Intermolecular Stretch Research Articles

H2O-HCN complex: A new potential energy surface and intermolecular rovibrational states from rigorous quantum calculations.

In this work the H2O-HCN complex is quantitatively characterized in two ways. First, we report a new rigid-monomer 5D intermolecular potential energy surface (PES) for this complex, calculated using the symmetry-adapted perturbation theory based on density functional theory method. The PES is based on 2833 abinitio points computed employing the aug-cc-pVQZ basis set, utilizing the autoPES code, which provides a site-site analytical fit with the long-range region given by perturbation theory. Next, we present the results of the quantum 5D calculations of the fully coupled intermolecular rovibrational states of the H2O-HCN complex for the total angular momentum J values of 0, 1, and 2, performed on the new PES. These calculations rely on the quantum bound-state methodology developed by us recently and applied to a variety of noncovalently bound binary molecular complexes. The vibrationally averaged ground-state geometry of H2O-HCN determined from the quantum 5D calculations agrees very well with that from the microwave spectroscopic measurements. In addition, the computed ground-state rotational transition frequencies, as well as the B and C rotational constants calculated for the ground state of the complex, are in excellent agreement with the experimental values. The assignment of the calculated intermolecular vibrational states of the H2O-HCN complex is surprisingly challenging. It turns out that only the excitations of the intermolecular stretch mode can be assigned with confidence. The coupling among the angular degrees of freedom (DOFs) of the complex is unusually strong, and as a result most of the excited intermolecular states are unassigned. On the other hand, the coupling of the radial, intermolecular stretch mode and the angular DOFs is weak, allowing straightforward assignment of the excitation of the former.

From Local Hydration Motifs in Aqueous Acetic Acid Solutions to Macroscopic Mixing Thermodynamics: A Quantitative Connection from THz-Calorimetry.

We report the results of THz measurements (30-440 cm-1) of aqueous acetic acid solutions over the full mixing range (XAcAc = 0-1). We recorded spectroscopic observables as a function of the acetic acid concentration in the frequency range of the intermolecular stretch at 150 cm-1 and of the librational modes at 350-440 cm-1. This allowed us to unravel changes in hydrophobic and hydrophilic hydration motifs, respectively. By means of a novel THz-calorimetry approach, we quantitatively correlated these changes in local hydration motifs to excess mixing entropy and enthalpy. We find that ΔHmix is determined by both hydrophobic and hydrophilic solvation contributions. In contrast, ΔSmix is governed by hydrophobic cavity formation. Our results further suggest that acetic acid-water mixtures are systems at the edge of phase separation due to endothermic contributions from both hydrophilic and hydrophobic solvation in a large portion of the miscibility range. Our work establishes a quantitative relationship between the balance of local hydrophobic and hydrophilic solvation motifs and the macroscopic mixing thermodynamic properties.

Unified view of the hydrogen-bond structure of water in the hydration shell of metal ions (Li+, Mg2+, La3+, Dy3+) as observed in the entire 100–3800 cm−1 regions

Hydrogen-bond (H-bond) structure of water in the hydration shell of an anion is fairly well understood, but the corresponding knowledge for cation hydration shell is sparse, mainly because of the overwhelming effect of anion on water that easily masks the subtle effect of cation. Moreover, most of the hydration shell investigations are confined to the OH stretch band of water. Using Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF), we have retrieved the spectrum of water pertaining to the hydration shell of different metal ions (Li+, Mg2+, La3+, Dy3+) in the entire spectral region of 100–3800 cm−1, encompassing the intermolecular H-bond stretch (HOH---OH2; 200 cm−1), librational (300–1000 cm−1), HOH bend (1640 cm−1), bend-librational combination (2120 cm−1) and intramolecular O–H stretch (3000–3700 cm−1) of water. The hydration shell spectra and their polarization dependence (Polarized Raman) are compared for different metal ions and with the hydration shell of Cl− anion and also the high temperature water component (80 °C; extracted by Raman-DS-SCF). These results reveal that the hydration shells of high charge density metal ions have “enhanced” as well as “reduced” H-bond structure. The intermolecular H-bonds become stiffer and the librational freedom of the water gets restricted in the hydration shell. The bend-librational combination band blue-shifts to a large extent and appears toward the tail of the OH stretch, which is expected to facilitate the ultrafast energy dissipation of higher vibrational modes of water.

Spectroscopic Fingerprints of Cavity Formation and Solute Insertion as a Measure of Hydration Entropic Loss and Enthalpic Gain.

