Insights into the structure and stability of the carbonic acid dimer.

In this paper we report the geometries and properties of 40 structural isomers located on the MP2/6-311++G** PES of the carbonic acid dimer. All six possible combinations of carbonic acid monomers were considered. The dimers are divided into six geometrical motifs. Our data suggests that combinations of anti-anti monomers do not necessarily lead to larger stabilization energies in the formation of the dimers. MP2 underestimates the relative binding energies with respect to CCSD(T) by as much as 3.2 kcal mol(-1). At least 3 different dimers which may contribute to the stability of carbonic acid are predicted to have significant populations. Binding energy is only directly related to relative stability when comparing dimers formed from the same monomers. Overall stabilization is mainly dictated by attractive electrostatic interactions via cooperative polarization by virtue of the spatial arrangement of the dipole moment components along the polar bonds. Shorter OH bond distances and larger bond orders predicted for the hydrogen bonds directed towards carbonyl groups make for stronger hydrogen bonding than in O...H bonds directed towards hydroxyl groups.