V. P. Bulychev

Study of the ν1 band shape of the H2O⋯HF, H2O⋯DF, and H2O⋯HCl complexes in the gas phase

The band shape of the ν1 hydrogen fluoride stretch in H2O⋯HF and H2O⋯DF complexes was studied in the gas phase. The spectra of H2O/HF mixtures at 293 K in cells 20 and 1200 cm long were recorded in the range 4200–3000 cm−1 at a resolution of 0.2−0.02 cm−1. The spectra of the 1 : 1 complex in the region of the ν1(HF) absorption band were obtained by subtracting the calculated spectra of free H2O and HF molecules from the experimental spectra. The asymmetric ν1 band of H2O⋯HF has a low-frequency head, an extended high-frequency wing, and a characteristic vibrational structure. The ν1 band shape was reconstructed nonempirically as a superposition of rovibrational bands of the ν1(HF) fundamental transition and hot transitions from excited states of low-frequency modes. The reconstruction was based on an ab initio calculation of the potential energy and dipole moment surfaces and subsequent variational multidimensional anharmonic calculations of the vibrational energy levels, the frequencies and intensities of the transitions considered, and the rotational constants. The calculated spectrum reproduces the structure of the experimental spectrum, in particular, the relative intensities of the peaks. However, the assignment of spectral features differs from that generally accepted. The central, most intense, peak is associated with the transition from the ground state, while the lowest-frequency peak with the P branch head of transition from the v6(B2) = 1 state. This leads to a value of 3633.8 cm−1 for the ν1(HF) stretch frequency of H2O⋯HF, which is higher than the commonly adopted value of 3608 cm−1. Similar calculations of H2O⋯DF predict a value of 2689 cm−1 for the ν1(DF) stretch and a less structured band shape. On formation of a 1 : 1 complex with water the frequency is shifted by −331.8 cm−1 and −229.4 cm−1 and the intensity is increased by a factor of 3.87 and 3.51 for HF and DF, respectively. Similar calculations of H2O⋯HCl predicted a value of 2726.5 cm−1 for the ν1 fundamental, a lower frequency for the hot transition from the v6(B2) = 1 excited state, and a ν1(HCl) band shape in agreement with the results of recent low-temperature experiments.

Experimental and theoretical study of the ν(HF) absorption band structure in the H2O…HF complex

The νHF absorption band shape of the H2O…HF complex is studied in the gas phase at a temperature of 293 K. The spectra of H2O/HF gaseous mixtures in the range 4000–3400 cm−1 are recorded at a resolution of 0.2–0.02 cm−1 with Bruker IFS-113v and Bruker IFS-120 HR vacuum Fourier spectrometers in a 20-cm cell. The spectra of the H2O…HF complex in the region of the ν1(HF) absorption band are obtained by subtracting the calculated spectra of free H2O and HF molecules from the experimental spectrum. The ν1 band of the H2O…HF complex has an asymmetric shape with a low-frequency head, an extended high-frequency wing, and a characteristic vibrational structure. Two approaches are used to calculate the ν1 band shape as a superposition of rovibrational bands of the fundamental and hot transitions involving the low-frequency modes of the complex. The first approach is based on a simplified semiempirical procedure. The second approach relies on a nonempirical anharmonic calculation of the vibrational energy levels, the frequencies and intensities of the corresponding transitions, and the rotational constants. These parameters are obtained by calculating ab initio the potential energy and dipole moment surfaces in the second-order Möller-Plesset approximation and using the variational method to solve one-, two-, and three-dimensional anharmonic vibrational problems. The absorption spectrum of the complex in the range 3600–3720 cm−1, reconstructed using the nonempirical electro-optical parameters, reproduces rather well the main features of the experimental spectrum, including the relative intensities of peaks of the vibrational structure. However, the interpretation of most of the structural features of the spectrum differs from that adopted in the semiempirical scheme. First of all, it follows from the results of nonempirical calculation that the central, most intense, maximum of the experimental spectrum should correspond to the v1=1←0 transition from the ground vibrational state. This fact gives rise to a new value of the vibrational transition frequency ν10 in the H2O…HF complex equal to 3635 cm−1, which is higher than the commonly accepted value of 3608 cm−1.

Theoretical study of the spectral and structural parameters of van der Waals complexes of the Li+ cation with the H2, D2, and T2 isotopomers of the hydrogen molecule

The three-dimensional vibrational problem for the isolated binary complexes formed by the Li+ cation with all the isotopomers of the hydrogen molecule is solved by the variational method using sufficiently exact nonempirical adiabatic surfaces of the potential energy and the dipole moment. Information on the largeamplitude vibrations was obtained for the first time, and the anharmonic effects caused by the interaction of the different internal degrees of freedom in these weakly bound van der Waals complexes were consistently taken into account. The frequencies and intensities of many spectral transitions are determined, and the average values of geometrical parameters and the dipole moment are calculated for the ground and excited vibrational states.

