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

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 XH→⋯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 XH→⋯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.