Hydration free energies are dictated by a subtle balance of hydrophobic and hydrophilic interactions. We present here a spectroscopic approach, which gives direct access to the two main contributions: Using THz‐spectroscopy to probe the frequency range of the intermolecular stretch (150–200 cm−1) and the hindered rotations (450–600 cm−1), the local contributions due to cavity formation and hydrophilic interactions can be traced back. We show that via THz calorimetry these fingerprints can be correlated 1 : 1 with the group specific solvation entropy and enthalpy. This allows to deduce separately the hydrophobic (i.e. cavity formation) and hydrophilic contributions to thermodynamics, as shown for hydrated alcohols as a case study. Accompanying molecular dynamics simulations quantitatively support our experimental results. In the future our approach will allow to dissect hydration contributions in inhomogeneous mixtures and under non‐equilibrium conditions.

Spectroscopic Fingerprints of Cavity Formation and Solute Insertion as a Measure of Hydration Entropic Loss and Enthalpic Gain

AbstractHydration free energies are dictated by a subtle balance of hydrophobic and hydrophilic interactions. We present here a spectroscopic approach, which gives direct access to the two main contributions: Using THz‐spectroscopy to probe the frequency range of the intermolecular stretch (150–200 cm−1) and the hindered rotations (450–600 cm−1), the local contributions due to cavity formation and hydrophilic interactions can be traced back. We show that via THz calorimetry these fingerprints can be correlated 1 : 1 with the group specific solvation entropy and enthalpy. This allows to deduce separately the hydrophobic (i.e. cavity formation) and hydrophilic contributions to thermodynamics, as shown for hydrated alcohols as a case study. Accompanying molecular dynamics simulations quantitatively support our experimental results. In the future our approach will allow to dissect hydration contributions in inhomogeneous mixtures and under non‐equilibrium conditions.

N2O–Ar and N2O–Kr: Intermolecular vibrations of N2O–Kr and symmetry breaking of the N2O bending mode in the presence of a rare gas

Rotationally-resolved infrared spectra of N2O–Ar and N2O–Kr are studied in the region of the N2O ν1 vibration (≈2224 cm−1) using a tunable Quantum Cascade laser source to probe a pulsed supersonic jet. The N2O–Kr ν1 fundamental band is re-analyzed, together with previous ν3 band data, using a unified scheme to fit the (small) observed Kr isotope splittings. This scheme is then transferred to analyze the bending combination band of N2O–Kr near 2257 cm−1 where isotope effects are much larger due to stretch-bend interactions. As a result, N2O–Kr intermolecular bend (33.29 cm−1) and stretch (34.48 cm−1) frequencies are directly determined for the first time. Weak spectra are detected for both N2O–Ar and –Kr corresponding to the (ν1, ν2l2, ν3) = (1110) ← (0110) hot band of N2O. Their analysis yields the magnitude of the splitting of the N2O ν2 bending mode into in-plane and out-of-plane components. These splittings are found to be significantly smaller than those observed in the analogous CO2-containing dimers.

Ab initio molecular dynamics study on energy relaxation path of hydrogen-bonded OH vibration in bulk water.

The vibrational energy relaxation paths of hydrogen-bonded (H-bonded) OH excited in pure water and in isotopically diluted (deuterated) water are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The present study extends the previous NE-AIMD simulation for the energy relaxation of an excited free OH vibration at an air/water interface [T. Ishiyama, J. Chem. Phys. 154, 104708 (2021)] to the energy relaxation of an excited H-bonded OH vibration in bulk water. The present simulation shows that the excited OH vibration in pure water dissipates its energy on a timescale of 0.1ps, whereas that in deuterated water relaxes on a timescale of 0.7ps, consistent with the experimental observations. To decompose these relaxation energies into the components due to intramolecular and intermolecular couplings, constraints are introduced on the vibrational modes except for the target path in the NE-AIMD simulation. In the case of pure water, 80% of the total relaxation is attributed to the pathway due to the resonant intermolecular OH⋯OH stretch coupling, and the remaining 17% and 3% are attributed to intramolecular couplings with the bend overtone and with the conjugate OH stretch, respectively. This result strongly supports a significant role for the Förster transfer mechanism of pure water due to the intermolecular dipole-dipole interactions. In the case of deuterated water, on the other hand, 36% of the total relaxation is due to the intermolecular stretch coupling, and all the remaining 64% arises from coupling with the intramolecular bend overtone.

