Temperature-dependent ESR studies of radical pairs in single crystals of barbituric acid.

After irradiation of single crystals of barbituric acid with X-rays at 77 K different types of radical pairs are found, which are composed of only one type of monoradical. The properties of radical pairs of the unpaired electrons are studied using frequency-variable ESR methods at various temperatures. In addition to the radical pairs AD and BC two further pairs A'D' and XY were identified. Measurements of the fine structure parameter D showed a linear temperature dependence in some regions between 77 K and 290 K. At 240 K the radical pairs AD and A'D' changed reversibly into A*D* and this pair remained stable up to 290 K. A level anticrossing effect was observed with the pair AD. The exchange energy J between the singlet state and the triplet state was determined as -(15.1 +/- 0.6) GHz at 77 K and its temperature coefficient as -(3.8 +/- 0.8) x 10(-3)K-1.