The frequencies and intensities for the vibrational bands of absorption spectra of hydrogen-bonded (D2CO)2 and D2CO∙∙∙DF dimers, two D2CO∙∙∙(DF)2 trimers and four (D2CO)2∙∙∙DF trimers are calculated in the MP2/aug-cc-pVTZ approximation with the basis set superposition error taken into account. Anharmonic values of spectral parameters were obtained using the vibrational second-order perturbation theory. The influence of hydrogen bonds on the spectral parameters was determined from comparison of the values calculated for monomers, dimers, and trimers in the same approximation. The data obtained were compared with the results of previous calculations of (H2CO)2 and H2CO∙∙∙HF dimers and H2CO∙∙∙(HF)2 and (H2CO)2∙∙∙HF trimers. It was shown that one D2CO∙∙∙(DF)2 trimer and two (D2CO)2∙∙∙DF trimers have significant binding energies and strong absorption bands, which makes them promising candidates for detection by spectroscopic methods.

The frequencies and intensities for the vibrational bands of absorption spectra of hydrogen-bonded (D2CO)2 and D2CO∙∙∙DF dimers, two D2CO∙∙∙(DF)2 trimers and four (D2CO)2∙∙∙DF trimers are calculated in the MP2/aug-cc-pVTZ approximation with the basis set superposition error taken into account. Anharmonic values of spectral parameters were obtained using the vibrational second-order perturbation theory. The influence of hydrogen bonds on the spectral parameters was determined from comparison of the values calculated for monomers, dimers, and trimers in the same approximation. The data obtained were compared with the results of previous calculations of (H2CO)2 and H2CO∙∙∙HF dimers and H2CO∙∙∙(HF)2 and (H2CO)2∙∙∙HF trimers. It was shown that one D2CO∙∙∙(DF)2 trimer and two (D2CO)2∙∙∙DF trimers have significant binding energies and strong absorption bands, which makes them promising candidates for detection by spectroscopic methods.

We analysed the problem of approximation of the potential function of a diatomic molecule by a Morse model function with constant anharmonicity ν x using the Birge-Sponer extrapolation. The analysis of the approximations used in the derivation of the Morse equation shows that the solution of this problem is ambiguous. A scheme for optimizing the selection of initial parameters is proposed, which is illustrated by examples taken from the literature. The advantages of delineation of anharmonicity in the excitation of vibrational levels by deviations of the value ν x from the constant value according to Morse are demonstrated. An attempt is made to use the dimensionless anharmonicity parameter x* as a universal characteristic of the shape features of the electronic term of the molecule. Keywords: Morse potential, diatomic molecule, Birge-Sponer extrapolation, anharmonicity, electronic terms, vibrational structure.

The frequencies and intensities of IR absorption bands of the H2O...HF, H2O...DF, D2O...HF, and D2O...DF hydrogen-bonded complexes are calculated using the second-order vibrational perturbation theory. The MP2/aug-cc-pVTZ method with the basis set superposition error taken into account is used to calculate the electronic wave functions in determining the equilibrium configuration, the potential energy and dipole moment surfaces of these complexes, as well as in calculating spectral parameters. It is shown that upon complexation the frequencies and intensities of the stretching vibration of HF (DF) molecules and the intensities of stretching vibrations of H2O (D2O) molecules change most significantly. A variational calculation of librational motion of the water molecule in the two-well potential explained the reason for the higher value of the fundamental transition frequency of the ν1(H-F) mode as compared to the frequency of this mode upon hot transition from the first excited librational state. The dependence of the intermode anharmonic interaction on isotope substitution was analyzed. Keywords: hydrogen bond, calculations of spectra of molecular complexes, anharmonic interactions, isotope effects.

