A Monte Carlo-quantum mechanics study of the spectroscopic properties of molecules in solution

A combination of Monte Carlo simulation and quantum mechanics calculation i s used to study properties, structure and spectroscopy of molecules in solu tion. All properties are obtained as statistical averages using uncorrelate d structures generated by the Monte Carlo simulation. The auto-correlation function of the energy is calculated and used in the correlation analysis. Examples include the solvatochromic shift of the n-pi* transition of formal dehyde and pyridine in water. A detailed and systematic analysis is made st arting from the hydrogen bond shell and extending up to the bulk limit. Lon g range effects are found to be of importance for the total solvatochromic shift. Short range, hydrogen bond, effects are seen to alter the dipole mom ent of the chromophore both in direction and module, having an effect on th e outer solvation shells. A careful analysis is made of the possible structural changes induced by a solvent in merocyanine dyes. The hyperpolariz ability of phenol blue is sho wn to vary with the resonance between the two, neutral and charge-separated , forms, having a pronounced maximum as obtained experimentally. This indic ates that phenol blue undergoes structural changes in polar solvents. (C) 2 001 Elsevier Science B.V. All rights reserved.