Reinvestigation of the microwave spectrum of acetamide

About 50 jet-cooled Fourier transform lines for acetamide have been recorde d using a new version of our spectrometer, which has been upgraded with a h eated nozzle and an expanded automatic scanning range. Nuclear quadrupole h yperfine structure arising from the nitrogen atom was removed theoretically to yield hyperfine-free center frequencies. In addition, about 30 millimet er measurements were carried out. When hyperfine structure was observed for these lines. it was also removed theoretically. A set of 115 A-species and E-species rotational transitions in the torsional ground state. obtained b y combining our new measurements with the literature data, have been fit to a model involving 28 torsion, rotation, and torsion-rotation interaction p arameters to near experimental uncertainty (i.e., to a weighted unitless st andard deviation of 1.5), significantly improving on previous fits. Various theoretical problems associated with K labels for E-species levels in this very low barrier molecule are briefly discussed and used to justify a vari ant of the signed K-a labels frequently used for internal-rotor E states. ( C) 2001 Academic Press.