THEORETICAL-STUDY OF THE CRYSTAL-FIELD EFFECTS ON THE TRANSITION DIPOLE-MOMENTS IN METHYLATED ADENINES

The effects of the crystal environment on the electronic spectral para meters in 9-methyladenine and N-6-methyladenine have been investigated . We have included the electrostatic effects of the crystal environmen t, a probable source of discrepancy between the experiment and theory, in the semiempirical molecular orbital calculations using the INDO/S method. The fields and potentials at atomic centers of the molecules i n the crystal were calculated using the ground-state charge distributi on and were included in the INDO/S Hamiltonian in an iterative process , until self-consistency was attained. The crystal field polarizes the ground state, leading to an increase in the net atomic charges, signi ficantly increasing the magnitude of the ground-state dipole moment in N-6-methyladenine, with the direction unaltered. In 9-methyladenine, a significant change was predicted in the direction of the ground-stat e dipole moment, with only a slight increase in the magnitude. The cha nges in the ground-state dipole moment depend on the relative directio ns of the crystal field and the gas phase dipole moment. The predicted gas phase spectra of both molecules are comparable due to the small e ffects of methyl substitution on the electronic structure. The crystal field introduces mixing of n pi and pi pi* transitions. This leads t o a slight red shift in the energy of the transitions, changes in inte nsities, and rotation of the transition dipole moment directions. The interactions between the excited states in the crystal were evaluated by a perturbation treatment. Excited-state mixing leads to extensive t ransfer of intensities and a slight blue shift in the energy of the tr ansitions. The predicted transition moment directions are in general a greement with experiment, although relative intensities differ in some cases, notably in the two lowest energy transitions.