Fourier transform infrared spectroscopy and ab initio theory of acid-hydrogen sulfide clusters: H2S-HCl, D2S-DCl and H2S-(HCl)(2)

The rotationally resolved Fourier transform infrared (FTIR) spectrum of the nu(s) HCl and DCl stretching bands for the hydrogen bonded complex H2S-HCl and its isotopomer D2S-DCl have been observed in a supersonic jet at 0.02 cm(-1) resolution. In the same experimental conditions, two additional bands observed without rotational structure in the HCl range of the dimer have been assigned to the cyclic trimer H2S-(HCl)(2). The multidimensional coupling picture involving the donor stretch mode nu(s) and low frequency intermolecular modes already evidenced in several medium strength hydrogen bonded complexes is beautifully confirmed by the observation of completely separated hot band progressions in the 198 K cell spectrum of both dimers. Based on our anharmonic adiabatic approach for the treatment of the coupled vibrations, absolute vibrational frequencies, diagonal and off-diagonal anharmonicities as well as rovibrational coupling constants obtained from analyses of several 2-D subspaces at MP2 and CCSD(T) level are in excellent agreement with spectroscopic results. In the case of small light complexes, the combination of elevated rotational constants and a negligible contribution of intramolecular vibrational redistribution (IVR) improve the reliability of predissociation lifetime measurements, estimated to 180 ps for H2S-HCl and above 200 ps for D2S-DCl.     

Rovibrational and dynamical properties of the hydrogen bonded complex (CH2)2S-HF: a combined free jet, cell, and neon matrix-Fourier transform infrared study

Fourier transform infrared spectra of the nu(s) (HF stretching) band of the (CH(2))(2)S-HF complex have been recorded at 0.1-0.5 cm(-1) resolution in a cooled cell, in a supersonic jet expansion seeded with argon and in a neon matrix at 4.5 K. The combination of controlled temperature effects over a range of 40-250 K and a sophisticated band contour simulation program allows the separation of homogeneous and inhomogeneous contributions and reveals significant anharmonic couplings between intramolecular and intermolecular vibrational modes similar to our previous work on (CH(2))(2)S-DF. The sign of the coupling constants is consistent with the expected strengthening of the hydrogen bond upon vibrational excitation of HF which also explains the observed small variations of the geometrical parameters in the excited state. The analysis of sum and difference combination bands involving nu(s) provides accurate values of intermolecular harmonic frequencies and anharmonicities and a good estimate of the dissociation energy of the complex. Frequencies and coupling parameters derived from gas phase spectra compare well with results from neon matrix experiments. The effective linewidth provides a lower bound for the predissociation lifetime of 10 ps. The comparison between effective linewidths and vibrational densities of states for (CH(2))(2)S-HF and -DF complexes highlights the important role of intramolecular vibrational redistribution in the vibrational dynamics of medium strength hydrogen bonds.     

Theoretical study of the polarized infrared spectra of the hydrogen bond in 2‐furoic acid crystal dimer

This work presents a theoretical simulation of ν_{O? H} and ν_{O? D} band shapes in the polarized infrared spectra of 2‐furoic acid dimer crystals measured at liquid‐nitrogen temperature. The line shapes are studied theoretically within the framework of the anharmonic couplings between low‐frequency hydrogen‐bond vibrations and degenerate excited states of high‐frequency hydrogen vibrations in hydrogen‐bonded dimers and the anharmonic coupling between the first excited state of the fast mode and the harmonics or band combinations of some low‐frequency bending modes, which lead to Fermi resonances.This approach takes into account the adiabatic approximation, the intrinsic anharmonicity of the low‐frequency mode through a Morse potential, Davydov coupling triggered by resonance exchange between the excited states of the fast modes of the two hydrogen bonds involved in the cyclic dimer, and the direct and indirect damping of the fast‐stretching modes of the hydrogen bonds and of the bending modes. The infrared spectral density was calculated within the linear response theory by Fourier transform of the autocorrelation function of the transition dipole moment operator of the fast mode. Numerical results show that mixing of all these effects allows satisfactory reproduction of the main features of the experimental IR line shapes of crystalline H‐ and D‐bonded 2‐furoic acid at liquid‐nitrogen temperature and for different polarizations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

The (CH2)2O-H2O hydrogen bonded complex. Ab Initio calculations and Fourier transform infrared spectroscopy from neon matrix and a new supersonic jet experiment coupled to the infrared AILES beamline of synchrotron SOLEIL

