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 shifts on vertical electronic excitation energies. The model, called vertical electrostatic model 42 (VEM42), is based on representing the solute by a set of distributed atomic monopoles obtained by charge model 2 (CM2) and representing the solvent by its static and optical dielectric constants. The theory is applied here with intermediate neglect of differential overlap for spectroscopy-parameterization 2 (INDO/S2) configuration interaction wave functions. The model is implemented in the ZINDO electronic structure code package. We present illustrative applications to the singlet n → π* excitation of acetone in nine solvents. When the electrostatics are augmented by one-parameter estimates of dispersion and hydrogen-bonding contributions, the experimental solvatochromic shifts in the nine solvents are reproduced with a mean unsigned error of 65 cm^-1 (0.2 kcal/mol). These calculations present a compelling picture of the quantitative origin of the solvatochromic red and blue shifts in this prototype n → π* excitation.