GAS-PHASE STRUCTURES OF DIMETHYLBORON AZIDE AND DIMETHYLBORON ISOCYANATE - ELECTRON-DIFFRACTION AND ABINITIO STUDY

The gas-phase structures of (CH3)2BN3 and (CH3)2BNCO have been determined by electron diffraction. Both molecules have planar heavy-atom skeletons with bent BN3 and BNCO moieties. Their geometries differ mainly in the BNY bond angle (Y = NN or CO), which is small (~ 120°) in the azide and large (~150°) in the isocyanate. The following skeletal distances (ra) and bond angles (∠α) with 3σ estimated uncertainties have been derived. (CH3)2BN3:BC = 1.569 (4) Å, BNα = 1.436 (6) Å, NαNβ = 1.252 (4) Å, NβNω= 1.128 (5) Å, CBC = 122.8 (5)°, NαBC1, = 115.8 (8)°, NαBC2 = 122.1 (8)°, BNαNβ = 118.9 (8)°, and NαNβNω = 175.0 (18)°. (CH3)2BNCO: BC = 1.563 (4) Å, BN = 1.425 (5) Å, NC = 1.206 (6) Å, CO = 1.167 (5) Å, CBC = 123.6 (7)°, NBC1, = 115.3 (11)°, NBC2 = 121.1 (11)°, and BNC = 153.8 (26)°. (C1 is the methyl carbon trans to Y.) Structure analyses with a large amplitude model for (CH3)2BNCO and a double minimum potential for the BNCO in-plane bending motion suggest that the barrier at the linear configuration is ≥ 0.2 kcal/mol. HF calculations with 6-31G* basis sets reproduce the experimental structure of the azide satisfactorily. Relative to the ground state, a configuration with a linear BN3 group and one with the azide group pointing out of the plane are respectively 15.0 and 12.1 kcal/mol less stable. For (CH3)2BNCO the ab initio method predicts a linear BNCO structure, which is at variance with the experimental result. © 1990, American Chemical Society. All rights reserved.