A series of hydrogen bonded complexes involving oxirane and water molecules have been studied. In this paper we report on the vibrational study of the oxirane-water complex (CH(2))(2)O-H(2)O. Neon matrix experiments and ab initio anharmonic vibrational calculations have been performed, providing a consistent set of vibrational frequencies and anharmonic coupling constants. The implementation of a new large flow supersonic jet coupled to the Bruker IFS 125 HR spectrometer at the infrared AILES beamline of the French synchrotron SOLEIL (Jet-AILES) enabled us to record first jet-cooled Fourier transform infrared spectra of oxirane-water complexes at different resolutions down to 0.2 cm(-1). Rovibrational parameters and a lower bound of the predissociation lifetime of 25 ps for the v(OH)(b) = 1 state have been derived from the rovibrational analysis of the ν(OH)(b) band contour recorded at respective rotational temperatures of 12 K (Jet-AILES) and 35 K (LADIR jet).     

Axial and equatorial hydrogen-bond conformers between (CH2)3S and H(D)F: Fourier transform infrared spectroscopy and ab initio calculations

The coexistence of axial and equatorial hydrogen-bonded conformers of 1?:?1 (CH(2))(3)S-HF (and -DF) has been observed in the same adiabatic expansion of a supersonic jet seeded with argon and in a static absorption cell at room temperature. High level calculations computed the axial conformer to be the most stable one with a small energy difference with respect to the equatorial one, in full agreement with previous microwave experiments. On the grounds of band contour simulations of FTIR spectra and ab initio energetic and anharmonic vibrational calculations, two pairs of ν(s) HF donor stretching bands, observed in a series of jet-FTIR spectra at 3457.9 and 3480.5 cm(-1) have been respectively assigned to the axial and equatorial forms of the 1?:?1 complex. In the jet-FTIR spectra series with HF, the assignment of an additional broad band (about 200 cm(-1) higher in frequency with respect to ν(s)) to a 1?:?2 complex has been supported by theoretical investigations. Experimental detection of both axial and equatorial forms of a cyclic trimer has been confirmed by calculated energetic and vibrational properties. The nature of hydrogen bonding has been examined within topological frameworks. The energetic partitioning within the 1?:?1 dimers has been elucidated with SAPT techniques. Interestingly, the interconversion pathway between two 1?:?1 structures has been explored and it was seen that the formation of the 1?:?1 complex affects the interconversion barrier on the ring puckering motion. The band contour analysis of gas phase FTIR experiments provided a consistent set of vibrational frequencies and anharmonic coupling constants, in good agreement with ab initio anharmonic vibrational calculations. Finally, from a series of cell-FTIR spectra recorded at different partial pressures of (CH(2))(3)S and HF monomers, the absorption signal of the 1?:?1 complex could be isolated which enabled to estimate the equilibrium constant K(p) = 0.023 at 298 K for the dimerization.     

Potential curves for proton motion in H-bonded systems: HF and H2O polymers

The presence of long range coupling between hydrogen atoms is shown for the HF and H₂O hydrogen bonded systems. The coupling of H atoms critically depends on the spatial orientation of the H atoms being considered. Explicit calculations of the potential curves of the protons are performed using as a model a ring of six HF, or H₂O, molecules. The method of calculation is the CNDO/2. The strong similarities of the results for H₂O and HF polymers supports the conclusion that the coupling is essentially due to factors such as the asymmetric equilibrium position of the H atoms, the high electronic polarizability of the system, etc.     

Vibrational spectroscopy of protonated imidazole and its complexes with water molecules: ab initio anharmonic calculations and experiments

The results of anharmonic frequency calculations on neutral imidazole (C3N2H4, Im), protonated imidazole (ImH+), and its complexes with water (ImH+)(H2O)n, are presented and compared to gas phase infrared photodissociation spectroscopy (IRPD) data. Anharmonic frequencies are obtained via ab initio vibrational self-consistent field (VSCF) calculations taking into account pairwise interactions between the normal modes. The key results are: (1) Prediction of anharmonic vibrational frequencies on an MP2 ab initio potential energy surface show excellent agreement with experiment and outstanding improvement over the harmonic frequencies. For example, the ab initio calculated anharmonic frequency for (ImH+)(H2O)N2 exhibits an overall average percentage error of 0.6% from experiment. (2) Anharmonic vibrational frequencies calculated on a semiempirical potential energy surface fitted to ab initio harmonic data represents spectroscopy well, particularly for water complexes. As an example, anharmonic frequencies for (ImH+)H2O and (ImH+)(H2O)2 show an overall average deviation of 1.02% and 1.05% from experiment, respectively. This agreement between theory and experiment also supports the validity and use of the pairwise approximation used in the calculations. (3) Anharmonic coupling due to hydration effects is found to significantly reduce the vibrational frequencies for the NH stretch modes. The frequency of the NH stretch is observed to increase with the removal of a water molecule or replacement of water with N2. This result also indicates the ability of the VSCF method to predict accurate frequencies in a matrix environment. The calculation provides insights into the nature of anharmonic effects in the potential surface. Analysis of percentage anharmoncity in neutral Im and ImH+ shows a higher percentage anharmonicity in the NH and CH stretch modes of neutral Im. Also, we observe that anharmonicity in the NH stretch modes of ImH+ have some contribution from coupling effects, while that of neutral Im has no contribution whatsoever from mode-mode coupling. It is concluded that the incorporation of anharmonic effects in the calculation brings theory and experiment into much closer agreement for these systems.     

