A Theoretical Study of Excited State Properties of Adenine−Thymine and Guanine−Cytosine Base Pairs

Abstract

Geometries of the Watson−Crick adenine−thymine (AT) and guanine−cytosine (GC) base pairs were optimized in the ground and some selected low-lying singlet π−π* and n−π* excited states. Ground-state geometries were optimized at the Hartree−Fock level of theory without symmetry restriction. Excited states were generated by employing a configuration interaction technique involving singly excited configurations (CIS method) using the ground-state optimized geometry, and this was followed by excited-state geometry optimization under planar symmetry. The standard 6-31++G(d,p) basis set was employed in all calculations. Ground-state geometries are found to be planar; the predicted planarity was validated by the harmonic vibrational frequency calculations. Electronic excitations are found to be localized at either of the monomeric units. The existence of higher energy charge-transfer type excited states is also revealed. These states are characterized by the excitation of electrons from occupied orbitals of one mo...