Theoretical study of the structures and vibrational spectra of the hydrogen-bonded systems of 4-cyanophenol with N-bases

The structural and vibrational features of the hydrogen bonded complexes of 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) with one and two 4-CNPhOH molecules have been studied extensively by ab initio SCF/6-31G(d,p) and BLYP calculations with various basis sets: 6-31G(d,p), 6-31+G(d,p) and 6-31++G(d,p). Full geometry optimization was made for the complexes studied. The nature of the hydrogen bonding and the influence of the hydrogen bonding on the structural and vibrational characteristics of the monomers have been investigated. The corrected values of the dissociation energy for the hydrogen-bonded complexes have been calculated in order to estimate their stability. The calculated values of the dissociation energy per phenol molecule indicate that the complex: TBD: 4-CNPhOH (1:1) is more stable than the complex: TBD: 4-CNPhOH (1:2). The changes in the structural and vibrational characteristics upon hydrogen bonding depend on the strength of the hydrogen bonds. In agreement with the experiment, the calculations show that the complexation between TBD and 4-CNPhOH leads to considerably changes in the vibrational characteristics of the stretching O-H vibration. The vibrational frequency of the O-H stretching vibration is shifted to lower wave numbers upon hydrogen bonding. The predicted frequency shifts Deltanu(O-H) for the complexes--TBD: 4-CNPhOH (1:1) and TBD: 4-CNPhOH (1:2) are in the range from -190 cm(-1) to -586 cm(-1). In the same time the IR intensity of the O-H stretching vibration increases dramatically in the hydrogen-bonded complexes.     

Structure, stability and vibrational spectrum of the hydrogen-bonded complex between HNO3 and H2O. Ab initio and DFT studies

The structure, stability and vibrational spectrum of the binary complex between HONO2 and H2O have been investigated using ab initio calculations at SCF and MP2 levels with different basis sets and B3LYP/6-31G(d,p) calculations. Full geometry optimization was made for the complex studied. It was established that the hydrogen-bonded H2O...HONO2 complex has a planar structure. The corrected values of the dissociation energy at the SCF and MP2 levels and B3LYP calculations are indicative of relatively strong OH...O hydrogen-bonded interaction. The changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding between HONO2 and H2O have been estimated by using the ab initio calculations at SCF and MP2 levels and B3LYP/6-31G(d,p) calculations. It was established that the most sensitive to the complexation is the stretching O-H vibration from HONO2. In agreement with the experiment, its vibrational frequency in the complex is shifted to lower wavenumbers. The predicted frequency shift with the B3LYP/6-31G(d,p) calculations (-439 cm(-1)) is in the best agreement with the experimentally measured (-498 cm(-1)). The intensity of this vibration increases dramatically upon hydrogen bonding. The ab initio calculations at the SCF level predict an increase up to five times; at the MP2 level up to 10 times and the B3LYP/6-31G(d,p) predicted increase is up to 17 times. The good agreement between the predicted values of the frequency shifts and those experimentally observed show that the structure of the hydrogen-bonded complex H2O...HONO2 is reliable.     

Theoretical study of the changes in the vibrational characteristics arising from the hydrogen bonding between Vitamin C (L-ascorbic acid) and H2O

The vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) for the hydrogen-bonded system of Vitamin C (L-ascorbic acid) with five water molecules have been predicted using ab initio SCF/6-31G(d,p) calculations and DFT (BLYP) calculations with 6-31G(d,p) and 6-31++G(d,p) basis sets. The changes in the vibrational characteristics from free monomers to a complex have been calculated. The ab initio and BLYP calculations show that the complexation between Vitamin C and five water molecules leads to large red shifts of the stretching vibrations for the monomer bonds involved in the hydrogen bonding and very strong increase in their IR intensity. The predicted frequency shifts for the stretching vibrations from Vitamin C taking part in the hydrogen bonding are up to -508 cm(-1). The magnitude of the wavenumber shifts is indicative of relatively strong OH...H hydrogen-bonded interactions. In the same time the IR intensity and Raman activity of these vibrations increase upon complexation. The IR intensity increases dramatically (up to 12 times) and Raman activity increases up to three times. The ab initio and BLYP calculations show, that the symmetric OH vibrations of water molecules are more sensitive to the complexation. The hydrogen bonding leads to very large red shifts of these vibrations and very strong increase in their IR intensity. The asymmetric OH stretching vibrations of water, free from hydrogen bonding are less sensitive to the complexation than the hydrogen-bonded symmetric OH stretching vibrations. The increases of the IR intensities for these vibrations are lower and red shifts are negligible.     

