Ab Initio Molecular Dynamics Study of Crystalline Nitric Acid Trihydrate

Ab initio based Car-Parrinello molecular dynamics calculations (CPMD) have been used to investigate the properties of crystalline nitric acid trihydrate (NAT) at 195 K. The crystal unit cell consists of four formula units of NAT and nominally contains the ions NO3- and H3O+ plus H2O molecules. Reversible proton-transfer events were observed between unique H3O+ and neighboring H2O pairs, which occur throughout the crystal. Full proton transfer to form a new neighboring H3O+ molecule was a rare event. The motion of this mobile proton was associated with hyperextension of the covalent oxygen−hydrogen of H3O+, whose stretch frequency was observed in corresponding power spectra. Calculations showed that a classical free energy of 3.3 kBT was required for the proton to reach the center of the oxygen−oxygen pair. Crystalline NAT was further characterized by calculated equilibrium averages, structural properties, and vibrational spectra.