Conformational preferences of carbonic acid and its sulfur derivatives, H2C(X)O2−nSn(X = O/S; n = 0-2)

Ab initio calculations have been performed to investigate the conformational stability of the carbonic acid and its five possible sulfur derivatives, H2C(X)O2−nSn (X = O/S, n = 0-2). Transition states have been calculated for the interconversion of the different isomers by internal rotation or intramolecular proton transfer reactions. The structures and vibrational frequencies for the global and local minima as well as for the transition states were obtained at the MP2/6-311++G(3df, 3p) level, and energies were calculated at the QCISD(T)/6-311++G(3df, 3p) level. The natural bond orbital analysis method is used to rationalize the conformational preferences of the molecules in terms of the stabilizing orbital interactions of the nπ, and nσ lone pairs of oxygen and sulfur atoms with the OCX or SCX moiety.