Microwave spectrum and structure of difluoro(trifluorosilyl)borane, SiF3BF2

Abstract

The microwave spectrum of ²⁸SiF₃¹¹BF₂ has been thoroughly examined in the region 24–40 GHz. Transitions have been assigned within the five lowest-lying torsional states and the earlier analysis of the spectrum refined. An improved value for the barrier to internal rotation about the Si—B bond has been determined as V₆= 2.178 ± 0.005 cal mol^–1(9.11 ± 0.02 J mol^–1).

The spectra of the isotopic species ²⁸SiF₃¹⁰BF₂ and ²⁹SiF₃¹¹BF₂ have also been assigned allowing the zero-point average Si—B bond length to be determined as 2.027 ± 0.005 Å, significantly longer than that expected for a single bond. The remaining r_z structural parameters are determined to be: Si—F = 1.567 ± 0.005 Å, B—F = 1.312 ± 0.005 Å, ∠FBF = 118.5 ± 0.5°, ∠FSiB = 110.9 ± 0.2°. The structure and bonding of SiF₃BF₂ are compared with those of related molecules.