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 i n 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 C H2=C(CH3)-(NO)-N-14 (normal species) and A = 8664.36(5), B = 4822.15(3), an d C = 3157.04(3) for CH2=C(CH3)-(NO)-N-15 (N-15 species) in the ground vibr ational state. The values of the planar moment (P-cc = (I-a + I-b - I-c)/2) obtained for the normal and N-15 species were 1.525(1) and 1.526(1) u Angs trom (2) respectively. This suggests that the nitrogen atom lies in or is c lose to the nb inertial plane of the molecule and shows also that only two hydrogen atoms are located symmetrically out of the symmetry plane. The rea ction product was determined to be syn-2-nitrosopropene by comparing the ob served and calculated rotational constants, kappa (Ray's asymmetry paramete r) and r(s) coordinates of the nitrogen atom. The dipole moments (D) were d etermined to be mu (a) = 2.43(5), mu (b) = 1.12(7), and mu (total) = 2.67(7 ). The barrier heights of the internal rotation owing to the methyl group o f 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), r espectively. The N-14 nuclear quadrupole coupling constants (MHz) were dete rmined to be chi (aa) = 0.25(21), chi (bb) = -7.11(40), and chi (cc) = 6.85 (61). Two vibrational excited states were observed and the vibrational freq uencies (cm(-1)) of the C-N and C-C torsional modes were determined to be 1 60(40) and 175(40), respectively. The lifetime of syn-2-nitrosopropene was found to be ca. 2 min in the waveguide cell. (C) 2000 Academic Press.