Density functional theoretical studies of the isomers of croconic acid and their dimers

The gas-phase structures of croconic acid and its dimers have been examined using the density functional methods B3LYP, M06 and M06-2X in conjunction with 6-311++G** basis set. The calculations revealed that among the two isomers of croconic acid, the one in which both hydroxyl groups are oriented in opposite directions (A) has a lower energy (more negative) than the other isomer in which both hydroxyl groups are oriented in the same direction (B). The binding energy of the dimer calculated with respect to the monomers is larger for that formed from B than formed from A. The formations of dimers by hydrogen bonding from all possible sites have been examined.