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Showcasing work from the Germany-Japan research team
by Profs. Dopfer, Mitric, Miyazaki and Fujii
Real-time observation of photoionization-induced water
migration dynamics in 4-methylformanilide–water by
picosecond time-resolved infrared spectroscopy and
ab initio molecular dynamics simulations
A novel time-resolved pump-probe spectroscopic
approach that keeps high resolution in both the time
and energy domain, nanosecond excitation–picosecond
ionization–picosecond infrared probe spectroscopy,
has been applied to the trans-4-methylformanilide–water
cluster. Water migration dynamics from the CO to the NH
sites in a peptide linkage triggered by photoionization is
directly monitored by the ps time evolution of IR spectra.
A significant acceleration of water migration has been
found by substitution of the methyl group.
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Otto Dopfer, Masaaki Fujii et al .,
Phys . Chem . Chem . Phys .,
2022, 24 , 73.
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Physical Chemistry Chemical Physics
Volume 24
Number 1
7 January 2022
Pages 1–596
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Cite this: Phys. Chem. Chem. Phys.,
2022, 24, 73
Real-time observation of photoionization-induced
water migration dynamics in
4-methylformanilide–water by picosecond
time-resolved infrared spectroscopy and ab
initio molecular dynamics simulations†
Mitsuhiko Miyazaki, *
ab
Tairiku Kamiya,
b
Matthias Wohlgemuth,
c
Kuntal Chatterjee,
d
Roland Mitric
´, *
c
Otto Dopfer *
de
and Masaaki Fujii *
be
A novel time-resolved pump–probe spectroscopic approach that enables to keep high resolution in
both the time and energy domain, nanosecond excitation–picosecond ionization–picosecond infrared
probe (ns–ps–ps TRIR) spectroscopy, has been applied to the trans-4-methylformanilide–water
(4MetFA–W) cluster. Water migration dynamics from the CO to the NH binding site in a peptide linkage
triggered by photoionization of 4MetFA–W is directly monitored by the ps time evolution of IR spectra,
and the presence of an intermediate state is revealed. The time evolution is analyzed by rate equations
based on a four-state model of the migration dynamics. Time constants for the initial to the
intermediate and hot product and to the final product are obtained. The acceleration of the dynamics by
methyl substitution and the strong contribution of intracluster vibrational energy redistribution in the
termination of the solvation dynamics is suggested. This picture is well confirmed by the ab initio on-the-fly
molecular dynamics simulations. Vibrational assignments of 4MetFA and 4MetFA–W in the neutral (S
0
and S
1
)
and ionic (D
0
) electronic states measured by ns IR dip and electron-impact IR photodissociation spectroscopy
are also discussed prior to the results of time-resolved spectroscopy.
1. Introduction
Realignment of solvent molecules is an initial process of
chemistry in solution, and it significantly affects the reaction
rates and yields.
1–3
Thus, the solvation dynamics after photo-
excitation of a solute molecule has attracted strong attention,
even from the early stage of ultrafast spectroscopy.
4–6
For
biomolecules, hydration dynamics around proteins is one of
the most important dynamical processes because the biological
functions of proteins take place in water, and the surfaces of
proteins are usually covered by water molecules. Protein
folding obviously triggers rearrangement of surrounding water
molecules.
7–10
Protein transport is also strongly dependent on
how tightly bound the surrounding water molecules are. Due to
their importance, water molecules surrounding proteins are
often called ‘‘biological water’’.
11,12
The dynamics of biological water has been interrogated by
several methods such as NMR spectroscopy, neutron scattering,
and dynamic Stokes shifts.
13–18
However, the interpretation of
these studies on dynamics of biological water in solution are
often controversial,
8,19
and the lifetime of initial dynamical
processes of biological water spreads over the sub-nanosecond
to picosecond range.
13,14
This discrepancy may be due to the
fact that biological water is inhomogeneous, and each of the
above experimental techniques probes the dynamics of water
molecules in different environments. A recent review about
biological water describes this unfortunate situation by the
famous poem of John Godfrey Saxe ‘‘The Blind Men and the
Elephant’’.
19
To circumvent the problem of inhomogeneous water
molecules, we have been working on hydration dynamics at a
a
Natural Science Division, Faculty of Core Research, Ochanomizu University,
2-1-1 Ohtsuka, Bunkyo-ku, Tokyo 112-8610, Japan.
E-mail: miyaza[email protected]
b
Laboratory for Chemistry and Life Science, Institute of Innovative Research,
Tokyo Institute of Technology, Yokohama 226-8503, Japan
c
Institut fu
¨r Physikalische und Theoretische Chemie, Julius-Maximilians-Universita
¨t
Wu
¨rzburg, 97074 Wu
¨rzburg, Germany. E-mail: roland[email protected]
d
Institut fu
¨r Optik und Atomare Physik, Technische Universita
¨t Berlin,
10623 Berlin, Germany. E-mail: [email protected]rlin.de
e
World Research Hub Initiatives, Institute of Innovative Research, Tokyo Institute of
Technology, 4259-R1-15, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan.
E-mail: [email protected]ech.ac.jp
†Electronic supplementary information (ESI) available: [DETAILS]. See DOI:
10.1039/d1cp03327a
Received 21st July 2021,
Accepted 1st October 2021
DOI: 10.1039/d1cp03327a
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single solvent molecular level by probing solvated clusters in
the gas phase using time-resolved infrared (TRIR) spectroscopy
and advanced ab initio molecular dynamics (MD)
simulations.
20–25
This approach offers size- and isomer-
selectivity as well as full control over solvation structure and
energy in the cluster. Hydrated clusters generated in a molecular
beam consist of solute and water molecules. The clusters are
isolated at low temperature and thus the initial geometry and
number of solvent molecules in the clusters are precisely
defined. As a result, we can directly watch single water migration
dynamics without ambiguity. The dynamics is typically initiated
by UV laser excitation/photoionization of the clusters, and
migration of the water molecule is monitored by changes in
theTRIRspectraatthepicosecond(ps)timescale.Thistechnique
of UV-UV(0)-IR ps time-resolved IR (ps-TRIR) spectroscopy
combined with mass spectrometry has been successfully applied
to a variety of monohydrated clusters, including the single water
CO -NH migration around the peptide linkage in trans-
acetanilide (tAA)
20,21
and trans-formanilide (tFA),
24
the CN -
NH migration around 4-aminobenzonitrile,
22
and the NH -
OHmigrationin5-hydroxyindole.
