Ab initio study of the effect of solvation on the electronic spectra of formamide and N-methylacetamide

Calculations on the electronic spectra of formamide and N-methylacetamide in solution are presented. Solvents are modeled by macroscopic continua characterized by their dielectric constant and refractive index. The complete-active-space self-consistent-field model implemented within a self-consistent reaction field model is used. Dynamical correlation is accounted for using multiconfigurational perturbation theory. The Rydberg states are shown to be destabilized by the Pauli repulsion of the solvent. An advantageous side effect of this destabilization is the reduction of mixing between valence and Rydberg states encountered in gas-phase calculations on amides. A red-shift of 0.5-1.0 eV relative to comparable gas-phase calculations is seen for the pi pi* transitions. For formamide in cyclohexane, n pi* and pi pi* vertical transitions are computed to lie at 5.56 eV (oscillator strength f = 0.0006) and 6.94 eV (f = 0.274). For N-methylacetamide, the corresponding transitions are found at 5.55 eV (f = 0.001) and 6.59 eV (f = 0.256). In addition, higher energy excitations have been studied, including excitations from the lower-lying oxygen lone-pair orbital. Good agreement with available experimental data is achieved throughout, indicating the method used provides an accurate treatment of valence excited states in solution.