Multireference configuration interaction calculations of electronic states of N-methylformamide, acetamide, and N-methylacetamide

The electronic excited states of N-methylformamide (NMF), acetamide, and N-methylacetamide (NMA) have been computed using multireference configuration interaction methods. The amide spectra are dominated by the valence pi pi* state, computed for the different molecules to be in the range 7.46-8.21 eV. The Rydberg pi 3p(pi), state also features prominently in the spectra of acetamide and NMA. The computed vertical energies of the pi pi* transition appear to be 0.3-0.6 eV too high, suggesting that either the observed bands do not correspond to vertical transitions (in analogy to ethylene) or that the interaction between the valence and Rydberg states is artificially too strong. Our state-averaged calculations provide a balanced treatment, but may overestimate the valence-Rydberg interaction. Alternative CASPT2 calculations (Serrano-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 spectra of formamide and NMA but underestimated the pi pi* transition energy for NMF and acetamide. The Rydberg-valence interaction appears to be important in the amide spectra, and a completely adequate treatment is still to be found. Nevertheless, the MRCI calculations reproduce well the amide spectra and we report the computed electronic properties of NMA that provide a compact parameterization of the amide chromophore.