25
Surprisingly, the lifetimes of
these single water migrations vary over four orders of magnitude,
ranging from 4 ps to 53 ns. This wide variety of the migration
time-constants has been rationalized by the differences in excess
energies and potential energy surfaces for both the initial and
final binding sites, such as (a) repulsive -bound, (b) flat -
bound, and (c) bound -bound.
23,25
The slow migration lifetime in
the bound -bound potential surface in 5-hydroxyindole–water is
well reproduced by RRKM theory, and thus the intracluster
vibrational energy redistribution (IVR) is one of the key factors to
understand the variety of the hydration dynamics at the molecular
level. Analysis of the fast migration regime relies mostly on MD
simulations.
For example, the ab initio ‘‘on-the-fly’’ MD simulations
reproduce the time-evolutions (TEs) obtained by the ps-TRIR
spectra on monohydrated clusters of tAA and tFA (tAA–W and
tFA–W), and visualize the migration trajectories of water as
shown in Fig. 1.
21,24
The water migration in tFA
+
–W(CO)
(hereafter, W(CO) and W(NH) mean the water molecule bound
to the CO and NH sites, respectively) triggered by photoionization
of the aromatic chromophore proceeds in the molecular plane.
The water molecule released from the initial CO binding site
migrates toward the final NH site, but overshoots and moves like
a large-amplitude damped pendulum around the NH destination
site. This overshooting motion is settled down within around
10 ps by vibrational energy transfer to low-frequency intra-
molecular vibrations of tFA
+
.
24
The migration dynamics in the
related tAA
+
–W(CO) cluster has been also simulated, and the twice
fastermigrationthanintFA
+
–W is well reproduced (Fig. 1).
21
tAA
is a methyl-substituted derivative of tFA and thus the faster
migration can be rationalized by the effect of the CH
3
group.
The CH
3
group provides low-energy vibrational levels due to its
(hindered) internal rotation, which accelerates the IVR.
26
Another
effect of CH
3
substitution is its direct interaction with the water
migration because the pathway is now split into in-plane and
out-of-plane paths in tAA
+
–W due to steric hindrance. In either
case, it is clear that IVR from inter- to intramolecular vibrations
controls the migration dynamics. However, it is difficult to
evaluate the importance of the two effects, namely the density
of states and the steric interaction.
In the present work, we apply ps-TRIR spectroscopy and
ab initio MD simulation to the monohydrated clusters of
Fig. 1 Trajectories of the ionization-induced water migration derived from the trans-FA
+
–W (left, 40 trajectories) and trans-AA
+
–W (right, 50
trajectories) MD simulations, starting from the CO-bound isomer (top row) and ending at the NH-bound isomer (bottom row) at 10 and 5 ps after
ionization, respectively (see movies in ESI†). The yellow curves for trans-FA
+
–W indicate the in-plane migration path of water. The out-of-plane pathways
for trans-AA
+
–W correspond to the slow (blue) and fast (red) migration channels, respectively.
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trans-4-methylformanilide (4MetFA–W) to determine the effect of
IVR acceleration by a CH
3
group. The ps-TRIR spectra and time
evolutions are measured herein by a new spectroscopic approach,
namely a nanosecond excitation–picosecond ionization–
picosecond IR probe (ns–ps–ps) technique. An increased spectral
resolution realized by replacement of the excitation laser from a
ps to ns laser allows us to achieve a higher species-selectivity in
ps-TRIR spectroscopy, which is crucial in cluster spectroscopy.
The CH
3
group in 4MetFA is located at the remote para-position to
the peptide linkage (–CONH–), and thus does not interfere with
the water migration pathway.Fromthecomparisontothe
dynamics of tFA
+
–W, we can discuss the effect of the density of
states on the single water migration dynamics.
2. Experimental and
theoretical methods
2.1. Experimental
2.1.1. Stationary UV and IR spectroscopies. The stationary
UV and IR spectroscopies were carried out by utilizing
resonance-enhanced multiphoton ionization (REMPI) and
double resonance IR-UV (IR dip and IR hole-burning) and UV-
IR depletion schemes utilizing nanosecond lasers. Briefly,
target molecules/clusters were ionized by REMPI via the S
1
’
S
0
origin transitions using two UV laser pulses for excitation
and ionization (n
exc
and n
ion
). With a certain delay time (Dt)to
the UV pulses, an IR laser pulse (n
IR
) was introduced (Dt=50 ns
for S
0
,Dt= +50 ns for D
0
). For measurements in the S
1
state, n
exc
and n
ion
were delayed ca. 10 ns, and n
IR
was introduced in
between them to minimize temporal overlap among the pulses.
4MetFA (Tokyo Chemical Industry Co., LTD, 498%) was
used without further purification. The sample of 4MetFA was
put in a glass tube with molecular sieves and set in a holder
behind a pulsed valve, both of which were heated to 343 K.
Water was stored in a vessel located between the carrier gas
cylinder and the valve, and cooled down to 243 K. Vapor of
4MetFA and water was seeded in He (1–2 bar), and the resulting
mixture was expanded into vacuum through the pulsed valve.
The jet expansion was collimated by a skimmer (f= 2 mm)
and introduced into the acceleration region of a time-of-flight
mass spectrometer. The molecular beam was irradiated by the
ns laser pulses, and the resulting ions were detected by a Daly
detector at the end of the flight tube. Pulsed-field extraction
was employed for introducing the ions into the flight tube to
minimize field shifts in the ionization energy. The field shift of
14 cm
1
resulting from field ionization of Rydberg states was
determined on the basis of the adiabatic ionization energy (IE
0
)
of phenol
27
recorded under the same condition. The n
exc
and
n
ion
pulses were generated by second harmonic generation of
dye laser outputs pumped by Nd:YAG lasers. The n
IR
pulse was
obtained by difference-frequency generation as described
elsewhere,
28,29
but the nonlinear crystal was changed from
LiNbO
3
to KTA. Laser frequencies were calibrated to within
1cm
1
by a wavemeter, while relative frequencies are
determined to within 0.5 cm
1
. The n
exc
and n
ion
beams were
focused on the molecular beam by a lens with f= 1500 mm after
adjusting focusing by telescopes equipped with a spatial filter.
The n
IR
beam was introduced in a counter propagating manner
to the UV pulses and focused by a BaF
2
lens with f= 350 mm.
