Determination of the singlet-triplet separation of a weakly interacting radical pair from the E.S.R. spectrum

An anomaly in the hyperfine structure of the electron spin resonance spectrum of the triplet state is expected to occur for a weakly interacting radical pair whose singlet-triplet separation is comparable to or smaller than the electronic Zeeman energy. This is caused by the mixing between the singlet and triplet states. We develop a general theory for the E.S.R. spectrum of a weakly interacting pair and derive the effective nuclear spin hamiltonian. Applying the theory to the radical pair of dimethylglyoxime, we succeeded in determining the singlet-triplet separation to be -0·100 ± 0·010 cm-1.