Computer simulations of the solvatochromism of betaine-30

Monte Carlo simulations of the pyridinium N-phenolate dye "betaine-30" in 1 2 solvents (20 solvent representations) were performed in oi-der to explore the molecular basis of the E-T(30) scale of solvent polarity. Ab initio (H F/6-31G*) and senniempirical (AMI ana INDO/S) electronic structure calculat ions were used to determine the geometry and charge distribution of betaine -30 in its S-0 and S-1 states. The solvent effect on the betaine absorption spectrum was assumed to derive from electrostatic interactions between the effective charge distributions of solvent molecules and the charge shift b rought about by the S-0 --> S-1 transition. Two models for this charge shif t, one obtained from INDO/S calculations and the other an idealized two-sit e model, were used for the spectral calculations. Good agreement between si mulated and observed Delta E-T shifts (E-T(30) values measured relative to the nonpolar standard tetramethylsilane) was found for both charge-shift mo dels. In water and other hydroxylic solvents, the O atom of the betaine sol ute was observed to form moderately strong hydrogen bonds to between one an d two solvent molecules. The contribution of these specifically coordinated molecules to the Delta E-T shift was found to be large, (30-60%) and compa rable to experimental estimates. Additional simulations of acetonitrile and methanol in equilibrium with the S1 state of betaine-30 were used to deter mine reorganization energies in these solvents and to decide the extent to which the solvent response to the S-0 <----> S-1 transition conforms to lin ear response predictions. In both solvents, the spectral distributions obse rved in The S-0 state simulations were similar to 15% narrower than those i n the S-1 simulations, indicating only a relatively small departure from li near behavior. Reorganization energies were also estimated for a number of other solvents and compared to values reported in previous experimental and theoretical studies.