Infrared spectroscopy of small protonated water clusters at room temperature: an effective modes analysis.

We perform infrared vibrational analysis on small protonated water clusters H(+)(H(2)O)(n), with n = 2, [ellipsis (horizontal)], 6, at room temperature. The absorption spectra are calculated based on classical trajectories obtained by the multistate empirical valence bond method. The analysis is carried out based on the effective modes analysis, which has been recently developed [Martinez et al., J. Chem. Phys. 125, 144106 (2006)] as generalization of the normal modes analysis. This technique enables us to decompose the full spectrum in maximally localized bands which are obtained by accounting for temperature and anharmonic effects. These effects are especially considered in the determination of the modes coupling. The spectra of the small clusters are interpreted by identifying the behavior of the excess charge, by understanding the role of hydrogen bonds, and by considering the effect of (micro-)solvation. Our results are presented by showing comparisons with other numerical methods and experimental measurements which are available in the literature.