Infrared-vacuum ultraviolet pulsed field ionization-photoelectron study of C2H4(+) using a high-resolution infrared laser

The infrared (IR)-vacuum ultraviolet (VUV)-pulsed field ionization-photoelectron (IR-VUV-PFI-PE) spectrum for C2H4(X1A(g), v11 = 1, N'(Ka'Kc') = 3(03)) in the VUV range of 83,000-84,800 cm(-1) obtained using a single mode infrared laser revealed 24 rotationally resolved vibrational bands for the ion C2H4(+)(X2B(3u)) ground state. The frequencies and symmetry of the vibrational bands thus determined, together with the anharmonic frequency predictions calculated at the CCSD(T)/aug-cc-pVQZ level, have allowed the unambiguous assignment of these vibrational bands. These bands are mostly combination bands. The measured frequencies of these bands yield the fundamental frequencies for v8+ = 1103 +/- 10 cm(-1) and v10+ = 813 +/- 10 cm(-1) of C2H4(+)(X2B(3u)), which have not been determined previously. The present IR-VUV-PFI-PE study also provides truly rovibrationally selected and resolved state-to-state cross sections for the photoionization transitions C2H4(X1A(g); v11, N'(Ka'Kc')) --> C2H4(+)(X2B(3u); vi+, N+(Ka+Kc+)), where N'(Ka'Kc') denotes the rotational level of C2H4(X1A(g); v11), and vi+ and N+(Ka+Kc+) represent the vibrational and rotational states of the cation.     

Interaction potential and infrared absorption of endohedral H2 in C60

We have measured the temperature dependence of the infrared spectra of a hydrogen molecule trapped inside a C(60) cage, H(2)@C(60), in the temperature range from 6 to 300 K and analyzed the excitation spectrum by using a five-dimensional model of a vibrating rotor in a spherical potential. The electric dipole moment is induced by the translational motion of endohedral H(2) and gives rise to an infrared absorption process where one translational quantum is created or annihilated, ΔN = ±1. Some fundamental transitions, ΔN = 0, are observed as well. The rotation of endohedral H(2) is unhindered but coupled to the translational motion. The isotropic and translation-rotation coupling part of the potential are anharmonic and different in the ground and excited vibrational states of H(2). The vibrational frequency and the rotational constant of endohedral H(2) are smaller than those of H(2) in the gas phase. The assignment of lines to ortho- and para-H(2) is confirmed by measuring spectra of a para enriched sample of H(2)@C(60) and is consistent with the earlier interpretation of the low temperature infrared spectra [Mamone et al., J. Chem. Phys. 130, 081103 (2009)].     

Ab initio calculations of anharmonic vibrational spectroscopy for hydrogen fluoride (HF)n (n = 3, 4) and mixed hydrogen fluoride/water (HF)n(H2O)n (n = 1, 2, 4) clusters

Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)n, with n = 3, 4 and mixed clusters of hydrogen fluoride with water (HF)n(H2O)n where n = 1, 2. For the (HF)4(H2O)4 complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)n, good agreement is found with experimental data. This agreement shows that the M?ller-Plesset (MP2) potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)n(H2O)n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.     

Far‐ and Mid‐Infrared Spectroscopic Analysis of the Substrate‐Induced Structural Dynamics of Respiratory Complex I

The catalytic activity of the respiratory NADH:ubiquinone oxidoreductase (complex I) is based on conformational reorganizations. Herein we probe the effect of substrates on the conformational flexibility of complex I by means of ¹H/²H exchange kinetics at the level of the amide proton in the mid‐infrared spectral range (1700–1500 cm^?1). Slow, medium, and fast exchanging domains are distinguished that reveal different accessibilities to the solvent. Whereas amide hydrogens undergo rapid exchange with the solvent in an open structure, hydrogens experience much slower exchange when they are involved in H‐bonded structures or when they are sterically inaccessible for the solvent. The results indicate a structure that is more open in the presence of both NADH and quinon. Complementary information on the overall internal hydrogen bonding of the protein was probed in the far infrared (300–30 cm^?1), a spectral range that includes a continuum mode of the hydrogen bonding signature.     

