Electron Spin Resonance of Radical Pairs in Single Crystals of Oxalic Acid, Dihydrate and Anhydrous

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Abstract
The electron-spin-resonance spectra of free-radical pairs have been observed in single crystals of oxalic acid, both anhydrous and dihydrate, produced by irradiation with 3-MeV x rays at 77°K. The anisotropic dipole—dipole coupling due to the unpaired electrons observed was fit to theoretical curves for unpaired electrons 6.33 Å apart in the dihydrate and 9.93 Å apart in the anhydrous oxalic acid. It is concluded that these electrons are located on the hydroxyl oxygen atoms. In the dihydrate they are in adjacent molecules of the unit cell, and in the anhydrous they are in molecules separated by 1 unit cell length in the b direction plus 1 unit cell length in the c direction.