HYBRID SIMULATIONS OF SOLVATION EFFECTS ON ELECTRONIC-SPECTRA - INDOLES IN WATER

Solute-solvent interactions and dynamics are simulated with a fully molecular hybrid method consisting of a semiempirical quantum mechanical method with singly excited configurations for the solute and classical molecular dynamics (MD) for the solvent(H2O). The interactions are purely electrostatic, with the solute being polarizable and sharing its charge information with the MD at 5 fs intervals. The solvent charges are fixed and the results are not sensitive to the point charges used. For the solute, the results depend on the;dipole moment much more than on the point charge magnitudes leading to a given dipole. This method is applied to the spectral shifts, dynamics, linewidths, and free energies of indole and 3-methylindole (3MI) in water at 300 K, including the effect of geometry changes and clarifications concerning vertical vs 0-0 transition predictions. Large fluorescence Stokes shifts are predicted, in fair agreement with observed values. The (1)L(a) excited state dipole is calculated to be about 12 D after solvent relaxation following excitation. This increase of about 5 D above that calculated in vacuum is caused by the solvent reaction field, and approximately doubles the calculated shift compared to that using the vacuum dipoles. There does not seem to be a need to invoke a solute-solvent excited state charge transfer complex (exciplex) to account for the large shifts. About 50% of the Stokes shift occurs in similar to 15 fs with a Gaussian response function, and the remainder is approximately an exponential with tau=400 fs. The fast component is created by small rotational deviations in the trajectories of a few nearby waters. The change in free energy of solvation upon excitation is found to be half the sum of the absorption and fluorescence shifts.