The E.S.R. spectra and structure of the B[Fdot]₄ radical formed in a single crystal of KBF₄

Abstract

Exposure of a single crystal of KBF₄ to γ-rays at 77 K gave E.S.R. spectra assigned to B[Fdot]₄ radicals. The spectra comprise a hyperfine structure due to four fluorine nuclei, two of which, F₃ and F_3′, have small coupling constants and are equivalent in the magnetic field applied perpendicular to the b axis of the crystal. Analysis of the spectra indicate that the unpaired electron possesses an orbital composed largely of co-linear p-orbitals on the F₁ and F₂ atoms with which the central B atom makes a plane perpendicular to the b axis. The two p-orbitals lie in this plane, making an angle of 16° with the a axis, the dominant spin density being in the p-orbital on F₁.

INDO calculations support the identification of the B[Fdot]₄ radical, although perfect accordance with observed results has not been obtained. According to these calculations, the bond angle of F₁, B, F₂ is reduced to 50 ∼ 70°, while that of F₃, B, F₃ ^′ is increased to c. 120°. The B-F₁ bond is long compared with the other B-F bonds, and there is incipient ·F₂ ^- formation.