Hindered Internal Rotation and ESR Spectroscopy

Abstract

A theory of the effect of hindered internal rotation on β-proton coupling constants in free radicals is proposed and discussed. The results of both classical and quantum-mechanical treatments are applied to recent ESR data on the anion radicals of several nitroalkanes. It is shown that a twofold barrier to internal rotation of magnitude 1.4 kcal/mole can account for the observed variation of proton coupling constants between the nitroethane and 2-nitropropane anion radicals. The temperature dependence of the coupling constants is predicted. In the equation aβH = (B0+B2 cos2θ)ρπ, it is shown that the term B0 may be attributed to residual torsional motion of β protons in the crystal lattice.