Generation, Microwave Spectrum, Barrier to Internal Rotation of Methyl Group, and ab Initio MO Calculation of syn-2-Nitrosopropene, syn-CH2C(CH3)NO

syn-2-Nitrosopropene was generated, in the gas phase, by chemical reaction of 1-chloro-2-(hydroxyimino)propane with K2CO3 and identified by microwave spectroscopy. The microwave spectrum of the reaction product was observed in the frequency range from 8.0 to 40.0 GHz. The rotational constants (MHz) were determined as A = 8744.09(6), B = 4846.07(2), and C = 3177.84(3) for CH2C(CH3)14NO (normal species) and A = 8664.36(5), B = 4822.15(3), and C = 3157.04(3) for CH2C(CH3)15NO (15N species) in the ground vibrational state. The values of the planar moment (Pcc = (Ia + Ib − Ic)/2) obtained for the normal and 15N species were 1.525(1) and 1.526(1) u Å2, respectively. 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 reaction product was determined to be syn-2-nitrosopropene by comparing the observed and calculated rotational constants, κ (Ray's asymmetry parameter) and rs coordinates of the nitrogen atom. The dipole moments (D) were determined to be μa = 2.43(5), μb = 1.12(7), and μtotal = 2.67(7). The barrier heights of the internal rotation owing to the methyl group of the normal species in the ground and first excited torsional states were determined to be 1750(50) and 1740(50) cal/mol (1 cal/mol = 4.184 J/mol), respectively. The 14N nuclear quadrupole coupling constants (MHz) were determined to be χaa = 0.25(21), χbb = −7.11(40), and χcc = 6.85(61). Two vibrational excited states were observed and the vibrational frequencies (cm−1) of the C–N and C–C torsional modes were determined to be 160(40) and 175(40), respectively. The lifetime of syn-2-nitrosopropene was found to be ca. 2 min in the waveguide cell.