Electronic States of the Amide Group

Abstract

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→V₁) 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.