The methyl/trifluoromethyl substitution effect in boranes. A gas-phase electron diffraction and vibrational study of bis(dimethylamino)-(trifluoromethyl)borane and dimethylaminobis(trifluoromethyl)borane

Abstract

The gas-phase structures of CF₃B(NMe₂)₂ and (CF₃)₂BNMe₂ have been determined by electron diffraction. The vibrational spectra have been recorded by i.r. (gas) and Raman (liquid) and valence force fields have been derived. The following skeletal geometric parameters (r_a values with 3σ uncertainties) have been obtained: for CF₃B(NMe₂)₂, B–C = 164.8(8), B–N = 142.2(9), C–N = 145.3(4), C–F = 135.2(3) pm, NBN = 125.5(31), FCF = 105.3(4), and τ_BN= 24(3)°(τ_BN is the torsional angle around the B–N bond from the planar configuration); for (CF₃)₂BNMe₂, B–C = 162.3(4), B–N = 142.5(18), C–N = 145.3(7), C–F = 134.3(2) pm, CBC = 122.2(15), FCF = 106.5(2)°. The B–C bonds in the trifluoromethyl compounds are considerably longer than in the analogous methyl derivatives and this CH₃/CF₃ substitution effect is confirmed by the force constants.