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

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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 F3′, 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.