Рассчитаны частоты и интенсивности ИК полос поглощения комплексов с водородными связями H2O...HF, H2O...DF, D2O...HF и D2O...DF с использованием колебательной теории возмущений второго порядка. При определении равновесной конфигурации, поверхностей потенциальной энергии и дипольного момента этих комплексов, а также при вычислении спектральных параметров электронные волновые функции вычислялись по методу MP2/aug-cc-pVTZ с учетом ошибки наложения базисных наборов мономеров. Показано, что при образовании комплексов более всего меняются частоты и интенсивности валентного колебания молекул HF (DF) и интенсивности валентных колебаний молекул H2O (D2O). ариационный расчет либрации молекулы воды в потенциале с двумя минимумами объяснил причину более высокого значения частоты фундаментального перехода по моде ν1(H-F) по сравнению с частотой горячего перехода по этой моде с первого возбужденного состояния либрации. Выполнен анализ зависимости межмодового ангармонического взаимодействия от изотопного замещения. Ключевые слова: водородная связь, расчеты спектров молекулярных комплексов, ангармонические взаимодействия, изотопные эффекты.

The equilibrium geometry, the binding energy, and harmonic and anharmonic spectral parameters of H2CO and HF monomers and the H2CO center dot center dot center dot HF complex are calculated in the MP2/6-311 ++ G(3df,3pd) approximation with the basis set superposition error taken into account. Anharmonic calculations are carried out using the second-order vibrational perturbation theory. The changes in transition frequencies and intensities of monomeric IR bands upon formation of the 1:1 complex are analyzed. A high intensity value (53 km mol(-1)) is predicted for the overtone band of the out-of-plane HF libration. The variational method was used to solve anharmonic vibrational problems in 1D-4D subspaces involving the H-F stretch. The influence of anharmonicity on spectral and structural parameters was studied. The variational calculation confirmed the high intensity value for the overtone of the HF librational mode from the perturbative calculation. Matrix-isolation experiments were performed in N-2 at T = 8 K to record the absorption spectra of pure H2CO and HF and H2CO/HF mixtures in the IR region. The spectral features related to the H-F, C = O, and C-H stretches and the in-plane HF librational motion were reliably identified. The predictions of our perturbative calculations for isolated compounds, especially for the band shifts upon complexation, are in satisfactory agreement with the experimental matrix-isolation findings. The only exception is the H-F stretching mode in the complex because of the high anharmonicity of HF and strong interaction of free HF molecules with a matrix environment. Of particular interest is the observation of a rather strong band at 1069 cm(-1), which is close to 1072 cm(-1) predicted for the overtone of HF out-of-plane librational mode. (C) 2020 Elsevier B.V. All rights reserved.

Features of the high-resolution IR spectra of the SiН4 molecule in low-temperature matrices of N2 and Ar at 6.6–20 K, depending on the experimental conditions, are analyzed. It was found that, in the nitrogen matrix, in the stretching region of 28SiH4, three narrow bands are observed and, in the region of bending vibration (ν4), there are two monomer bands instead of one, which is explained by the change in the symmetry of the molecule from Td in gas to $${{C}_{{3v}}}$$ in solid nitrogen. The spectra in the argon matrix change even more, where, in addition to narrow bands, rather wide components are also detected. The spectrum of SiН4 in an Ar matrix was calculated based on the QM/MM approach, which confirms the reality of the change in the symmetry of the molecule as a result of its interaction with the matrix environment.

The frequencies and intensities of IR absorption bands of symmetric and asymmetric H-bonded complexes [FL1FL2F]- (L1, L2 = K-meson Ka, proton H, deuton D, and triton T) are calculated. The equilibrium configuration and potential energy and dipole moment surfaces of isotopologues [FL1FL2F]- were calculated by the MP2/6-311++G(3df,3pd) method with the basis set superposition error taken into account. The calculations of spectral parameters with allowance for anharmonic interactions of all vibrations were carried out using the second-order vibrational perturbation theory. Variation of Li and L2 masses in wide regions allowed significant changes in the forms of normal vibrations and values of anharmonic interaction constants upon isotopic substitution to be obtained. The trends in the changes of spectral parameters were determined upon transition from one symmetric isotopologue to another and upon transition from symmetric to asymmetric isotopologues. The D-F stretching band frequency predicted for [FHFDF]- is in good agreement with the experimental value. The assignment of this band was improved.

