ENDOR study of long‐range spin interactions in molecular crystals. I. 1‐methyl uracil

ENDOR spectroscopy has been used for the precise measurements of the proton couplings in two different radicals derived from 1‐methyl uracil by gamma ray irradiation. Couplings of the protons located in as many as seven molecules in the host crystal around the free radical have been identified. The intermolecular spin interaction is predominantly of dipolar character. For the remote protons it is well described by the point‐dipole approximation. Intramolecular proton couplings are more complex because of the significant isotropic component and delocalization of the unpaired electron.