2.1.2. Electron impact ionization (EI)-IR spectroscopy.
EI-IR spectroscopy was carried out to obtain the IR spectrum
of the most stable isomer of the 4MetFA
+
–W cation cluster.
EI-IR spectra of size-selected clusters were recorded in a
quadrupole–octopole–quadrupole tandem mass spectrometer
coupled to an electron ionization source.
30,31
Clusters were
generated in a pulsed supersonic plasma expansion by electron
and/or chemical ionization of 4MetFA close to the nozzle orifice
followed by aggregation/cooling, which produces predomi-
nantly the most stable cluster isomer in the cationic ground
state (D
0
). The expanding gas mixture was produced by passing
Ar carrier gas (8–10 bar) through a reservoir filled with 4MetFA,
which was heated to 418 K. Cluster ions of interest were mass-
selected by the first quadrupole and subsequently irradiated
with n
IR
from a tunable optical parametric oscillator pumped by
a Nd:YAG laser. The IR laser frequency was calibrated to better
than 1 cm
1
using a wavemeter. Resonant vibrational excitation
induces the loss of the most-weakly bound ligand(s). Fragment
ions produced by the IR absorption were mass-selected with
the second quadrupole and monitored by a Daly detector.
To separate the signal of laser-induced dissociation from the
metastable decay background, the ion source was triggered at
twice the laser frequency, and signals from alternating triggers
were subtracted. To reduce the width of the transitions caused
by internal energy of the cluster ions, tagging with loosely
bound Ar was also performed.
2.1.3. TRIR spectroscopy. The nanosecond excitation–
picosecond ionization–picosecond IR probe (ns–ps–ps) scheme
(Fig. 2) was adopted to achieve a higher species-selectivity in
ps-TRIR spectroscopy, which is restricted by the energy-time
uncertainty relation. In this scheme, a population in a S
1
vibronic level is prepared by a ns UV pulse (n
exc
(ns)) as the
first step. The population in this level is then ionized by a ps UV
pulse (n
ion
(ps)) to trigger a reaction in the cationic D
0
state, and
the population of the produced ion is monitored as ion current.
The dynamics is traced by a ps IR pulse (n
IR
(ps)) with a certain
delay time (Dt) from the ionization event through changes
in the monitored ion population caused by resonant IR
absorption. Coupling of the first ns excitation process offers a
much better state selectivity for the subsequent ps pump–probe
spectroscopy in the D
0
state, even for the case of a cluster
system with a congested excitation spectrum resulting from
close-lying or overlapping transitions of isomers and fragments
from higher clusters, which is illustrated in Fig. S1 (ESI†)in
detail.
The 4MetFA–W clusters were produced in a similar manner
to that of the stationary spectroscopy but in a pulsed free jet
expansion. The clusters were irradiated by laser pulses ca. 1cm
downstream from the pulsed valve, and resulting ions were
turned by 901into a quadrupole mass filter before the ion
current in the 4MetFA
+
–W mass channel was detected by a
channeltron. Picosecond laser pulses were generated by a
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high-energy ps laser setup described in detail elsewhere.
32–34
Briefly, tunable ps UV (n
ion
(ps)) and IR (n
IR
(ps)) pulses were
generated through optical parametric and nonlinear frequency
conversion processes of a ps pulse obtained from regenerative
amplification of a femtosecond Ti/Sapphire laser pulse. The
energy and time resolutions of the n
ion
(ps) and n
IR
(ps) pulses
were ca. 12 cm
1
and 3 ps, respectively. The second harmonic
of a ns dye laser pumped by a Nd:YAG laser was used as
excitation UV pulse (n
exc
(ns)). These pulses were then combined
by beam combiners and focused on the jet expansion by a CaF
2
lens with f= 300 mm after adjusting the focusing by telescopes.
The delay time between the ps UV and IR pulses (Dt
IR
)was
controlled by optical delay stages. The delay between n
exc
(ns)
and n
ion
(ps) (Dt
UV
)wasseparatedbyca. 4 ns (adjusted by a
digital delay generator) to avoid unexpected photoprocesses
caused by their overlap. This delay is shorter than the lifetime
of the cluster measured as 13 ns (see Fig. S2 in ESI†).
2.2. Theoretical methods
Quantum chemical calculations were carried out at the CAM-
B3LYP/aug-cc-pVTZ level of theory with the GD3BJ empirical
dispersion correction using Gaussian16.
35
Relative energies
(E
rel
) of structural isomers of 4MetFA–W were evaluated including
the (non-scaled) harmonic zero-point energy (ZPE) correction
estimated from the normal mode analysis. The theoretical
vibrational frequencies were scaled by 0.95 for comparison with
experimental IR spectra.
In order to generate the initial conditions for the MD
simulations of 4MetFA–W in the D
0
state, a 50 ps long trajectory
in the S
1
excited electronic state at a constant temperature of
T= 100 K was run employing the TD-CAM-B3LYP functional.
This temperature is chosen to reproduce the experimental
excess energy after photoionization and is the same as the
one used for the tAA
+
–W simulations. After initial equilibration,
structures and velocities in regular time intervals were
sampled, thus generating an ensemble of 50 initial conditions.
The MD simulations starting from these initial conditions were
propagated for 10 ps in the D
0
cationic state. The integration of
the classical equations of motions was performed using the
velocity Verlet algorithm with a time step of 0.2 fs, which is
sufficiently small to obtain adequate conservation of energy.
At each step of the trajectories, electronic and nuclear dipole
moments are calculated in order to serve as basis for simulating
the ps-TRIR spectra. The latter are simulated according to the
procedure that has been presented previously (for details see
ref. 21, 24, 36 and 37). Briefly, the ps-TRIR spectrum is
expressed as a trajectory average of the windowed Fourier
transform of the dipole derivative function of each trajectory,
where the probe pulse envelope serves as a window. The finite
width of the pump pulse is obtained by convoluting the
transient IR spectra with the experimental time resolution
obtained from measuring the cationic state population of the
NH isomer, which does not undergo any isomerisation
dynamics. In the simulations, Gaussian-shaped laser pulses
with the envelope of a FWHM of 3 ps were employed. The pulse
widths correspond to the experimentally determined values.
3. Results and discussion
3.1. UV spectra
3.1.1. REMPI spectra. Fig. 3 shows 1 + 102-color REMPI
spectra of 4MetFA and 4MetFA–W. The REMPI spectrum of
4MetFA in Fig. 3(A) reveals an intense band at 35 192 cm
1
,
which is red-shifted by 812 cm
1
from the S
1
’S
0
origin of tFA
(36 004 cm
1
).
