Anharmonic effects on theoretical IR line shapes of medium strong H(D) bonds

We extend a quantum nonadiabatic treatment of damped H‐bonds involving combined effects of anharmonicities of both the fast and the slow modes, Fermi resonances and relaxation [Rekik et al., J Mol Liq (in press)] in order to account for stronger H‐bonds. For this purpose, we introduce, in the model, the quadratic modulation of both the angular frequency and the equilibrium position of the X − $\mathop H\limits^{\rightarrow}$ … Y stretching mode on the intermonomer $\mathop X\limits^{\leftarrow}- H \ldots \mathop Y\limits^{\rightarrow}$ motions to refine the structure of the spectrum, whereas we have considered in our previous work only the linear modulation of the angular frequency of the fast mode. In this approach, the strong anharmonic coupling theory is used through second‐order expansion in the slow‐mode coordinate Q of the angular frequency and the equilibrium position of the fast mode. The relaxations of the fast mode (direct damping) and of the H‐bond bridge (indirect damping) are incorporated by aid of previous results [Rekik et al., J Mol Struct 2004, 687, 125]. The spectral density is obtained by Fourier transform of the autocorrelation function of the dipole moment of the fast stretching mode. The numerical calculation shows that the modulation of the angular frequency of the fast mode and its equilibrium position by the slow mode coordinate generate an improvement of the fine structure of the spectrum and also provide a direct evidence of the increase of the level density and the spectral broadening. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009