The unusual torsion–inversion dynamics of carbodi-imide, HNCNH

Abstract

The rotation–torsion spectrum of carbodi-imide (HNCNH) has been measured in the region from 20 GHz to 330 cm^–1. Aside from a small doublet splitting (<0.016 cm^–1) detected for most of the transitions, the spectral pattern for a perpendicular band, expected for skew-chain molecules, was observed. Carbodi-imide was found to be one of the most nearly symmetric-top molecules known with the b-rotational axis parallel to the C₂ symmetry axis. Molecular constants were obtained by fitting the positions of the doublet centres with Watson's Hamiltonian in S-reduction: A= 379 244.455 (27) MHz, B= 10366.940 8 (20) MHz, C= 10366.090 7 (26) MHz, κ=–0.999 995 390 (18).

The analysis of the unusual dependence of the torsional splitting on K_a allowed some conclusions concerning the dynamics of carbodi-imide. The energy separation due to the cis torsional tunnelling was found to be somewhat larger than that dur to the trans torsional tunnelling. The strong dependence of the torsional splittings on K_a indicates a large rotation–HNC bending interaction affecting the barriers V_cis and V_trans, hindering internal rotation.