Ab initio and DFT studies of the vibrational spectra of hydrogen-bonded PhOH...(H2O)4 complexes

The vibrational characteristics (vibrational frequencies and infrared intensities) for the hydrogen-bonded complex of phenol with four water molecules PhOH...(H2O)4 (structure 4A) have been predicted using ab initio and DFT (B3LYP) calculations with 6-31G(d,p) basis set. The changes in the vibrational characteristics from free monomers to a complex have been calculated. The ab initio and B3LYP calculations show that the observed four intense bands at 3299, 3341, 3386 and 3430 cm(-1) can be assigned to the hydrogen-bonded OH stretching vibrations in the complex PhOH...(H2O)4 (4A). The complexation leads to very large red shifts of these vibrations and very strong increase in their IR intensity. The predicted red shifts for these vibrations with B3LYP/6-31G(d,p) calculations are in very good agreement with the experimentally observed. It was established that the phenolic OH stretching vibration is the most sensitive to the hydrogen bonding. The predicted red-shift with the B3LYP/6-31G(d,p) calculations for the most stable ring structure 4A (-590 cm(-1)) is in better agreement with the experimentally observed than the red-shift, predicted with SCF/6-31G(d,p) calculations. The magnitude of the wavenumber shift is indicative of relatively strong OH...H hydrogen-bonded interaction. The complexation between phenol and four water molecules leads to strong increase of the IR intensity of the phenolic OH stretching vibration (up to 38 times).     

Hydrogen-bonded complexes of nitrous and nitric acids with ethene Ab initio and DFT studies

The complexes formed by ethene with nitric and nitrous (trans and cis) acids have been investigated by ab initio (SCF and MP2) and B3LYP calculations with 6-311++G(d,p) basis set. Full geometry optimisation has been performed for the complexes studied. The most stable structures of the complexes are established. Bearing in mind the corrected values of the dissociation energy the studied hydrogen-bonded complexes can be ordered as follows: C(2)H(4)...HONO(2)>C(2)H(4)...HONO-trans>C(2)H(4)...HONO-cis. In the complexes the acids act as proton donors forming the pi-type of hydrogen bond with ethene. The predicted changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding are in good agreement with the experimentally measured. The predicted frequency shift of the stretching OH vibration in the nitric acid is largest (-210 cm(-1)), followed by the shifts in the trans-HONO (-141 cm(-1)) and cis-HONO (-109 cm(-1)). The calculations predict an increase of the IR intensity of the stretching O-H vibration in the complexes from 6 to 10 times.     

Ab initio prediction of the vibrational spectrum of the hydrogen bonded complex O2N...HONO2

The changes in the structural parameters and vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) arising from the hydrogen bonding between NO(2) and HONO(2) have been studied employing ab initio 6-31G(d, p)/UHF and 6-31+G(d, p)/UHF, and B3LYP/6-31G(d, p) calculations. The charge rearrangement upon hydrogen bonding have been, estimated using the Mulliken population analyses. It was established that the complexation between NO(2) and HONO(2) leads to changes in the structural parameters and the vibrational characteristics of the monomers. The most sensitive to the hydrogen bond formation are the vibrational characteristics of the normal modes of the monomer bonds participating in the hydrogen bonding. The predicted shifts in the vibrational frequencies by ab initio and B3LYP/6-31G(d, p) calculations are in very good agreement with the experimentally observed, which is an evidence for the reliance of the studied structure.     

