Generation, Microwave Spectrum, and Ab Initio MO Calculation of trans-1-Nitrosopropene, CH3CH&dbond;CH-NO (syn form).

trans-1-Nitrosopropene (syn form) 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 as A = 34 025(390), B = 2315.62(2), and C = 2198.54(2) for CH3CH&dbond;CH-NO (normal species) and A = 34 012(530), B = 2300.04(3), and C = 2182.70(2) for CH3CH&dbond;CH-15NO (15N isotopic species) in the ground vibrational state. The values of planar moment (Pcc = (Ia + Ib - Ic)/2) obtained for the normal and 15N 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 the ab inertial 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 be trans-1-nitrosopropene by comparing the observed and calculated rotational constants, kappa (Ray's asymmetric parameter), and rs coordinates 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 of trans-1-nitrosopropene is found to be ca. 5 s in the waveguide cell. Copyright 1999 Academic Press.