Thermal fluctuations of the unusually symmetric and stable superoxide tetrahydrate complex: An ab initio molecular dynamics study

We report an ab initio molecular dynamics simulation study of a complex composed of the superoxide anion and four water molecules. We probe the symmetry and stability of the superoxide tetrahydrate anion cluster at a low temperature (25 K) where there are vibrational spectra and a higher temperature (111 K) where experimental data are not available. From the ab initio simulation trajectory, we found the superoxide tetrahydrate complex to be highly symmetric, exhibiting a collective breathing motion on the picosecond time scale at low temperature that subsequently disappears at higher temperatures. Furthermore, we establish that superoxide tetrahydrate is a highly stable species at the higher temperature with little or no distortion to its geometry. The vibrational spectrum calculated, from our ab initio trajectory is in reasonable agreement with experimental spectra, and we were able to assign individual peaks in the spectra to specific modes in the superoxide tetrahydrate anion cluster.