Theoretical study of the vibrational spectra of the hydrogen-bonded systems between pyridine-3-carboxamide (nicotinamide) and DMSO

The vibrational characteristics (vibrational frequencies and infrared intensities) for the hydrogen-bonded systems of nicotinamide (NA(Z) and NA(E)) with dimethyl sulfoxide (DMSO) have been predicted using ab initio SCF/6-31G(d,p) and DFT (BLYP/6-311++G(d,p)) calculations. The changes in the vibrational characteristics from free monomers to a complex have been calculated. The ab initio and BLYP calculations show that the complexation between nicotinamide (NA(Z) and NA(E)) and DMSO leads to large red shifts of the stretching vibrations for the hydrogen-bonded N-H bonds of nicotinamide and very strong increase in their IR intensity. The results from the BLYP/6-311++G(d,p) calculations show that the predicted red shifts of the nu(s)(NH) and nu(as)(NH) vibrations for the complex NA(E)-DMSO (1:2) (Deltanu(as)(NH)=-186 cm(-1) and Deltanu(s)(NH)=-198 cm(-1)) are in better agreement with the experimentally measured. The magnitudes of the wavenumber shifts are indicative of strong NH...O hydrogen-bonded interactions in both complexes. The calculations predict an increase of the IR intensity of nu(s)(NH) and nu(as)(NH) vibrations in the complexes up to 14 times. Having in mind that in more cases the predicted changes in the vibrational characteristics for the complexes studied are very near, it could be concluded that both conformers of nicotinamide, Z-conformer and E-conformer, are present in the solution forming the hydrogen-bonded complexes with DMSO.     

Aromaticity-controlled tautomerism and resonance-assisted hydrogen bonding in heterocyclic enaminone-iminoenol systems

The contribution of aromaticity and intramolecular hydrogen bonding to relative stability, for a set of (1H-azahetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs, has been investigated by means of quantum chemical DFT and ab initio methods up to the MP4(SDTQ)/AUG-cc-pVDZ and MP2/AUG-cc-pVTZ levels of theory. It is found that the relative energy of the tautomers is governed by the change in the degree of heterocycle aromaticity upon intramolecular hydrogen transfer. An analysis of geometrical parameters of a hydrogen-bonded system reveals a clear relationship between the aromaticity of the heterocycle, the conjugation in a resonant spacer, and the strengths of the intramolecular hydrogen bonds. This allows the conclusion to be drawn that intramolecular N-H...O and O-H...N hydrogen bonds formed are found to be resonance-assisted and their strength is dependent on the pi-donating/accepting properties of the heterocycle. On the basis of the results of the calculations, a simple model describing the mechanism of resonance assistance of hydrogen bonding has been suggested.     

Complexes of atmospheric alpha-dicarbonyls with water: FTIR matrix isolation and theoretical study

The complexes of glyoxal (Gly), methylglyoxal (MGly), and diacetyl (DAc) with water have been studied using Fourier transform infrared (FTIR) matrix isolation spectroscopy and MP2 calculations with 6-311++G(2d,2p) basis set. The analysis of the experimental spectra of the Gly(MGly,DAc)/H2O/Ar matrixes indicates formation of one Gly...H2O complex, three MGly...H2O complexes, and two DAc...H2O ones. All the complexes are stabilized by the O-H...O(C) hydrogen bond between the water molecule and carbonyl oxygen as evidenced by the strong perturbation of the O-H, C=O stretching vibrations. The blue shift of the CH stretching vibration in the Gly...H2O complex and in two MGly...H2O ones suggests that these complexes are additionally stabilized by the improper C-H...O(H2) hydrogen bonding. The theoretical calculations confirm the experimental findings. They evidence the stability of three hydrogen-bonded Gly...H2O and DAc...H2O complexes and six MGly...H2O ones stabilized by the O-H...O(C) hydrogen bond. The calculated vibrational frequencies and geometrical parameters indicate that one DAc..H2O complexes, two Gly...H2O, and three MGly...H2O ones are additionally stabilized by the improper hydrogen bonding between the C-H group and water oxygen. The comparison of the theoretical frequencies with the experimental ones allowed us to attribute the calculated structures to the complexes present in the matrixes.     

