Microwave Spectrum and Molecular Conformation of δ-Valerolactam

Microwave rotational spectra for normal and N -deuterated species of δ-valerolactam (2-piperidinone) were observed in the frequency range from 8 to 40 GHz. The spectra of the ground vibrational states for two isotopic species and the excited vibrational states for normal species were assigned, and the rotational constants (MHz) in ground vibrational state were determined using the Watson A -reduced Hamiltonian: A = 4590.96(11), B = 2495.03(2), C = 1731.06(2) for normal species and A = 4436.79(13), B = 2484.49(1), C = 1703.83(1) for deuterated species, respectively. The comparison of the rotational constants and r s coordinates of the imino hydrogen atom with ones from the ab initio MO calculation at the MP2/6-31G( d , p ) level of theory led to the conclusion that the spectra assigned were due to a half-chair conformer. One vibrationally excited state was observed and its vibrational frequency was 95(64) cm −1 .