Spectral density of medium strength H-bonds. Direct damping and intrinsic anharmonicity of the slow mode. Beyond adiabatic approximation

The present theory is a new step in our attempt to obtain a flexible tool susceptible to be used by experimentalists working in the realm of the spectra of the infrared nu(X-H...Y) mode of weak and medium strength hydrogen bonds: the spectral density is studied within the linear response theory by Fourier transform of the autocorrelation function of the transition dipole moment of the fast mode. The strong anharmonic coupling theory is used through second-order expansion in the slow-mode coordinate Q of the angular frequency and the equilibrium position of the fast mode. The theory is working beyond the adiabatic approximation. It takes into account the intrinsic anharmonicity of the low frequency mode through Morse potential, and assumes a direct damping of the fast mode. At last, indirect damping, Fermi resonances and Davydov coupling are ignored. When the Morse potential is expanded up to the harmonic approximation, the theoretical spectral density reduces [O. Henri-Rousseau, P. Blaise, Chem. Phys. 250 (1999) 249]. The theory reproduce not only the experimental features obtained in this previous paper but also the increase in magnitude of the first moment with temperature. (C) 2000 Elsevier Science B.V. All rights reserved.