MULTIREFERENCE CONFIGURATION-INTERACTION CALCULATIONS OF ELECTRONIC STATES OF N-METHYLFORMAMIDE, ACETAMIDE, AND N-METHYLACETAMIDE

The electronic excited states of N-methylformamide (NMF), acetamide, a nd N-methylacetamide (NMA) have been computed using multireference con figuration interaction methods. The amide spectra are dominated by the valence pi pi state, computed for the different molecules to be in t he range 7.46-8.21 eV. The Rydberg pi 3p(pi), state also features prom inently in the spectra of acetamide and NMA. The computed vertical ene rgies of the pi pi transition appear to be 0.3-0.6 eV too high, sugge sting that either the observed bands do not correspond to vertical tra nsitions (in analogy to ethylene) or that the interaction between the valence and Rydberg states is artificially too strong. Our state-avera ged calculations provide a balanced treatment, but may overestimate th e valence-Rydberg interaction. Alternative CASPT2 calculations (Serran o-Andres, L.; Fulscher, M. P. J. Am. Chem. Sec. 1996, 118, 12190-12199 ), which involved separate optimizations of different types of states and ignored the Rydberg-valence interaction, agreed well with the spec tra of formamide and NMA but underestimated the pi pi transition ener gy for NMF and acetamide. The Rydberg-valence interaction appears to b e important in the amide spectra, and a completely adequate treatment is still to be found. Nevertheless, the MRCI calculations reproduce we ll the amide spectra and we report the computed electronic properties of NMA that provide a compact parameterization of the amide chromophor e.