Solution epr studies have been carried out on acid-base complexes of the form R2NO: MX3 (M = B, Al and X = F, Cl, Br), where a stable nitroxide free radical serves as a Lewis base and the acids are aluminum chloride and the boron halides. The hyperfine interaction of the nitroxide 14N nucleus increases upon complexation and is shown to depend on the electron-withdrawing strength of the particular Lewis acid in the complex. This variation is interpreted in terms of simple MO theory. Analyses of the g factors and epr line-width variations in these adducts indicate that the nitroxide retains the major portion of the unpaired electron spin density. A scale of electron-withdrawing strengths, based on the 14N spin density in the complexed nitroxide, is proposed and enables the Lewis acids studied here to be placed in the following order of acceptor strengths: A1C13 = BF3 < BC13 ≈ BBr3. Temperature dependences of the hyperfine interactions in these adducts provide some insight into this anomalous ordering for the boron halides.
ASJC Scopus subject areas
- Colloid and Surface Chemistry