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 a β H = (B 0+B 2 cos2θ)ρπ, it is shown that the term B 0 may be attributed to residual torsional motion of β protons in the crystal lattice.