ACCURATE ABINITIO QUARTIC FORCE-FIELDS FOR THE IONS HCO+ AND HOC+

The quartic force fields of HCO+ and HOC+ have been computed using augmented coupled cluster methods and basis sets of spdf and spdfg quality. Calculations on HCN, CO, and N2 have been performed to assist in calibrating the computed results. Going from an spdf to an spdfg basis shortens triple bonds by about 0.004 angstrom, and increases the corresponding harmonic frequency by 10-20 cm-1, leaving bond distances about 0.003 angstrom too long and triple bond stretching frequencies about 5 cm-1 too low. Accurate estimates for the bond distances, fundamental frequencies, and thermochemical quantities are given. HOC+ lies 37.8 +/- 0.5 kcal/mol (0 K) above HCO+; the classical barrier height for proton exchange is 76.7 +/- 1.0 kcal/mol.