38–42
This red-shift is in line with shifts induced
by CH
3
substitution at the para-position of mono-substituted
benzenes as summarized in Table S1 (ESI†), and no other
prominent band was observed below this transition. Thus, this
band is assigned to the S
1
’S
0
origin of 4MetFA. This assignment
is confirmed by IR spectroscopy discussed in Section 3.2. From the
analogy to tFA and tAA,
26,43–45
other weaker features are assigned to
S
1
vibronic bands (Table 1).
Fig. 2 Excitation scheme of the ns excitation–ps ionization–ps IR time resolved spectroscopy. A single vibronic level in the excited state, which is
prepared precisely by the ns excitation pulse (n
exc
(ns)), is ionized by n
ion
(ps) to trigger solvent dynamics in the cationic D
0
state, and the dynamics is
probed by the tunable n
IR
(ps) pulse through time-resolved IR photodissociation spectroscopy.
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The complicated appearance of the REMPI spectrum of
4MetFA–W shown in Fig. 3(A) suggests the existence of several
isomers. In the case of tFA, hydration occurs at either the NH or
CO sites, and the S
1
origins are shifted by 218 and +113 cm
1
from the tFA monomer, respectively.
39–41
The REMPI spectrum
of 4MetFA–W shows two pronounced bands shifted by 229
and +135 cm
1
from the origin band of the 4MetFA monomer.
Thus, these features are attributed to the origin bands of the
NH- and CO-bound isomers of 4MetFA–W, respectively.
These assignments are confirmed below by IR and IR-UV
hole-burning (IR-HB) spectroscopies. Major assignments
including vibronic bands are summarized in Table 1.
The conformation of the amide group in FA keeps a small
population in the cis form even under jet-cooled conditions.
40,42
The S
1
’S
0
origin band of cis-FA (34 914 cm
1
) shows a red-shift
of 1091 cm
1
from that of tFA (36 004 cm
1
) accompanied
with a long progression arising from a change in planarity of
the amide group.
40
The cis-4MeFA is also detected by REMPI
spectroscopy as shown in Fig. S3 in ESI.†The spectrum
shows much more complicated features than for cis-FA, prob-
ably due to coupling with the CH
3
internal rotation.
The detection of cis-FA–W in a jet expansion was reported
recently by microwave spectroscopy.
46
However, no transition
assignable to cis-4MeFA–W was detected by REMPI spectro-
scopy in the present work. A much larger hydration-induced
origin shift and/or some fast dynamics from the S
1
state for the
cis isomers may be reasons for the absence in the investigated
spectral range.
3.1.2. Photoionization efficiency (PIE) and photodissociation
efficiency (PDE) spectra. To deduce the energetics of the hydration
of 4MetFA, PIE and PDE spectra were measured via the respective
S
1
origin bands (Fig. S4 and S5 in ESI†). PIE spectra of 4MetFA
and 4MetFA–W(NH) demonstrate sharp steps at 65 135 and
62 167 cm
1
corresponding to their adiabatic ionization energies.
These are field-corrected values, which was 14 cm
1
in the
measurements. Appearance of the sharp steps means similarity
of the structures between the S
1
and D
0
states. The red-shift of IE
0
(2237 cm
1
) induced by the p-methyl substitution is consistent
to other mono-substituted benzenes (see Table S2 in ESI†). On the
other hand, the PIE spectrum of 4MetFA–W(CO) shows a gradual
increase, and no clear step is seen. Thus, only an upper bound of
IE
0
, 64 800 cm
1
, was estimated from the position at which a
detectable signal level could be obtained. The broad onset of the
PIE spectrum is indicative of a large structural change upon
ionization (including water migration) of 4MetFA–W(CO), similar
to those observed for tFA–W(CO) and tAA–W(CO).
PDE spectra of 4MetFA–W(NH/CO) were recorded to
determine the binding energies of the water ligand in a similar
manner to that of PIE measurements by monitoring 4MetFA
+
produced from photodissociation of the clusters (Fig. S5 in
ESI†). The binding energy was estimated only for 4MetFA–
W(CO), while that of 4MetFA–W(NH) was not determined
because no clear dissociation threshold was observed.
Transition energies of 4MetFA and 4MetFA–W(NH/CO)
obtained in this work are summarized in the energy diagram
shown in Fig. 4. Corresponding values for tFA(–W) and tAA(–W)
are compared in Fig. S6 (ESI†), and for a detailed discussion of
this diagram, we refer to previous work.
23,47
3.2. Stationary IR spectra
All stationary IR spectra measured in this study are compared
in Fig. 5, including 4MetFA and 4MetFA–W in the S
0
(A–C), S
1
(D–F) and D
0
states (G, H). These spectra were obtained by IR
Fig. 3 1+1
0REMPI spectra of (A) trans-4MeFA and (B) trans-4MeFA–W
obtained by ns lasers.
Table 1 Transition frequencies and vibrational assignments of REMPI
spectra of the trans-4MeFA and trans-4MeFA–W clusters
Species n
obs
/cm
1
n
vib
/cm
1
Assignments
bcd
trans-4MeFA 35 192 0 0
0
35 207 15 2e0
Me rot
35 244 52 3a
1
00
Me rot
(?)
35 367 175 b
amide
35 561 369 n
18 or 16
35 739 547 n
6b
35 975 783 n
12
or g
NH
trans-4MeFA–W 34 963 (229)
a
00
0
(NH)
34 998 35 2e0
Me rot
(NH)
35 013 50 3a
1
00
Me rot
(NH)
35 088 125 s
H2O
(NH)
35 142 179 b
amide
(NH)
35 327 (+135)
a
00
0
(CO)
35 336 373 n
18
or n
16
(NH)
35 371 407 ? (NH)
35 511 547 n
6b
(NH)
35 748 785 n
12
or g
NH
(NH)
833 ? (NH)
911 n
1
? (NH)
36 111 784 n
12
or g
NH
(CO)
36 156 829 ? (CO)
a
Shift from the monomer 0
0
transition.
b
Wilson notation.
c
(NH) and
(CO) mean the NH- and CO-bound isomers, respectively. The isomer
assignment is assured from IR-UV hole-burning spectra.
d
The ?
symbols indicate tentative or unknown vibrational assignments.
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dip spectroscopy. On the other hand, Fig. 5(I–L) show EI-IR
spectra of 4MetFA
+
–W, 4MetFA
+
–W–Ar, and 4MetFA
+
–Ar
1,2
.
