Bending Overtone Research Articles

The strength of the OH-bend/OH-stretch Fermi resonance in small water clusters.

A novel Raman jet-spectrometer is used to study the Fermi resonance between the OH bending overtone and OH stretching fundamental in small cyclic water clusters (H2O)n with n = 3, 4, 5. The new setup features a recirculating vacuum system which reduces the gas consumption by 2 to 3 orders of magnitude and enables long-term measurements of very weak Raman signals. Raman spectra measured from highly diluted expansions with unprecedented signal-to-noise ratio are presented and cluster-specific intensity ratios and effective coupling constants are derived using Markov-Chain Monte-Carlo methods, yielding a high probability for an almost "perfect" resonance for the tetramer and pentamer, i.e. a close frequency match of bend overtone and stretch fundamental with intensity ratios close to 1, but a larger coupling constant for the trimer, with best estimates close to W5 ≲ 50 cm-1 < W4 ≲ 60 cm-1 < W3 ≈ 65 cm-1.

Vibrational properties of OH groups associated with divalent cations in corundum (α-Al2O3)

Abstract. The infrared spectra of synthetic corundum (α-Al2O3) samples either doped directly with divalent cations (Mg2+) or containing divalent cations formed by reduction of trivalent cations in H2 gas (Co2+, Ni2+) may display broad OH stretching bands at ∼3000 cm−1 due to the structural incorporation of trace amounts of hydrogen. Experimental spectra recorded from some natural sapphires display a similar absorption band associated with a dominant absorption at 3161 cm−1, and some beryllium-diffused corundum crystals show a band at 3060 cm−1. All of these also display smaller and generally narrower bands between 1900 and 2700 cm−1, whose natures are poorly defined. In this work, the atomic-scale structure, relative stability and infrared spectroscopic properties of a series of OH defects in corundum (α-Al2O3) are theoretically investigated at the density-functional-theory level. The investigated defects consist of interstitial H+ ions forming OH groups and compensating for the charge imbalance related to the presence of divalent cations (Be2+, Mg2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+) substituted for Al3+ at nearby octahedral sites. Bands occurring at ∼3000 cm−1 in experimental spectra are assigned to the OH stretching modes of some of these defects, with bands observed around 1900 and 2700 cm−1 being assigned to overtones of corresponding OH bending modes. The results also support the assignment of the so-called “3161 cm−1 series”, observed in experimental spectra of some rubies and yellow sapphires, to structural OH groups in association with Fe2+ ions, rather than Si4+, as has been previously proposed. These inferences are also supported by analysis of correlations between band areas in experimental infrared spectra extracted from a database of corundum gemstones. A qualitative explanation relating the anomalous intensity and the polarisation properties of the OH bending overtone bands to the electrical anharmonicity of OH groups involved in medium-strength H bonds is proposed.

Combinational Vibration Modes in H2O/HDO/D2O Mixtures Detected Thanks to the Superior Sensitivity of Femtosecond Stimulated Raman Scattering.

Overtones and combinational modes frequently play essential roles in ultrafast vibrational energy relaxation in liquid water. However, these modes are very weak and often overlap with fundamental modes, particularly in isotopologues mixtures. We measured VV and HV Raman spectra of H2O and D2O mixtures with femtosecond stimulated Raman scattering (FSRS) and compared the results with calculated spectra. Specifically, we observed the mode at around 1850 cm-1 and assigned it to H-O-D bend + rocking libration. Second, we found that the H-O-D bend overtone band and the OD stretch + rocking libration combination band contribute to the band located between 2850 and 3050 cm-1. Furthermore, we assigned the broad band located between 4000 and 4200 cm-1 to be composed of combinational modes of high-frequency OH stretching modes with predominantly twisting and rocking librations. These results should help in a proper interpretation of Raman spectra of aqueous systems as well as in the identification of vibrational relaxation pathways in isotopically diluted water.

