Electronic states of the amide group

The presence of a moderately strong band falling between the n→-π* and π-→π* (N→V₁) transitions of the amide group has been observed for the first time. All-electron SCF calculations in Gaussian bases aimed at assigning the bands of formamide suggest that the new band is an n→σ* excitation in which the σ* orbital is Rydberg-like. A second π→σ * big-orbit transition follows the π→π* (N→V ₁) excitation. In a basis containing expanded orbitals, the usual virtual orbital-configuration interaction calculations predict that every upper state will be of Rydberg-orbital dimensions. However, this obvious error can be avoided by indirect SCF calculations on the upper states, which show that in formamide, all states below 13 eV terminating at π* are valence-shell transitions, whereas all states terminating at σ* are big-orbit Rydberg states.