Infrared spectral density of H-bonds within the strong anharmonic coupling theory: Indirect relaxation effect

The IR spectral density (SD) of the high frequency stretching mode of H-bonded complexes involving both the intrinsic anharmonicity of the fast and slow mode, together with direct and indirect relaxations is studied within the linear response theory. For this aim, we extend a quantum non-adiabatic treatment of H-bonds involving intrinsic anharmonicity of the fast mode [N. Rekik, A. Velescu, P. Blaise, O. Henri-Rousseau, Chem. Phys. 273 (2001) 11.] which is described by an asymmetric double well potential by accounting for the anharmonicity of the slow mode and the indirect relaxation. In addition, the repulsive potential intervening in the asymmetric double well potential is described by the sum of three Gaussian whereas in the previous model, only one Gaussian was taken into account. The anharmonic coupling between the high frequency X–H→…Y and the low frequency X←–H…Y→ modes is treated inside the strong anharmonic coupling theory. The relaxation of the fast mode (direct damping) and of the H-bond bridge (indirect damping) is incorporated by aid of our previous results [N. Rekik, B.Ouari, P. Blaise, O. Henri-Rousseau, J. Mol. Struc. (Theochem.) 687 (2004) 125–133.]. The IR SD is obtained by Fourier transform of the autocorrelation function of the dipole moment operator of the fast mode.The numerical calculation shows that the indirect damping plays a specific role in the features of the lineshapes of hydrogen bonds systems by favouring more the fine structure of the low frequency tail than that of the high frequency one.