Theoretical study of the uncatalyzed hydration of carbon dioxide in the gas phase

The reaction energy surface and pathway of the reversible gas-phase reaction of CO/sub 2/ with H/sub 2/O and OH/sup -/ have been evaluated with PRDDO and 4-31G SCF-MO methods, including one MP2 correlation calculation. In the most favorable pathway, H and O of water approach respectively O and C of CO/sub 2/ simultaneously, and after the transition state is passed the new OH bond is formed followed by formation of the new CO bond. The major components of the energy barrier are analyzed, and the ways to lower the energy barrier are discussed. The deformation energies of CO/sub 2/ and H/sub 2/O contribute most to the barrier, and the exchange repulsive interactions are also important. After corrections for zero-point vibrational energies and thermal energies and inclusion of entropic effects, the free energy barrier of the hydration reaction is about 62 kcal/mol at the 4-31G level, 64.6 kcal/mol at the 4-31G+ level, and about 60.7 kcal/mol when 4-31G**/MP2 correlation corrections are made at the 4-31G geometry. Entropic changes, included in these values, contribute some 15% to the activation free energy. A population analysis indicates locations of charges which may be stabilized by external charges in hydration and catalysis. For the reactionmore » of CO/sub 2/ with OH/sup -/ no barrier is found; here the deformation energy for CO/sub 2/ is smaller than the charge transfer and electrostatic interactions as CO/sub 2/ and OH/sup -/ react, owing to the extra negative charge on OH/sup -/.« less