Excited-State Molecular Electric Properties of Some Biologically Important Purines, Pyrimidines, and Azines: An ab Initio Study

Abstract

Electronic charge distributions and molecular electrostatic potentials (MEP) in the ground and lowest singlet π−π* and n−π* excited states of certain azines, biologically important pyrimidines, and purines were studied using ab initio calculations where excited-state wave functions were generated using configuration interaction involving singly excited configurations (CIS). Further, these results were obtained by optimizing ground- and excited-state geometries of the molecules, and the 3-21G basis set was used. The MEP-fitted atomic point charges were obtained using the CHelpG procedure, and these charges were then distributed in three dimensions in the forms of the squares of the corresponding valence Slater ns (n = principal quantum number) atomic orbitals. It is found that while π−π* excited-state charges and MEP patterns are usually not too different from those of the ground states, the corresponding patterns in the n−π* excited states are drastically different. The charge redistributions in the azine...