Calculation of Vibrations of the H-bonds and Electrooptical Parameters of the (F(HF) 2 ) - Complex

The problem of calculating the vibrations of the F(HF)2]− complex with hydrogen bonds is considered with allowance for the anharmonicity and interaction of motions in different degrees of freedom. A systematic solution of this problem is proposed which consists in separating the total vibrational system into subsystems, obtaining sufficiently exact vibrational wave functions of subsystems, and expanding the vibrational wave functions of the total system in basis functions constructed from the wave functions of subsystems. At the first stage of our study, the stretching and bending modes of two F...HF hydrogen bonds are considered with the use of an exact kinetic energy operator and a nonempirical three-dimensional potential energy surface. It is shown that these vibrational modes of the complex are characterized by significant mechanical and electric anharmonicities. The calculated values of frequencies of the symmetric and antisymmetric vibrations of hydrogen bonds are in good agreement with the experimental findings.

Influence of isotopic substitution on spectral and structural parameters of an isolated [F(HF)2]— complex. Substitution of a proton by a kaon and a triton

This paper continues the theoretical study (see V. P. Bulychev and M. V. Buturlimova, J. Mol. Struct. 928, 32 (2009)) of the isotopic effects in the H-bonded anionic complex [F(HF)2]-. Isotopomers of the complex with significant differences between the masses of the light atoms are considered. The four-dimensional anharmonic vibrational problem are solved by the variational method for the symmetric complex [F(KaF)2]-, in which both protons are substituted by a positive kaon (positive K-meson), and for the asymmetric complex [FKaFTF]-. Variables related to the changes in the lengths of molecular fragments LF (L = Ka and T) and the distances between the F- anion and the centers of mass of LF are used as the vibra-tional coordinates. The potential energy surfaces are calculated in the MP2/6-311++G(3df,3pd) approximation taking into account the basis set superposition error. The vibrational energy levels, frequencies, and absolute intensities for spectral transitions are determined. To study the isotope effect on the geometrical parameters of the complex, the values of internuclear separations and the asymmetry parameter of the F-…L-F bridge averaged over the ground state and several excited vibrational states are calculated, as well as their standard deviations. The calculated results are compared to the data obtained previously for the symmetric complexes [F(HF)2]-, [F(DF)2]-, and [F(TF)2]-.

Theoretical study of structural and energy parameters of the van der Waals complex of the Li+ cation with the N2 molecule

The three-dimensional vibrational problem for the isolated van der Waals complex formed by the Li+ cation with the N2 molecule is solved by the variational method. The potential energy and dipole moment surfaces are calculated using different basis sets of atomic functions and different approaches for taking electron correlation into account. The anharmonic effects caused by the interaction of vibrational degrees of freedom of the complex are consistently considered for the first time. The energy levels of three-dimensional vibrational states are determined. The frequency shift of the N2 molecule vibration upon complexation and the fundamental transition intensity for this vibration in the complex are calculated. The frequencies and intensities for a number of spectral transitions between the states associated with excitation of low-frequency modes are determined. The average values of geometrical parameters and their variances are calculated for the ground state and excited vibrational states of the complex.

Calculation of interaction of the stretching and bending vibrations of HF in the hydrogen-bonded complex [F(HF)2]−

Vibrations of the [F(HF)2]− complex are calculated with allowance for the anharmonic interactions of the stretching vibrations of HF monomers and their rotations about the centers of gravity of HF in the plane of the complex. A four-dimensional vibrational Schrödinger equation is solved using a potential energy surface calculated in the MP2/6-311++G(3df,3pd) approximation with the superposition of atomic functions of the monomers taken into account. The equilibrium and vibrationally averaged structures of the complex are determined. The frequencies and intensities for spectral transitions from the ground state to a number of excited vibrational states are calculated. It is shown that, due to resonances between the excited states of the stretching modes and doubly excited states of the bending modes, the overtone transitions associated with the bending modes borrow a significant part of the intensity of fundamental stretching transitions.

Calculation of vibrational spectroscopic and geometrical characteristics of the [F(HF)2]− and [F(DF)2]− complexes using the second-order vibrational perturbation theory and a 6D variational method

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 structure and vibrational spectral parameters of a complex of HF with the planar (H 2 CO) 2 dimer

Equilibrium nuclear configurations of the planar formaldehyde homodimer (H2CO)2 and the (H2CO)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 (H2CO)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.

A study of the structure of the ν1(HF) absorption band of the СH3СN…HF complex

The ν1(HF) absorption band shape of the CH3CN…HF complex is studied in the gas phase at a temperature of 293 K. The spectra of gas mixtures CH3CN/HF are recorded in the region of 4000–3400 cm–1 at a resolution from 0.1 to 0.005 cm–1 with a Bruker IFS-120 HR vacuum Fourier spectrometer in a cell 10 cm in length with wedge-shaped sapphire windows. The procedure used to separate the residual water absorption allows more than ten fine-structure bands to be recorded on the low-frequency wing of the ν1(HF) band. It is shown that the fine structure of the band is formed primarily due to hot transitions from excited states of the low-frequency ν7 librational vibration. Geometrical parameters of the equilibrium nuclear configuration, the binding energy, and the dipole moment of the complex are determined from a sufficiently accurate quantum-chemical calculation. The frequencies and intensities for a number of spectral transitions of this complex are obtained in the harmonic approximation and from variational solutions of anharmonic vibrational problems.