Theoretical modeling of the O-H stretching IR bands of hydrogen-bonded dimers of benzoic acid in S(0) and S(1) electronic states

Theoretical model for vibrational interactions in the hydrogen-bonded dimer of benzoic acid is presented. The model takes into account anharmonic-type couplings between the high-frequency O-H and the low-frequency O[cdots, three dots, centered]O stretching vibrations in two hydrogen bonds, resonance interactions (Davydov coupling) between two hydrogen bonds in the dimer, and Fermi resonance between the O-H stretching fundamental and the first overtone of the O-H in-plane bending vibrations. The vibrational Hamiltonians and selection rules for the C(2h) geometry in the S(0) state and for the C(s) in-plane bent geometry in the S(1) state of the dimer are derived. The model is used for theoretical simulation of the O-H stretching IR absorption bands of benzoic acid dimers in the gas phase in the electronic ground and first excited singlet states. Ab initio CIS and CIS(D)6-311++G(d,p) calculations have been performed to determine geometry, frequencies, and excited state energies of benzoic acid dimer in the S(1) state.     

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.     

Anharmonic vibrational frequency shifts upon interaction of phenol+ with the open shell ligand O2. The performance of DFT methods versus MP2

Anharmonic vibrational frequency shifts of the phenol(+) O-H stretching mode upon complex formation with the open-shell ligand O(2) were computed at several DFT and MP2 levels of theory, with various basis sets, up to 6-311++G(2df,2pd). It was found that all DFT levels of theory significantly outperform the MP2 method with this respect. The best agreement with the experimental frequency shift for the hydrogen-bonded minimum on the potential energy surfaces was obtained with the HCTH/407 functional (-93.7 cm(-1) theoretical vs -86 cm(-1) experimental), which is a significant improvement over other, more standard DFT functionals (such as, e.g., B3LYP, PBE1PBE), which predict too large downshifts (-139.9 and -147.7 cm(-1), respectively). Good agreement with the experiment was also obtained with the mPW1B95 functional proposed by Truhlar et al. (-109.2 cm(-1)). We have attributed this trend due to the corrected long-range behavior of the HCTH/407 and mPW1B95 functionals, despite the fact that they have been designed primarily for other purposes. MP2 method, even with the largest basis set used, manages to reproduce only less than 50% of the experimentally detected frequency downshift for the hydrogen-bonded dimer. This was attributed to the much more significant spin contamination of the reference HF wave function (compared to DFT Kohn-Sham wave functions), which was found to be strongly dependent on the O-H stretching vibrational coordinate. All DFT levels of theory outperform MP2 in the case of computed anharmonic OH stretching frequency shifts upon ionization of the neutral phenol molecule as well. Besides the hydrogen-bonded minimum, DFT levels of theory also predict existence of two other minima, corresponding to stacked arrangement of the phenol(+) and O(2) subunits. mPW1B95 and PBE1PBE functionals predict a very slight blue shift of the phenol(+) O-H stretching mode in the case of stacked dimer with the nearly perpendicular orientation of oxygen molecule with respect to the phenolic ring, which is entirely of electrostatic origin, in agreement with the experimental observations of an additional band in the IR photodissociation spectra of phenol(+)-O(2) dimer [Patzer, A.; Knorke, H.; Langer, J.; Dopfer, O. Chem. Phys. Lett. 2008, 457, 298]. The structural features of the minima on the studied PESs were discussed in details as well, on the basis of NBO and AIM analyses.     

Ab initio study of the vibrational spectra of the hydrogen-bonded nitric acid dimer.

The changes in the vibrational characteristics characterizing the dimerization of nitric acid have been investigated by ab initio calculations at the MP2 level, with 6-31G(d,p) and 6-31 + G(d,p) basis sets, and B3LYP/6-31G(d,p) calculations. The most consistent agreement between the computed values of the frequency shifts for the planar fully symmetric structure (2A) and those experimentally observed suggests that this structure is preferred. It was established that the most sensitive to the complexation is the stretching O-H vibration. The values of the frequency shift (-306 cm(-1)) is indicative for the formation of the relatively strong hydrogen bonds. The calculations predict an increase of the infrared intensity of the stretching O-H vibration in the nitric acid dimer more than 26 times.     

