A Monte Carlo-quantum mechanical study of the solvatochromism of pyrimidine in water and in carbon tetrachloride

A sequential combination of Monte Carlo simulation and quantum mechanics ca lculation is used to study the solvatochromic shift of the n --> pi* absorp tion transition of pyrimidine in water and in carbon tetrachloride. Super-m olecular configurations are generated from NVT Monte Carlo simulations and are used for subsequent extensive quantum mechanical calculations. The auto -correlation function of the energy is used to analyze the statistical corr elation between the configurations used in the quantum mechanical calculati ons. The total number of molecules used in the super-molecules is obtained after analysis of the radial distribution function that defines the solvati on shells. For the case of pyrimidine in water, full quantum mechanical IND O/CIS calculations are performed in the super-molecular clusters correspond ing to the first, second and third solvation shells, extending up to nearly 11.5 Angstrom away from the center of mass of pyrimidine. For the largest calculation, made for the third solvation shell, it includes 1 pyrimidine a nd 213 water molecules, with a total of 1734 valence electrons explicitly i ncluded. Using the results obtained for the different solvation shells the solvatochromic shift is extrapolated to the bulk limit. This gives our best result of 2223 +/- 60 cm(-1), in good agreement with the experimental valu e of 2700 +/- 300 cm(-1) and explicitly confirming that the polarization ef fects of pyrimidine in protic solvents extend to a very long distance from the solute. For pyrimidine in carbon tetrachloride, a non-polar and aprotic solvent, the use of only the first solvation shell gives a stable result o f similar to 100 cm(-1) for the n --> pi* blue shift.