Comparative ab initio study of the structures and stabilities of the ethane dication C2H6(2+) and its silicon analogues Si2H6(2+) and CSiH6(2+)

Ab initio MP2/6-311G and QCISD(T)/6-311G levels as well as Gaussian-2 theory were used to perform a comparative study of the structures and stabilities of the ethane dication C(2)H(6)(2+) and its silicon analogues Si(2)H(6)(2+) and CSiH(6)(2+). Similar to previous HF/6-31G results, our present calculations also indicate that the two-electron three-center (2e-3c) bonded carbonium-carbenium structure 1 is more stable than the doubly hydrogen bridged diborane-type structure 2 by about 12 kcal/mol. For the silicon analogue Si(2)H(6)(2+) the calculations, however, indicate that the 2e-3c bonded siliconium-silicenium structure 8 is about 9 kcal/mol less stable than doubly hydrogen bridged structure 9. Similar results were also computed for carbon-silicon mixed CSiH(6)(2+) dication structures. These studies are in agreement with the more electropositive character of silicon compared to carbon. Possible dissociation paths of the minimum structures were also calculated.     

Observation of rovibrational transitions of HCl, (HCl)2, and H2O-HCl in liquid helium nanodroplets

We report the infrared spectra of HCl, (HCl)2, and H2O-HCl in liquid helium nanodroplets in the frequency region between 2680 and 2915 cm(-1). For the HCl monomer a line width of 1.0 cm(-1) (H35Cl) corresponding to a lifetime of 5.3 ps was observed. The line broadening indicates fast rotational relaxation similar to that previously observed for HF. For (HCl)2 the free HCl as well as the bound HCl stretching band has been observed. The nu2+ bands of (HCl)2 could be rotationally resolved, and rotational constants were deduced from the spectra. We observed both the allowed and the symmetry forbidden transition. However, the forbidden "broken symmetry" tunneling transition of the mixed dimer shows an intensity that is considerably enhanced compared to the gas phase. Upon the basis of the present measurements we were able to calculate the tunneling splitting in the excited state. The tunneling splitting is found to be reduced by 28% compared to the gas phase. Transitions from the ground state to the Ka=1 level of the free HCl stretch (nu1) are recorded and show considerable line broadening with a line width of 2 cm(-1). The excited state Ka=1 has an additional rotational energy of about 10 cm(-1), thereby allowing fast rotational relaxation by coupling to helium excitations. In addition we observed the HCl stretch of the HCl-H2O dimer, which exhibits an unusually large width (1.7 cm(-1) for H35Cl)) and large red shift (8.5 cm(-1)), compared to the gas-phase values. The large-amplitude motion originating from the libration mode of the HCl-H2O complex is supposed to act as a fast relaxation manifold.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.     

Intramolecular H-Bond Dynamics of Catechol Investigated by THz High-Resolution Spectroscopy of Its Low-Frequency Modes

Catechol is an oxygenated aromatic volatile organic compound and a biogenic precursor of secondary organic aerosols. Monitoring this compound in the gas phase is desirable due to its appreciable reactivity with tropospheric ozone. From a molecular point of view, this molecule is attractive since the two adjacent hydroxy groups can interchangeably act as donor and acceptor in an intramolecular hydrogen bonding due to the tunnelling between two symmetrically equivalent structures. Using synchrotron radiation, we recorded a rotationally-resolved Fourier Transform far-infrared (IR) spectrum of the torsional modes of the free and bonded -OH groups forming the intramolecular hydrogen bond. Additionally, the room temperature, pure rotational spectrum was measured in the 70–220 GHz frequency range using a millimeter-wave spectrometer. The assignment of these molecular transitions was assisted by anharmonic high-level quantum-chemical calculations. In particular, pure rotational lines belonging to the ground and the four lowest energy, vibrationally excited states were assigned. Splitting due to the tunnelling was resolved for the free -OH torsional state. A global fit combining the far-IR and millimeter-wave data provided the spectroscopic parameters of the low-energy far-IR modes, in particular those characterizing the intramolecular hydrogen bond dynamics.     

