an
extension of the H-bonded hydration network by forming an OHO H-bond of the third W to one ligand
in the first hydration subshell (n= 3), like in the W
2
dimer. For n= 2, a minor population corresponds to
Ama
+
W
2
structures with a W
2
unit attached to Ama
+
via aNHW
2
H-bond. Although the N–H proton
donor bonds are progressively destabilized by gradual microhydration, no proton transfer to the W
n
sol-
vent cluster is observed in the investigated size range (nr3). Besides the microhydration structure, we
also obtain a first impression of the structure and IR spectrum of bare Ama
+
, as well as the effects of
both ionization and hydration on the structure of the adamantyl cage. Comparison of Ama
+
with alipha-
tic and aromatic primary amine radical cations reveals differences in the acidity of the NH
2
group and
the resulting interaction with W caused by substitution of the cycloalkyl cage.
1. Introduction
Amantadine (Ama, 1-tricyclo[3.3.1.1
3,7
]decylamine, 1-adamantyl-
amine, 1-aminoadamantane, C
16
H
15
NH
2
) is the amino derivative
of adamantane (C
10
H
16
,Ad),whichistheparentmoleculeofthe
diamondoids (also called polymantanes),
1,2
a fundamental class
of saturated sp
3
-hybridized hydrocarbon molecules. These rigid
and stress-free cycloalkanes are nanometer-sized H-passivated
nanodiamonds, which are thermodynamically and chemically
highly stable and perfectly size-selectable.
3–5
Diamondoids are
therefore an important class of hydrocarbon molecules, with a
variety of applications in materials and polymer sciences, mole-
cular electronics, astrochemistry, chemical synthesis, and medical
sciences.
6–14
Concerning pharmaceutical applications, Ama is
one of the best known commercially available diamondoid,
because it is successfully marketed as an antiviral and anti-
parkinson drug under the brand names Symmetrel
s
(AmaHCl),
Gocovri
s
,Symadine
s
, and Osmolex ER
s
.
15–18
The antiviral
properties result from the ability of the drug to prevent the virus
from entering the host cell by blocking the ion channel. The
drug interferes with a viral protein, M2 (an ion channel), which is
needed for the virus to become ‘‘uncoated’’ as soon as it is
absorbed into the cell by endocytosis.
19
However, its use for
influenza is no longer recommended due to drug resistance.
20,21
The mechanism of its antiparkinsonian effect is not yet fully
understood at the molecular level, but the drug increases dopamine
release from the nerve endings of brain cells, together with a
stimulation of the norepinephrine response.
22–24
Moreover, it also
has NMDA receptor antagonistic effects.
25,26
Ama is also under
discussion for the treatment of fatigue in multiple sclerosis,
depression, and cocaine dependence.
27–29
The methylated deriva-
tive, memantine or Namenda
s
, was amongst the top 100 sold
drugs worldwide, with sales of more than 10
9
US$ in the year
2013.
30
Herein, we study microhydrated clusters of the Ama
+
cation,
Ama
+
W
n
(W=water=H
2
O), to characterize the intermolecular
Institut fu
¨r Optik und Atomare Physik, Technische Universita
¨t Berlin,
Hardenbergstr. 36, 10623 Berlin, Germany. E-mail: dopfer@physik.tu-berlin.de
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cp05299j
Received 8th October 2020,
Accepted 18th November 2020
DOI: 10.1039/d0cp05299j
rsc.li/pccp
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interaction of this biomolecular cation with W at the molecular
level. In addition to its pharmaceutical importance, Ama and
particularly its radical cation is also of relevance in the context
of astrochemistry. In particular, due to their high stability,
diamondoids are postulated to be highly abundant in the inter-
stellar medium.
31–36
The large fraction of primary amines
detected in interstellar space suggests a significant abundance
of Ama and its radical cation, whereby the latter is produced by
ionizing radiation. Microhydrated clusters of Ama
+
are relevant to
investigate the chemical reactivity of diamondoid radical cations
in interstellar ice grains, in which ionizing radiation may induce
photochemical reactions of organic molecules leading to mole-
cules with higher complexity (e.g., ethers, alcohols).
37,38
Neutral Ama has a structure with C
s
symmetry and consists
of a primary pyramidal amino group (NH
2
) attached to the
adamantyl backbone (C
10
H
15
), which grants the molecule the
special chemical properties of diamond-like structures.
26,39
The geometric, vibrational, and electronic properties of Ama have
been characterized by infrared (IR), Raman, and electron momen-
tum spectroscopy.
40–42
Moreover, the AmaDNA interaction has
been studied by Raman spectroscopy.
43
In contrast to neutral Ama,
there are no studies on the Ama
+
cation so far, with the notable
exception of low-resolution electron momentum spectra yielding
approximate energies for the electronic states.
41
The latter study
reveals that ionization of Ama into the ground electronic state of
the cation occurs by removal of one electron from the nonbonding
N lone pair of the amino group, with a rough estimation of the
vertical ionization energy of 8.6 eV.
Herein, we analyse IR photodissociation (IRPD) spectra of
size-selected Ama
+
W
n
clusters with n= 1–3 generated in a
molecular beam with the aid of quantum-chemical dispersion-
corrected density functional theory (DFT) calculations at the
B3LYP-D3/cc-pVTZ level, with the major goal of characterizing
the initial steps of the microhydration process of this important
diamondoid cation. This dual experimental and computational
approach has been well established in our laboratory for the
study of microhydrated cations,
38,44–68
and provides direct access
to the interaction potential between the cation and the solvent
molecules. The Ama
+
W
n
cluster system has been chosen for the
following reasons. (1) No spectroscopic studies are available for
both Ama
+
and any of its clusters. Hence, our Ama
+
W
n
data
provide a first impression of the interaction of this biological ion
with neutral solvent molecules at the molecular level. The IRPD
spectra recorded in the informative OH, NH, and CH stretch
range serve as a very sensitive probe of the ion–ligand interaction
potential with respect to both binding site and interaction
strength, as well as the structure of the hydrogen-bonded
(H-bonded) solvent network. In addition, the cluster data also
provide information about the properties of the Ama
+
mono-
mer ion, such as the acidity of its N–H bonds. (2) In previous
work, we characterized the structure and bonding in the bare
adamantane cation (Ad
+
)
35
and its monohydrated cluster
(Ad
+
W).
38
Ionization of Ad with tetrahedral symmetry (T
d
) from
the triply-degenerate bonding t
2
highest occupied molecular
orbital (HOMO), which is delocalized over the Ad cage, results
in a Jahn–Teller distorted cation with C
3v
symmetry, with one
very acidic C–H bond along the C
3
axis.
35
Formation of
the CHO H-bond in the Ad
+
W monohydrate activates this
C–H bond even further. Thus, microhydration of cationic
diamondoids can have a profound impact on their structure,
IR spectrum, and chemical reactivity, and the magnitude of
these effects may differ largely from that observed for related
simple cationic hydrocarbons such as CH
4+
.
38
As ionization of
Ama into the cation ground state occurs by removal of an
electron from the N lone pair and not from the adamantyl
cage, H -NH
2
substitution will have a big impact on the
structure of the diamondoid cation, as well as its interaction
with ligands. (3) Microhydration of aromatic amine radical
cations has been studied before by IRPD spectroscopy and
computations, and examples relevant for the present work
include aniline (AN
+
W
n
)
69,70
and aminobenzonitrile (ABN
+
W
n
).
55
Surprisingly, no experimental data are available for the micro-
hydrated methylamine cation (CH
3
NH
2+
), the most simple aliphatic
amine cation, so that all information relies on recent computa-
tional data for CH
3
NH
2+
W
nr5
.
71
Comparison of Ama
+
with
AN
+
, ABN
+
, and CH
3
NH
2+
reveals the differences of an aromatic,
aliphatic, and cycloaliphatic substituent on the acidity and
intermolecular interaction of the NH protons of primary amine
cations. Cationic NH
2
and NH groups of primary and secondary
amines are known to form strong NHO ionic H-bonds with
W ligands and larger W
n
clusters.
54,55,57,69–73
A particular ques-
tion to be addressed is thus the competition between the
formation of the H-bonded solvent network (which is strongly
favoured by nonadditive cooperative three-body interactions)
and interior cation hydration with individual ligands binding
by simple charge–dipole forces (suffering from noncooperative
three-body forces).
44
A further issue to be addressed is the
question of proton transfer to solvent. As the proton affinity of
W
n
clusters strongly increases with cluster size n(PA = 691, 808,
862, 900, 904, and 908 kJ mol
1
for n= 1–6),
74–78
proton transfer
from the NH group of the cation to the W
n
solvent cluster can
occur above a critical size n
c
.
44,56,67,79
The critical size depends
strongly on the structure of the molecule, functional groups,
the charge and protonation state, and the way of generating the
clusters. For example, for protonated benzaldehyde (C
7
H
7
O
+
W
n
)
proton transfer to the solvent is observed for nZn
c
=3,
56
and for
protonated benzonitrile at n
c
=2,
67
whereas in AN
+
W
n
it occurs
for nZ6orn412, depending on the way the clusters are
generated.
69,70
Calculations for CH
3
NH
2+
W
n
predict no proton
transfer up to n=5.
71
Our analysis of the IRPD spectra of
Ama
+
W
n
provides information on the interaction strength of
the NHO H-bonds, the structure of the microhydration shell
(H-bonded solvent network vs. interior ion solvation), and the
degree of N–H bond activation by hydration.
2. Experimental and
computational techniques
IRPD spectra of Ama
+
W
n
with n= 1–3 are recorded in a tandem
quadrupole mass spectrometer coupled to an electron ioniza-
tion cluster source described elsewhere.
44,80
Briefly, the clusters
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are produced in a pulsed supersonic plasma expansion by
electron and/or chemical ionization of Ama (Sigma Aldrich,
497%) close to the nozzle orifice and subsequent three-body
clustering reactions. The expanding gas mixture is generated by
passing Ar carrier gas (5 bar) through a reservoir containing
solid Ama. To produce hydrated Ama
+
clusters, distilled water
is added into the gas line just before the sample reservoir.
