Has the Dimer of Carbonic Acid a Lower Energy Than Its Constituents Water and Carbon Dioxide?

Abstract

On the basis of high-level molecular quantum mechanics, the zero-point-corrected energy for formation of the anti-anti conformer of carbonic acid from water and carbon dioxide is compensated by formation of the most stable dimer. Therefore, the zero-point-corrected energy for formation of the carbonic acid dimer from its constituents water and carbon dioxide is close to zero, and even slightly negative values seem to be possible. These results include new CCSD(T)/6-311++G(3df,3pd) calculations of the formation energy of carbonic acid from water and carbon dioxide, which gave an energy difference of between 8.2 and 9.2 kcal/mol which is lower than those reported before. Furthermore, the dimerization energy of carbonic acid and formic acid is reported up to the MP2/aug-cc-pVTZ level of theory, leading to a best estimate of −16.9 kcal/mol for the carbonic acid dimer relying on convergence considerations for the MP2/aug-cc-pVxZ level of theory that also clearly reveal the 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.