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We examine the spectroscopic red shifts that occur when benzene is dissolved in ͑liquid͒ 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 Ϫ306 cm Ϫ1 , Ϫ268 cm Ϫ1 , Ϫ456 cm Ϫ1 , and Ϫ122 cm Ϫ1 , in excellent agreement with the experimentally observed shifts of Ϫ332 cm Ϫ1 , Ϫ308 cm Ϫ1 , Ϫ458 cm Ϫ1 , and Ϫ143 cm Ϫ1 , 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.

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Supermolecular calculations that treat both the solute and the solvent quantum mechanically are performed to analyze the n-π * transition of formaldehyde in water. The liquid structures are generated by canonical (constant volume, temperature, and number of particles) (NVT) Metropolis Monte Carlo simulation. Autocorrelation function is calculated to obtain efficient ensemble average. Full quantum mechanical intermediate neglect of differential overlap/singly excited configuration interaction (INDO/CIS) calculations are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The largest cluster, corresponding to the third solvation shell, includes 1 formaldehyde and 80 water molecules. INDO/CIS calculations are performed on a properly antisymmetric reference ground-state wave function involving all valence electrons. The results are then extrapolated to the bulk limit. The estimated limit value for the solvatochromic shift of the n-π * transition of formaldehyde in water, compared to gas phase, is 2200 cm −1 .

Supermolecular calculations that treat both the solute and the solvent quantum-mechanically are performed to analyze the solvatochromism of the first emission transition of formaldehyde in water. The liquid structures are generated by NVT Metropolis Monte Carlo simulation assuming a fully relaxed excited state. The autocorrelation function is calculated to obtain an efficient ensemble average. A detailed analysis of the hydrogen bonds and their contribution to the solvation shift is presented. On average, 0.7 hydrogen bonds are formed in the excited state, about three times less than in the ground state. Quantum-mechanical calculations using the intermediate neglect of differential overlap with singly excited configuration interaction (INDO/CIS) are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The third solvation shell extends up to 10 Å from the center of mass of formaldehyde, showing the very long-range effects on the solvation shift of this polar molecule. The largest cluster includes one formaldehyde and 142 water molecules. INDO/CIS calculations are performed on this cluster with a properly antisymmetric reference ground state wave function involving all valence electrons. The estimated limit value for the solvatochromic shift of the n-π* emission transition of fully relaxed formaldehyde in water, compared to the gas phase, is ≈1650 cm−1. The total Stokes shift of formaldehyde in water is calculated as ≈550 cm−1.

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