Long-range proton hyperfine coupling in bicyclic radicals

Abstract

Long-range isotropic proton hyperfine coupling in rigid bicyclic systems is investigated using a non-empirical configuration interaction study of an appropriate a-bonded fragment in conjunction with a semi-empirical description of a particular spin label. The coupling is found to depend on the symmetry of the highest occupied molecular orbital of the spin label and upon the stereochemistry of a-bonded moiety. Relationships are obtained linking the hyperfine coupling to the adjacent spin density of the label for both cases wherein the highest occupied molecular orbital is either symmetric or anti-symmetric relative to the reflection plane containing the C-H bond. Numerical calculations are carried out for the particular case of the bicyclo-[2.2.1]heptane semidione radical anion wherein the reflection plane contains the y--CH 2 group as well as bisecting the molecular orbitals of the spin label.