A typical mass spectrum of the ion source in the mass range m/z
115–220 is shown in the ESI†(Fig. S1). The chemical composi-
tion of the mass-selected Ama
+
W
n
clusters is confirmed by
collision-induced dissociation (CID) spectra (Fig. S2, ESI†), which
show the exclusive loss of W ligands. The generated Ama
+
W
n
parent clusters are mass selected by a first quadrupole and
irradiated in an adjacent octopole with a tunable IR laser pulse
(n
IR
) of an optical parametric oscillator pumped by a nanosecond
Q-switched Nd:YAG laser. The IRPD spectra are recorded in the
spectral range of 2700–3800 cm
1
to cover the informative CH,
NH, and OH stretch modes. The IR laser pulses are characterized
by an energy of 2–5 mJ in the considered spectral range, a
repetition rate of 10 Hz, and a bandwidth of 2 cm
1
.Thelaser
frequency is calibrated to better than 1 cm
1
using a wavemeter.
Resonant vibrational excitation of Ama
+
W
n
induces evaporation
of a single W ligand. Resulting Ama
+
W
n1
fragment ions are
selected by a second quadrupole and monitored as a function of
the laser frequency to obtain the IRPD spectra of Ama
+
W
n
.The
ion source is triggered at twice the laser frequency to allow for a
separation of laser-induced Ama
+
W
n1
fragments from those
produced by metastable decay. All IRPD spectra are normalized
for frequency-dependent variations in the photon flux.
Dispersion-corrected DFT calculations are carried out at the
B3LYP-D3/cc-pVTZ level for the Ama, Ama
+
,andWmonomers
and the Ama
+
W
n
clusters to determine their structural, vibra-
tional, and energetic properties.
81
This computational approach
has proven to provide reliable results for the monohydrated
adamantane cation (Ad
+
W) and is an efficient compromise
between computing time and accuracy.
38
In particular, spin
contamination for the open-shell Ama
+
radical cation is negli-
gible at this DFT level, with values of hS
2
i0.75 below 0.005 and
10
4
before and after annihilation, respectively. This is in
contrast to MP2/aug-cc-pVTZ calculations, with a significantly
higher contamination of hS
2
i0.75 = 0.04 and 0.0006 before and
after annihilation, respectively. All coordinates are relaxed in the
search for stationary points, and their nature as minima or
transition states is verified by harmonic frequency analysis.
Relative energies and equilibrium dissociation energies (E
e
,D
e
)
are corrected for harmonic zero-point vibrational energy to yield
E
0
and D
0
values. If not stated otherwise, the binding energies
are computed for dissociation into the relevant Ama
+
W
n1
fragment isomer and W. The total binding energies (D
total
0
)
are calculated with respect to Ama
+
and separated W ligands
(Table S1, ESI†). Free energies (G) are obtained at room
temperature (T= 298.15 K, Table S1, ESI†). Cartesian coordi-
nates and energies of all structures are available in the ESI.†
Harmonic frequencies are scaled by factors of 0.9630 and
0.9491 for OH and NH/CH stretch frequencies, respectively.
These factors are derived from fitting calculated harmonic OH
stretch frequencies of W and CH/NH stretch frequencies of Ama
to available experimental values.
42,82
Such a dual scaling factor
procedure accounts for the somewhat different anharmonicities
of the OH and CH/NH stretch modes. To calculate harmonic
combination bands and overtones, harmonic frequencies below
2000 cm
1
(fingerprint range) are scaled by a factor of 0.9672.
This factor results from fitting the calculated harmonic OH and
NH bending frequencies of W and Ama to experimental
values.
42,82
For Ama
+
W, additional anharmonic calculations are
performed.
81
Computed IR stick spectra are convoluted with
Gaussian line profiles (fwhm = 10 cm
1
). Natural bond orbital
(NBO) analysis is employed to evaluate the charge distribution
and charge transfer in Ama
+
W
n
, as well as the second-order
perturbation energies (E
(2)
) of donor–acceptor orbital interac-
tions involved in the H-bonds.
83,84
3. Experimental results
A typical mass spectrum of the electron ionization source is
shown in Fig. S1 (ESI†). The molecules present in the source
(Ama, W, Ar) form different cluster combinations including the
desired Ama
+
W
n
clusters. The mass spectrum is dominated by
W
n
H
+
, Ama
+
and its fragment ions, and hydrated Ama
+
W
n
clusters. As mentioned above, the CID spectrum of Ama
+
W
n
reveals only the loss of W ligands, which confirms the cluster
composition and excludes unwanted isobaric mass contamina-
tions. While the CID spectra reveal the loss of 1 to nW ligands,
the IRPD process leads to the loss of only a single W ligand,
because the relatively high hydration energies are comparable
to the IR photon energy (D
0
Bhn
IR
).
The IRPD spectra of Ama
+
W
n
with n= 1–3 are compared in
Fig. 1, and the observed transitions are listed in Table 1, along
with their vibrational assignments. The investigated spectral
range (2700–3800 cm
1
) covers free and bound OH stretch
modes of the W ligands (A–D, n
OH
, 3400–3800 cm
1
), free and
bound NH stretch modes of the amino group of Ama
+
(E–H,
n
NH
, 2800–3500), aliphatic CH stretch modes of the Ama
+
cage
(I and J, n
CH
, 2800–3000 cm
1
), and combination and overtone
bands of the OH, NH, and CH
(2)
bend fundamentals (K and L,
2900–3300 cm
1
). The IRPD spectra show a clear dependence
on the cluster size nand thus provide insight into the details of
the hydration motif with respect to both the water binding site
and the structure of the solvation network.
The IRPD spectrum of the Ama
+
W monohydrate contains
clear signatures for the predominant presence of a single H-
bonded isomer, in which W binds as an acceptor in an NHO
ionic H-bond to one of the acidic NH groups of Ama
+
. Such a
structure is expected from the strong charge–dipole forces
between the Ama
+
cation and dipolar W, which dominate the
long-range part of the intermolecular attraction. The two
coupled free OH stretch modes of W appear at 3717 (A, n
3
=
n
OHa
, antisymmetric OH stretch) and 3627 cm
1
(C, n
1
=n
OHs
,
symmetric OH stretch) with a width of about 30 cm
1
. Their
redshifts of 39 and 30 cm
1
from the corresponding transitions
of bare W (n
3
= 3756 cm
1
,n
1
= 3657 cm
1
), as well as the IR
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enhancement of n
1
are typical for cation–W clusters.
44,51,63,64,67
Only a single band appears in the spectral range of the free NH
stretch modes of Ama
+
(3300–3450 cm
1
), which is readily
assigned to the remaining free NH group of the H-bonded
Ama
+
W dimer (E, n
NHf
, 3368 cm
1
). The absence of a second
intense free NH stretch band implies that nearly all detected
Ama
+
W isomers feature a strong NHO ionic H-bond invol-
ving the amino group, which shifts the resulting bound NH
stretch (n
NHb
) down to the spectral range 2800–3100 cm
1
.In
this range, it couples strongly with NH, OH, and CH
(2)
bend
overtones (via anharmonic Fermi resonances) as well as CH
stretch fundamentals (via local mode mixing), giving rise to a
broad vibrational multi-band pattern in the IRPD spectrum. A
similar complex pattern has previously been observed for a
variety of monohydrated cationic dimers with a NHO ionic
H-bond such as formanilide, acetanilide, hydroxyindole, or
pyrrole.
54,57,60–62
As the n
NHb
band is expected to carry the
highest IR oscillator strength, it is assigned to the broad
band H at 2992 cm
1
with a width of 66 cm
1
. Its large redshift
of 376 cm
1
from n
NHf
is a measure of the strength of the
NHO H-bond. The interpretation of the sharper bands I, J, and
K at 2937, 2858, and 3187 cm
1
arelessobvious.Bycomparison
with the Ad
+
W spectrum,
38
bands I and J are assigned to CH
stretch modes, while band K is attributed to a combination or
overtone band.
Most transitions in the IRPD spectrum of Ama
+
W
2
show only
small changes in frequency and intensity compared to the n=1
spectrum, with the major exception that the transitions in the
free NH stretch range disappear almost completely (e.g., E),
while the intense transition H in the bound NH stretch range now
splits into the doublet F and G. Thus, the coarse structure of the
Ama
+
W
2
spectrum demonstrates the predominant presence of an
isomer, in which the two W ligands bind separately to the two NH
groups of Ama
+
leading to the formation of two equivalent
NHO H-bonds. In line with this interpretation, bands A and
C at 3721 and 3642 cm
1
are attributed to n
OHa
and n
OHs
of the
individual uncoupled W ligands. Their redshifts from the transi-
tions of bare W, 35 and 15 cm
1
, are slightly smaller than those
for n= 1, because of the noncooperative (i.e.,anticooperative)
three-body effects typical for interior ion solvation.
44,55
Peaks F
and G at 3137 and 3027 cm
1
are then readily assigned to bound
antisymmetric and symmetric NH stretch modes, n
NHba
and
n
NHbs
, respectively. The centre of the two bands at 3082 cm
1
,
whichisameasureforthestrengthoftheN–Hbonds,is
blueshifted by 90 cm
1
from n
NHb
of n= 1 (H), confirming that
the NHOH-bondsinn= 2 are indeed weaker than in n=1.
Peaks J and I at 2874 and 2949 cm
1
assigned to CH stretch
modes are also slightly blueshifted from the corresponding n=1
transitions, consistent with their mixing with the NH stretch
modes. Band K at 3206 cm
1
also has a somewhat higher
frequency than in n= 1. The weak bands E and D at 3376 and
3413 cm
1
are clear spectral signatures of a less abundant isomer,
in which a H-bonded W
2
dimer is attached to one of the acidic NH
groups of Ama
+
. This isomer has a remaining free NH stretch
mode (n
NHf
)assignedtobandE,andthisbandisslightlyblue-
shifted by 8 cm
1
compared to that of n=1.BandDisattributed
to the bound OH stretch mode of the W
2
dimer corresponding to
the OH donor in the OHOH-bond(n
OHb
). Its frequency is
drastically reduced from the corresponding transition in bare W
2
(3601 cm
1
),
85
becauseofthelargecooperativityoftheH-bonded
network, which arises from the strong polarisation forces induced
bythenearbypositivechargeofAma
+
. The weak intensities of
bands D and E indicate that this isomer is far less abundant than
the isomer in which both NH groups of Ama
+
are solvated by a
single W. As a result, the free uncoupled OH stretch of the proton
donor (n
OHf
) expected near 3700 cm
1
is not visible.
