Generation, Microwave Spectrum, andAb InitioMO Calculation oftrans-1-Nitrosopropene, CH3CH[dbnd]CH[sbnd]NO (synform)

trans-1-Nitrosopropene (synform) was generated in the gas phase by pyrolysis of 1-chloro-1-methyl-2-(hydroxyimino)ethane and identified by microwave spectroscopy. The microwave spectrum of the pyrolysate was observed in the frequency range from 8.0 to 40.0 GHz. The rotational constants (MHz) were determined asA= 34025(390),B= 2315.62(2), andC= 2198.54(2) for CH3CHCHNO (normal species) andA= 34012(530),B= 2300.04(3), andC= 2182.70(2) for CH3CHCH15NO (15N isotopic species) in the ground vibrational state. The values of planar moment (Pcc= (Ia+Ib−Ic)/2) obtained for the normal and15N isotopic species were found to be 1.62(10) and 1.52(13) uÅ2, respectively. These values are almost the same within the limit of errors. This suggests that the nitrogen atom lies in or is close to theabinertial plane of the molecule and shows also that only two hydrogen atoms are located symmetrically out of the symmetry plane. The pyrolysate was determined to betrans-1-nitrosopropene by comparing the observed and calculated rotational constants, κ (Ray's asymmetric parameter), andrscoordinates of the nitrogen atom. One vibrationally excited state was observed and assigned to the C–N torsional mode (158(50) cm−1). Interesting pyrolysates such as hydrogen cyanide and acetaldehyde were also detected during the pyrolysis of the precursor. The lifetime oftrans-1-nitrosopropene is found to be ca. 5 s in the waveguide cell.