An ab initio investigation of a pyranosyl oxocarbenium ion with an alpha-carboxylate substituent is presented. Geometry, energetics, and isotope effects have been determined in the gas phase and with solvation models. RHF/6-31G* geometry optimizations identified a minimum for the oxocarbenium ion in which the ring atoms comprising the oxocarbenium ion lie in a plane, and the carboxylate group is twisted by 82 degrees relative to this plane. A transition state conformer was identified which placed the carboxylate group in the oxocarbenium ion plane. The transition state conformer was similar to 5 kcal/mol higher in energy than the twist conformer (B3LYP/6-31G*//RHF/6-31G*). Single point calculations using density functional theory on the structures optimized at the RHF level indicate that in the gas phase an a-carboxylate stabilizes an oxocarbenium ion by 110 kcal/mol relative to an Ii-substituted oxocarbenium ion. Inclusion of solvation in the energy calculation by use of a self-consistent isodensity polarized continuum model lowers the stabilization to 17 kcal/mol in water. Equilibrium C-14 and H-2 isotope effects have been calculated for formation of the carboxylate-substituted oxocarbenium ion and are compared to the experimentally determined kinetic isotope effects for acid-catalyzed solvolysis of cytidine monophosphate N-acetylneuraminate (CMP-NeuAc) (Horenstein, B.A.; Bruner, M. J. Am. Chem; Sec. 1996, 118, 10371-10379). The calculations support a transition state model which features a nearly fully formed sialyl oxocarbenium ion with the carboxylate group planar or near planar to the oxocarbenium ion plane. A rationale is provided to explain the origin of an unusually large secondary C-14 KIE at the carboxylate carbon of CMP-NeuAc.