Classical Intermolecular Hydrogen Bonding Motifs of Heterocyclic rac-2-Amino-3-carbonitrile Derivatives: Linking Hirshfeld Surface Analysis, CT-DNA Binding Affinity, and Molecular Docking

Four 2-amino-4-(aryl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (Series I) and three 2-amino-1-phenyl-7,7-dimethyl-5-oxo-4-(aryl)-1,4,5,6,7,8-hexahydroquinoline-3-carbonitrile (Series II) derivatives were formed via multicomponent microwave reactions as racemic mixtures. The solid-state IR spectra revealed bands in the region between 3190 and 3414 cm–1 attributed to the amino functional group’s asymmetric, symmetric, and bending overtone vibration modes. Furthermore, the IR spectra of some of the compounds in each series were similar and this served as an indication of similar structural features in the solid state. This was confirmed via single-crystal X-ray diffraction. A detailed survey of the related 2-amino-3-carbonitrile derivatives in the Cambridge Structural Database revealed 12 classical intermolecular hydrogen-bonding motifs involving the amino functional group. The vast variation in the hydrogen bonding motifs was found to be caused by the type of group bonded to the 4H-pyran (in Series I) or dihydropyridine (in Series II) moiety. In this work, an unprecedented hydrogen bonding motif comprising a non-classical hydrogen bond acceptor was found in one instance. For both Series I and II, the N–H···N and N–H···O hydrogen bonds were investigated using noncovalent interaction (NCI) plots and molecular pairwise energy calculations. The contributions of reciprocal N···H and O···H contacts (attributed to N–H···N and N–H···O, respectively) toward the Hirshfeld surface range between 9.9 and 15.3% in Series I and between 10.8 and 15.0% in Series II. In addition, the compounds showed moderate to good binding affinity to calf thymus DNA (CT-DNA) with intrinsic binding constants (Kb) between 8.00 × 104 and 5.00 × 105 M–1 for Series I compounds and between 6.60 × 104 and 1.45 × 105 M–1 for Series II. A relationship was established between the reciprocal N···H to O···H contributions ratio from the Hirshfeld surface analysis and Kb. Molecular docking studies of each compound’s respective enantiomers into DNA were done using canonical–DNA dodecamer (B-DNA) as a model for CT-DNA. The compounds were found to interact with DNA via a minor groove-binding mode. The S-enantiomer for most of the compounds bound favorably compared to their respective R-enantiomers. The ratio of calculated ΔG for the DNA binding of the R-enantiomer to that of the S-enantiomer (ΔGR/ΔGS) follows the same positive trend as the experimental Kb value for each compound in Series I while a negative trend was observed for Series II. Finally, a relationship between the Hirshfeld surface analysis and calculated ΔG values was also established in this work.

Structural and vibrational characterization of HCO+ and Rg-HCO+, Rg = {He, Ne, Ar, Kr, and Xe}.

The structures of the formyl ion (HCO+) and its rare gas tagged counterparts (Rg-HCO+, Rg = He, Ne, Ar, Kr, and Xe) were studied at the coupled-cluster singles, doubles, and perturbative triples [CCSD(T)]/aug-cc-pVTZ level of theory and basis set. A linear structure for these tagged complexes was predicted. The Rg binding energies for Rg-HCO+ are also examined at the CCSD(T) level. It was found that the binding interaction increases from He-HCO+ to Xe-HCO+. A multilevel potential energy surface built at the CCSD(T) and second-order Møller-Plesset perturbation levels of theory were used to study these species' vibrational spectra. By changing the Rg in the first-solvation shell for HCO+, the Fermi resonance interaction between the first H+ bend overtone and the asymmetric and symmetric H-C-O stretches can be modulated. This Fermi resonance modulation is demonstrated by examining a series of rare gas solvated HCO+.

