Analysis of the rotational spectrum of pyruvonitrile up to 324 GHz

The room-temperature rotational spectrum of pyruvonitrile (acetyl cyanide, CH₃COCN) was measured up to 324 GHz, and additional measurements were also made in supersonic expansion in the region 7-19 GHz. The available data sets for the A and E torsional sublevels were extended to over 1200 transitions, J = 65 and K_a = 38 for the ground vibrational state, and to comparable numbers of transitions for first excited states of the methyl torsional mode ν₁₈, and the in-plane CCN bending mode ν₁₂. The collected experimental measurements were fitted with several different computer programs for dealing with the effects of methyl torsional motion on the rotational spectrum and many spectroscopic constants have been determined. The experimental results are discussed in detail and are augmented by ab initio computations. Stark effect measurements in supersonic expansion were used to precisely determine the electric dipole moment of pyruvonitrile, ∣μ_a∣ = 2.462(2) D, ∣μ_b∣ = 2.442(2) D, μ_tot = 3.468(2) D. Pyruvonitrile, as an 8-atom molecule with a sizable dipole moment, is a possible candidate for astrophysical detection and the present work provides the laboratory data necessary for that purpose.