Unpaired electron density in an indolinonic nitroxide radical as determined by polarized neutron diffraction

The spin density distribution in the indolinonic nitroxide radical 1,2-dihydro-2-methyl-2-phenyl-3H-indole-3-oxo-1-oxyl has been determined by a multipole expansion analysis of polarized neutron diffraction data. A large spin delocalization from the NO group to the benzene ring of the indole nucleus has been found. The results are in agreement with those previously obtained from EPR and ENDOR spectroscopy and by INDO calculations. The experimental spin density distribution has been used to calculate the dipolar electron proton interaction tensors and the dipolar electron electron interaction between pairs of neighbour radical molecules. Comparisons with the hyperfine and fine structure tensors determined by ENDOR and EPR spectroscopy experiments are presented.