MICROWAVE-SPECTRUM AND ROTATIONAL-ISOMERISM OF GASEOUS NITROSOETHANE, CH3CH2NO

The microwave spectrum of gaseous nitrosoethane has been measured in the 7-40 GHz region. The monomeric vapour is shown to exist as two rotational isomers, one the cis form with a planar heavy-atom structure, the other a staggered gauche conformer. The microwave spectrum of gauche-nitrosoethane shows large centrifugal distortion shifts and additional splitting arising from tunnelling through the trans barrier. Relative intensity measurements show the cis to be more stable than the gauche form by 2.1 +/- 0.4 kJ mol-1. The dipole moments were determined through the Stark effect: mu(a) = 2.316 (2), mu(b) = 0.623 (4) and mu(t) = 2.398 (2) D (1 D almost-equal-to 3.33564 x 10(-30) C m) for the N-15 cis, and mu(a) = 2.288 (4), mu(b) = 0.814 (5), mu(c) = 0.460 (9) and mu(t) = 2.471 (4) D for the N-15 gauche form. Nitrogen-14 quadrupole coupling constants have been determined to be chi(aa) = -2.525 (56) and (chi(bb) - chi(cc)) = -7.311 (97) MHz for the cis, and (chi(bb) - chi(cc)) = 3.518 (40) MHz for the gauche conformer.

Molecular structures have been calculated for both conformers on the basis of the oxygen-18, nitrogen-15 and main species data. The angle CCN angle opens up by 8-degrees on going from gauche- to cis-nitrosoethane. The barrier hindering internal rotation of the methyl group is the same for both conformers within experimental error, V3 = 10.9 +/- 0.3 kJ mol-1 for the cis form and 10.9 +/- 0.3 kJ mol-1 for the gauche conformer.