Towards complete assignment of the infrared spectrum of the protonated water cluster H+(H2O)21

The spectroscopic features of protonated water species in dilute acid solutions have been long sought after for understanding the microscopic behavior of the proton in water with gas-phase water clusters H+(H2O)n extensively studied as bottom-up model systems. We present a new protocol for the calculation of the infrared (IR) spectra of complex systems, which combines the fragment-based Coupled Cluster method and anharmonic vibrational quasi-degenerate perturbation theory, and demonstrate its accuracy towards the complete and accurate assignment of the IR spectrum of the H+(H2O)21 cluster. The site-specific IR spectral signatures reveal two distinct structures for the internal and surface four-coordinated water molecules, which are ice-like and liquid-like, respectively. The effect of inter-molecular interaction between water molecules is addressed, and the vibrational resonance is found between the O-H stretching fundamental and the bending overtone of the nearest neighboring water molecule. The revelation of the spectral signature of the excess proton offers deeper insight into the nature of charge accommodation in the extended hydrogen-bonding network underpinning this aqueous cluster.

Hydration and Seamless Integration of Hydrogen Peroxide in Water.

Raman multivariate curve resolution is used to decompose the vibrational spectra of aqueous hydrogen peroxide (H2O2) into pure water, dilute H2O2, and concentrated H2O2 spectral components. The dilute spectra reveal four sub-bands in the OH stretch region, assigned to the OH stretch and Fermi resonant bend overtone of H2O2, and two nonequivalent OH groups on water molecules that donate a hydrogen bond to H2O2. At high concentrations, a spectral component resembling pure H2O2 emerges. Our results further demonstrate that H2O2 perturbs the structure of water significantly less than either methanol or sodium chloride of the same concentration, as evidenced by comparing the hydration-shell spectra of tert-butyl alcohol dissolved in the three aqueous solutions.

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.

Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion.

Vibrational predissociation processes of the H2O+Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H2O+ unit were studied by photofragment ion imaging. The anisotropy parameters, β, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, β > 0 for the P-branch excitations, while β < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H2O+ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H2O+ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H2O+ fragment ions was discussed based on the steric configuration of the H2O+ and Ar units at the moment of dissociation.

Line strengths, widths and shifts analysis of the [formula omitted][formula omitted] and [formula omitted] bands in [formula omitted]SiH[formula omitted][formula omitted]SiH[formula omitted] and [formula omitted]SiH[formula omitted

The high resolution infrared spectra of SiH4 in its natural abundance were recorded with a Bruker IFS125 HR Fourier transform infrared spectrometer at an optical resolution of 0.003 cm−1 and analyzed in the region of 1600–2100 cm−1 where the first bending overtone 2ν2,2ν4 and combinational ν2+ν4 bands are located. A line strength analysis of the 2018 (1636/211/171) experimentally recorded transitions of the 28SiH4,29SiH4, and 30SiH4 species was made by the fit of their line shapes with the Hartmann-Tran profile and 9 effective dipole moment parameters (7/1/1 fitted parameters for the 28SiH4,29SiH4 and 30SiH4 species, respectively) were obtained from the weighted fit which reproduce initial experimental line strengths of 28SiH4,29SiH4 and 30SiH4 with the drms=3.65%,3.77% and 3.46%. Self–broadening coefficients of 1064 lines (typically lying between (8 and 10)×10−2 cm−1·atm−1) and self–shift coefficients of 785 lines (typically lying between (5 and 10)×10−3 cm−1·atm−1) of all three isotopologues of silane were determined from the multi–spectrum analysis of these lines.

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.

Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect.

Intriguing vibrational features of solvated protonated methanol between 2400-3800 cm-1 are recorded by infrared predissociation spectroscopy. Positions of absorption bands corresponding to OH stretching modes are sensitive to changes in solvation environments, thus leading to changes in these vibrational features. Two anharmonic coupling mechanisms, Fermi resonance (FR) contributed by bending overtones and combination band (CB) associated with intermolecular stretching modes, are known to lead to band splitting of OH stretching fundamentals in solvated hydronium and ammonium. Theoretical analyses based on the ab initio anharmonic algorithm not only well reproduce the experimentally observed features but also elucidate the magnitudes of such couplings and the resulting interplay between these two mechanisms, which provide convincing assignments of the spectral patterns. Moreover, while the hydroxyl group plays the leading role in all the above-mentioned features, the role of the methyl group is also analyzed. Through the H/D isotope substitution, we identify overtones of the methyl-hydroxyl rocking modes and their participation in FR.

