A Monte Carlo-quantum mechanics study of the solvatochromic shifts of the lowest transition of benzene

We examine the spectroscopic red shifts that occur when benzene is dissolve d in (liquid) benzene, in cyclohexane, in carbon tetrachloride, and in wate r. For this we develop a mixed classical/quantum model in which uncorrelate d structures are obtained from Monte Carlo simulation, and these structures are then used for quantum chemical calculations including the chromophore and all solvent molecules within the first radial distribution maxima. We d iscuss the effects of different sampling techniques and the inclusion of mo re, or less, solvent molecules in the quantum chemical supermolecule calcul ation. We obtain shifts of -306 cm(-1), -268 cm(-1), -456 cm(-1), and -122 cm(-1), in excellent agreement with the experimentally observed shifts of - 332 cm(-1), -308 cm(-1), -458 cm(-1), and -143 cm(-1), respectively. We not e that the larger shift observed in carbon tetrachloride that is not expect ed on the basis of larger dielectric constant results from small contributi ons of the charge transfer type from solvent to solute. (C) 2000 American I nstitute of Physics. [S0021-9606(00)51716-4].