Comparative analysis of the H–F stretching band in absorption spectra of gas-phase complexes of HF with water, dimethyl ether, and acetone

Abstract

Comparative analysis of the ν 1(H–F) stretching band shapes in the absorption spectra of complexes of HF with water, dimethyl ether, and acetone and the mechanisms of formation of this band is performed by comparing the experimental spectra and results of nonempirical quantum–mechanical calculations. The experimental spectra of complexes considered were obtained by subtracting the spectra of monomers from the experimental spectra of gaseous mixtures recorded at different temperatures in the region of the ν 1(H–F) band at a high resolution with Bruker IFS-113v and Вruker IFS-125 HR vacuum Fourier spectrometers. These spectra are compared with the band shapes reconstructed theoretically with the use of electro-optical parameters obtained from variational solutions of multidimensional anharmonic vibrational problems. The equilibrium geometries of the complexes, their potential energy and dipole moment surfaces necessary for solving these problems were calculated ab initio at a high level of theory. In this approach the internal anharmonicity of the ν 1(H–F) mode and its interaction with all low-frequency intermolecular modes are considered explicitly. Anharmonic interactions between different intermolecular motions are also analyzed in detail. The inter-mode interactions are found to have different effects on the frequency and absolute intensity of the ν 1(H–F) fundamental transition. Comparison of the data for different complexes shows that the resulting spectral manifestations of the anharmonic effects depend on the interplay between the H-bond strength, the barrier height for tunneling, the magnitudes of frequencies of intermolecular vibrations, rotational constants, etc. The reconstruction of spectra as a superposition of rovibrational bands of the fundamental, hot, and combination transitions with calculated values of absolute intensities can reproduce the shape and separate details of the experimental spectra. Analysis of the experimental and calculated data provides reliable values of the fundamental ν 1(H–F) transition frequency, indicates its position in the spectrum, and elucidates the nature of different structural features.