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

We extend a quantum non-adiabatic treatment of damped weak H-bonds [P. Blaise, O. Henri-Rousseau, Chem. Phys., Vol. 243 (1999) 229] in order to account 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 arrow intermonomer motion. We obtain the spectral density corresponding to this Hamiltonian by Fourier transform of the autocorrelation function of the dipole moment of the fast stretching mode. The changes which are due to the adiabatic corrections are discussed by comparing with previous adiabatic studies [O. Henri-Rousseau, D. Chamma, Chem. Phys. 229 (1998) 37]. The theoretical handling of Fermi resonances is analyzed, which leads us to define an 'exchange approximation' which was implicitly used in these adiabatic studies. The non-adiabatic treatment proposed here is a starting point toward the 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 interpretation of experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.