Ab initio molecular dynamics simulation of H5 O2+ and H7 O3+ gas phase clusters based on density functional theory

Results of ab initio molecular dynamics simulations of protonated water clusters H+(H2O) n n=2,3 are presented. The simulations are based on Born–Oppenheimer molecular dynamics with ab initio forces obtained from density functional theory. Stationary points on the potential enery surfaces of these clusters are discussed and in particular the performance of the chosen approximation to the exchange correlation functional has been considered. Infrared spectra are obtained from trajectories at 80 K and 270 K and are compared with experimental spectra generated by molecular beam experiments. While the terminal hydrogen stretch frequencies are perturbed only weakly, vibrations of the hydrogen bonded protons are red shifted strongly and broadened. The trajectories generated are interpreted in terms of molecular rearrangements. In the case of H5O2 + a coupled rotational-translational motion leads to an intermolecular rearrangement, while for H7O3 + a simple umbrella motion describes the reaction coordinate best.