Solvation and the excited states of formamide

Excited state geometries of formamide have been explored using the multiconfiguration self-consistent-held method. Optimized equilibrium geometries for the S1 and T1 states are nonplanar with the C-O and C-N bond distances substantially increased from the ground state values. The excitation energies at the ground and excited state geometries are calculated to vary dramatically with nonplanar rotation. Raman scattering from the S2 state depends on the transition moment which is shown to vary strongly with geometry. Experimental analyses that project out restricted planar conformations can fit the Raman vibrational pattern but do not inform us about the complicated energy surface for the S2 state which is a resonance embedded in a Rydberg series. Constrained optimizations are used to explore this surface and the variation in the oscillator strength with geometry. Effective fragment potentials (EFP) model the waters in the solvation models. Comparison of the EFP and all-electron structures and energy of binding shows that the EFP adequately replace the all-electron waters. The use of constrained C-2v geometries for the EFP water does not significantly affect either the optimized structure or the energetics of the complex. (C) 1997 American Institute of Physics.