A quantum-mechanical analysis of the manifestations of the NHN hydrogen bond in the vibrational spectra of the [HCN.H.NCH]+ linear complex along the profile of the proton transition reaction path is given, and the laws governing the diagnostic parameters, that is, potential descriptors of the dynamics of this process, are determined. We calculated the surface of the potential energy and harmonic frequencies of normal vibrations along the profile of the reaction path of the proton transition in the system under study. When the fragments [HCNH]+ and NCH approach each other, a noticeable distortion of the forms of their skeletal vibrations occurs, up to complete mixing into the symmetric and antisymmetric forms. The frequency of the longitudinal vibration of the central proton ν(NH) varies along the reaction path from ~3600 to ~500 cm–1. An abrupt frequency change is observed in the region of intersection of the terms ν(NH) and ν(CN).

The frequencies and intensities for IR absorption bands of symmetric and asymmetric hydrogen-bonded complexes [FL1FL2F]– (L1, L2 = K-meson Ka, proton H, deuteron D, and triton T) are calculated. The equilibrium configuration and the potential energy and dipole moment surfaces of [FL1FL2F]– isotopologues were calculated by the MP2/6-311++G(3df,3pd) method with the basis set superposition error taken into account. The calculations of spectral parameters with allowance for anharmonic interactions of all vibrations were carried out using the second-order vibrational perturbation theory. Variation of L1 and L2 masses over wide limits led to significant changes in the forms of normal vibrations and anharmonic interaction constants upon isotopic substitution. The trends in the changes of spectral parameters were determined both upon transition from one symmetric isotopologue to another and upon transition from symmetric to asymmetric isotopologues. The frequency of the D–F stretching band of [FHFDF]– was predicted in good agreement with experimental information, and the assignment of this band was improved.

The features of high resolution IR spectra of the SiH4 molecule in N2 and Ar low-temperature matrices at 6.6-20 K were analyzed depending on experimental conditions. It was found, that in the nitrogen matrix in region of stretching vibration of 28SiH4 three narrow bands are observed, in the region of bending vibration (ν4) two bands are observed instead of one, which is explained by a change in the symmetry of the molecule from Td in the gas phase to C3v in solid nitrogen. In the argon matrix spectra change even more, where, in addition to narrow bands, the broad enough components are also recorded. The calculations of the SiH4 spectrum in the Ar matrix were performed based on the QM/MM approach, which confirm the reality of the change in the symmetry of the molecule as a result of its interaction with the matrix environment.

Our research is focused on the ab initio calculations of the equilibrium structures, binding energies, harmonic and anharmonic vibrational frequencies of a hydrogen-bonded complex, which is formed between formaldehyde H2CO and hydrogen fluoride HF, using the Gaussian 09 package of programs with full 6311++G(3df, 3pd) basis sets in the MP2 second-order perturbation theory and CCSD(T) methods. Harmonic and anharmonic vibrational frequencies and intensities of the H2CO···HF complex were calculated by the Gaussian 16 package programs within the same approximation. Geometric changes and frequency shifts at the complex formation were evaluated. The H2CO···HF complex formation energy and the dipole moment were calculated in the CCSD(T)6311++G(3df, 3pd) approximation to be equal, respectively, to 7.78 kcal/mol and 4.2 D. Changes of the geometric, spectral, and energetic parameters of the complex proved the existence of a stable hydrogen bond F–H···O=CH2 between the components.

