Ab initio GB study of solvent effect on the cis-trans isomerization of 4-dimethylamino-4′-nitroazobenzene

Ab initio RHF and ROHF methods including solvent effects based on the generalized Born formula (GB) were applied to a typical push-pull azobenzene, 4-(dimethylamino)-4'-nitroazobenzene (DMANAB), and the solvent effect on the inversion and rotation mechanisms of thermal cis-trans isomerization of DMANAB was examined. The pull-group inversion path was more favorable than the push-group one in the gas phase and in solution, and the calculated activation energies were compared well with experimental values. Analysis of two-dimensional potential energy map and electron configurations of the transition states indicated that the transition state of the rotation path, if it exists, resembles closely to that of the inversion path, and it is unlikely that both transition states are found separately in the potential energy surface of the closed-shell electronic state. Although large stabilization by solvent was expected for the rotation transition state, searching for such a transition state on the closed-shell electronic state gave the inversion transition state even in polar solvent. The transition state for the rotation path in the diradical electronic state was calculated to be comparable in terms of energy to the transition state for the pull-group inversion path in the closed-shell electronic state. (C) 1999 Elsevier Science B.V. All rights reserved.