Theoretical study of the intermolecular H-bonding and intermolecular proton transfer between isocytosine tautomeric forms and R,S-lactic acid.

Eight H-bonded complexes between isocytosine (isoC) tautomeric forms and R/S-lactic acid (LA) have been studied at the B3LYP and HF levels of theory using 6-31+G(d) basis set. The energy barriers of the intermolecular proton transfers were also estimated as the results showed that they are several times lower than those of the intramolecular proton transfers of isoC in the gas phase. Furthermore, the energy barriers of the tautomerizations in which the carboxylic H-atom takes part are several times lower than those in which the LA OH group assists the proton transfer.