A quantum-mechanical analysis of the manifestations of the NHN hydrogen bond in the vibrational spectra of the [HCN.H.NCH] + linear complex along the proton transfer reaction profile is given, and the laws governing the diagnostic parameters — potential descriptors of the dynamics of this process — are established. The surface of the potential energy and harmonic frequencies of normal vibrations along the profile of the proton transfer reaction path in the system studied are calculated. It has been shown that when the [HCNH] + and NCH fragments come closer together, a noticeable distortion of the forms of their skeletal vibrations occurs, up to complete mixing into the symmetric and antisymmetric forms. The frequency of the longitudinal vibration of the central proton ν (NH) varies along the reaction path from ~ 3600 to ~ 500 cm-1. In the region of intersection of the terms ν (NH) and ν (CN) an abrupt nature of frequency change is detected.

IR absorption spectra of mixtures (12CH3) $$_{2}^{{13}}$$ CO/HF and free (12CH3) $$_{2}^{{13}}$$ CO molecules are recorded in the region of 4000−800 cm−1 with a Bruker IFS-125 HR vacuum Fourier spectrometer at room temperature with a resolution of 0.05 cm−1. Absorption bands of the (12CH3) $$_{2}^{{13}}$$ CO…HF complex are obtained by subtracting the absorption bands of free HF and acetone molecules and absorption lines of atmospheric water from the experimental spectrum of mixtures. Spectral characteristics of the 2ν(13C=O) overtone band of free acetone were also recorded. Comparison of the spectral data obtained with the analogous data measured earlier for the (12CH3) $$_{2}^{{12}}$$ CO…HF complex shows changes in the absorption spectra of complexes caused by 12C → 13C isotopic substitution. The frequencies and intensities for absorption bands of both complexes are calculated using the perturbation theory and methods MP2/6-311++G(2d,2p) and MP2/6-311++G(3df,3pd) with allowance for the error of superposition of basis functions of the monomers. The calculated results are in good agreement with the experimental data and are used to interpret the observed spectra. The influence of anharmonic effects on the frequencies and intensities of the strongest ν(H–F) and ν(13C=O) bands is examined with the help of variational calculations. It is shown that, unlike the (12CH3) $$_{2}^{{12}}$$ CO…HF complex, the 2ν(13C=O)/ν(H–F) resonance is virtually absent in the (12CH3) $$_{2}^{{13}}$$ CO…HF complex.

Four stable hydrogen- and lithium-bonded heterodimers containing the HF molecule and an electride-like Li4C4H2N2 molecule were predicted by ab initio calculations. Equilibrium geometries, binding energies, and dipole moments were determined. The interaction-induced contributions to dipole moments and natural bond orbital charges were analyzed. Infrared transition frequencies and intensities of two stronger heterodimers were calculated in the harmonic approximation. Variational 1D anharmonic calculations performed for all stable heterodimers predicted negative frequency shifts up to -1000 cm(-1) and an order of magnitude increase in intensity of the H-F stretching band upon formation of these systems.

AbstractIR absorption spectra of mixtures (^12CH_3) $$_{2}^{{13}}$$ CO/HF and free (^12CH_3) $$_{2}^{{13}}$$ CO molecules are recorded in the region of 4000−800 cm^−1 with a Bruker IFS-125 HR vacuum Fourier spectrometer at room temperature with a resolution of 0.05 cm^−1. Absorption bands of the (^12CH_3) $$_{2}^{{13}}$$ CO…HF complex are obtained by subtracting the absorption bands of free HF and acetone molecules and absorption lines of atmospheric water from the experimental spectrum of mixtures. Spectral characteristics of the 2ν(^13C=O) overtone band of free acetone were also recorded. Comparison of the spectral data obtained with the analogous data measured earlier for the (^12CH_3) $$_{2}^{{12}}$$ CO…HF complex shows changes in the absorption spectra of complexes caused by ^12C → ^13C isotopic substitution. The frequencies and intensities for absorption bands of both complexes are calculated using the perturbation theory and methods MP2/6-311++G(2d,2p) and MP2/6-311++G(3df,3pd) with allowance for the error of superposition of basis functions of the monomers. The calculated results are in good agreement with the experimental data and are used to interpret the observed spectra. The influence of anharmonic effects on the frequencies and intensities of the strongest ν(H–F) and ν(^13C=O) bands is examined with the help of variational calculations. It is shown that, unlike the (^12CH_3) $$_{2}^{{12}}$$ CO…HF complex, the 2ν(^13C=O)/ν(H–F) resonance is virtually absent in the (^12CH_3) $$_{2}^{{13}}$$ CO…HF complex.

