Ab initio study of the electronic spectrum of formamide with explicit solvent

Of the many roles that solvent plays, its influence on molecular electronic structure is perhaps one of the more challenging phenomena to study. In this study, the effect of solvation on the electronic spectrum of formamide is investigated. Ab initio complete-active-space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) methods are used to compute the ground- and excited-. state energies of formamide complexed with one, two, and three water molecules. In addition, a semicontinuum approach is employed, in which formamide-(H2O)(n) (n = 1, 3) complexes are studied within a continuum solvent model. The presence of the explicit water molecules destabilizes the Rydberg states of formamide by approximately 0.5 eV. In the case of the pi(nb)pi* transition,a red shift from 7.41 eV (gas phase) to 7.16 eV is observed, and its oscillator strength increases by similar to 10%. The n pi* transition undergoes a blue shift which is dependent on the O- - -H formamide-water hydrogen bond distance. The physical origin of these solvatochromic shifts is investigated. The former effects have been well reproduced in a previous ab initio study with a continuum model. In contrast, at least one explicit water molecule is needed to observe the blue shift in the n pi* transition. The semicontinuum approach provides a description of the electronic spectrum of solvated formamide that captures important local and bulk solvent effects.