Solvent-induced shifts in electronic spectra of uracil.

Highly accurate excitation spectra are predicted for the low-lying n-π* and π-π* states of uracil for both the gas phase and in water employing the complete active space self-consistent field (CASSCF) and multiconfigurational quasidegenerate perturbation theory (MCQDPT) methods. Implementation of the effective fragment potential (EFP) solvent method with CASSCF and MCQDPT enables the prediction of highly accurate solvated spectra, along with a direct interpretation of solvent shifts in terms of intermolecular interactions between solvent and solute. Solvent shifts of the n-π* and π-π* excited states arise mainly from a change in the electrostatic interaction between solvent and solute upon photoexcitation. Polarization (induction) interactions contribute about 0.1 eV to the solvent-shifted excitation. The blue shift of the n-π* state is found to be 0.43 eV and the red shift of the π-π* state is found to be -0.26 eV. Furthermore, the spectra show that in solution the π-π* state is 0.4 eV lower in energy than the n-π* state.