Vibrational spectroscopic and average geometrical parameters of the strong H-bonded complexes [F(HF)2]- and [F(DF)2]- are determined for the first time from nine-dimensional (9D) perturbative and 6D variational calculations. The frequencies and intensities for all fundamental and some combination and overtone transitions obtained by the method of second-order vibrational perturbation theory (VPT2) are reported. A two-fold decrease in the H-F (D-F) stretching band frequency and a more than ten-fold increase in the intensity of this band upon complexation are predicted. The theoretical frequencies for both isolated isotopologues are in satisfactory agreement (to better than 70 cm-1) with the scarce experimental data obtained in condensed phases. The main purpose of variational calculations is to analyze the intermode anharmonic coupling and the changes in the geometrical parameters upon vibrational excitation and H/D isotopic substitution. The equilibrium nuclear configuration and the 2D potential energy surface (PES) of [F(HF)2]- for H-F stretches are calculated in the MP2/6-311++G(3df,3pd), CCSD(T)/6-311++G(3df,3pd), CCSD(T)/aug-cc-pVTZ, and CCSD(T)/d-aug-cc-pVTZ approximations with the basis set superposition error taken into account. Anharmonic vibrational problems are solved by the variational method for 2D, 4D, and 6D systems of H-bond and H-F (D-F) stretches and in-plane bends. The VPT2 calculations and calculations of the PESs for 4D and 6D systems are performed in the MP2/6-311++G(3df,3pd) approximation. Comparison of variational anharmonic solutions for different vibrational subsystems demonstrates the influence of intermode anharmonic coupling on the mixing of wave functions and spectroscopic and geometrical characteristics. The inverse Ubbelohde effect is predicted and substantiated.

The geometrical parameters, the frequencies, and absolute intensities of vibrational transitions of H2O···trans-HONO hydrogen-bonded complex are calculated using the approach earlier tested in calculations of isolated molecules of nitrous acid and complexes of this acid with ammonia. The equilibrium nuclear configuration and potential energy and dipole moment surfaces are calculated by the MP2/aug-cc-pVTZ method with the basis set superposition error taken into account. The fundamental transition frequencies and intensities of the complex are first obtained in the harmonic approximation, and then the energy values, vibrational wave functions, and transition frequencies and intensities are determined from variational solutions of one- to four-dimensional anharmonic equations. The results obtained are compared with the data calculated in the same approximation for an isolated trans-HONO molecule and the NH3···trans-HONO complex. The average discrepancy between the anharmonic frequency values and five available experimental data is 15 cm-1. Three absorption bands of trans-HONO with the highest intensity are recommended for detecting the presence of H2O···trans-HONO.

Equilibrium nuclear configurations of the planar formaldehyde homodimer (H 2 CO) 2 and the (H 2 CO) 2 ···HF complex are determined in the MP2/6-311++G(3df, 3pd) approximation taking into account the superposition error of basis sets of monomers. Harmonic values of the frequencies and intensities of fundamental transitions between vibrational states of these hydrogen-bonded complexes were calculated using the Gaussian 09 package of programs. Anharmonic values of the frequencies and intensities of the ν(H–F) stretching vibration and several intermolecular vibrations in the (H 2 CO) 2 ···HF trimer were obtained from variational solutions of one-, two-, and three-dimensional vibrational Schrödinger equations. The anharmonic influence of the C=O and hydrogen bond O···H–F stretching vibrations, as well as of librational vibrations of monomers, on the spectral parameters of the strongest ν(H–F) absorption band of trimer was studied.