Fourier transform microwave spectra and ab initio calculation of N-ethylformamide

A peptide molecule: N-ethylformamide HCONHCH2CH3 (NEFA) was investigated by Fourier transform microwave spectroscopy in order to determine molecular structure, potential barrier to methyl internal rotation, and nuclear quadrupole coupling constant of the nitrogen atom. All the three (a, b and c) types of transitions were observed; they were split into hyperfine structure components due to nitrogen nuclear quadrupole coupling. The rotational constants of NEFA were determined to be A=9904.8373(6), B=3521.0995(2) and C=2984.9808(2) MHz, with three standard deviations in parentheses. The inertial defect Δ=Icc−Iaa−Ibb was calculated from the rotational constants to be −25.24492(2) uÅ2, which indicates the ethyl group to be bent out of the peptide linkage plane. A comparison of the observed rotational constants with those calculated by an ab initio molecular orbital method also led us to conclude that the most stable form of NEFA is trans-sc, a conformer with a nonplanar heavy atom skeleton. No evidence has so far been obtained for the existence of other conformers, as was the case for a related molecule: N-ethylacetamide. We have also observed spectra of five singly substituted isotopomers, three 13C and one for each of 15N and 18O, from which we derived a partial rs structure, in fair agreement with an ab initio result.