Theory of weak damped H-bonds: relative influence of relaxation mechanismson IR spectra

We revisit numerically the roles played by relaxation mechanisms an the lin e shapes of the IR spectral density of weak H-bonds. This is performed by m eans of three theories already published. The tools common to these theorie s are the strong anharmonic coupling theory (between the high- and low-freq uency stretching modes of the H-bond), and the linear response theory (acco rding to which the spectral density is the Fourier transform of the autocor relation function). The theories are those of: (1) G. Robertson and J. Yarw ood [Chem. Phys. 32 (1978) 267], taking into account (semiclassically) indi rect damping; (2) N. Rosch and M. Ratner [J. Chem. Phys. 61 (1974) 3444] de aling (quantum mechanically) with direct damping; and (3) B. Boulil, J.-L. Dejardin, N, El-Ghandour, O. Henri-Rousseau [J. Mel. Struct. (Theochem) 314 (1994) 83] involving (quantum mechanically) slow-mode damping. The quantum direct damping induces a broadening, and the quantum slow-mode damping (in contrast with the semiclassical indirect relaxation) a weak narrowing, whe n they are both occurring. The direct damped quantum spectral density leads to Lorentzian (fast modulation limit) or Gaussian (slow modulation limit) shapes as does the spectral density of the semiclassical model of indirect relaxation. The dephasing of the fast mode should be predominant for line s hapes with broadened sub-bands (obeying the Franck-Condon progression law), or without sub-bands (but with nearly symmetric profiles intermediate betw een Gaussian and Lorentzian). Both the dephasing of the fast mode and the d amping of the slow mode should occur by similar amounts if the line shapes are without sub-bands but with asymmetry, or with sub-bands but with intens ity anomalies in the Franck-Condon progression. (C) 1999 Published by Elsev ier Science B.V. All rights reserved.