Conformational Analysis of ϵ-Caprolactam, N -Acetyl-ϵ-caprolactam and N -Vinyl-ϵ-caprolactam Using Rotational Spectroscopy

The rotational spectra of ε-caprolactam, N-vinyl-ε-caprolactam, and N-acetyl-ε-caprolactam were measured in the 26.5–40 GHz region using a chirped-pulse Fourier transform microwave spectrometer, providing the first rotational spectroscopic characterization of two N-substituted seven-membered lactams and extending a previous study of the parent compound. Supported by geometry optimizations and frequency calculations at the ωB97X-D/def2-TZVP level, the chair conformer was assigned for all three molecules, and the boat conformer was additionally observed for vinylcaprolactam. Rotational, centrifugal distortion, and 14N nuclear quadrupole coupling constants were determined for each species. For the chair conformer of vinylcaprolactam, all eight 13C isotopologues and the 15N isotopologue were assigned in natural abundance, yielding a Kraitchman substitution structure of the complete heavy-atom framework in close agreement with the calculated geometry. The calculations show that N-substitution narrows the chair–boat energy gap from 15.0 kJ mol–1 in caprolactam to 9.8 and 9.2 kJ mol–1 in the vinyl and acetyl derivatives, respectively, while leaving the ring scaffold nearly unchanged, with the structural effects of substitution concentrated in the amide unit. The conformers observed for caprolactam and vinylcaprolactam are consistent with pre-expansion thermal equilibrium, whereas the boat conformer of acetylcaprolactam was not detected despite a comparable predicted abundance.