Vibrational frequencies and infrared intensities of the hydrogen-bonded complexes of nitrous acid with ethers: ab initio and DFT studies

The vibrational spectra of the binary complexes formed by HONO-trans and HONO-cis with dimethyl and diethyl ethers have been investigated using ab initio calculations at the SCF and MP2 levels with 6-311++G(d,p) basis set and B3LYP calculations with 6-31G(d,p) and 6-31+G(d,p) basis sets. Full geometry optimisation was made for the complexes studied. The accuracy of the ab initio calculations have been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP calculations are very near to the results, obtained with 6-311++G(d,p)/MP2. The ab initio and DFT calculations show that the changes in the vibrational characteristics (vibrational frequencies and infrared intensities) upon hydrogen bonding for the hydrogen-bonded complex (CH3)2O...HONO-trans are larger than for the complex (CH3)2O...HONO-cis.     

Effect of hydrogen bonding and cooperativity on stretching force constants of formamide

Stretching force constants for formamide and its seven associated species involving two to four molecules hydrogen-bonded through linear and cyclic configurations and 10 structures containing formamide hydrogen-bonded with one to five water molecules are reported. Since ab initio calculations are rather inconvenient to perform on such big clusters and are time-consuming, CNINDO MO calculations were carried out using the gradient method. The results demonstrate, on the one hand, the feasibility of semiempirical calculations for the evaluation of trends in force constants for big clusters where generally ab initio calculations become much involved and, on the other hand, explain the effect of hydrogen bonding and cooperativity on force constants and vibrational spectra of biologically important systems composed of formamide in the condensed phase and its aqueous solutions. The C?O and N? H stretching force constants are found to reduce significantly on hydrogen bonding. The reduction in force constant is further enhanced when two cyclic dimers become associated through a linear hydrogen bond. The results indicate justification for the stabilization of the formamide structure with two cyclic dimers hydrogen-bonded together. The reduction in the force constants on hydrogen bonding also reflect the cooperativity contribution. The C?O and C? N stretching force constants for the structures corresponding to formamide in liquid and aqueous solution phases are in agreement with the experimental vibrational frequencies reported.     

Contrast effect of hydrogen bonding on the acceptor and donor OH groups of intramolecularly hydrogen-bonded OH pairs in diols

We studied the influence of hydrogen bonding on the fundamental and overtone bands of the OH-stretching vibration of each OH group in the intramolecularly hydrogen-bonded OH(I)::OH(II) pair in 1,2-, 1,3- and 1,4-diols. The hydrogen bonding between the two OH groups significantly increases in strength from the five-membered ring of a 1,2-diol to the seven-membered ring of a 1,4-diol. Although the hydrogen bonding does not affect the vibrational property of the OH(II) (or acceptor), it significantly influences the OH(I) (or donor). As the hydrogen bonding becomes stronger from a 1,2- to a 1,4-diol, the fundamental band of the OH-stretching shifts downwards by from about 50 to 140 cm(-1), and the overtone band markedly decreases in intensity, although the effect on the intensity and bandwidth of the fundamental band varies among 1,2-, 1,3- and 1,4-diols. The quantum-mechanically calculated normal frequencies of the acceptor and donor OH groups in the hydrogen-bonded ring are in good agreement with the observed frequencies. The calculated interatomic distance between the O of an acceptor OH and the H of a donor OH is the shortest for a 1,4-diol, which is consistent with the largest frequency shift caused by the hydrogen bonding.     

