Showing papers by "Houcine Ghalla published in 2007"

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 XH→⋯Y and the low frequency X←H⋯Y→ modes on the υX–H infrared lineshapes of H-bonds systems are considered. The anharmonic coupling between the high frequency XH→⋯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 Rosch 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 υX–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.

Infrared spectral density of hydrogen bonds within the strong anharmonic coupling theory: Quadratic dependence of the angular frequency and the equilibrium position of the fast mode

Abstract: The paper presents extension of a quantum non-adiabatic treatment of H-bonds involving intrinsic anharmonicity of the fast mode [Rekik et al. Chem. Phys. 273 (2001) 11] by accounting for quadratic dependence of both the angular frequency and the equilibrium position of the X H → ⋯ Y stretching mode on the X ← H ⋯ Y → motion, in order to account for stronger H-bonds. Attention is focused on the study of effects induced by incorporation of such dependence on the IR spectral density of the high frequency stretching mode. The spectral density is obtained, within the linear response theory, by Fourier transform of the direct damped autocorrelation function of the dipole moment of the fast stretching mode. The anharmonic coupling between the high frequency X H → ⋯ Y and the low frequency X ← H ⋯ Y → modes is treated by the strong anharmonic coupling theory. Intrinsic anharmonicity of the fast mode is described by a double well potential, whereas the slow mode is considered to be harmonic. The relaxation of the fast mode (direct damping) is considered.