TheappearanceoftheAma
+
W
3
spectrum is consistent with a
predominant isomer, in which one NH group of Ama
+
is solvated
by W and the other one by W
2
. As a result, the bands associated
with the W
2
ligand become very prominent in the OH stretch
range, and the characteristic bound and free OH stretch modes of
the proton donor W molecule are clearly apparent at 3426 (D,
n
OHb
) and 3707 cm
1
(B, n
OHf
), while the free OH stretch modes of
the acceptor occur at 3730 (A, n
OHa
) and 3644 cm
1
(C, n
OHs
). The
n
OHa/s
modes of the single W ligand overlap with bands A and C.
Fig. 1 IRPD spectra of Ama
+
W
n=1–3
in the 2700–3800 cm
1
range
recorded in the W loss channel. The spectral range investigated covers
CH, NH, and OH stretch fundamentals. The positions, widths, and assign-
ments of the transitions observed (A–L) are listed in Table 1.
Table 1 Positions, widths (fwhm in parenthesis) and vibrational assignments
of the transitions observed in the IRPD spectra of Ama
+
W
n=1–3
(Fig. 1)
a
Peak Mode Ama
+
W Ama
+
W
2
Ama
+
W
3
An
OHa
3717 (28) 3721 (24) 3730 (20)
Bn
OHf
3707 (12)
Cn
OHs
3627 (31) 3642 (17) 3644 (21)
Dn
OHb
3413 (30) 3426 (86)
En
NHf
3368 (27) 3376 (28)
Fn
NHba
3137 (68) 3116 (97)
Gn
NHbs
3027 (58) 2944 (45)
Hn
NHb
2992 (66)
In
CH2
2937 (35) 2949 (37) 2944 (45)
Jn
CH
2858 (20) 2874 (29) 2840 (40)
K
b
3187 (20) 3206 (32) 3247 (15)
L
b
2993 (20)
a
All values are given in cm
1
.
b
Combination or overtone band.
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The fact that band E disappears completely for n=3impliesthat
at this cluster size both NH bonds of the NH
2
group are fully
solvated, and that isomers with a free N–H bond and one N–H
bond solvated with W
3
are below the detection limit. The bound
NH stretch modes are now quite decoupled, because of asym-
metric solvation of the NH
2
group with W and W
2
.Because
W
2
has a much higher proton affinity than W (808 and
691 kJ mol
1
),
74,75
the NHW
2
H-bond is much stronger than
the NHW H-bond, giving rise to a larger redshift in n
NHb
.Thus,
band F at 3116 cm
1
is assigned to n
NHb
of the NHWmoiety,
while band G at 2944 cm
1
is attributed to n
NHb
of the NHW
2
unit. Band J at 2840 cm
1
is broader and redshifted compared to
the n= 2 spectrum, while band I overlaps with band G. There are
weaker bands L and K at 2993 and 3247 cm
1
, whose interpreta-
tion is again less obvious. In summary, the IRPD spectra of
Ama
+
W
n
with n= 1–3 can be interpreted with a predominant
sequential cluster growth, in which the two acidic NH protons of
the amino group of Ama
+
are first solvated by single W ligands via
NHOionicH-bonds(n= 1–2), before further W molecules are
attached to the initial W ligands via the formation of an OHO
bonded network (n= 3). In order to identify more precise
structural motifs, DFT calculations are employed in Section 4.
4. Computational results and
assignments
4.1. Ama, Ama
+
, and W
In an effort to determine the structures responsible for the
IRPD spectra, quantum chemical calculations are performed at
the B3LYP-D3/cc-pVTZ level. The calculated structures of the
Ama, Ama
+
, and W monomers are shown in Fig. 2. The
structure of Ama (C
s
) in its
1
A0ground electronic state consists
of a pyramidal NH
2
group (y
CNX
= 1241with the dummy atom X
located on the C
2
axis of the NH
2
group) attached to an
adamantyl cage (C
10
H
15
), which has slightly different binding
parameters compared to Ad arising from H -NH
2
substitution
(Fig. S3, ESI†).
35,38
Especially the C–C bond adjacent to the NH
2
group in the C
s
plane is significantly longer than in Ad (r
CC
=
1.544 vs. 1.538 Å). The NH
2
group in Ama has similar binding
parameters (y
HNH
= 106.41,r
NH
= 1.015 Å, r
CN
= 1.467 Å) as in
CH
3
NH
2
(y
HNH
= 106.41,r
NH
= 1.013 Å, r
CN
= 1.465 Å, Fig. S4,
ESI†). The calculated structure and IR spectrum of Ama (Fig. 3)
is consistent with previous computational and experimental
data.
41,42,86,87
Concerning W, the O–H bond parameters in its
1
A
1
ground state (r
OH
= 0.961 Å, n
1/3
= 3658/3754 cm
1
) agree
well with the experimental data (0.9578 Å, 3657/3756 cm
1
).
88,89
Ionization of Ama into the ground electronic state of Ama
+
(
2
A0,C
s
) occurs by removal of one electron from the HOMO,
which is largely described by the nonbonding a0lone pair
orbital of N, as illustrated by the orbitals of Ama
(+)
shown in
Fig. S5 (ESI†).
41
As a result, the pyramidal NH
2
group becomes
essentially planar upon ionization (y
CNX
= 170.71, close to the
NH
3
case with y
HNX
= 1801), with the NH
2
plane being perpendi-
cular to the symmetry plane. As a result, the NH
2
angle opens
up from 106 to 1171. Due to the increasing conjugation of the
nitrogen lone pair orbital, the bond order of the C–N bond
increases substantially from 1 to B1.5, as illustrated by the
massive bond contraction of 83 mÅ. Ionization increases the
charge on the NH
2
group by Dq= 474 me, which is substantially
Fig. 2 Calculated equilibrium structures (in Å and degrees) of W, W
2
, Ama, Ama
+
,andAma
+
W(I–III) in their ground electronic state (B3LYP-D3/cc-pVTZ).
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smaller than in the related CH
3
NH
2(+)
molecule, because
of enhanced charge delocalization into the adamantyl cage
(Fig. S6, ESI†). Nonetheless, q
NH2
= 313 meremains on the
NH
2
group in Ama
+
(as compared to 597 mein CH
3
NH
2+
),
making this binding site highly attractive for the W ligands.
Ionization slightly contracts the N–H bonds by 3 mÅ, leading to
significantly higher frequencies (B40/80 cm
1
) for the free
symmetric and antisymmetric NH stretch normal modes
(n
NHs
= 3322 vs. 3285 cm
1
and n
NHa
= 3438 vs. 3358 cm
1
,
Fig. 3 and Table 2). Ionization also increases the positive charge
of the NH protons to q
H
= 405 me, which drastically enhances
the IR intensity of the NH stretch modes (by two orders of
magnitude). Because the charge on the NH protons is much
higher than on all CH protons (206–232 me), the resulting
rather acidic N–H bonds are excellent H-bond donors for
NHO ionic H-bonds with W. Most of the C–C bonds of the
Ama cage remain nearly unchanged. The notable exception is
the C–C bond adjacent to the C–N bond denoted C–C
ext
(which
elongates by 153 mÅ), because it lies in the symmetry plane and
thus can conjugate with the in-plane lone pair of N. The next
adjacent C–C
adj
bond in the C
s
plane shortens by a smaller amount
(16 mÅ) because of the increased distance from the N lone pair,
thus damping the conjugation effects. Ionization increases the
variation of the C–H bond lengths drastically from 1.092–1.095 Å
(Dr
CH
= 3 mÅ) in Ama to 1.087–1.100 Å in Ama
+
(Dr
CH
=13mÅ).As
a result, the single broad and unresolved intense convoluted band
of the coupled n
CH(2)
stretch vibrations in the computed IR
spectrum of Ama splits for Ama
+
into four better resolved and
more widely spread bands, including one intense peak of the
isolated n
CH
mode at 2803 cm
1
, two broader peaks of n
CH/CH2
modes at 2882 and 2914 cm
1
, and one weak peak of the isolated
n
CH2
mode at 2984 cm
1
(Table 2). The isolated low and high
frequency transitions result from the long and short C–H bonds at
the C atoms in the symmetry plane of Ama
+
,wheretheHOMOis
concentrated. The adiabatic computed ionization energy of 7.95 eV
corresponds well with the measured vertical value of 8.6 eV, derived
from low-resolution electron momentum spectra.
41
4.2. Ama
+
W
The Ama
+
cation offers three main attractive binding sites for
dipolar W, which can be attached either to the NH
2
group
(isomer I) or to the side (II) or bottom (III) of the Ama
+
cage
(Fig. 2 and Table S1, ESI†). The computed IR spectra of all
Ama
+
W isomers are compared in Fig. 3 to the experimental
IRPD spectrum and to those calculated for Ama, Ama
+
, and W.
The position and widths of the transitions observed in the IRPD
spectrum of Ama
+
W are listed in Table 2, along with their
vibrational assignments. While W forms in the most stable
isomer Ia strong NHO ionic H-bond to the NH
2
group (E
0
=0),
it is attached in II and III to the Ama
+
cage by substantially
Fig. 3 IRPD spectrum of Ama
+
W compared to linear IR absorption spectra
of W, Ama, Ama
+
,andAma
+
W(I–III) calculated at the B3LYP-D3/cc-pVTZ
level. The positions of the transition observed in the IRPD spectrum of
Ama
+
W and their vibrational assignment are listed in Table 2. Differences in
relative energy (DE
0
) are given in kJ mol
1
for isomers I–III.
