ABSTRACT The present theoretical paper is dealing with the infrared vX-H stretching absorption band of weak hydrogen bonded species. The hydrogen bond (H-bond) is considered through a full quantum treatment within the strong anharmonic coupling theory according to which the vX-H high frequency mode corresponding to the H-bond bridge. The theoretical lineshape is situated within the linear response theory according to which the spectral density is the Fourier transform of the autocorrelation function of the dipole moment operator of the mode of interest. The damping of the H-bond bridge i.e. the indirect relaxation is studied by aid of the usual quantum theories of damping. The first part is dealing with rigorous analytical treatment of indirect damping applying only to bare H-bonds and working within the harmonic approximation for the H-bond bridge and the adiabatic approximation allowing to separate the high and low frequency motions. The second part is concerned by quantum approximate treatments of indirect damping, allowing, because of their simplicity, the study of more complex and physical situations: introduction of Davydov coupling for cyclic H-bonds dimers, Fermi resonances, remove of the adiabatic and harmonic approximations… |
