Electron-spin resonance and theoretical studies of radicals formed from trimethylamine borane : the radicals Me2[Ndot]-BH3and (Me3N-BH3)-

Abstract

Exposure of trimethylamine borane to 60Co γ-rays at 77 K gave, initially, a four-line E.S.R. spectrum assigned to the radical anion, Me3[Ndot]-BH3 -, having a large (58 G) hyperfine coupling to boron. Theoretical calculations suggest that the radical resembles a perturbed .BH3 - in which the normally planar unit still has considerable pyramidal character. After annealing above 77 K this species was lost and a multiline, isotropic spectrum grew in which, by comparison with spectra obtained from Me3N-BD3, is shown to belong to Me2[Ndot]-BH3 radicals. This species is of interest because of the large 1H coupling to the -BH3 protons (ca. 45 G). This coupling, qualitatively ascribed to hyperconjugation, is correctly reproduced by theoretical (ab initio UHF) calculations.