Gas‐phase IR spectra of substituted alkylamides

The source of the splittings observed in the amide A, I, II, and III absorption bands of N‐methylacetamide (NMA) have been determined by high‐resolution gas‐phase FTIR. Analysis of isotopically labeled NMA, along with simpler amides (formamide, acetamide) show that the observed splitting arise from rotational transitions within the vibrational manifold. The ir spectra of a homologous series of monosubstituted alkylamides are also examined. The observed effects on the ir spectra from incorporating substituents from methyl to t‐butyl on both sides of the amide linkage are discussed in terms of rotational transitions and coupled oscillations. During these studies we did not observe any evidence for the presence of cis conformations of the amide bond.