IR spectral density of H-bonds. Both intrinsic anharmonicity of the fast mode and the H-bond bridge. Part I: Anharmonic coupling parameter and temperature effects

Abstract

The paper presents extension of a quantum non-adiabatic treatment of H-bonds in which effects of anharmonicities of the high frequency X H → ⋯ Y and the low frequency X ← H ⋯ Y → modes on the υ X– H infrared lineshapes of H-bonds systems are considered. The anharmonic coupling between the high frequency X H → ⋯ Y and the low frequency X ← H ⋯ Y → modes is treated within strong anharmonic coupling theory and the relaxation is included following quantum treatment of Rösch and Ratner. The intrinsic anharmonicity of the fast frequency mode is described by a double well potential and of the slow frequency mode by Morse potential. IR spectral density is obtained within the linear response theory by the Fourier transform of the autocorrelation function of the X–H transition dipole moment operator. The main feature brought by the anharmonicity of the H-bond bridge X ← H ⋯ Y → is the increase of the average frequency of the υ X– H IR band with temperature for asymmetrical H-bonds, and decrease for symmetrical and weakly asymmetrical H-bonds. The numerical results are in fairly good agreement with the experimental behaviour of the first and the second moment of the X– H bands, observed when varying the temperature.