IR theory of weak H-bonds: Davydov coupling, Fermi resonances and direct relaxations. II. General trends, from numerical experiments

The theoretical model proposed in the precedent paper [Chem. Phys. 248 (199 9) 53] which was dealing with the X-(H) over right arrow...Y stretching mod e of a cyclic dimer susceptible of Davydov coupling, and involving weak H-b onds and Fermi resonances, is applied. The basis of this theory was an exte nsion of the quantum model of Wojcik [Mol. Phys. 36 (1978) 1757] in which h as been incorporated the direct damping of the fast mode and that of the be nding mode involved in the Fermi resonance mechanism. The Fourier transform of the autocorrelation function of the dipolar moment operator is computed as a function of nine basic physical parameters. Fermi resonances ought to be an universal phenomenon for H-bonded dimers, since the lineshape is sen sitive to the Fermi coupling over a frequency range that is one order of ma gnitude greater than the Fermi coupling parameter. The spectral lineshapes exhibit a rich polymorphism, mainly with respect to the coupling and dampin g parameters, which was some unpredictable and must lead to be extremely ca utious in the interpretation of experimental spectra. In contrast, the deta ils of the lineshapes appear to be very stable with respect to large temper ature changes, even if the half width is smoothly increased by around 12% b y raising the temperature from 10 to 300 K, that is in qualitative agreemen t with experiment. (C) 1999 Elsevier Science B.V. All rights reserved.