Ab initio based Car-Parrinello molecular dynamics calculations (CPMD) have
been used to investigate the properties of crystalline nitric acid trihydra
te (NAT) at 195 It. The crystal unit cell consists of four formula units of
NAT and nominally contains the ions NO3- and H3O+ plus H2O molecules. Reve
rsible proton-transfer events were observed between unique H3O+ and neighbo
ring 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-hyd
rogen of H3O+, whose stretch frequency was observed in corresponding power
spectra. Calculations showed that a classical free energy of 3.3 k(B)T was
required for the proton to reach the center of the oxygen-oxygen pair. Crys
talline NAT was further characterized by calculated equilibrium averages, s
tructural properties, and vibrational spectra.