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JP18

Twisting of molecular structures during intermolecular hydrogen transfer of conjugated 1,2-diketone enol dimers

�›Fumio Imashiro�iGraduate School of Science, Kyoto University�j

DFT calculations on the proton transfer in the dimers of the enol form of conjugated 1,2-diketones, tropolone and 2-hydroxycyclopentadienone, were performed. Although the two conjugated carbon rings in both dimers were essentially coplanar in the ground state, they were severely twisted to each other in the transition state of the simultaneous, intermolecular double proton transfer process. From close examination of the hypothetical coplanar structure for the transition state the twisted transition-state structures were ascribed to the tendency of the linear arrangement of the intermolecular O...H...O hydrogen bonds. The coplanar structures of the ground state were attributed to that the intramolecular hydrogen bonds are important than the intermolecular hydrogen bonds, which were supported the results that the dimerization energies for the tropolone and the 2-hydroxycyclopentadienone dimers (5.9 and 8.9 kcal/mol, respectively) were much smaller than that (16.4 kcal/mol) for the benzoic acid dimer, which is a structural isomer of the tropolone dimer. The proton transfer process accompanied with inversion of the one carbon ring was also discussed.