Comprehensive study of the pentad bending triad region of germane: Positions, strengths, widths and shifts of lines in the [formula omitted][formula omitted] and [formula omitted] bands of [formula omitted]GeH[formula omitted][formula omitted]GeH[formula omitted][formula omitted]GeH[formula omitted][formula omitted]GeH[formula omitted][formula omitted]GeH[formula omitted

The high resolution infrared spectra of GeH4 in its natural abundance were recorded with a Bruker IFS125 HR Fourier transform infrared spectrometer at an optical resolution of 0.003 cm−1 and analyzed in the region of 1400–2000 cm−1 where the first bending overtone 2ν2,2ν4 and combinational ν2+ν4 bands are located. Ro–vibrational line positions and energies of the 70GeH4 species were analysed for the first time and line positions and energies of the 72GeH4 and 74GeH4 species were improved considerably in comparison with the preceding studies. The numbers of 2316/2406/873/3007/2257 transitions with Jmax = 21/23/18/22/19 of the 2ν2 (A1− and E−type sub–bands), 2ν4 (A1−,E− and F2−type sub–bands) and ν2+ν4 (F1− and F2−type sub–bands) bands of the 70GeH4,72GeH4,73GeH4,74GeH4, and 76GeH4 molecules were used in the joint weighted fit of experimentally assigned transitions with the Hamiltonian model which takes the resonance interactions between the seven, (0200,A1),(0200,E),(0101,F1),(0101,F2),(0002,A1),(0002,E) and (0002,F2), vibrational states into account. From our preceding studies 5563 hot band transitions were also taken into account. As the result of a joint fit, a set of 129 fitted parameters was obtained which reproduce the initial 16422 experimental (including “hot”) transitions of five isotopologues with the drms=3.26×10−4 cm−1. A line strength analysis of the 1697 experimentally recorded transitions of all species was made by the fit of their line shapes with the Hartmann-Tran profile and 13 effective dipole moment parameters were obtained from the weighted fit which reproduce the initial experimental line strengths with the drms=3.4%. Self–broadening coefficients of 993 lines and self–shift coefficients of 674 lines were determined from the multi–spectrum analysis of these lines.

A Rather Universal Vibrational Resonance in 1:1 Hydrates of Carbonyl Compounds.

When the lower frequency OH stretching fundamental of a water molecule is shifted to the 3500 cm-1 spectral range by the solvation of a carbonyl compound, in this case a ketone, its infrared intensity is shared with a dark state. It is shown by chemical and isotope substitution for more than a dozen systems that the location of this resonance is remarkably substitution-independent. Harmonic and anharmonic model calculations support its assignment to a combination of the water bending overtone and in-plane water libration. This previously unrecognized intramolecular-intermolecular coupling in single solvent water has a strength of 7-10 cm-1. It may have been sporadically observed before in a few other carbonyl compounds such as amides, without any previous exploration of its potential universality. The resulting generic picosecond energy redistribution channel for aqueous solutions may represent a slow counterpart and doorway model of what happens on a subpicosecond time scale when the hydrogen bonds become stronger, such as in carboxylic acid dimers or protonated water clusters.

