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

The molecular geometries in the ground and lowest singlet pi-pi* and n-pi* excited states of three tautomers of isocytosine (keto-N3H, keto-N1H, and e nol) and their complexes with a water molecule were optimized at the ab ini tio level of theory applying the 6-311 + + G(d,p) basis set. Excited-state calculations were carried out using configuration interaction involving sin gly 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 t o be highly nonplanar in the pi-pi* excited state while in the second singl et (n-pi*) excited state, the geometries were found planar, including the a mino group which was pyramidal in the ground as well as in the pi-pi* excit ed states. in the lowest singlet pi-pi* 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 origin ate from the lowest singlet excited state of the keto-N3H tautomer. (C) 200 0 John Wiley & Sons, Inc.