Further analysis of the dynamically averaged vibrational spectrum for the 'magic' protonated 21-water cluster

摘要

In a recent publication, the structure, dynamics, and ro-vibrational spectrum of the hydrated proton in the "magic" 21-water cluster (H~+(H_2O)__(21)) was studied. It was found that dynamical effects were critical in determining the vibrational properties of such clusters. This result was gathered from an analysis of the vibrational density of states including (a) the Fourier transform of the nuclear velocity-velocity autocorrelation (FT-VAC) function, (b) the Fourier transform of the dipole-dipole autocorrelation (FT-DAC) function, and (c) harmonic frequencies from the isomeric structures sampled in atom-centered density matrix propagation (ADMP). The spectra obtained in (a) and (b) were directly computed from ab initio ADMP (Refs. 6-11) and the spectra in (c) were obtained through optimization and frequency calculations. Furthermore, the spectrum obtained from the FT-VAC was in good agreement with previous experiment through the reproduction of the sharp free OH stretch peak at approx=3700 cm~(-1) and the almost complete lack of intensity in the 2000-3000 cm~(-1) region. The latter was perhaps most puzzling since the harmonic spectra do reveal sharp peaks with large intensities in this region (Fig. 4 in Refs.), resulting from the OH stretch of the protonated (Eigen-like) species. Based on the agreement of our dynamical results with experiment we concluded the importance of dynamics.