Molecular structure of N-trimethylsilylaziridine in the gas phase

Abstract

N-Trimethylsilylaziridine, Me₃SiNC₂H₄1, prepared by a literature procedure has been studied by gas-phase IR and NMR spectroscopy of the nuclei ¹H, ¹³C, ¹⁵N, ²⁹Si. The molecular structure of 1 has been determined in the gas phase by analysis of electron diffraction (GED) data augmented by geometric and vibrational amplitude restraints derived from ab initio calculations employing the second order Møller–Plesset (MP2) level of theory and the 6-311G(d,p) basis set. The molecule was found to adopt C_s symmetry and has a pyramidal arrangement around the nitrogen atom with a slightly elongated Si–N bond [1.770(4) Å], relative to those in other Si–N species having a planar nitrogen co-ordination. The results for the principal distances (r_a) and angles from the combined GED/ab initio study of Me₃SiNC₂H₄ are (uncertainties: one sigma): r(Si–N) = 1.770(4), r(Si–C_in-plane) = 1.868(4), r(Si–C_out-of-plane) = 1.881(2), r(C–N) = 1.459(3) Å, C–N–C 61.1(5), N–Si–C_in-plane 109.7(7), N–Si–C_out-of-plane 107.8(10), Si–N–C 121.9(4)°. The structure data are complemented by natural bond orbital analyses, and calculation of the barrier to inversion at the nitrogen centre, which is predicted to be 21.4 kJ mol⁻¹. Calculations on the simpler model compound H₃SiNC₂H₄ [up to QCISD/6-311G(d,p)] gave geometrical and energetic results similar to those of 1. The steeply pyramidal nitrogen co-ordination is attributed to the strain of the aziridine ring, as was found by comparison with the results of other calculations on acyclic systems and those with larger rings.