Electron spin resonance in X-irradiated single crystal of strontium tartrate tetrahydrate

The electron spin resonance spectra of an irradiated single crystal of strontium tartrate tetrahydrate (SrC₄H₄O₆·4H₂O) grown from silica gel have been investigated. Two species of free radical were observed at room and liquid nitrogen temperatures. The free radicals were found to be the result of the splitting of a C-H bond of the tartrate ions contained in the unit cell. The unpaired electron of each radical specie interacts with two nonequivalent protons giving rise to hyperfine splittings of relatively small magnitude. The g factors of the radicals were found to be almost isotropic and have similar values and it was very difficult to determine the ESR parameters from experimental patterns directly. A least-squares method, proposed earlier, was used to analyze the observed experimental patterns at various orientations.The existence of two nonequivalent radicals is believed to be the result of the nonplanarity of the two halves of the tartrate ions. The H nuclei giving rise to the hyperfine splittings are the protons of the hydroxyl groups attached directly to the unsaturated carbon atoms and those attached directly to the other asymmetric carbon atoms (β protons). The principal elements of the hyperfine splittings for the observed radicals were: Radical I: ?9.02, ? 2.84, ?0.47 and ?1.66, ?1.54, ?0.89. Radical II: ?10.16, ?7.83, ? 1.51 and 0.00, ?1.58, ?1.06. The radicals were found to be very stable, the ESR patterns being undiminished for more than six months after the irradiation.