Abstract
Molecular dynamics simulations have been used to compute the adiabatic free energy difference between the electronic ground and 'L, excited states of 3-methylindole in water. Partial atomic charges for the two electronic states were derived from fits to the (gas phase) quantum mechanical electrostatic potentials produced by semiempirical INDO/S wave functions. The computed solvent reorganization energy (- 3 kcal/mol) is smaller than values deduced from experimental spectroscopic data for various indole derivatives in polar solvents (4-12 kcal/mol). The solvent reorganization energy computed from the simulations is close to the value estimated from the Onsager dipole cavity reaction field model when a previously suggested, but ad hoc, molecular volume parameter is used with the model. However, this model is shown to generally not approximate reaction field energies well for a large planar molecule like indole, since it does not distinguish among diffmnt charge distributions which produce the same change in the magnitude of the chromophore dipole moment but very different reaction fields. Additional free energy simulations are carried out to demonstrate this point. In contrast, a recently proposed molecular dielectric model, which explicitly incorporates the effects of the solute structure and charge on the solvent dielectric response, is found to agree with the simulated free energy changes to within a few percent.
Published Version
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