New infrared spectra of CO2–Xe: modelling Xe isotope effects, intermolecular bend and stretch, and symmetry breaking of the CO2 bend

The infrared spectrum of the weakly bound CO2–Xe complex is studied in the region of the carbon dioxide ν3 fundamental vibration (≈2350 cm−1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. The Xe isotope dependence of the spectrum is modelled by scaling the vibrational and rotational parameters, with the help of previous microwave data. The scaling model provides a good simulation of the observed broadening and (partial) splitting of transitions in the fundamental band, and it is essential for understanding the intermolecular bending combination band where some transitions are completely split by isotope effects. The combination band is influenced by a significant bend–stretch Coriolis interaction and by the relatively large Xe isotope dependence of the intermolecular stretch frequency. The weak CO2–Xe spectrum corresponding to the (0111) ← (0110) hot band of CO2 is also detected and analysed, providing a measurement of the symmetry breaking of the CO2 bending mode induced by the nearby Xe atom. This in-plane/out-of-plane splitting is determined to be 2.14 cm−1.

Fermi resonance switching in KrH+Rg and XeH+Rg (Rg = Ne, Ar, Kr, and Xe).

Matrix isolation experiments have been successfully employed to extensively study the infrared spectrum of several proton-bound rare gas complexes. Most of these studies have focused on the spectral signature for the H+ stretch (ν3) and its combination bands with the intermolecular stretch coordinate (ν1). However, little attention has been paid to the Fermi resonance interaction between the H+ stretch (ν3) and H+ bend overtone (2ν2) in the asymmetric proton-bound rare gas dimers, RgH+Rg'. In this work, we have investigated this interaction on KrH+Rg and XeH+Rg with Rg = (Ne, Ar, Kr, and Xe). A multilevel potential energy surface (PES) was used to simulate the vibrational structure of these complexes. This PES is a dual-level comprising of second-order Møller-Plesset perturbation theory and coupled-cluster singles doubles with perturbative triples [CCSD(T)] levels of ab initio theories. We found that when both the combination bands (nν1 + ν3) and bend overtone 2ν2 compete to borrow intensity from the ν3 band, the latter wins over the former, which then results in the suppression of the nν1 + ν3 bands. The current simulations offer new assignments for the ArH+Xe and KrH+Xe spectra. Complete basis set (CBS) binding energies for these complexes were also calculated at the CCSD(T)/CBS level.

Observation of the Low‐Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces

AbstractUsing the helium nanodroplet isolation setup at the ultrabright free‐electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm−1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in‐plane and out‐of‐plane librational modes. This experimental data set provides a sensitive test for state‐of‐the‐art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high‐level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full‐dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.

Observation of the Low-Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces.

Using the helium nanodroplet isolation setup at the ultrabright free‐electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm−1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in‐plane and out‐of‐plane librational modes. This experimental data set provides a sensitive test for state‐of‐the‐art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high‐level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full‐dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.

Assignment of Infrared-Active Combination Bands in the Vibrational Spectra of Protonated Molecular Clusters Using Driven Classical Trajectories: Application to N4H+ and N4D+

We investigate the utility of the driven molecular dynamics (DMD) approach to complex molecular vibrations by applying it to linear clusters with several degenerate vibrational modes and infrared (IR) intense combination bands. Here, the prominent features in N4H+ and N4D+ IR spectra, reported and described by others previously, have been characterized for the first time by DMD using recently published high-level potential and dipole moment surfaces. Namely, the calculations closely correlate the parallel proton stretch vibration in N4H+, at 750 cm-1, with the one observed experimentally at 743 cm-1. Second, the intense IR-active combination bands found in experimental spectra within 900-1100 cm-1 have been properly recovered by DMD at 950 cm-1 as strongly IR-active and confirmed as consisting of H+ asymmetric stretch and N2···N2 intermolecular symmetric stretch modes. Furthermore, we show that certain combination bands involving overtone transitions may be recovered by DMD using a hard-driving regime, such as the 1409 cm-1 band measured in N4H+, revealed by DMD at 1375 cm-1, and assigned to a progressive combination of the parallel H+ stretch and two quanta of N2···N2 stretch, in agreement with quantum mechanical studies reported previously by others.

Effect of alkali ions on optical properties of flavins: vibronic spectra of cryogenic M+lumichrome ions (M = Li-Cs) in the gas phase.

