Temperature-Dependent Resonance between Electronic Exchange and Zeeman Energies in ESR Spectra of Paired Radicals

The electronic exchange energy $J$ of a radical pair trapped in crystals of 1-methyl uracil has been measured precisely by observing the effect on the ESR spectra of mixing the singlet and triplet states. The interradical distance and exchange energy were varied by thermal expansion of the crystal. Values of 0.3105 ± 0.001 ${\mathrm{cm}}^{-1\mathrm{}}$ were obtained for $\frac{J}{\mathrm{hc}}$ at 95 K and - (3.29 ± 0.16) × ${10}^{-3\mathrm{}}$ ${\mathrm{K}}^{-1\mathrm{}}$ for its thermal variability between 95 and 111 K.