Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: an ab initio mixed quantum-classical study.

The infrared spectrum of phenol-water cationic cluster, [PhOH.H2O]+, taken by Sawamura et al. [J. Phys. Chem. 100, 8131 (1996)] is puzzling in that the peak due to the stretching mode of the phenolic OH (3657 cm-1 for a neutral monomer and 3524 cm-1 for PhOH.H2O) seemingly disappears and instead an extremely broad tail extending down to 2900 cm-1 is observed. The present authors theoretically ascribe this anomalous spectrum to an inhomogeneous broadening of the OH stretching peak caused by the hydrogen bond, the strength of which has been greatly enhanced by ionization of the phenyl ring. Indeed they estimate that the peak position is at 2300 cm-1 and the spectral width can become as wide as 1000 cm-1 at the cluster energy of 32 kcal/mol. This surprisingly wide broadening can be generic in hydrogen-bond systems, which in turn is useful to study the nature of the hydrogen-bond assisted dynamics in various systems such as those in DNA and proteins. To study the present system quantitatively, the authors have developed an ab initio mixed quantum-classical method, in which the nuclear motions on an adiabatic ab initio potential surface are treated such that only the OH stretching motion is described quantum mechanically, while all the other remaining modes are treated classically with on-the-fly scheme. This method includes the implementation of many numerical methodologies, which enables it to deal with a relatively large molecular system. With this theoretical method, the authors analyze the present anomalous broadening in a great detail. In particular, they suggest that one can extract direct information about the hydrogen-bond dynamics with respect to the clear correlation between the vibrational excitation energy of the OH stretching and intermolecular distance by means of a time-resolved infrared spectroscopy: Reflecting the slow and wide-range variation of the intermolecular distance of the relevant hydrogen bond, the time-resolved spectrum is predicted to vary (shift) largely covering the wide range of frequency domain. Thus, it is found that the short-time average along a selected trajectory sensitively reflects the change of the intermolecular distance. The authors also study the effect of internal energy on the hydrogen bonding and the OH spectrum.