The photochemical properties of flavins depend sensitively on their environment and are strongly modified by coordination with metal ions. Herein, the electronic spectra of cold complexes of the smallest flavin molecule (lumichrome, LC, C12N4O2H10) with alkali ions (M+LC, M = Li-Cs) are measured by photodissociation in the visible range (VISPD) in a cryogenic ion trap coupled to a tandem mass spectrometer and an electrospray ionization source. The observed vibronic spectra of all ions are assigned to the optically bright S1 ← S0 (ππ*) transition of the most stable O4 isomer of M+LC by comparison with quantum chemical calculations at the PBE0/cc-pVDZ level coupled to multidimensional Franck-Condon simulations. The rich vibronic spectra indicate substantial geometry changes upon S1 excitation. Large red shifts of the S1 origins upon metal complexation and progressions in the intermolecular in-plane metal stretch and bend modes demonstrate that the strength of the metal-flavin interaction in M+LC(O4) strongly increases by S1 excitation. The stronger M+LC bond in the S1 state of M+LC(O4) is rationalized by the charge reorganization upon ππ* excitation of the LC chromophore. The computations confirm that the optical properties of LC can be strongly modulated by metalation via both the type and binding site of the metal ion.

Near-infrared spectroscopy and anharmonic theory of the H2O+Ar1,2 cation complexes

Weakly bound complexes of the water radical cation with argon (H2O+Arn, n = 1,2) were generated by an electrical discharge/supersonic expansion and probed with mid- and near-infrared photodissociation spectroscopy in the 2050-4550 and 4850-7350 cm-1 regions. To elucidate these spectra, these complexes were studied computationally at the CCSD(T) level including anharmonicity with the VPT2 method. The comparison between experiment and predicted spectra demonstrates that the VPT2 method is adequate to capture most of the vibrational band positions and their intensities. In addition to the fundamentals, overtones of the symmetric and the asymmetric water stretches and their combination were detected. Additional bands were assigned to combinations of the overtone of the bound O-H stretch with multiple excitation levels of the intermolecular Ar-H stretch. H2O+Ar2 could not be dissociated in the higher frequency region (4850-7350 cm-1).

Solvation Dynamics of Trimethylamine N-Oxide in Aqueous Solution Probed by Terahertz Spectroscopy.

We have studied the hydration dynamics of trimethylamine N-oxide (TMAO) in aqueous solution using a combination of concentration-dependent terahertz/far-infrared (THz/FIR) and Raman spectroscopic techniques. Terahertz/FIR absorption was measured using narrowband (76-93 cm(-1)) p-Ge laser and broad band (30-400 cm(-1)) Fourier transform spectroscopy. We used principal component analysis in combination with a semi-ideal chemical equilibrium model to dissect the spectra into linear and nonlinear contributions of the solvated solute extinction. We attribute the linear part to the average extinction and Raman scattering of TMAO-water aggregates with approximately 3-4 water strongly hydrogen bonded to TMAO. An additional nonlinear concentration dependence indicates a decrease of the number of attached water molecules with increasing TMAO concentrations due to a shift in association equilibria. The Raman spectra reveal a frequency shift of the (narrowband) intramolecular vibrations with decreasing dilution. Based on the results of a detailed analysis and isotopic substitution, the experimentally observed absorption bands at 0, 176, and 388 cm(-1) could be assigned to water relaxation modes, an intermolecular TMAO-H2O stretch, and the C-N-C bending mode, respectively. Our results provide evidence for a local modification of the water structure.

Structure and Intermolecular Vibrations of Perylene·trans-1,2-Dichloroethene, a Weak Charge-Transfer Complex

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene trans-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S0 ↔ S1 spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(–1) (S1). The intermolecular Tz stretch and the Ra and Rb bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a(u) “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from D2h to D2. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of D(e) = −20.8 to −26.1 kJ/mol. The 000 band of the S0 → S1 transition is red-shifted by δν = −301 cm(–1) relative to that of perylene, implying that the D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb vibrations by comparing to the observed/calculated frequencies and S0 ↔ S1 Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results.