Characterization of hydrated Na+(phenol) and K+(phenol) complexes using infrared spectroscopy

Hydrated alkali metal ion-phenol complexes were studied to model these species in aqueous solution for M=Na and K. IR predissociation spectroscopy in the O-H stretch region was used to analyze the structures of M+(Phenol)(H2O)n cluster ions, for n = 1-4. The onset of hydrogen bonding was observed to occur at n=4. Ab initio calculations were used to qualitatively explore the types of hydrogen-bonded structures of the M+(Phenol)(H2O)4 isomers. By combining the ab initio calculations and IR spectra, several different structures were identified for each metal ion. In contrast to benzene, detailed in a previous study of Na+(Benzene)n(H2O)m [J. Chem. Phys. 110, 8429 (1999)], phenol is able to bind directly to Na+ even in the presence of four waters. This is likely the result of the sigma-type interaction between the phenol oxygen and the ion. With K+, the dominant isomers are those in which the phenol O-H group is involved in a hydrogen bond with the water molecules, while with Na+, the dominant isomers are those in which the phenol O-H group is free and the water molecules are hydrogen-bonded to each other. Spectra and ab initio calculations for the M+(Phenol)Ar cluster ions for M=Na and K are reported to characterize the free phenol O-H stretch in the M+(Phenol) complex. While pi-type configurations were observed for binary M+(Phenol) complexes, sigma-type configurations appear to dominate the hydrated cluster ions.     

Infrared spectroscopy of hydrated alkali metal cations: evidence of multiple photon absorption

Infrared predissociation spectra of M(+)(H(2)O)(4-7), where M = alkali metal, are presented. Hydrogen bonding O-H stretching features are strongly dependent on which fragmentation channel is monitored. Spectra recorded by monitoring the loss of multiple waters show a preference for one absorption feature in the hydrogen-bonded region centered at ～3430-3500 cm(-1), which is assigned to linear-type hydrogen bonded OH stretches. Cyclic- and bent-type hydrogen bonded OH stretches have diminished photodissociation cross sections in the multiple ligand loss channels. Evidence from Rice-Ramsperger-Kassel-Marcus-evaporative ensemble calculations and laser fluence dependence experiments indicates that the multiple water loss channels are primarily the result of multiple photon absorption which we propose could be due to multiple, independent oscillators within a cluster ion each absorbing a photon during a single, 10 ns laser pulse.     

Structural correlations in liquid water: a new interpretation of IR spectroscopy

We present a new and alternative interpretation of the structure of the IR vibrational mode (nu(OH) band) of pure water. The re-interpretation is based on the influence of the cooperative hydrogen bonding arising from a network of hydrogen bonds in the liquid. The nu(OH) band has six components that are dominated by differences in their O-H bond lengths but deviate from thermodynamically average values due to interactions with the hydrogen bond network. The physical origin of the structure in the nu(OH) band is directly related to the O-H bond length, and variations in this bond length are caused by the influence of the surrounding hydrogen-bonded network of water molecules.     

Hydrogen bond, dimerization and vibrational modes in 2-chloro-3-hydroxybenzaldehyde and 3-chloro-4-hydroxybenzaldehyde from vibrational and ab initio studies

Ab initio conformers and dimers have been computed at RHF and B3LYP/6-31G* levels for isomers 2-chloro-3-hydroxybenzaldehyde and 3-chloro-4-hydroxybenzaldehyde to explain the observed infrared absorption and Raman vibrational spectral features in the region 3500-50 cm(-1). The position of the chlorine in ortho position with respect to aldehyde group in 2-chloro-3-hydroxybenzaldehyde yields four distinct conformers; whereas the chlorine in meta position in 3-chloro-4-hydroxybenzaldehyde yields effectively only three conformers. Major spectral features as strong absorptions near 3160-80 cm(-1), down-shifting of the aldehydic carbonyl stretching mode and up-shifting of hydroxyl group's in-plane bending mode are explained using ab initio evidence of O-H?O bond-aided dimerization between the most stable conformers of each molecule. Absorption width of about 700 cm(-1) (～8.28 kJ/mol) of O-H stretching modes suggests a strong hydrogen bonding with the ab initio bond lengths, O-H?O in the range of 2.873-2.832 ?. A strong Raman mode near 110-85 cm(-1) in each molecule is interpreted to be coupled vibrations of pseudo-dimeric trans and cis structures.