Hyperfine coupling tensors for 24 protons have been determined from ENDOR measurements of the C103 centre trapped in y-irradiated single crystals of barium perchlorate trihydrate at 120 K. The tensors have small isotropic components and their dipolar components are close to axial so that their interpretation in the point-dipole approximation to reconstruct the proton geometry.in the vicinity of the trapped radical is justifiable. The model thus obtained indicates that there is no severe distortion of the crystal structure in the immediate vicinity of the trapped radicals. There are, two chemically identical but ENDOR-distinguishable sites related by a reflection in the u1a2 p!ane of the hexagonal crystal. This observation supports the assignment of the P63/m space group to the crystal: X-ray diffraction measurements had indicated that the P63 space group was also a possibility.

Irradiation of ammonium paramolybdate yields a paramagnetic centre which is trapped uniquely on one of the molybdenum atoms in the {Mo7024} 6-ion. The centre has low symmetry so that its location cannot be deduced from the E.S.R. parameters. Hyperfine coupling tensors for six protons have been obtained from ENDOR measurements at 15 K and interpreted in the point-dipole approximation to construct the proton geometry in the vicinity of the paramagnetic centre. Comparison of this model with the heavy atom positions, known from X-ray diffraction, has enabled the location of the paramagnetic centre to be identified.

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