Comparison with theoretical IR spectra is shown in Fig. S7
and S8 in the ESI.†
3.2.1. IR dip spectra in the S
0
state. IR dip spectra in the S
0
states show nearly the same appearance as those of tFA and
tFA–W,
39
except for additional transitions arising from the
additional CH
3
group. The similarity between the IR spectra
of tFA and 4MetFA and their hydrated clusters means that the
CH
3
group in the para-position does not have any significant
effect on interactions around the amide group. Thus, the
observed transitions are assigned as denoted in the figures
(Table 2). The IR spectrum of cis-4MetFA was also recorded
(Fig. S7 in the ESI†), though we do not discuss it in detail here.
Fig. 5(B and C) shows IR spectra of 4MetFA–W recorded by
probing at the 34 963 and 35 327 cm
1
origin bands in the
REMPI spectrum, respectively. They are assigned to 4MetFA–
W(NH) and 4MetFA–W(CO) isomers according to the
appearance of the hydrogen-bonded and free NH stretching
bands (n
HB
NH
and n
f
NH
, respectively) consistent with the S
1
origin
band shifts. IR-HB spectra shown in Fig. S9 in ESI†confirm
that these bands are indeed the S
1
’S
0
origin transitions of
these isomers, because they are the lowest-energy transitions in
each spectrum.
3.2.2. IR dip spectra in the S
1
state. Fig. 5(D–F) shows IR
dip spectra in the S
1
states recorded by exciting the S
1
’S
0
origin transitions. These spectra preserve the same features as
those in the S
0
states, and band positions almost match with
those of tFA and tFA–W in the S
1
states, respectively.
39
This
observation means that the S
1
states keep the same structures
as those in the S
0
states, and therefore that no dynamics
of the water ligand occurs by excitation to the S
1
origin.
The observed bands and their assignments are summarized
in the Table 3.
3.2.3 IR dip spectra in the D
0
state. Fig. 5(G and H) shows
IR dip spectra of 4MetFA
+
–W in the D
0
state ionized via the
S
1
’S
0
origin bands of the NH and CO isomers, respectively.
Both IR spectra closely resemble each other, even though
different hydration isomers are ionized. The nearly identical
IR spectra suggest that the structures of both isomers become
the same 50 ns after ionization to the D
0
state. The structures in
D
0
are assigned to 4MetFA
+
–W(NH), because of the appearance
of the free OH stretching vibrations (n
n1
OH
,n
n3
OH
) around
3700 cm
1
and the broad hydrogen-bonded NH stretching
vibration (n
HB
NH
) around 3200 cm
1
, as well as the disappearance
of the free NH stretching band (n
f
NH
). The observed bands and
their assignments are summarized in Table 4. This assignment
means that the water molecule in the CO-bound isomer migrates
to the NH site after ionization. Such an ionization-induced
hydration site switching has been reported for several mono-
hydrated amides (tFA,
24,48,49
tAA,
20,21,50
N-(2-phenylethyl)
acetamide)
51
and also for monohydrated 4-
aminobenzonitrile,
22,52
5-hydroxyindole,
25,53,54
tryptamine,
55
aminophenol,
56,57
and phenylglycine.
58
The isomerization
dynamics is investigated in Section 3.3 by ps-TRIR
spectroscopy.
3.2.4. Electron impact ionization (EI)-IR spectra in the D
0
state. Fig. 5(I) shows IR spectra of 4MetFA
+
–W obtained by EI-IR
spectroscopy, which predominantly probes the spectrum of the
most stable isomer in the D
0
state.
30
The spectrum matches with
the IR dip spectra in Fig. 5(G and H), confirming that the most
stable hydration structure in D
0
is the NH-bound isomer. This
result also means that the isomerization of water starting from
the CO-site terminates at the most stable NH binding site in D
0
.
Fig. 4 Energy diagram of cis/trans-4MeFA and trans-4MeFA–W(NH/CO) clusters derived mostly from experimental values. Values in parentheses
represent theoretical estimations (CAM-B3LYP/aug-cc-pVTZ).
a
The binding energy of water was estimated from the PDE spectrum.
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Fig. 5(K and L) shows EI-IR spectra of 4MetFA
+
–Ar
1,2
.Ar
tagging enables us to probe vibrational spectra of the monomer
cation, for which single-photon IRPD does not work. Also,
attachment of weakly-bound Ar atom(s) reduces the internal
energy of the cation, and definite band positions can be
determined by avoiding influences from hot bands. These
spectra are again nearly identical to those of tFA
+
–Ar
1,2
,
59
reflecting the negligible perturbation of the CH
3
group in the
para-position on the amide vibrations. The NH stretching
vibration appears at 3380–3390 cm
1
as a doublet reflecting
the two possible binding sites for Ar. The higher frequency
component stems from n
f
NH
of the p-bound isomer, in which Ar
sticks on the aromatic ring, while the lower one arises from n
HB
NH
of the NH-bound isomer, in which Ar is hydrogen-bonded to
the NH group.
59,60
The n
f
NH
band disappears in 4MetFA
+
–Ar
2
because the most stable NH binding site is occupied by one of
the Ar atoms (the other sits on the pring).
3.2.5. IR hole-burning (IR-HB) spectra. Using IR
transition(s) specific to individual species, IR-UV hole-burning
spectroscopy was performed to confirm the number of isomers
present in the jet expansion, and to extract vibronic bands
belonging to each isomer. IR transitions marked by asterisks in
Fig. 5A–C were used to probe (burn) species, and REMPI spectra
were recorded. Obtained spectra are compared with REMPI
spectra in Fig. S9 (ESI†). The hole-burning spectrum of 4MetFA
reproduces all bands in the REMPI spectrum, indicating that a
Fig. 5 IR dip spectra of trans-4MeFA, trans-4MeFA–W(NH), and trans-
4MeFA–W(CO) clusters in the S
0
states (A–C), trans-4MeFA, trans-
4MeFA–W(NH), and trans-4MeFA–W(CO) clusters in the S
1
states (D–F),
and trans-4MeFA
+
–W ionized via NH and CO bound clusters (G and H).
Panels (I–L) show EI-IR spectra of trans-4MeFA
+
–W, trans-4MeFA
+
–W–
Ar, trans-4MeFA
+
–Ar, and trans-4MeFA
+
–Ar
2
. Asterisks in panels (A–C)
represent bands used by IR-HB spectroscopy. n
ion
for measurement of the
spectra (G and H) was fixed at 30 295 cm
1
.