Table 2 Computed vibrational frequencies (in cm
1
, B3LYP-D3/cc-pVTZ)
of Ama, Ama
+
, W, and Ama
+
W(I) compared to experimental values of
Ama
+
W (Fig. 3)
a
Mode Ama Ama
+
W Ama
+
W(I) Ama
+
W exp.
n
CH
2845 (13) 2803 (76) 2831 (59) J 2858 (20)
n
CH2
2848 (30) 2870 (4) 2869 (11)
n
CH2
2853 (39) 2874 (16) 2879 (12)
n
CH2
2854 (29) 2882 (16) 2881 (14)
n
CH2
2855 (18) 2884 (20) 2882 (22)
n
CH2
2856 (25) 2887 (10) 2884 (14)
n
CH2
/n
CH
2875 (119) 2910 (5) 2906 (23)
n
CH2
/n
CH
2876 (105) 2911 (28) 2909 (14)
n
CH2
/n
CH
2880 (66) 2912 (1) 2911 (14)
n
CH2
/n
CH
2884 (26) 2913 (10) 2915 (7)
n
CH2
2888 (12) 2922 (6) 2919 (5)
n
CH2
2889 (33) 2922 (11) 2922 (7)
n
CH2
2894 (102) 2927 (4) 2923 (12)
n
CH2
2898 (5) 2932 (27) 2927 (36) I 2937 (35)
n
CH2
2903 (80) 2984 (7) 2973 (7)
n
NHb
3022 (1267) H 2992 (66)
2b
CH2
2960
b
I 2937 (35)
2940 (12)
c
b
NH
+b
OH
3187
b
K3187 (20)
3167 (136)
c
n
NHf
3378 (95) E 3368 (27)
n
NHs
3285 (3) 3322 (223)
n
NHa
3358 (0.2) 3438 (67)
n
OHs
3658 (3) 3650 (51) C 3627 (31)
n
OHa
3754 (40) 3736 (110) A 3717 (28)
a
IR intensities in km mol
1
are given in parentheses. The computa-
tional data for isomers II and III are available in Table S2 (ESI). The
experimental frequencies with width (fwhm in parenthesis) are
assigned to the most dominant vibrations.
b
Harmonic value.
c
Result
of anharmonic calculation.
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weaker charge–dipole forces resulting in much higher relative
energies (E
0
= 27.8 and 30.9 kJ mol
1
). All structures have a
favourable charge–dipole configuration, with the electro-
negative O atom of W pointing toward the Ama
+
cation. Due to
the low ionization energy of Ama and the higher proton affinity
of the amantandinyl radical as compared to W (IE = 7.95 vs.
12.6 eV, PA = 960 vs. 691 kJ mol
1
)
42,74
neither charge nor proton
transfer occurs upon monohydration.
In the Ama
+
W(I) global minimum, W forms a strong and
nearly linear NHO H-bond (170.51) to the NH
2
group, with a
bond length of 1.768 Å and a high binding energy of D
0
=
61.4 kJ mol
1
(Fig. 2). The barrier for internal W rotation around its
C
2
rotational axis is relatively high (V
b
=3.28kJmol
1
or 274 cm
1
)
and the pyramidal angle of the amino group is nearly unchanged
from that in bare Ama
+
(y
CNX
= 170.11). Upon monohydration,
the N–H bond is stretched by 21 mÅ and the NH
2
bond angle
increases by 1.41to 118.21. As a result, its bound NH stretch
frequency shifts down to n
NHb
= 3022 cm
1
along with a drastic
enhancement in IR intensity (by a factor 6), as is typical for
RNH
2+
-W dimers.
55
The redshift of 358 cm
1
from the average
NH stretch frequency of Ama
+
(n
NHav
= 3380 cm
1
) is a direct
measure of the strength of the H-bond. The remaining free N–H
bond does not change its strength (r1 mÅ) resulting in a less
intense n
NHf
mode at 3378 cm
1
, essentially unshifted from
n
NHav
(Dn
NH
=2cm
1
). The NBO charge analysis shows that
formation of the H-bond involves a charge transfer of 38 me
from Ama
+
to W and a second order perturbative energy of E
(2)
=
33.9 kJ mol
1
for the donor–acceptor interaction between the
lone pairs of O and the antibonding s* orbital of the N–H
donor bond. As a result, the O–H bonds of W elongate by 2 mÅ
and the W bond angle increases by 1.21. Further, the free OH
stretch modes n
OHs
and n
OHa
experience redshifts of 8 and
18 cm
1
and a substantial IR intensity enhancement (factor
17 and 3). These effects are typical for the formation of cation–W
clusters.
44
Apart from the NH
2
group, monohydration has also a
certain impact on the structure of the Ama
+
cage. Because
of partial charge transfer from Ama
+
to W, the molecule gets
more neutral again, and as a result the structural effects
upon ionization of Ama are somewhat reduced. For example,
the strongly contracted C–N bond upon ionization is slightly
elongated again by 8 mÅ upon hydration, the C–C
ext
bond
contracts by 39 mÅ, and the C–C
adj
bond elongates by 7 mÅ.
As a result, the low-frequency n
CH
mode is blueshifted by
28 cm
1
to 2831 cm
1
, while the remaining CH and CH
2
stretch
frequencies do not change much (Fig. 3).
There is a large energy gap of B30 kJ mol
1
between the
global minimum Iand the next two local minima II and III. Due
to their high relative energy, they are not observed experimen-
tally and we do not discuss them in detail here. Briefly, W binds
in II and III via CHO contacts to two CH
2
groups of Ama
+
with low binding energies of D
0
= 33.6 and 30.6 kJ mol
1
,
respectively. The Ama
+
W bonds in II and III are essentially
based on electrostatic charge–dipole forces with long intermo-
lecular distances of more than 2.4 Å. Because of the strongly
nonlinear CHO contacts, there is little contribution from
H-bonding. As a result, charge transfer is minor (Dqo9me)
and E
(2)
r2 kJ mol
1
. Due to this weak intermolecular
bonding, complex formation in II and III has much less impact
on the structures of both monomer units than for I. Conse-
quently, the resulting IR spectra are essentially an addition of
those of W and Ama
+
in the considered spectral range, with
characteristic intense free NH stretch bands at B3330 and
B3440 cm
1
(Fig. 3 and Table S2, ESI†).
The comparison of the measured IRPD spectrum of Ama
+
W
with the linear IR spectra computed for isomers I–III in Fig. 3
immediately demonstrates the predominant abundance of the
most stable structure I, because of the observation of only a
single free intense NH stretch band between 3300 and 3450 cm
1
.
Based on the achieved signal-to-noise ratio and the computed
oscillator strengths, the absence of the two bands of the free and
fully coupled intense NH stretch modes of the unperturbed NH
2
group of II and III near B3330 and B3440 cm
1
provides an
estimate of the upper limit of their abundance as B15%. Thus,
the whole observed spectrum is fully attributed to isomer Iwhose
computed spectrum agrees well with the measured one. Specifi-
cally, bands E and H at 3368 and 2992 cm
1
are readily assigned
to the free and bonded NH stretch bands predicted at n
NHf
=
3378 cm
1
and n
NHb
=3022cm
1
. Both frequencies are close to
those found for the related ABN
+
W dimer at 3433 and 3040 cm
1
,
acomplexwithasimilarNHO H-bond between a cationic
amine and W.
55
Moreover, due to the larger redshifts of the free
OH stretch frequencies of isomer I, the observed bands C and A at
3627 and 3717 cm
1
agree better with those predicted for I
(3650 and 3736 cm
1
) than for II and III. Peak J (2858 cm
1
) and
I (2937 cm
1
) are attributed to the n
CH/CH2
modes of the
adamantyl cage, and these agree indeed well with the corres-
ponding bands measured for Ad
+
(2868/2875 and 2954/
2942 cm
1
).
35,38
While J is assigned to the intense n
CH
mode
predicted at 2831 cm
1
, peak I is associated with several over-
lapping CH
2
modes. If only the most intense CH
2
mode
(2927 cm
1
,I= 36 km mol
1
) is taken into account, the
deviation is only 10 cm
1
. The high intensity of peak I cannot
be explained by a fundamental mode of isomer Iand is
probably enhanced from signal of the intense band H (n
NHb
).
Possibly, an overtone of a CH
2
bending mode at 1480 cm
1
(anharmonic: 2b
CH
= 2940 cm
1
) may gain intensity by a Fermi
resonance with the strongly IR active n
NHb
mode at 3022 cm
1
and thus contribute further to the intensity of band I. The
isolated peak K at 3187 cm
1
can also not be explained by a
fundamental transition of isomer Ibut may be associated with
a combination band b
NH
+b
OH
at 3187 cm
1
of the symmetric
(1578 cm
1
) and antisymmetric (1609 cm
1
) coupled OH and
NH bending modes. The anharmonic vibrational calculation of
the IR spectrum of Ama
+
W(I) supports such a scenario and
predicts the combination band b
NH
+b
OH
at 3167 cm
1
with
high intensity (I= 136 km mol
1
).
4.3. Ama
+
W
2
The IRPD spectrum of Ama
+
W
2
detailed in Fig. 1 (Table 1)
shows only very weak activity in the free NH stretch range,
providing a strong indication that both NH donors of the NH
2
group of Ama
+
are hydrated in the predominant isomer. The
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weak band E near 3350 cm
1
(n
NHf
) may be taken as evidence
for minor isomers in which only one of the N–H bonds is
solvated. The absence of any signal in the range of n
NHs
and
n
NHa
near 3350 and 3450 cm
1
suggests that the population of
isomers with a completely free NH
2
group is below the detec-
tion limit. Consequently, we investigated computationally only
isomers in which one or both NH groups are hydrated. Indeed,
based on the Ama
+
W dimer potential, such isomers are
expected to be by far lowest in energy. The structures of the
five isomers found are shown in Fig. 4 and Table S1, ESI.†Their
calculated IR spectra are compared in Fig. 5 to the IRPD
spectrum of Ama
+
W
2
and the IR spectra computed for Ama
+
,
W, and W
2
(Table 3 and Table S3, ESI†).
