Two-response-time model based on CM2/INDO/S2 electrostatic potentials for the dielectric polarization component of solvatochromic shifts on vertical excitation energies

A model is presented for the electrostatic component of solvatochromic shif ts on vertical electronic excitation energies. The model, called vertical e lectrostatic model 42 (VEM42), is based on representing the solute by a set of distributed atomic monopoles obtained by charge model 2 (CM2) and repre senting the solvent by its static and optical dielectric constants. The the ory is applied here with intermediate neglect of differential overlap for s pectroscopy-parameterization 2 (INDO/S2) configuration interaction wave fun ctions. The model is implemented in the ZINDO electronic structure code pac kage. We present illustrative applications to the singlet n --> pi* excitat ion of acetone in nine solvents. When the electrostatics are augmented by o ne-parameter estimates of dispersion and hydrogen-bonding contributions, th e experimental solvatochromic shifts in the nine solvents are reproduced wi th a mean unsigned error of 65 cm(-1) (0.2 kcal/mol). These calculations pr esent a compelling picture of the quantitative origin of the solvatochromic red and blue shifts in this prototype n --> pi* excitation. (C) 2000 John Wiley & Sons, Inc.