Table 2 Infrared bands and assignments of 4MeFA and 4MeFA–W in the
S
0
state
n
CHO
CH
n
Me
CH
n
arom
CH
n
HB
NH
n
f
NH
n
HB
OH
n
f
OH
trans-4MeFA 2770
a
2936 3033 — 3463 — —
2859
a
2962 3053
2993 3061
cis-4MeFA 2871 2934 3035 — 3441 — —
2963 3062
2996
trans-4MeFA–W(NH) 2766
a
2934 3033 3402 — — 3650 (n
1
)
2852
a
2960 3050 3745 (n
3
)
2985 3063
2993
trans-4MeFA–W(CO) 2792
a
2936 3035 — 3464 3512
a
3724
2886
a
2963 3054 3520
a
2995 3062 3534
a
a
Splitting due to anharmonic coupling.
Table 3 Infrared bands and assignments of 4MeFA and 4MeFA–W in the
S
1
state
Species n
CHO
CH
n
Me
CH
n
arom
CH
n
HB
NH
n
f
NH
n
HB
OH
n
f
OH
trans-4MeFA 2780
a
2935 3051 — 3415 — —
2860
a
2966 3065
2873
a
2988 3074
trans-4MeFA–W(NH) 2782
a
2996 3058 3344 — — 3649 (n
1
)
2858
a
3088 3744 (n
3
)
2873
a
trans-4MeFA–W(CO) 2894 2932 3052 — 3420 3555 3726
a
Splitting due to anharmonic coupling.
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single conformation contributes to the REMPI spectrum in
this range.
Transitions observed in the IR-UV hole-burning spectra of
4MetFA–W(NH/CO) reproduce the total REMPI spectrum, con-
firming that the two isomers are the only species observed in
the jet expansion. From the lowest transition of each spectrum,
the S
1
’S
0
origin transitions of the NH/CO bound clusters are
determined to be 34 963 and 35 327 cm
1
, respectively, as
deduced from the shifts from that of the monomer. Major
vibronic features are tentatively assigned in Table 1.
Table 4 Infrared bands and assignments of 4MeFA, 4MeFA–W, and 4MeFA–W–Ar in the D
0
state
Species n
CHO
CH b
n
Me
CHb
n
arom
CH
n
HB
NH
n
f
NH
n
HB
OH
n
f
OH
trans-4MeFA
+
–Ar (EI-IR) 2937 (?) 2903 (?) 3378 (NH-bound) 3393 (p-bound)
trans-4MeFA
+
–W (via NH) 2931 (?) 3060
a
— — 3634 (n
1
)
3180
a
3718 (n
3
)
3215
a
trans-4MeFA
+
–W (via CO) 2932 (?) 3060
a
3634 (n
1
)
3180
a
3717 (n
3
)
3215
a
trans-4MeFA
+
–W–Ar (EI-IR) 2915
a
3634 (n
1
)
3150
a
3719 (n
3
)
3175
a
a
Fermi resonance multiplets arising from anharmonic coupling of strongly IR active NH fundamental and overtones.
b
The ? symbols indicate
tentative assignments.
Fig. 6 (A) The ps-TRIR spectra of trans-4MeFA
+
–W recorded by the ns-excitation–ps-ionization scheme. n
ion
was fixed to 33213 cm
1
.Thetopand
bottom traces show ns stationary IR spectra in the S
1
and D
0
states, respectively. The EI-IR spectrum of trans-4MeFA
+
–Ar is also shown above the D
0
spectrum to indicate the n
f
NH
position of bare trans-4MeFA
+
. (B) Theoretical TRIR spectra obtained by the ab initio on-the-fly MD simulations (see Section 4).
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3.3. Picosecond time-resolved IR spectroscopy
3.3.1. ps-TRIR spectra. Fig. 6 shows ps-TRIR spectra
obtained by ionizing 4MetFA–W(CO) via the S
1
’S
0
origin.
Before ionization (Dto0), a strong band at 3555 cm
1
and a
weak band at 3726 cm
1
appear, which are assigned to n
HB
OH
and
n
f
OH
in the S
1
state by comparison to the stationary IR spectra in
the S
1
state (Fig. 5(F), which is reproduced at the top of Fig. 6).
Immediately after the ionization event, the S
1
state features
(n
HB
OH
and n
f
OH
) disappear, and instead, a new band comes up at
3393 cm
1
. This band position matches with that of n
f
NH
of
4MetFA
+
obtained from the EI-IR spectrum of 4MetFA
+
–Ar
(Fig. 5(K) and reproduced in the second bottom panel of
Fig. 6 for comparison). After Dt4B5 ps, the n
f
NH
band
gradually decreases in intensity, and a broad feature centered
at B3200 cm
1
grows. This broad absorption corresponds to
n
HB
NH
of the NH-bound structure. After 15 ps, the ps-TRIR spectra
do not change anymore, and the water molecule seems to have
reached a final stationary state, the NH-bound structure.
The ps-TRIR spectra prove that the O–HOQC hydrogen
bond is cleaved by ionization, and the water molecule starts the
isomerization. This process is caused by the strong repulsion
between the created positive charge of the cation and the
partial positive charges on the water hydrogens. The ps-TRIR
spectra also illustrate the existence of an intermediate during
the course of the isomerization process of the water ligand,
because n
f
NH
appears right after ionization and disappears after
10 ps, while n
HB
NH
continuously grows after 3 ps. This assignment
of the intermediate means that the water molecule is released
from the initial CO site but has not yet arrived at the final NH
site in 4MetFA
+
.
3.3.2. Time evolutions (TEs). The TEs of the n
f
NH
(3393 cm
1
) and n
HB
NH
(3200 cm
1
) vibrations in the ps-TRIR
spectra were traced to reveal dynamics of the hydration site
switching, and they are plotted in Fig. 7(A) and (B), respectively.
The n
f
NH
band, which represents the population of the intermedi-
ate, appears at Dt= 0 (ionization event) and immediately decays
until DtB15 ps, while n
HB
NH
gradually rises with the corresponding
rate of the n
f
NH
decay. This observation directly proves that the
water molecule is transferred from the CO site to the NH site after
ionization through an intermediate within B15 ps.
This behavior of the TEs is quite similar to those of tFA
+
–W
and tAA
+
–W, both of which show water migration motions with
an intermediate.