In the Ama
+
W
2
(I) global minimum structure with C
s
sym-
metry, both N–H bonds of the NH
2
group are symmetrically
monohydrated by a single W ligand in form of nearly linear
NHO ionic H-bonds (170.51). Due to noncooperative effects
typical for internal ion solvation, the binding energy of D
0
=
56.9 kJ mol
1
is slightly lower (by 7.3%) than that of the
corresponding Ama
+
W(I) dimer (D
0
= 61.4 kJ mol
1
). As a result,
the H-bond is slightly longer (1.802 vs. 1.768 Å) and described
by a lower orbital interaction energy (E
(2)
=29.4vs. 33.9 kJ mol
1
)
and smaller charge transfer (34 vs. 38 me, Fig. S7, ESI†).
Consequently, the intramolecular N–H bonds in n= 2 are less
stretched than in n= 1 (1.030 vs. 1.033 Å) and the NH
2
angle
increases further (119.61vs. 118.21). The pyramidal character of
the amino group is not much affected (y
CNX
= 171.11). Due to
symmetric water solvation of both N–H bonds, both NH stretch
frequencies shift down to 3034 (n
NHbs
) and 3139 cm
1
(n
NHba
)
along with a strong increase in IR intensity. Their centre
frequency (3137 cm
1
) is somewhat higher than n
NHb
in n=1
(3022 cm
1
) due to the weaker NHO H-bonds in n= 2. Just as
Fig. 4 Calculated equilibrium structures (in Å and degrees) of Ama
+
W
2
(I–V) in their ground electronic state (B3LYP-D3/cc-pVTZ).
Fig. 5 IRPD spectrum of Ama
+
W
2
compared to linear IR absorption spectra
of Ama
+
,W,W
2,
and Ama
+
W
2
(I–V) calculated at the B3LYP-D3/cc-pVTZ
level. The positions of the transition observed in the IRPD spectrum of
Ama
+
W
2
and their vibrational assignment are listed in Table 3. Differences in
relative energy (DE
0
)aregiveninkJmol
1
for isomers I–V.
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with Ama
+
W(I), the O–H bonds in W are elongated by 2 mÅ and
the W bond angle is increased by 1.21. This effect shifts the n
OHs
and n
OHa
modes to 3653 and 3741 cm
1
along with an increase
in IR intensity. Upon addition of two W molecules to Ama
+
, the
C–N bond becomes further elongated (1.398 Å), while the C–C
ext
bond contracts further (1.635 Å). These two geometry effects are
amplified by the second W ligand.
In the slightly less stable isomers II and III of Ama
+
W
2
(E
0
= 5.4 and 5.7 kJ mol
1
), a H-bonded W
2
dimer is attached to
the NH
2
group via aNHO ionic H-bond. These isomers
correspond to the formation of a H-bonded solvent network,
which is strongly cooperative in nature. The orientation of the
W
2
unit differs slightly in the two isomers, because they are
stabilized by weak secondary CHO contacts involving two
different CH
2
groups adjacent to the CNH
2
moiety (2.579 and
2.626 Å for II and III). Because the proton affinity of W
2
is much
larger than for W (808 vs. 691 kJ mol
1
),
74,75
the NHW
2
H-bonds in II and III are much stronger and shorter than the
NHW H-bonds in isomer Iof n= 1 (and also n=2),withR=
1.685/1.679 (II/III)vs. 1.768 Å, respectively. As a result, charge
transfer to the solvent is increased from 38 to 57/60 me,whileE
(2)
increases from 33.9 to 46.8/51.8 kJ mol
1
. Due to the stronger
H-bond, the bound N–H bond elongates from 1.033 to 1.043/
1.044 Å, which shifts its n
NHb
frequency from 3022 cm
1
further
down to 2870/2853 cm
1
. In case of III, this leads to strong
mode mixing with the CH stretch mode at 2837 cm
1
, which
thus gains substantial IR activity from the NH stretch mode.
The free N–H bond of II and III elongates only slightly by the
addition of the second W ligand (by 1 mÅ), leading to a small
decrease in n
NHf
(by 4 and 9 cm
1
). The presence of the nearby
Ama
+
charge causes, via strongly cooperative three-body polar-
ization forces, the OHO H-bond in the W
2
unit in II and III to
be much stronger and shorter than in bare W
2
(1.799/1.772 Å vs.
1.946 Å). The binding energy of the terminal W ligand of D
0
=
51.6/51.3 kJ mol
1
exceeds by far the one in bare neutral W
2
(19.7 kJ mol
1
) and reaches almost the strength of the NHO
ionic bonds in isomer I(56.9kJmol
1
). The large cooperativity in
total binding energy amounts to 11% for II/III,andmakesthese
isomers energetically competitive with I.TheOHOH-bondin
III is somewhat stronger and more linear (168.1 vs. 163.41)thanin
II, with a larger redshift in the bound OH stretch frequency from
bare W
2
(n
OHb
= 3383/3415 vs. 3557 cm
1
),whichplacesthisband
with high intensity into the vicinityofthefreeNHstretchbandsof
Ama
+
. These frequencies are in line with the bond lengths of the
O–Hprotondonorbond(0.979/0.977vs. 0.969 Å). The corres-
ponding free O–H bond remains largely unchanged and produces
an uncoupled n
OHf
band at 3727/3722 cm
1
,whilethen
OHs
and
n
OHa
bands of the terminal W acceptor ligands are predicted at
3647/3651 and 3735/3740 cm
1
. The C–N bonds (1.394 Å, 1.396 Å)
are shorter and the extended C–C
ext
bonds (1.647 Å, 1.642 Å) are
longer compared to Ama
+
W
2
(I), which however has no big influ-
ence on the CH
2
frequencies.
There is a large energy gap of 23 kJ mol
1
between III and
the higher energy Ama
+
W
2
isomers IV and Vat E
0
= 28.6 and
Table 3 Computed vibrational frequencies (in cm
1
, B3LYP-D3/cc-pVTZ) of Ama
+
,W
2
, and Ama
+
W
2
(I–III) compared to experimental values of Ama
+
W
2
(Fig. 5)
a
Mode Ama
+
W
2
Ama
+
W
2
(I) Ama
+
W
2
(II) Ama
+
W
2
(III) Ama
+
W
2
exp.
n
CH
2803 (76) 2849 (36) 2839 (201) 2837 (896) J 2874 (29)
n
CH2
2870 (4) 2876 (0.4) 2868 (199) 2868 (15) J 2874 (29)
n
CH2
2874 (16) 2876 (17) 2877 (75) 2878 (14)
n
CH2
2882 (16) 2877 (27) 2879 (13) 2879 (33)
n
CH2
2884 (20) 2880 (25) 2881 (16) 2882 (16)
n
CH2
2887 (10) 2881 (9) 2887 (7) 2885 (8)
n
CH2
/n
CH
2910 (5) 2904 (39) 2904 (28) 2903 (27)
n
CH2
/n
CH
2911 (28) 2907 (23) 2909 (10) 2909 (12)
n
CH2
/n
CH
2912 (1) 2909 (1) 2911 (16) 2911 (23)
n
CH2
/n
CH
2913 (10) 2915 (3) 2912 (10) 2912 (7)
n
CH2
2922 (6) 2917 (0.03) 2918 (7) 2917 (10)
n
CH2
2922 (11) 2919 (12) 2919 (7) 2919 (5)
n
CH2
2927 (4) 2921 (24) 2924 (40) 2924 (39)
n
CH2
2932 (27) 2923 (46) 2933 (15) 2935 (8) I 2949 (37)
n
CH2
2984 (7) 2962 (12) 2970 (4) 2967 (10)
2b
CH2
2958 I 2949 (37)
n
NHb
2870 (1094) 2853 (711) J 2874 (29)
n
NHbs
3034 (1065) G 3027 (58)
n
NHba
3139 (901) F 3137 (68)
b
NH
+b
OH
3191 K 3206 (32)
n
NHf
3374 (75) 3369 (51) E 3376 (28)
n
NHs
3322 (223)
n
NHa
3438 (67)
n
OHb
3557 (290) 3415 (494) 3383 (612) D 3413 (30)
n
OHs
3653 (10) 3652 (48) 3647 (33) 3651 (33) C 3642 (17)
n
OHs
3653 (41) C 3642 (17)
n
OHf
3728 (65) 3727 (124) 3722 (111) A 3721 (24)
n
OHa
3747 (68) 3741 (5) 3735 (100) 3740 (100) A 3721 (24)
n
OHa
3741 (201) A 3721 (24)
a
IR intensities in km mol
1
are given in parentheses. The experimental frequencies with width (fwhm in parenthesis) are assigned to the most
dominant vibrations. The computational data for isomers IV and Vare available in Table S3 (ESI).
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31.0 kJ mol
1
, so that we do not discuss them in detail. Briefly,
both isomers have one W attached to the NH
2
group (D
0
= 59.2
and56.8kJmol
1
for IV and V)andonetotheCH
2
groups on the
opposite side of the Ama
+
cage (D
0
= 28.4 and 25.9 kJ mol
1
). They
result essentially by an addition of isomers I+II and I+III of the
n= 1 cluster. Because the two W ligands do not interact much,
their intermolecular bonding and resulting IR spectra in the NH
and OH stretch range are similar to that of isomer Iof n=1.
ComparisonoftheIRPDspectrumofAma
+
W
2
to the calcu-
lated spectra in Fig. 5 demonstrates good agreement with that of
the most stable isomer Iwith respect to both band positions and
intensities. The lack of strong signals in the free NH stretch range
(3300–3500 cm
1
) provides strong evidence that in the predomi-
nantly observed isomer both N–H bonds of Ama
+
are solvated,
which exclusively occurs in I.TheresultingtwoboundNHstretch
bands n
NHbs
and n
NHba
of the two equivalent NHOH-bondsare
assigned to the intense bands G (3027 cm
1
) and F (3137 cm
1
),
with deviations of only 7 and 2 cm
1
from the predicted values.
These transitions are also consistent with n
NHbs
(3073 cm
1
)and
n
NHba
(3195 cm
1
)ofABN
+
W
2
.
