Microwave spectrum and structure of difluoromethylborane, CH3BF2

Abstract

Microwave spectra of CH₃¹¹BF₂, CH₃¹⁰BF₂ and ¹³CH₃¹¹BF₂ in the ground torsional state are reported. Moments of inertia derived from the analysis of these spectra together with published data for CD₃BF₂ have been used to determine the following structure: C—H = 1.102 ± 0.007, C—B = 1.564 ± 0.005, B—F = 1.315 ± 0.005 Å HCB = 110.4 ± 0.5°, FBF = 116.8 ± 0.5°. The structure of the BF₂ group in CH₃BF₂ and other BF₂ compounds is shown to be relatively invariant in comparison to that of the isoelectronic NO₂ group. Nuclear quadrupole coupling constants at boron are discussed in terms of the structure. The range of transitions studied for the species CH₃¹¹BF₂ and CH₃¹⁰BF₂ has enabled a centrifugal distortion analysis to be carried out for the ground torsional state using Watson's reduced Hamiltonian. This has revealed an anomaly for high J, Q-branch transitions which are not satisfactorily fitted by the present treatment. Stark effect measurements give a ground-state dipole moment of µ= 1.668 ± 0.003 D.