20,21,24
The structures of the intermediates,
however, are different depending on the migration pathways.
In the case of tFA
+
–W, the water molecule travels along an
in-plane path around the CHO group, while in tAA
+
–W it can
take two competing pathways. One is an in-plane path similar
to that of tFA
+
–W, while the other is out-of-plane via an
intermediate above the amide group.
20,21
These migration
mechanisms were concluded through fruitful combination
between the experimental observations and ab initio MD
simulations. The determination of the rate constants of the
observed dynamics in 4MetFA
+
–W by a kinetic analysis requires
a reaction model for the water migration process. Thus, we
again deduced the migration model with the help from the
ab initio MD simulation discussed in detail in Section 4.
3.4. Kinetic analysis of TEs
3.4.1. Reaction model. Fig. 8 shows the kinetic model
used for the analysis of the dynamics observed for 4MetFA
+
–
W. The model is based on four states proposed by the MD
simulations. The water migration starts from the CO site of the
CO-bound structure, the initial state (reactant R
+
). It is
generated by ionization from the S
1
state, which has the same
CO-bound structure as the S
0
state. The water molecule is
repelled after ionization from the CO site into the in-plane
isomerization path around the formyl (CHO) group toward the
NH site. After the NH site has been reached for the first time,
the water molecule passes over the NH site due to its large
translational energy. Then, it oscillates around the NH site until
the translational energy is lost by IVR, which distributes the
excess kinetic energy among low-frequency inter- and
intramolecular vibrations. This damped oscillating state is
represented in the model by two intermediates, a hot NH-
bound structure (P
+
(Hot)) and a hot NH-free structure (I
+
),
which are assumed to be in equilibrium. The final product
state (P
+
(Cold)) is the cold NH-bound structure formed when
the translational energy is reduced down to below the barrier
for the well at the NH site. Thus, the model is expressed in total
by four states, the initial reactant, two intermediates, and the
final product.
The initial state corresponds to the Franck–Condon (FC)
state of ionization from the S
1
origin, and has the n
f
NH
of
4MetFA
+
as characteristic mode, but its contribution to the
ps-TRIR spectra is low due to its short lifetime. Thus, the n
f
NH
band consists mainly of the intermediate of the hot NH-free
structure (I
+
). The hot NH-bound intermediate (P
+
(Hot)) and
the cold NH-bound product (P
+
(Cold)) both contribute to n
HB
NH
.
Their band positions and intensities are somewhat different
Fig. 7 TEs probed at the (A) n
f
NH
(3393 cm
1
) and (B) n
HB
NH
(3200 cm
1
)
vibrations of trans-4MeFA
+
–W obtained by the ns-excitation–ps-
ionization scheme (Fig. 6). The best fits obtained by the kinetic analysis
are also shown. The background signal in (A) after 30 ps arises from the tail
of the strong n
HB
NH
band. See Section S-13 in ESI†for the detained
explanation of the fitting model.
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because of their different effective hydrogen bond strength.
However, the extensive broadening of the n
HB
NH
band makes
them indistinguishable in the experiment. The reaction time
constants between these four states of the model are repre-
sented by t
1
–t
4
(Fig. 8). The TEs were fitted by functions derived
from the rate equations, details of which are given in the ESI.†
In the fitting, functions are convoluted by a Gaussian with a
FWHM of 2.8 ps, which represents the time response of the
measurement.
3.4.2. Results of fits. The best fits of the model are compared
in Fig. 7 to the experimental TEs, and the parameters are given in
Table 5. The fits reproduce the experimental TEs well, suggesting
that the reaction model contains the essential states of the water
migration dynamics. Thetimeconstantt
1
is much shorter than
the instrumental resolution. This fast dynamics corresponds to
the immediate formation of the intermediates, being consistent
with the strong initial repulsion between the water molecule and
the generated cation. The time constants t
2,3
for interconversion
oftheintermediatesarealsocomparabletotheinstrumental
resolution, and thus we do not discuss the differences of these
constants in detail. Comparison of the more significant and
reliable t
4
constant of 4MetFA with tFA will be discussed in the
next section.
3.4.3. Comparison to tFA–W. In this work, the ionization-
induced water migration dynamics in tFA
+
–W and tAA
+
–W were
re-measured by the ns-excitation–ps-ionization TRIR
spectroscopy to directly compare the results on the same
basis. The obtained ps-TRIR spectra and TEs are given in
Fig. S10 and S11 (ESI†), respectively. The ps-TRIR spectra
reproduce previous measurements well,
20,21,24
however with a
better spectral quality. The time constants obtained from the
kinetic fits to the TEs are compared in Table 5 for tFA
+
and
4MetFA
+
. The case of tAA
+
is not included because of its
different reaction model.
The time constants t
1
–t
3
are similar for 4MetFA
+
–W and
tFA
+
–W, confirming also similar initial migration dynamics in
both clusters. Indeed, the MD simulations predict similar
in-plane paths around the CHO group with overshooting at
the NH site. The time constants reveals however a marked
shortening of t
4
for 4MetFA
+
–W from 4.2 to 2.1 ps, indicating
faster translational cooling of the water molecule at the NH site
by IVR. This acceleration of IVR can be attributed to an
increased vibrational density of states (vDOS) in 4MetFA
+
arising from the additional CH
3
group. The anharmonic cou-
pling between the in-plane motion of the water molecule and
the NH group in tFA
+
/4MetFA
+
is assumed to be the same
because of the remote para-substitution of the CH
3
group.
To estimate the influence of the vDOS on the IVR rate, Fig. 9
compares the vDOSs of 4MetFA
+
–W, tFA
+
–W, and tAA
+
–W versus
internal vibrational energy. The vDOSs were evaluated from
differentiation of the smoothed curve of the total number of
vibrational states obtained by integrating the number of
vibrational states (vNOS) at each vibrational energy. vNOS was
calculated using direct counting of the unscaled harmonic
vibrational states obtained by the DFT calculations at a given
vibrational energy by the Beyer–Swinehart algorithm.
61
The
internal rotation of the CH
3
group was treated separately as a
one-dimensional hindered rotation with an effective rotational
constants of 5.2 cm
1
on the internal rotation potential
(without ZPE correction) obtained by relaxed scans at the same
theoretical level. The rotational potentials and energy levels are
shown in Fig. S12 in ESI.†Contributions from rotational states
were neglected because the differences in rotational constants
are small for the structures concerning the isomerization. From
their IEs and photon energies used for ionization, the internal
vibrational energies of the cluster cations are expected to be in
the range of 3000 to 5000 cm
1
. In this excess energy range,
vDOS of 4MetFA
+
–W is ca. 80 times higher than that of tFA
+
–W.