ASTER mapping of gypsum deposits of Thumrait region of southern Oman

AbstractThis study demonstrates the use of ASTER data for the mapping of gypsum deposits and associated geological formations that occurred in the Thumrait region of southern Oman. The measurement of spectra over samples of gypsum in the 1,300–2,500 nm wavelength using a PIMA spectrometer showed the presence of distinct absorptions at 1400–1600, 1750, 1940, 2,100, and 2,400 nm characteristics to OH stretching, H2O combinations, and SO bending overtones and stretching, respectively. Studying the unique spectral absorption characters of gypsum samples, we developed a false color composite (FCC) and an image by decorrelation stretch using the spectral bands 7, 3, and 2 of ASTER. The results FCC showed the regions of gypsum occurrences, and the decorrelated image discriminated the gypsum occurrences from other geological formations of the area. The study of surface mineralogy of the region using the VNIR‐SWIR bands by the spectral angle mapper method showed the presence of sulfate, carbonate, and clay minerals of the geological formations in the study area. We compared the results of ASTER with the results obtained using spectral bands 12, 8, and 4 of Sentinel‐2A processed by the same methods. The study showed that the spectral bands of ASTER can be used for mapping the gypsum and associated geological formations.

Vibrational spectroscopic signatures of hydrogen bond induced NH stretch-bend Fermi-resonance in amines: The methylamine clusters and other N-H⋯N hydrogen-bonded complexes.

The appearance of multiple bands in the N-H stretching region of the infrared spectra of the neutral methylamine dimer and trimer is a sign of NH bend-stretch anharmonic coupling. Ab initio anharmonic calculations were carried out in a step-wise manner to reveal the origin of various bands observed in the spectrum of the methylamine dimer. A seven-dimensional potential energy surface involving symmetric and asymmetric stretching and bending vibrations of both the hydrogen bond donor and the acceptor along intermolecular-translational modes was constructed using the discrete variable representation approach. The resulting spectrum of the dimer shows five bands that can be attributed to the symmetric stretching (νsym D), asymmetric stretchin (νasym D), and bending overtone (2νbend D) of the donor moiety. These appear along with the combination band arising out of bending vibrations of the donor and acceptor (νbend D + νbend A) and with the combination of the intermolecular translational mode over the donor bending overtone (νtrans + 2νbend D). The spectrum of the trimer essentially consists of all the features seen in the dimer with marginal changes in band positions. The analysis of the experimental spectra based on the two-state deperturbation model and ab initio anharmonic calculations yield a matrix element of about 40 cm-1 for the N-H bend-stretch Fermi resonance coupling. In general, the IR spectra of the hydrogen-bonded amino group depict three sets of bands that arise due to bend-stretch Fermi resonance coupling.

Vibrational Coupling in Solvated H3O+: Interplay between Fermi Resonance and Combination Band.

Complex vibrational features of solvated hydronium ion, H3O+, in 3 μm enable us to look into the vibrational coupling among O-H stretching modes and other degrees of freedom. Two anharmonic coupling schemes have often been engaged to explain observed spectra: coupling with the OH bending overtone, known as Fermi resonance (FR), has been proposed to account for the splitting of the OH stretch band at ∼3300 cm-1 in H3O+···Ar3, but an additional peak in H3O+···(N2)3 at the similar frequency region has been assigned to a combination band (CB) with the low-frequency intermolecular stretches. While even stronger vibrational coupling is expected in H3O+···(H2O)3, such pronounced peaks are absent. In the present study, vibrational spectra of H3O+···Kr3 and H3O+···(CO)3 are measured to complement the existing spectra. Using ab initio anharmonic algorithms, we are able to assign the observed complex spectral features, to resolve seemingly contradictory notions in the interpretations, and to reveal simple pictures of the interplay between FR and CB.

High–resolution study of the pentad bending triad region of silane: The [formula omitted][formula omitted] and [formula omitted] bands of [formula omitted]SiH[formula omitted][formula omitted]SiH[formula omitted] and [formula omitted]SiH[formula omitted

