Investigations of the excited-state properties of isocytosine: An ab initio approach

The molecular geometries in the ground and lowest singlet π-π* and n-π* excited states of three tautomers of isocytosine (keto-N3H, keto-N1H, and enol) and their complexes with a water molecule were optimized at the ab initio level of theory applying the 6-311++G(d, p) basis set. Excited-state calculations were carried out using configuration interaction involving singly excited configurations (CIS). The effect of bulk aqueous solvation was studied applying the polarized continuum model (PCM) of the self-consistent reaction field (SCRF) theory. The geometries of the tautomers were found to be highly nonplanar in the π-π* excited state while in the second singlet (n-π*) excited state, the geometries were found planar, including the amino group which was pyramidal in the ground as well as in the π-π* excited states. In the lowest singlet π-π* state, keto-N1H tautomer was found to be dissociative in the gas phase. It has been shown that the isocytosine would phototautomerize in the excited state, and fluorescence would originate from the lowest singlet excited state of the keto-N3H tautomer.