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 dissolved in (liquid) benzene, in cyclohexane, in carbon tetrachloride, and in water. For this we develop a mixed classical/quantum model in which uncorrelated 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 discuss the effects of different sampling techniques and the inclusion of more, or less, solvent molecules in the quantum chemical supermolecule calculation. 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 note that the larger shift observed in carbon tetrachloride that is not expected on the basis of larger dielectric constant results from small contributions of the charge transfer type from solvent to solute. (C) 2000 American Institute of Physics. [S0021-9606(00)51716-4].