Ground and first excited torsional states of acetamide

Abstract

We present a first global study involving rotational levels in the lowest three torsional states of acetamide (CH 3CONH 2). New measurements of this spectrum, consisting of approximately 1600 lines and involving torsion–rotation transitions with J up to 20 and K a up to 11, have been carried out between 49 and 149 GHz using the millimeter-wave spectrometer in Kharkov. After removing the observed quadrupole hyperfine splittings, the new data were combined with previously published measurements and fitted using a rho-axis-method (RAM) torsion–rotation Hamiltonian in conjunction with a new computer-automated v t and K labeling algorithm. The final fit used 48 parameters to give an overall weighted standard deviation of 0.72 for 759, 587, and 265 lines belonging, respectively, to the ground, first, and second excited torsional states and 95 lines corresponding to Δ v t =1 transitions falling in the millimeter wave range. Separate root-mean-square (rms) deviations for the A (25 kHz) and E (27 kHz) species, as well as for the v t =0 state (26 kHz), v t =1 (25 kHz), v t =2 (22 kHz), and Δ v t =1 transitions (35 kHz) indicate a similar quality of the fit for the two symmetry species and for the three torsional states. The RAM Hamiltonian was found to have rather good predictive ability when fits of only v t =0 (or only v t =0 and 1) transitions were used to calculate v t =1 (or v t =2) lines. In addition, the combination of this Hamiltonian and the computer-automated v t and K labeling algorithm seems to provide a rather powerful tool for the precise assignment and fitting of torsion–rotation transitions in C 3 v internal-rotor molecules with low torsional potential barriers.