Quasi-classical Trajectory Study of F+H2O→HF+OH Reaction: Influence of Barrier Height,Reactant Rotational Excitation,and Isotopic Substitution (cited: 4)

The reaction dynamics of the F+H₂O/D₂O→HF/DF+OH/OD are investigated on an ac-curate potential energy surface (PES) using a quasi-classical trajectory method. For bothisotopomers, the hydrogen/deuterium abstraction reaction is dominated by a direct rebound mechanism over a very low “reactant-like” barrier, which leads to a vibrationally hot HF/DF product with an internally cold OH/OD companion. It is shown that the lowered reaction barrier on this PES, as suggested by high-level ab initio calculations, leads to a much better agreement with the experimental reaction cross section, but has little impact on the product state distributions and mode selectivity. Our results further indicate that rotational exci-tation of the H₂O reactant leads to significant enhancement of the reactivity, suggesting a strong coupling with the reaction coordinate.     

Coherent low-frequency motions of hydrogen bonded acetic acid dimers in the liquid phase

Ultrafast vibrational dynamics of cyclic hydrogen bonded dimers and the underlying microscopic interactions are studied in temporally and spectrally resolved pump-probe experiments with 100 fs time resolution. Femtosecond excitation of the O-H and/or O-D stretching mode gives rise to pronounced changes of the O-H/O-D stretching absorption displaying both rate-like kinetic and oscillatory components. A lifetime of 200 fs is measured for the v=1 state of the O-H stretching oscillator. The strong oscillatory absorption changes are due to impulsively driven coherent wave packet motions along several low-frequency modes of the dimer between 50 and 170 cm(-1). Such wave packets generated via coherent excitation of the high-frequency O-H/O-D stretching oscillators represent a clear manifestation of the anharmonic coupling of low- and high-frequency modes. The underdamped low-frequency motions dephase on a time scale of 1-2 ps. Calculations of the vibrational potential energy surface based on density functional theory give the frequencies, anharmonic couplings, and microscopic elongations of the low-frequency modes, among them intermolecular hydrogen bond vibrations. Oscillations due to the excitonic coupling between the two O-H or O-D stretching oscillators are absent as is independently confirmed by experiments on mixed dimers with uncoupled O-H and O-D stretching oscillators.     

Ultrafast Vibrational Dynamics of Membrane‐Bound Peptides at the Lipid Bilayer/Water Interface

Vibrational energy transfer (VET) of proteins at cell membrane plays critical roles in controlling the protein functionalities, but its detection is very challenging. By using a surface‐sensitive femtosecond time‐resolved sum‐frequency generation vibrational spectroscopy with infrared pump, the detection of the ultrafast VET in proteins at cell membrane has finally become possible. The vibrational relaxation time of the N−H groups is determined to be 1.70(±0.05) ps for the α‐helix located in the hydrophobic core of the lipid bilayer and 0.9(±0.05) ps for the membrane‐bound β‐sheet structure. The N−H groups with strong hydrogen bonding gain faster relaxation time. By pumping the amide A band and probing amide I band, the vibrational relaxation from N−H mode to C=O mode through two pathways (direct coupling and through intermediate states) is revealed. The ratio of the pathways depends on the NH⋅⋅⋅O=C hydrogen‐bonding strength. Strong hydrogen bonding favors the coupling through intermediate states.     

IR spectroscopy of hydrogen-bonded 2-fluoropyridine-methanol clusters

The electronic and infrared spectra of 2-fluoropyridine-methanol clusters were observed in a supersonic free jet. The structure of hydrogen-bonded clusters of 2-fluoropyridine with methanol was studied on the basis of the molecular orbital calculations. The IR spectra of 2-fluoropyridine-(CH3OH)n(n = 1-3) clusters were observed with a fluorescence-detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The structures of the clusters are similar to those observed for 2-fluoropyridine-(H2O)n (n = 1-3) clusters. The existence of weak hydrogen bond interaction through aromatic hydrogen was observed in the IR spectra. The theoretical calculation also supports the result. The vibrational frequencies of CH bonds in CH3 group are affected by hydrogen bond formation although these bonds do not directly relate to the hydrogen bond interaction. The B3LYP/6-311 ++G(d,p) calculations reproduce well the vibrational frequency of the hydrogen-bonded OH stretching vibrations. However, the calculated frequency of CH stretching vibration could not reproduce the IR spectra because of anharmonic interaction with closely lying overtone or combination bands for nu3 and nu9 vibrations. The vibrational shift of nu2 vibration is reproduced well with molecular orbital calculations. The calculation also shows that the frequency shift of nu2 vibration is closely related to the CH bond length at the trans position against the OH bond in hydrogen-bonded methanol.