SOLVATION AND THE EXCITED-STATES OF FORMAMIDE

Excited state geometries of formamide have been explored using the mul ticonfiguration self-consistent-held method. Optimized equilibrium geo metries for the S1 and T1 states are nonplanar with the C-O and C-N bo nd distances substantially increased from the ground state values. The excitation energies at the ground and excited state geometries are ca lculated to vary dramatically with nonplanar rotation. Raman scatterin g from the S2 state depends on the transition moment which is shown to vary strongly with geometry. Experimental analyses that project out r estricted planar conformations can fit the Raman vibrational pattern b ut do not inform us about the complicated energy surface for the S2 st ate which is a resonance embedded in a Rydberg series. Constrained opt imizations are used to explore this surface and the variation in the o scillator strength with geometry. Effective fragment potentials (EFP) model the waters in the solvation models. Comparison of the EFP and al l-electron structures and energy of binding shows that the EFP adequat ely replace the all-electron waters. The use of constrained C-2v geome tries for the EFP water does not significantly affect either the optim ized structure or the energetics of the complex. (C) 1997 American Ins titute of Physics.