A gas phase ab initio excited state geometry optimization study of thymine, cytosine and uracil

Molecular geometries of the nucleic acid bases thymine, cytosine and uracil in the ground and the lowest two singlet excited states were optimized usi ng the ab initio approach employing the 4-31G basis set for all the atoms e xcept the amino group of cytosine for which the 6-311 + G* basis set was us ed. The excited state calculations were performed employing configuration i nteraction involving singly excited configurations (CIS). Vibrational frequ encies were computed in order to examine the nature of the stationary point s on the potential energy surfaces obtained by geometry optimization. While the ground state geometries of uracil and thymine (except the methyl group hydrogens) are planar, the corresponding excited state geometries were fou nd to be significantly nonplanar. In the case of cytosine, the amino group is pyramidal and the rest of the molecule is only slightly nonplanar in the ground state, but the excited state geometries are appreciably nonplanar. In particular, consequent to the S-2(n-pi*) excitation of cytosine, the ami no group plane is strongly rotated. While thymine is stable in the S-2(pi-p i*) excited state, uracil appears to be dissociative in the corresponding e xcited state. (C) 1999 Elsevier Science B.V. All rights reserved.