55
The bands C and A at 3642 and
3721 cm
1
are then attributed to the free OH stretch modes n
OHs
and n
OHa
with deviations of 10 and 20 cm
1
, which is acceptable
concerning the widths of the transitions (17 and 24 cm
1
). Peak J
(2874 cm
1
) can again be assigned to the lowest CH stretch mode
predicted at 2849 cm
1
, and its observed blueshift of 16 cm
1
by
hydration with the second W is well reproduced by the calculated
shift (18 cm
1
). Band I is again composed of overlapping unre-
solved CH and CH
2
stretching modes. When considering the most
intense mode (2923 cm
1
, 46 km mol
1
), the deviation is merely
26 cm
1
.ItisalsopossiblethatagainanovertoneofaCH
2
bending
mode predicted at 2958 cm
1
may add to the intensity via Fermi
resonance with the n
NHbs
mode. Like for n=1,bandKcannotbe
explained by any fundamental mode of Ama
+
W
2
(I)andanyother
Ama
+
W
2
isomer but can again be attributed to a combination band
b
NH
+b
OH
at 3191 cm
1
of the symmetric (1577 cm
1
)and
antisymmetric (1614 cm
1
) coupled bending OH and NH modes.
The remaining weak but clearly discernible bands D and E at
3413 and 3376 cm
1
occur in the free NH stretch and/or bound OH
stretch range can clearly not be attributed to a fundamental mode
of isomer I. Hence, they are taken as spectroscopic signature for the
presence of the low-energy local minima II and III. While bands D
and E perfectly match n
OHb
(3415 cm
1
)andn
NHf
(3374 cm
1
)ofII,
with deviations of less than 3 cm
1
, their relative intensities suggest
an additional overlapping contribution of n
NHf
/n
OHb
of III (3369/
3383 cm
1
) to band E. The calculated free OH stretch modes of II
and III (n
OHs
,n
OHf
,n
OHa
) also match with peaks C and A. The bound
n
NHb
modes of II/III are strongly shifted into the CH stretch range
(2870 and 2853 cm
1
) and can possibly be assigned to peak J with a
deviation of only 4 cm
1
and unstructured signal between 2800 and
2850 cm
1
. The population of II and III may roughly be estimated
as o20% and o10% of Iby considering the achieved signal-to-
noise ratio and the computed and observed IR intensities of peaks
D, F, and the background in the 2800–2850 cm
1
range. While the
higher energy isomers IV and Vcannot readily be excluded spectro-
scopically, a significant population may be excluded by their rather
low stability (E
0
430 kJ mol
1
).
4.4. Ama
+
W
3
Increasing the number of W ligands to three leads to the nine
possible isomers I–IX for Ama
+
W
3
generated from the low
energy isomers of n= 2 (Table S1, ESI†). The most important
structures (I–VI) are shown in Fig. 6 and the remaining ones
Fig. 6 Calculated equilibrium structures (in Å and degrees) of Ama
+
W
3
(I–VI) in their ground electronic state (B3LYP-D3/cc-pVTZ). The structures of
isomers VII–IX are shown in Fig. S8 (ESI†).
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(VII–IX) are available in Fig. S8 (ESI†). Their computed IR
spectra are compared in Fig. 7 to the experimental IRPD
spectrum (Table 4 and Fig. S4, ESI†). The position of the
transitions observed in the IRPD spectrum of Ama
+
W
3
and
their vibrational assignment are listed in Table 4. Because of
the many isomers, we will not discuss their structures and
properties in detail. The cooperative and noncooperative three-
body effects resulting from the formation of a solvent network
and interior ion solvation, respectively, are discussed for n=2.
Their impact on structure, energy, charge transfer, orbital
interactions, and the IR spectrum of the n= 3 isomers are
consistent with these three-body interactions.
In the global minimum Iwith C
s
symmetry, the W ligands
form a cyclic ring involving the NH
2
group via four H-bonds,
leading to a total binding energy of D
total
0
= 167.6 kJ mol
1
,
accompanied by a charge transfer of 78 mefrom Ama
+
to W
3
(Fig. S9, ESI†). Two W ligands act as single-donor single-
acceptor molecules in NHO and OHO H-bonds, while
the bridging terminal W ligand closes the ring as double-
acceptor ligand in OHO H-bonds. To close the solvation
ring, the NHO and OHO H-bonds have to be quite non-
linear and the two single-donor single-acceptor ligands have to
rotate into the plane of the NH
2
group. Thus, although this
isomer has four intermolecular H-bonds, the steric strain
makes it only slightly more stable than the next isomers II–IV
by 0.2 to 1.5 kJ mol
1
, although the latter isomers have only
three H-bonds. The low binding energy of the outer W, D
0
=
49.3 kJ mol
1
, is consistent with the long OHO H-bonds
(1.955 Å). These correlate with relatively high bound OH stretch
frequencies of n
OHb
= 3519 and 3539 cm
1
, which makes them a
characteristic IR fingerprint of this cyclic isomer (Fig. 7).
In the only slightly less stable isomers II–IV with E
0
= 0.2, 0.6,
and 1.5 kJ mol
1
(D
total
0
= 166.1–167.4 kJ mol
1
), the two
N–H bonds of the NH
2
group are solvated by W and W
2
leading
to a weaker NHW and stronger NHW
2
ionic H-bond. They
differ mainly in the orientation and binding parameters of the
W
2
unit (r
OH
= 0.975–0.978 Å, R
OHO
= 1.783–1.821 Å), which
leads to somewhat different frequencies of their n
OHb
modes
(3442, 3407, 3391 cm
1
, Fig. 7). Moreover, they differ slightly
in their N–H bond lengths (1.030/1.038, 1.029/1.039, 1.029/
1.041 Å), which results in different n
NHb
frequencies. The
frequencies of n
NHba
(NHW) occur in a narrow range between
3108 and 3121 cm
1
, while n
NHbs
(NHW
2
) varies more
strongly between 2941 and 2896 cm
1
. Overall, the binding
energies of the terminal W ligand (D
0
= 47.7, 48.6, 49.0 kJ mol
1
)
and the total charge transfer from Ama
+
to the solvent are quite
similar (81, 86 and 87 me, Fig. S9, ESI†).
In isomers V–VII of Ama
+
W
3
with E
0
=4.3,5.5,and9.4kJmol
1
(D
total
0
= 163.3, 162.2, 158.3 kJ mol
1
), a W trimer is attached to the
NH
2
group via aNHOH-bondineitheralinear(V/VI)or
branched (VII) fashion, while the other N–H bond remains free.
Isomers Vand VI differ only slightly in the orientation of the W
3
chain, mainly by different ways to optimize the weak CHO
contacts (five between 2.5 and 3.0 Å), and thus have similar
bonding parameters and IR spectra. Hence, we will discuss only
Vin more detail. The linear W
3
chain has two OHOH-bonds
(1.752 and 1.802 Å), resulting in two intense n
OHb
modes at 3313
and 3415 cm
1
. The terminal W ligand has a lower binding energy
of D
0
=50.3kJmol
1
and the total charge transfer (70 me)tothe
solvent is lower than for II–IV. The rather short and strong NHO
H-bond (1.661 Å) results from the high proton affinity of W
3
(862 kJ mol
1
)
78
and produces a strongly redshifted n
NHb
band at
2789 cm
1
because of the substantial elongation of the N–H bond
(1.047 Å). The free n
NHf
mode occurs at 3369 cm
1
between the
two more intense n
OHb
bands. Isomer VII with the branched W
3
unit is slightly less stable than V/VI because it can develop less
CHO contacts. Due to reduced cooperativity, its OHObondsare
slightly longer and weaker (1.768 and 1.856 Å, D
0
B45 kJ mol
1
)
resulting in higher-frequency n
OHb
bands at 3406 and 3506 cm
1
,
which occur both to the blue of n
NHf
=3365 cm
1
. Because the
W ligands are closer to the Ama
+
charge, charge transfer to W
3
is somewhat higher (85 me, Fig. S9, ESI†). The resulting stronger
NHO H-bond (1.590 Å) causes a strong elongation of the
N–H donor bond to 1.060 Å, and the corresponding n
NHb
mode
(2589 cm
1
) is shifted out of the investigated frequency range.
For completeness, we also computed two structures VIII and
IX corresponding to interior ion solvation, in which three
individual W ligands bind to the NH
2
group (via NHWH-
bonds with 1.8–2.0 Å) and the Ama
+
cage (via charge–dipole
Fig. 7 IRPD spectrum of Ama
+
W
3
compared to linear IR absorption
spectra of Ama
+
and Ama
+
W
3
(I–IX) calculated at the B3LYP-D3/cc-pVTZ
level. The positions of the transition observed in the IRPD spectrum of
Ama
+
W
2
and their vibrational assignment are listed in Table 4. Differences
in relative energy (DE
0
) are given in kJ mol
1
.
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forces and CHW contacts with 2.3–2.5 Å). They result essen-
tially from adding one W to the cage of isomer Iof n= 2. These
are relatively high in energy (E
0
= 14.4 and 23.7 kJ mol
1
),
because they lack the cooperativity effect of the formation of a
H-bonded solvent network observed in isomers I–VII.VIII is
slightly more stable than IX because it has a weak OHO
contact (2.02 Å), producing a slightly redshifted n
OHs
band at
3603 cm
1
in the free OH stretch range. Otherwise, their IR
spectra are dominated by the two bound NH stretch bands
between 3000 and 3250 cm
1
(Fig. 7).
The IRPD spectrum of Ama
+
W
3
is compared in Fig. 7 to
those calculated for isomers I–IX. At first glance, the best match
between experimental and computed spectra is observed for
II–IV, in which the NH
2
group is hydrated by NHW and
NHW
2
H-bonds. These isomers are expected from the pre-
dominant observation of isomers Iand II of the n= 2 cluster.
Clearly, there is no spectroscopic signature for the formation of
isomer I, which is computed as global minimum on the n=3
potential at T=0K(E
0
= 0). In particular, the characteristic n
OHb
modes of the cyclic W
3
ring (3519 and 3539 cm
1
) and the most
intense n
NHbs
band at 3020 cm
1
are absent. The population of
Iis estimated to be below 20% of II by considering the achieved
signal-to-noise ratio and the computed IR intensities. The lack
of detection of the cyclic isomer can be rationalized by
considering entropic effects, which favour the more flexible
chain structures over the more rigid cyclic solvation ring.
