Studies of radical pairs in potassium hydrogen malonate I. E.S.R.

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Abstract
Irradiation of single crystals of potassium hydrogen malonate with X-rays at 77 K yields two types of radical pairs, which differ in the orientation of the vector r linking the radicals of a pair but are composed of the same monoradicals. This monoradical is the main radical species found at room temperature and is characterized by hydrogen abstraction from the methylene group. Each type of pair exists in three modifications with respect to the fine structure parameters D and E, depending on the temperature treatment of the crystals after irradiation. The principal values of the dipole splitting tensor and the direction cosines of r for all pairs were determined. By application of the appropriate theory the signs of D and E could be deduced from second order effects observed at 9·5 GHz and 35 GHz. The two types of pairs exhibit marked differences in their fine structure parameters, their microwave saturation and their temperature and light stability, all of which can be attributed to different orientations of the linking vectors r relative to the molecular planes. Finally temperature coefficients of the dipole interactions were measured yielding opposite signs for the two types of pairs.