This is significantly higher than the observed enhancement of
IVR rate (1/t
4
) by a factor 2. The higher vDOS in 4MetFA
+
arises
not only from the free rotational levels but also from other low-
Fig. 8 Reaction model used for the kinetic fits of the TEs and resulting
time constants. Values in parentheses represent those for trans-FA
+
–W
discussed in detail in Fig. S10 and S11 (ESI†). See Section S-13 in ESI†for the
detained explanation of the rate equations.
Table 5 Time-constants (in ps) obtained from the kinetic fits to the
reaction model. See Section S-13 in ESI for the detained explanation of
time constants in the rate equations. The errors of the values are estimated
to be 0.5 ps
Time constant trans-4MeFA
+
–W trans-FA
+
–W
t
1
0.5 0.3
t
2
0.4 1.0
t
3
0.6 0.6
t
4
2.1 4.2
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frequency vibrations associated with the CH
3
group, such as its
bending vibrations. Actually, contributions from these vibrations
account for B40% of the increase in vDOS, meaning that the
existence of the CH
3
group itself is essential for the higher
density of low-frequency bath modes. Symmetry restriction due
to the nuclear spin statistics of the CH
3
group reduce the
effective vDOS. However, the difference between vDOS and the
observed increase in the IVR rate is difficult to be rationalized
only by the symmetry restriction. From the view point of the tier
model (Fig. S14, ESI†),
62
the methyl substitution at the para
position mainly increases the density of the second tier in the
bath, while the doorway states in tFA
+
–W and 4MetFA
+
–W are
similar. Thus, the acceleration of IVR occurs only from the
second tier, and thus the effective IVR enhancement is reduced
compared to the increase in vDOS.
4. Ab initio on-the-fly MD simulations
The MD simulations show that the water isomerization in
4MetFA–W proceeds along the same in-plane reaction pathway
as in the case of tFA
+
–W (Fig. 1 and the movie in ESI†).
24
In the
simulation, the NH site is reached after 500 fs for the first time.
The water molecule moves over to the opposite side of the
phenyl ring, but finally stabilizes near the final destination, i.e.,
the NH site in both systems. No out-of-plane migration, which
is dominant in tAA
+
–W, is found.
20,21
The simulated TR-IR spectra based on the MD simulations
are presented in Fig. 6B. The marker bands of the water
migration dynamics, n
HB
OH
,n
free
NH
, and n
HB
NH
for the initial structure,
the intermediate, and the final product are indicated in blue,
green, and red color, respectively. The decrease of the n
HB
OH
band, the appearance and disappearance of the intermediate,
and the rise of the final product are qualitatively well
reproduced.
In order to analyze the influence of the CH
3
group on the
energy redistribution dynamics, we consider the calculated
average kinetic energies of the water molecule for the
translational motion along the reaction coordinate (Fig. 10).
Both tFA
+
–W and 4MetFA
+
–W exhibit a fast initial rise of the
translational kinetic energy, which reaches a maximum around
Dt= 500 fs followed by damped oscillations. The damping
constant is significantly shorter for 4MetFA
+
–W (0.96 ps) than
for tFA
+
–W (1.53 ps). The fast damping of the kinetic energy in
4MetFA
+
–W is consistent with the fast IVR as already concluded
from the observed time evolution. From the reasonable repro-
duction of the observed time-resolved spectra of 4MetFA
+
–W,
the effect of the CH
3
group on the acceleration of the water
migration based on IVR is confirmed.
5. Summary
Water migration dynamics induced by photoionization of
4MetFA–W is revealed by the novel ns–ps–ps TRIR spectroscopy
approach, which enables both the highly specific selection of
the species at high spectral resolution and a sufficiently high
time resolution in the ps part. The acceleration of the dynamics
by the methyl group is clearly shown by comparison to the
ps-TRIR spectra of tFA
+
–W. The time evolution is analyzed by
rate equations based on the four-state model of the migration
dynamics. The strong contribution of IVR in the termination of
the solvation dynamics is demonstrated. This picture is well
reproduced by the ab initio on-the-fly MD simulations. Thus,
acceleration of the dynamics by the methyl group and its
relation to IVR are confirmed.
From the energetics, the driving force of the water migration,
i.e. hydration dynamics at the single molecular level, originates
from the initial watercation repulsion and the stabilization
at the final solvation site by the waterNH hydrogen bond.
The kinetics of the migration motion is predominantly
controlled by the energy randomization process in the cluster.
Fig. 9 Vibrational excess energy dependence of vibrational density of
states (vDOS) of trans-4MeFA
+
–W and trans-FA
+
–W. In trans-4MeFA
+
–W,
contributions from the CH
3
internal rotation are estimated regarding the
CH
3
groups as independent rotors. Dashed lines indicate the excess
energy range (3000–5000 cm
1
) of the product P
+
after photoionization.
Fig. 10 Time profiles of the kinetic energy change in the water molecule
of the trans-4MeFA
+
–W and trans-FA
+
–W obtained by the MD simulation.
Single exponential curves representing the decay of the energy are shown
as dashed lines. The damping time constants are 0.96 ps for 4MetFA
+
–W
and 1.53 ps for tFA
+
–W.
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These results point out that both solvent–solute interaction and
solute-bath energy flow must be carefully considered in the
microscopic description. It demonstrates the advantage of
the application of ps-TRIR spectroscopy to solvated clusters for
the study of solvation dynamics at the molecular level, which is
still challenging in the condensed phase.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported in part by Bilateral Open Partnership
Joint Research Projects of Japan Society for the Promotion of
Science, research grant from World Research Hub Initiative
(WRHI) of Tokyo Institute of Technology, KAKENHI (19K05368,
19H05527, 20H00372), and the Cooperative Research Program
of the ‘‘Network Joint Research Center for Materials and
Devices’’, and Core -to-core program (JPJSCCA20210004) from
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan. This study was also supported by
Deutsche Forschungsgemeinschaft (DO 729/4-2). The theoretical
calculations were performed using the Research Center for
Computational Science (RCCS), Okazaki, Japan. M. M. and
M. F. are grateful for supports from the Alexander von Humboldt
Foundation. O. D. acknowledges travel support from the
World Research Hub Initiative (WRHI) of Tokyo Institute of
Technology.
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