Indeed, free energy calculations at 300 K show that Ilies above
II by DG= 5.1 kJ mol
1
. Such entropic effects changing the order
of isomers are not observed for the nr2clusters(TableS1,
ESI†). A major contribution of isomers V–VII to the IRPD
spectrum can be excluded because of the mismatch of their
predicted spectra in the range of the bound NH and OH stretch
modes (n
NHb
,n
OHb
). For example, the population of Vis esti-
mated to be r15% of that of II. Similarly, VIII and IX can be
excluded because of the disagreement in the n
NHb
range and
their high relative energy (E
0
414 kJ mol
1
).
Out of the energetically and spectroscopically favourable
isomers II–IV, the most stable structure II (concerning both
E
0
and G) shows by far the best agreement with the IRPD
spectrum, with maximum, mean, and summed deviations of
23, 11, and 10 cm
1
, respectively. The IR spectra of the similar
structures III and IV are also in good accord with the IRPD
spectrum, but the frequency deviations are larger and thus we
will not discuss them in further detail. Peaks A–C at 3730, 3707,
and 3644 cm
1
are attributed to the various free OH stretch
modes of II (n
OHa
,n
OHf
,n
OHs
). B and D at 3426 cm
1
corre-
sponds well to n
OHb
predicted at 3442 cm
1
, and is close to the
corresponding mode measured for ABN
+
W
3
(3402 cm
1
).
55
Bands F and G at 3116 and 2944 cm
1
are attributed to n
NHba
and n
NHbs
predicted at 3108 and 2941 cm
1
, and their observed
shifts upon addition of the third W ligand (21 and 83 cm
1
)
agree well with the computed trend (31 and 93 cm
1
).
Table 4 Computed vibrational frequencies (in cm
1
, B3LYP-D3/cc-pVTZ) of Ama
+
W
3
(II-IV,VIII) compared to experimental values of Ama
+
W
3
(Fig. 7)
a
Mode Ama
+
Ama
+
W
3
(II) Ama
+
W
3
(III) Ama
+
W
3
(IV) Ama
+
W
3
(VIII) Ama
+
W
3
exp.
n
CH
2803 (76) 2855 (36) 2856 (49) 2856 (84) 2840 (35) J 2840 (40)
n
CH2
2870 (4) 2875 (14) 2875 (16) 2874 (19) 2872 (4)
n
CH2
2874 (16) 2876 (11) 2875 (20) 2876 (21) 2875 (18)
n
CH2
2882 (16) 2876 (27) 2876 (16) 2878 (6) 2877 (21)
n
CH2
2884 (20) 2880 (21) 2879 (19) 2880 (21) 2878 (20)
n
CH2
2887 (10) 2885 (8) 2883 (11) 2883 (4) 2883 (22)
n
CH2
/n
CH
2910 (5) 2902 (43) 2901 (39) 2901 (91) 2900 (36)
n
CH2
/n
CH
2911 (28) 2906 (25) 2905 (23) 2904 (34) 2906 (26)
n
CH2
/n
CH
2912 (1) 2909 (15) 2907 (113) 2907 (128) 2914 (8)
n
CH2
/n
CH
2913 (10) 2912 (5) 2912 (8) 2913 (7) 2917 (6)
n
CH2
2922 (6) 2915 (4) 2915 (9) 2916 (3) 2918 (5)
n
CH2
2922 (11) 2918 (20) 2919 (14) 2920 (39) 2922 (31)
n
CH2
2927 (4) 2921 (56) 2920 (21) 2922 (16) 2923 (42) I 2944 (45)
n
CH2
2932 (27) 2930 (13) 2931 (11) 2925 (20) 2932 (1)
n
CH2
2984 (7) 2964 (6) 2958 (13) 2957 (15) 2990 (3)
n
NHbs
2941 (1072) 2917 (1137) 2896 (1172) 3097 (949) G 2944 (45)
L 2993 (20)
n
NHbs
3097 (949) F 3116 (97)
n
NHba
3108 (989) 3114 (967) 3121 (914) F 3116 (97)
n
NHba
3244 (588) K 3247 (15)
b
NH
+b
OH
3193 K 3247 (15)
n
NHs
3322 (223)
n
NHa
3438 (67)
n
OHb
3442 (404) 3407 (510) 3391 (571) D 3426 (86)
n
OHs
3603 (157)
n
OHs
3648 (32) 3653 (33) 3654 (46) 3645 (27) C 3644 (21)
n
OHs
3654 (42) 3654 (40) 3654 (24) 3655 (43) C 3644 (21)
n
OHf
3730 (114) 3726 (104) 3726 (100) B 3707 (12)
n
OHa
3735 (95) 3742 (90) 3742 (97) 3731 (105) A 3730 (20)
n
OHa
3742 (102) 3743 (108) 3743 (99) 3732 (113) A 3730 (20)
n
OHa
3744 (102) A 3730 (20)
a
IR intensities in km mol
1
are given in parentheses. The computational data for the other isomers are available in Table S4 (ESI). The
experimental frequencies with width (fwhm in parenthesis) are assigned to the most dominant vibrations.
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This assignment is also consistent with n
NHbs
(2919 cm
1
) and
n
NHba
(3180 cm
1
) measured for ABN
+
W
3
.
55
Similar to Ama
+
W
2
,
peak J at 2840 cm
1
is attributed to a n
CH
mode, while I
(2944 cm
1
)arisingfromn
CH2
modes overlaps with band G
(n
NHbs
)inthen= 3 spectrum. Again, the origin of peaks K
(3247 cm
1
) and L (2993 cm
1
) remains unclear. They cannot
be explained by fundamental modes and thus are ascribed to
combination bands or overtones. For example, K could again be
assigned to the combination band b
NH
+b
OH
of the symmetric
(1583 cm
1
)andantisymmetric(1610cm
1
) coupled NH and OH
bending modes, although the observed blueshift of peak K to
3247 cm
1
is not reproduced by the harmonic calculation.
5. Further discussion
The analysis of the IRPD spectra of Ama
+
W
n
in the CH, NH, and
OH stretch range with the aid of B3LYP-D3 calculations pro-
vides a consistent picture of the sequential cluster growth of
this fundamental diamondoid amine cation. All main bands in
the IRPD spectra can readily be assigned to the energetically
most stable isomer I, except for Ama
+
W
3
where free energies
must be considered and then isomer II becomes more stable
than Iat room temperature by 5 kJ mol
1
. For nr2, the W
ligands preferentially bind to the acidic protons of the NH
2
group via strong and nearly linear NHO ionic H-bonds,
corresponding to interior ion solvation. The H-bonds in n=2
are slightly weaker than in n= 1 due to noncooperative effects
typical for interior ion solvation. Hence, for n= 2 a local
minimum with a H-bonded W
2
dimer attached to the NH
2
group becomes competitive because of strong cooperative
effects of the H-bonded solvent network (E
0
= 5.4 kJ mol
1
)
and thus is observed as a minor isomer. Starting from n= 3, the
microhydration network expands as a H-bonded W
n
network
incorporating the acidic NH
2
group. Such H-bonded isomers
appear energetically favoured over other isomers, in which the
Ama
+
cage is progressively solvated by single W ligands via
cation–dipole forces supported by weak CHO contacts. The
calculated binding energies for the identified Ama
+
W
n
clusters
decrease as D
0
= 61.4 456.9 454.4 kJ mol
1
for n= 1–3, and a
linear extrapolation to larger cluster sizes would suggest that
for n48 internal ion solvation becomes more favourable
than the extension of the solvation network (Fig. S10, ESI†).
However, this prediction is in contradiction to the observed
sublimation enthalpy of bulk ice of 51.0 kJ mol
1
,
90
suggesting
that larger Ama
+
W
n
clusters also prefer structures with the
hydrophobic adamantyl cage residing at the surface of the
water solvent network. Clearly, structural isomers in which W
ligands are attached to the adamantyl cage via charge–dipole
forces supported by weak CHO contacts are much less stable
than those with hydration around the NH
2
group. The computed
hydration energies for n= 1–3 (54–61 kJ mol
1
) exceed by far the
absorbed IR photon energy (hno48 kJ mol
1
= 4000 cm
1
),
indicating that under the employed single-photon absorption
conditions, using an unfocused IR laser beam, only cluster ions
with significant internal energy can undergo the IRPD process.
This result can also explain the widths of the transitions and the
contributions of entropy necessary for the evaluation of the
energetic order of the isomers. Although there is progressive
partial charge transfer from Ama
+
to the W
n
solvent cluster, most
of the charge in [AmaW
n
]
+
remains on Ama
+
because of the large
disparity in the ionization energy of Ama (computed as 7.95 eV)
and W
n
(410 eV),
79
justifying the notation Ama
+
W
n
in the
considered size range. Similarly, the computed proton affinity
of the amantadinyl radical (PA = 960 kJ mol
1
) is substantially
higher than that of W
n
clusters (PA = 691, 808, 862, 900, 904,
and 908 kJ mol
1
for n= 1–6),
74–78
suggesting that no proton
transfer to solvent occurs for small polyhydrates of Ama
+
,
although the tendency for proton shift from Ama
+
to W
n
gradually increases with n.
The detailed evolution of the properties of the N–H bonds of
the predominantly observed Ama
+
W
n
clusters as a function of n
are compared in Fig. 8. These include the calculated individual
and averaged free and bound N–H bond lengths (r
NHf
,r
NHb
),
the corresponding calculated and experimental individual and
averaged free and bound NH stretch frequencies (n
NHf
,n
NHb
),
and the E
(2)
energies for the NHO H-bonds. In general, the
experimental and theoretical frequencies agree quantitatively
in terms of both, absolute values and incremental changes,
which confirms the given isomer assignments. These diagrams
show monotonous trends for all averaged values with increas-
ing degree of hydration. As nincreases, the intermolecular
NHO H-bonds become stronger due to the increasing proton
affinity of the solvent cluster, as illustrated by the rising E
(2)
energies. As a result, the intramolecular N–H bonds become
weaker and longer and this trend directly transfers in the
reduction of the NH stretch frequencies. However, the N–H
bonds are far from being broken by the solvent at the con-
sidered cluster size range.
