The sequential Monte Carlo-quantum mechanics methodology. Application to the solvent effects in the Stokes shift of acetone in water

The sequential Monte Carlo quantum mechanics methodology is used to obtain the solvent effects on the Stokes shift of acetone in water. One of the great advantages of this methodology is that all the important statistical information is known before running into the costly quantum mechanical calculations. This advantage is discussed not only with respect to the statistical correlation between the different structures generated by the simulation but also in the proper identification of hydrogen bonds in liquids. To obtain the solvent effects in the Stokes shift of the n-pi* absorption transition of acetone in water, quantum-mechanical calculations are performed in super-molecular structures generated by NVT Monte Carlo simulation. The statistical correlation between configurations is analyzed using the auto-correlation function of the energy. The largest calculations include one acetone and 170 water molecules. One-hundred INDO/CIS super-molecular calculations are performed for each solvation shell to obtain the statistical average value. The calculated solvatochromic shift of the n-pi* absorption transition of acetone in water, compared to gas phase, is similar to 1310 cm(-1) in good agreement with the experimental blue shift of 1500 +/- 200 cm(-1). For the emission of the relaxed excited state, the calculated shift is similar to 1850 cm(-1). The total calculated solvent effect on the Stokes shift of acetone in aqueous solution is thus 540 cm(-1). A detailed analysis of the sampling of the configurations obtained in the Monte Carlo simulation is made and it is shown that all results represent statistically converged values. (C) 2003 Elsevier B.V. All rights reserved.