The Cr+–D2 cation complex: Accurate experimental dissociation energy, intermolecular bond length, and vibrational parameters

The infrared spectrum of the T-shaped (52)Cr(+)-D(2) complex is measured over the 2742-2820 cm(-1) range by detecting Cr(+) photofragments. The main band, due to the D-D stretch excitation, is shifted at 215 cm(-1) to lower energy from the Q(0) (1) transition of the free D(2) molecule and displays clearly resolved rovibrational transitions. Observation of a photodissociation onset for the N(')=8 rotational level is used to infer that the dissociation energy of Cr(+)-D(2), with respect to ground-state Cr(+) and D(2) fragments, lies between 2839.7 and 2856.9 cm(-1). Perturbations to the upper state levels are presumed to arise from interactions with quasibound combination levels involving the intermolecular stretch and bend vibrational modes. A vibrationally averaged Cr(+)...D(2) separation of 2.023 A and an estimate of 394 cm(-1) for the intermolecular harmonic stretching frequency are derived from the measured rotational constants.

O−H···O versus O−H···S Hydrogen Bonding I: Experimental and Computational Studies on thep-Cresol·H2O andp-Cresol·H2S Complexes

The weak hydrogen bonding ability of sulfur-containing hydrides makes it difficult to study their complexes and has not been characterized experimentally so far. In this work, the hydrogen-bonded complexes of H(2)S and H(2)O with p-cresol (p-CR) were studied using a variety of techniques such as two-color resonant two-photon ionization (2c-R2PI) spectroscopy, single vibronic level fluorescence (SVLF) spectroscopy, resonance ion dip infrared spectroscopy (RIDIRS), and fluorescence dip infrared spectroscopy (FDIRS), with an aim of comparing the nature and strength of their respective hydrogen bonding abilities. The intermolecular stretch (sigma) and the shift in the O-H stretching frequency of p-CR in the complex were taken as the measures of the O-H...O and O-H...S hydrogen bonding strength. The experiments were complemented by the ab initio calculations, atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analyses carried out at different levels of theory. The experimental data indicates that in the p-CR x H(2)S complex, the phenolic OH group acts as a hydrogen bond donor, and sulfur as the acceptor. Further, it indicates that the p-CR x H(2)S complex was about half as strong as the p-CR x H(2)O complex. The AIM and NBO analyses corroborate the experimental findings. The energy decomposition analyses for the O-H...S hydrogen bond in the p-CR x H(2)S complex reveal that the dispersion interaction energy has the largest contribution to the total interaction energy, which is significantly higher than that in the case of the p-CR x H(2)O complex.

IR-REMPI Double Resonance Spectroscopy: The Near-IR Spectrum of NO−Ar Revisited

We describe a new approach to IR-UV double resonance spectroscopy of NO-containing van der Waals complexes. The basic idea combines REMPI detection through a hot band transition with a simultaneous frequency scan of the IR and UV lasers in such a way that the combined photon energy is kept constant throughout the scan, matching a UV resonance transition in the system. As a result, the two-dimensional frequency problem is reduced to a fixed number of one-dimensional frequency scans, each defined by a particular photon energy sum. The method is applied to the near-IR spectrum of NO-Ar using hot band detection via the electronic A state of the complex. In the frequency range from 3718 to 3765 cm(-1), we recorded the previously known vibrational bands with improved frequency resolution. The increased sensitivity of the present experiment allowed us to measure for the first time their overtone, combination, and hot bands. Through the comparison with results of a close-coupling (CC) calculation, we were able to assign most of the rovibrational structures of the spectrum. Except for the first intermolecular stretch level, the band positions and rotational structures of the observed bands are in good agreement with the predictions of the CC calculations.

Multidimensional Franck-Condon simulations of photodetachment spectra for the formate-water cluster anion: Investigating H atom transfer along the HCOOH+OH reaction coordinate

A new multidimensional Franck-Condon (FC) simulation methodology was applied to an anionic-neutral cluster transition for the first time to investigate the use of photodetachment spectroscopy of the HCOO(-).H(2)O anion as a means to study the HCOO.H(2)O and HCOOH.OH neutral clusters. For the HCOO(-).H(2)O to HCOO.H(2)O transition, vibrationally resolved simulated spectra were obtained across the threshold detachment region, indicating that photodetachment spectroscopy of the respective anionic cluster should provide detailed structural information on the bifurcated HCOO.H(2)O neutral cluster. The simulations predict that the photodetachment spectra should display prominent progressions of both the intermolecular stretch and the in-plane OCO bending mode. In contrast, for the HCOO(-).H(2)O to HCOOH.OH transition, the vibronic FC simulations resulted in transitions with negligible intensities, despite the fact that the geometries of the respective anionic and neutral systems were similar. The low FC intensities were traced to the large off-diagonal elements of the Duschinsky matrix for this transition, which arise due to the considerable differences in the vibrational wave functions following hydrogen transfer.