Apart from the NH
2
group, there are further structural changes
induced by hydration and the resulting charge transfer to the
solvent. To illustrate the major effects on the distortion of the
Ama
+
cage as a function of progressive solvation, the most affected
C–C and C–N bond lengths of Ama
(+)
and the observed most stable
Ama
+
W
n
clusters are plotted in Fig. 9 as a function of the partial
charge on the adamantyl cage. Ionization of Ama increases the
partial charge on this cage from 161 to 687 me.Asaresult,the
C–C
ext
bond in the C
s
planeisstronglyelongated,whiletheC–C
adj
and C–N bonds are shortened. The charge of the cage decreases
again with addition of each W ligand, and thus the trends induced
by ionization are progressively reversed. However, this process
slows down with each additional W ligand so that a complete
return to the properties of neutral Ama by hydration is unlikely,
because of the disparity in the ionization energies of Ama and W
n
(IE = 10.12 0.15 eV for liquid water).
91
It is instructive to compare the results of Ama
+
W
n
with those
obtained previously for microhydration of other primary amine
cations, such as aromatic amines (e.g., AN, ABN)
55,69
or aliphatic
amines (e.g.,CH
3
NH
2
).
71
Microhydration of AN
+
W
n
and ABN
+
W
n
shows the same preferential cluster growth as Ama
+
W
n
.How-
ever, the N–H bonds in AN
+
and ABN
+
are less acidic, leading to
weaker NHO H-bonds than in Ama
+
W
n
. For example, the
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redshifted bound NH stretch bands occurs at n
NHb
= 3105, 3040,
and 2992 cm
1
for the monohydrates of AN
+
,ABN
+
, and Ama
+
.
As a result, the IR spectra of AN
+
W and ABN
+
W are not
complicated by the anharmonic interaction of n
NHb
with CH
stretch modes. Furthermore, because of smaller charge transfer
to the solvent, the IRPD spectra of the dehydrates of AN
+
and
ABN
+
reveal isomer Ias a single isomer without any population
of the minor isomers II and III as observed for Ama
+
.
In an effort to study the effects of replacing the cycloalkyl
cage by a simple linear alkyl group on the hydration of aliphatic
amine cations, we compare in the following Ama
+
W
n
with
CH
3
NH
2+
W
n
, which is the simplest alkylamine cation with a
comparable microhydration process.
71
Although the latter clus-
ters are rather simple, no IRPD spectra have been reported yet,
and the only information available comes from computational
studies at the MP2/6-311+G(2d,p) level.
71
Because the MP2 level
often suffers from spin contamination when applied to radical
cations and because we like to compare directly with our data
on Ama
+
W
n
, we computed herein the properties of CH
3
NH
2+
W
n
at the B3LYP-D3 level (Fig. S4, ESI†). In general, the B3LYP-D3
results are consistent with previous MP2 results.
71
The ioniza-
tion energy of CH
3
NH
2
is substantially higher than for Ama
(8.9 vs. 7.9 eV) and the calculated proton affinity of CH
3
NH is
lower than for C
10
H
15
NH (896 vs. 960 kJ mol
1
).
92
The major
difference between Ama
+
and CH
3
NH
2+
is that the HOMO orbital
is largely delocalized over the Ama cage, while in CH
3
NH
2+
it is
essentially the localized nitrogen porbital perpendicular to the
CNH
2
plane. Hence, the positive charge on the NH
2
group is
much higher in CH
3
NH
2+
(q
NH2
=597vs. 313 me) leading to more
acidic NH protons (r
NH
=1.020vs. 1.012 Å). As a result, the
NHO H-bonds in CH
3
NH
2+
W
n
are substantially stronger than
in Ama
+
W
n
. For example, the NHOH-bondinCH
3
NH
2+
Whas
a higher binding energy than Ama
+
W(I), associated with a
shorter H-bond, larger orbital interactions, higher charge trans-
fer to solvent, and stronger intramolecular deformation (D
0
=
88.7 vs. 61.4 kJ mol
1
,R
NHO
= 1.613 Å vs. 1.768 Å, E
(2)
=62.4vs.
33.8 kJ mol
1
,Dq=68vs. 38 me,Dr
NH
=39vs. 21 mÅ). On the
Fig. 8 Plots of various calculated and experimental properties of the N–H bonds of the most stable Ama
+
W
n
clusters as a function of the cluster size n:
calculated N–H bond lengths (r
NH
); calculated second-order perturbation energies (E
(2)
) of donor–acceptor orbital interactions involved in the H-bonds;
calculated and experimental NH stretch frequencies (n
NH
). Crosses indicate individual data points, whereas connected points correspond to averaged values.
Fig. 9 Plot of the bond lengths of the extended C–C bond (C–C
ext
), the
adjacent C–C bond (C–C
adj
), and the C–N bond of Ama, Ama
+
, and the
most stable structures of Ama
+
W
1–3
as a function of the partial charge of
the adamantyl cage to illustrate that the bond lengths of Ama
+
approach
those of neutral Ama upon progressive hydration.
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other hand, the C–N bond elongation is reduced (Dr
NH
=3vs.
8 mÅ), because of the lacking orbital delocalization. The
hydration motif in CH
3
NH
2+
W
2
is quite similar to that of
Ama
+
W
2
(I), but again with shorter and stronger NHO
H-bonds (D
0
=73.0vs. 56.9 kJ mol
1
,R
NHO
=1.695Åvs.
1.802 Å, E
(2)
=44.7vs. 29.3 kJ mol
1
,Dq=97vs. 68 me,r
NH
=
1.045 Å vs. 1.030 Å). A similar trend is also observed for n=3,
where in CH
3
NH
2+
W
3
the NHO(1.587vs. 1.722 Å) and OHO
H-bonds (1.715 vs. 1.821 Å) of the W
2
unit are also shorter than
in Ama
+
W
3
(II), based on the higher total binding energy (D
total
0
=
216 vs.167 kJ mol
1
) and the larger charge transfer to the solvent
(Dq=120vs. 81 me).
6. Concluding remarks
In summary, we combined IRPD spectroscopy of size-selected
Ama
+
W
n
clusters with nr3 in the sensitive OH, NH, and CH
stretch range with DFT calculations at the B3LYP-D3/cc-pVTZ
level to determine the first steps in the microhydration process
around the radical cation of this important pharmaceutical
molecular drug. Significantly, these spectra provide the first
spectroscopic information about the Ama
+
cation and any
Ama
+
L
n
cluster with neutral ligands L. Thus, they yield a first
impression about the acidity of its NH
2
group and its inter-
action with solvent molecules. The salient results may be
summarized as follows.
(1) In agreement with previous experimental and computa-
tional evidence, ionization of Ama into the ground state of the
cation is achieved by removal of an electron from the non-
bonding lone pair of the N atom of the NH
2
group, which has a0
symmetry in the C
s
point group. As a result, the pyramidal NH
2
group in neutral Ama becomes essentially planar upon ioniza-
tion, much like for NH
3
. As this orbital is significantly deloca-
lized into the adamantyl cage, ionization also strongly affects
the lengths of the neighbouring C–N and C–C bonds lying in
the symmetry plane. These structural changes of the NH
2
group
and the adamantyl cage are further modulated by sequential
microhydration because the formation of the NHO H-bonds
is accompanied by partial charge transfer from Ama
+
to W
n
.
(2) The measured IRPD spectra of Ama
+
W
n
can essentially be
assigned to the energetically most favourable structures for a
given cluster size obtained by the DFT calculations, whereby for
n= 3 the free energies have to be considered. The preferred
sequential cluster growth starts with the hydration of the two
acidic NH protons of the NH
2
group via strong NHO ionic
H-bonds (n= 1–2) and continues with further extension of the
H-bonded solvent network by the formation of a W
2
unit via an
OHO H-bond (n= 3). Clearly, structural isomers in which W
ligands are attached to the adamantyl cage via charge–dipole
forces supported by weak CHO contacts are much less stable
than those with hydration around the NH
2
group. However, for
n= 2 a minor additional isomer with a W
2
dimer attached to the
NH
2
group is detected, because the nearby Ama
+
charge
strongly enhances the binding energy of the W
2
dimer via
polarization effects. The formation of a cyclic hydration
structure for n= 3, computed to be the global minimum at
T= 0 K, can be excluded by spectroscopy, and indeed this
structure becomes destabilized by 5 kJ mol
1
compared to the
observed structure when considering free energies. Overall, the
IRPD spectra and calculations reveal a similar solvation process
as observed previously for related RNH
2+
W
n
clusters,
55,69,70
in
which also the formation of a H-bonded network is favoured over
internal ion solvation.
55
While the formation of the H-bonded
solvation network is strongly favoured by cooperative three-body
interactions, the tendency for interior ion solvation is somewhat
reduced by noncooperative three-body induction forces.
(3) In general, the solvent increasingly attracts the acidic NH
protons of Ama
+
as the number of W ligands increases.
Although the N–H bonds are progressively destabilized by
gradual microhydration, no proton transfer to the solvent is
observed for nr3, which is in line with the proton affinities of
C
10
H
15
NH and W
n
. From the increasing proton affinity of the
W
n
, such a proton transfer may only occur for large n(n446).
Similarly, charge transfer from Ama
+
to W
n
increases with n,
but remains below 0.1 edue to the large discrepancy between
the ionization energies of Ama and W
n
.
(4) From comparison of computational spectroscopic data,
we conclude that the acidity of the NH
2
group in RNH
2+
cations
increases in the order aniline oamantadine omethylamine,
because of decreasing charge delocalization and resulting
stronger H-bonds.
For a more detailed investigation of the bare Ama
+
cation,
experiments with weakly bound rare gas ligands are currently
underway.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This study was supported by Deutsche Forschungsge-
meinschaft (grant DO 729/8).
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