The high resolution infrared spectra of SiH4were recorded with Bruker IFS120 and IFS125 HR Fourier transform infrared spectrometers at an optical resolution of 0.0021 and 0.0025 cm−1and analyzed in the region of 1600–2100 cm−1where the first bending overtone 2ν2,2ν4and combinational ν2+ν4bands are located. The number of 640/1114/2667 transitions with Jmax= 23 were assigned to the bands 2ν2(A1- and E-type sub–bands), 2ν4(A1-, E- and F2-type sub–bands) and ν2+ν4(F1- and F2-type sub–bands) of the 28SiH4molecule. The subsequent weighted fit of experimentally assigned transitions was made with the Hamiltonian model which takes the resonance interactions between seven vibrational states ((0200,A1),(0200,E),(0101,F1),(0101,F2),(0002,A1),(0002,E)and (0002,F2)) into account. As the result, a set of 73 fitted parameters was obtained which reproduces the initial 4421 experimental transitions with the drms=3.31×10−4 cm−1. On the basis of the obtained results, 4940 hot transitions were assigned in the high resolution spectra of the Dyad region studied before. The corresponding analysis was made for the 29SiH4and 30SiH4isotopologues. A line list of 12,698 experimental transitions for both the “cold” and “hot” transitions of all three isotopologues is presented.

Analysis of the Proton Transfer Bands in the Infrared Spectra of Linear N2H+···OC and N2D+···OC Complexes Using Electric Field-Driven Classical Trajectories.

In this work, we describe ab initio calculations and assignment of infrared (IR) spectra of hydrogen-bonded ion-molecular complexes that involve a fluxional proton: the linear N2H+···OC and N2D+···OC complexes. Given the challenges of describing fluxional proton dynamics and especially its IR activity, we use electric field-driven classical trajectories, i.e., the driven molecular dynamics (DMD) method that was developed by us in recent years and for similar applications, in conjunction with high-level electronic structure theory. Namely, we present a modified and a numerically efficient implementation of DMD specifically for direct (or "on the fly") calculations, which we carry out at the MP2-F12/AVDZ level of theory for the potential energy surface (PES) and MP2/AVDZ for the dipole moment surfaces (DMSs). Detailed analysis of the PES, DMS, and the time-dependence of the first derivative of the DMS, referred to as the driving force, for the highly fluxional vibrations involving H+/D+ revealed that the strongly non-harmonic PES and non-linear DMS yield remarkably complex vibrational spectra. Interestingly, the classical trajectories reveal a doublet in the proton transfer part of the spectrum with the two peaks at 1800 and 1980 cm-1. We find that their shared intensity is due to a Fermi-like resonance interaction, within the classical limit, of the H+ parallel stretch fundamental and an H+ perpendicular bending overtone. This doublet is also observed in the deuterated species at 1360 and 1460 cm-1.

Coupling of three intramolecular vibration modes of liquid H2O molecules in the framework of the fluctuation theory of hydrogen bonding.

Vibrational spectra of liquid water contain a wealth of information about its structure and dynamics but there are no generally acknowledged interpretation of their band profiles, unfortunately. We have tried to calculate them taking into account only the coupling of three intramolecular vibrations in the set of H2O molecules with different initial oscillator frequencies and intensities. The matter is that each water molecule forms hydrogen bonds of different strengths; thus the OH stretching band spans several hundreds of wave numbers (the fluctuation theory of hydrogen bonding). This distribution overlaps with the similar band of the first overtone of the HOH bending frequencies thus triggering a Fermi resonance between three vibrations. There were some problems causing some simplifications in previous theoretical modeling of vibrational transitions in condensed water. To solve them we extract the statistical distribution of OH frequencies of H2O molecules directly from the experimental spectra of HOD molecules at the same conditions instead of defining it theoretically. Also the bending overtone is allowed to have non-zero intrinsic intensity when calculating the Fermi resonance. The test calculation of the isotropic component of the Raman spectrum which is most critical for interpretation shows that our algorithm can reproduces the characteristic peculiarities of the experiment. The spectrum consists of three non-Gaussian contours. The overtone of bending vibration, being strengthened by Fermi resonance, makes the greatest contribution to the dominating low-frequency spectrum component that was previously attributed to intermolecular coupling of adjacent OH oscillators by some authors. Further we plan to calculate similar band profiles in the IR and also Raman isotropic and anisotropic spectra of Н2О and D2О molecules within a wide temperature range for their quantitative comparison with experiment.