SOLVATOCHROMISM OF THE PI-ASTERISK[-N TRANSITION OF ACETONE BY COMBINED QUANTUM MECHANICAL CLASSICAL MECHANICAL CALCULATIONS

The solvent shift of the pi <-- n transition of acetone in water, ace tonitrile, and tetrachloromethane was calculated in a combined quantum mechanical-classical mechanical approach, using both dielectric conti nuum and explicit, polarizable molecular solvent models. The explicit modeling of solvent polarizability allows for a separate analysis of e lectrostatic, induction, and dispersion contributions to the shifts. T he calculations confirm the qualitative theories about the mechanisms behind the blue shift in polar solvents and the red shift in nonpolar solvents, the solvation of the ground state due to electrostatic inter actions being preferential in the former, and favorable dispersion int eraction with the excited state, in the latter case. Good quantitative agreement for the solvent shift between experiment (+ 1700, + 400, an d -350 cm(-1) in water, acetonitrile, and tetrachloromethane, respecti vely) and the explicit solvent model ( + 1821, + 922, and - 381 cm(-1) ) was reached through a modest Monte Carlo sampling of the solvent deg rees of freedom. A consistent treatment of the solvent could only be r ealized in the molecular solvent model. The dielectric-only model need s reparameterization for each solvent. (C) 1996 John Wiley & Sons, Inc .