Theoretical studies of the unimolecular and bimolecular tautomerization of cytosine

Computational investigations of the unimolecular and bimolecular tautomerization of isolated and dimeric cytosine have been performed. Stationary and transition states of the isolated and dimeric cytosine systems were characterized at the MP2(full)/6-311+G(2d,2p)//MP2(full)/6-31G* and MP2(full)/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory, respectively. In the solid phase, cytosine exists in a single tautomeric state. In contrast, experiments conducted in the gas phase find that cytosine exists as a mixture of several tautomeric forms. The energy barriers for unimolecular tautomerization of the tautomeric form found in solids to those observed in the gas phase are high and vary between 142.2 and 169.9 kJ mol−1. The formation of dimers with dual hydrogen bonding interactions results in a significant lowering of the barriers to tautomerization, thus facilitating tautomerization during the sublimation process. Based on such bimolecular tautomerization mechanisms, we believe that the relative populations of the cytosine tautomers produced in the gas phase via thermal vaporization cannot be accurately predicted without considering intermolecular hydrogen bonding interactions present in the condensed phase.