Theoretical infrared spectral density of H-bonds in liquid and gas phases: Anharmonicities and dampings effects

Abstract

The main purpose of the present paper is to show how both anharmonicities of the fast and the slow modes, multiple Fermi resonances and damping mechanisms introduced within the strong anharmonic coupling theory, are susceptible to explain some analogies in the infrared spectra of hydrogen bonded systems, when passing from the condensed to the gas phase. The high-frequency mode X – H → ⋯ Y described by double well potential is supposed to be anharmonically coupled to the H-bond stretching mode X ← – H ⋯ Y → described by Morse potential and to first overtones of some bending modes through Fermi resonances. The relaxation of the fast and bending modes and of the H-bond bridge is incorporated by aid of previous results [N. Rekik, B. Ouari, P. Blaise, O. Henri-Rousseau, J. Mol. Struct. 687 (2004) 125]. The spectral density is obtained as the Fourier transform of the autocorrelation function of the dipole moment operator within linear response theory. Numerical results show that mixing of all these effects results in a broad and complicated structure and expects to provide efficient energy relaxation pathways by using large dampings parameters for the condensed phase and weaker dampings for the gas one.