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

The theoretical model proposed in the precedent paper [Chem. Phys. 248 (1999) 53] which was dealing with the X-H→⋯Y stretching mode of a cyclic dimer susceptible of Davydov coupling, and involving weak H-bonds and Fermi resonances, is applied. The basis of this theory was an extension of the quantum model of Wójcik [Mol. Phys. 36 (1978) 1757] in which has been incorporated the direct damping of the fast mode and that of the bending 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 sensitive to the Fermi coupling over a frequency range that is one order of magnitude greater than the Fermi coupling parameter. The spectral lineshapes exhibit a rich polymorphism, mainly with respect to the coupling and damping parameters, which was some unpredictable and must lead to be extremely cautious in the interpretation of experimental spectra. In contrast, the details of the lineshapes appear to be very stable with respect to large temperature changes, even if the half width is smoothly increased by around 12% by raising the temperature from 10 to 300 K, that is in qualitative agreement with experiment.