HAS THE DIMER OF CARBONIC-ACID A LOWER ENERGY THAN ITS CONSTITUTENTS WATER AND CARBON-DIOXIDE

On the basis of high-level molecular quantum mechanics, the zero-point -corrected energy for formation the anti-anti conformer of carbonic ac id from water and carbon dioxide: is compensated by formation of the m ost stable dimer. Therefore, the zero-point-corrected energy for forma tion of the carbonic acid dimer from its constituents water and carbon dioxide is close to zero, and even slightly negative values seem to b e possible. These results include new CCSD(T)/6-311++G(3df,3pd) calcul ations of the formation energy of carbonic acid from water and carbon dioxide, which gave an energy difference of between 8.2 and 0.2, kcal/ mol which is lower than those reported before. Furthermore, the dimeri zation energy of carbonic acid and formic acid is reported up to the M P2/ang-cc-pVTZ level gf theory, leading to a best estimate of -16.9 kc al/mol for the carbonic acid dimer relying on convergence consideratio ns for the MP2/aug-cc-pVxZ level of theory that also clearly reveal th e failure of counterpoise correction for basis set superposition error . We conclude that in our atmosphere formation of a dimer of carbonic acid is disfavored mainly by entropy, and not by enthalpy.