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 calculation is used to study the solvatochromic shift of the n --> pi* absorption transition of pyrimidine in water and in carbon tetrachloride. Super-molecular 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 correlation between the configurations used in the quantum mechanical calculations. The total number of molecules used in the super-molecules is obtained after analysis of the radial distribution function that defines the solvation shells. For the case of pyrimidine in water, full quantum mechanical INDO/CIS calculations are performed in the super-molecular clusters corresponding 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 and 213 water molecules, with a total of 1734 valence electrons explicitly included. 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 value of 2700 +/- 300 cm(-1) and explicitly confirming that the polarization effects 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 of similar to 100 cm(-1) for the n --> pi* blue shift.