The phase of the primary electron spin echo signal was analyzed on model radical pairs (RPs) in a singlet spin state. An interpretation is given of the fact known in the literature that the echo signal of such pairs may not have a component with a “normal” phase, which is expected for the echo signal of non-interacting unpaired electrons in the RP. For the first time, it has been theoretically shown that forbidden EPR transitions can make a significant contribution to the phase shift of the magnetization of RP spins, caused by the dipole–dipole hyperfine interaction of unpaired RP electrons with magnetic nuclei. The “anomalous” phase of the spin echo signal of interacting spins in radical pairs is a demonstration that spin dynamics is accompanied by reversible transitions of dipole polarization (spin magnetization) and quadrupole spin polarization.

在单重态自旋态的模型自由基对（RP）上分析了初级电子自旋回波信号的相位。对文献中已知的事实给出了解释，即此类对的回波信号可能不具有具有“正常”相位的分量，这对于RP中不相互作用的不成对电子的回波信号来说是预期的。首次从理论上证明，禁止的 EPR 跃迁可以对 RP 自旋磁化强度的相移做出重大贡献，这是由不成对的 RP 电子与磁核的偶极-偶极超精细相互作用引起的。自由基对中相互作用的自旋的自旋回波信号的“反常”相位证明了自旋动力学伴随着偶极极化（自旋磁化）和四极自旋极化的可逆转变。