Infrared spectra of weak H-bonds: beyond an adiabatic description of Fermiresonances

We extend a quantum non-adiabatic treatment of damped weak H-bonds [P. Blai se, O. Henri-Rousseau, Chem. Phys., Vol. 243 (1999) 229] in order to accoun t for Fermi resonances. We write the Hamiltonian whose potential part is a straight consequence of the linear modulation of the X-(H) over right arrow ...Y stretching frequency by the (X) over left arrow-H...(Y) over right arr ow intermonomer motion. We obtain the spectral density corresponding to thi s Hamiltonian by Fourier transform of the autocorrelation function of the d ipole moment of the fast stretching mode. The changes which are due to the adiabatic corrections are discussed by comparing with previous adiabatic st udies [O. Henri-Rousseau, D. Chamma, Chem. Phys. 229 (1998) 37]. The theore tical handling of Fermi resonances is analyzed, which leads us to define an 'exchange approximation' which was implicitly used in these adiabatic stud ies. The non-adiabatic treatment proposed here is a starting point toward t he description of stronger H-bonds for which the adiabatic separation must be overcome. The account for higher order anharmonicities will then give to the details of the theoretical spectra a meaningful role in the interpreta tion of experimental data. (C) 1999 Elsevier Science B.V. All rights reserv ed.