An ab initio study of the geometry,energy, and selected force constants for the three planar conformers of carbonic acid,and the bicarbonate ion; and of the energy for the reaction H2O + CO2 → H2CO3

Ab initio calculations using the unscaled 4-31G basis set have been carried out on the cc, tc, and tt conformers of carbonic acid and the bicarbonate ion, with full geometry optimization assuming the structures to be planar. The complete harmonic force field is reported for the (most stable) tt conformer and for the bicarbonate ion, also selected quadratic force constants for the cc and tc conformers. The changes in certain bond lengths and stretching force constants in the cc → tc, tc → tt, and cc → tt conformer conversion reactions are indicative of intramolecular hydrogen bonding, C?O…H? O and H? O…H? O, which is examined in greater detail by partitioning the overall conformer conversion energy into distortion and bonding energy components. The fundamental vibration frequencies for the tt conformer and the bicarbonate ion are calculated from the force constant matrices, and hence, using a scaling factor based on a comparison of calculated and experimental values for the bicarbonate ion and trans-formic acid, a value is predicted for the zero-point energy of the tt conformer. A new estimate of ΔH? for the hydration reaction, H₂O + CO₂ → H₂CO₃, at 298 K in the gas phase; is made from thermochemical data, +20.2 ± 3.4 kJ mol^?1, which, together with estimates of (H₂₉₈? – H₀?) and the zero-point energy for H₂CO₃, gives +8.1 ± 7.0 kJ mol^?1 for ΔE_T(expt). ΔE_T calculated from the 4-31G basis set data is -29.1 kJ mol^?1. Comparison of the experimental value, the Hartree–Fock limit value, and values calculated with a variety of basis sets for the bond separation reaction, CO₂ + CH₄ → 2H₂CO, suggests that the differences, ΔE_T(expt) minus ΔE_T(SCF ), are due mainly to basis set limitations and not substantial correlation energy contributions.