EXCITED-STATE MOLECULAR ELECTRIC PROPERTIES OF SOME BIOLOGICALLY IMPORTANT PURINES, PYRIMIDINES, AND AZINES - AN AB-INITIO STUDY

Electronic charge distributions and molecular electrostatic potentials (MEP) in the ground and lowest singlet pi-pi and n-pi* excited state s of certain azines, biologically important pyrimidines, and purines w ere studied using ab initio calculations where excited-state wave func tions 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 dist ributed in three dimensions the forms of the squares of the correspond ing valence Slater ns (n = principal quantum number) atomic orbitals. It is found that while pi-pi excited-state charges and MEP patterns a re usually not too different from those of the ground states, the corr esponding patterns in the n-pi excited states are drastically differe nt. The charge redistributions in the azines following their n-pi exc itations appear to follow an ortho, para directing effect. The compute d dipole moment changes following excitations of the molecules are in satisfactory agreement with experiment.