Magnetic isotope effect in the presence of external magnetic fields

In this chapter we will discuss one of the MIE’s most remarkable features: its dependence on constant and alternating external magnetic fields. The influence of the external magnetic fields on the isotope effect parameters suggests that MIE operates. The physical origin of the phenomenon was briefly discussed in Chap. 2. The constant field splits the triplet sublevels (see, e.g., Fig. 2.6) so that the transitions between the RP singlet S and T₊₁, T₋₁ triplet sublevels induced by the hyperfine interaction with the magnetic nuclei can be suppressed at high field intensities. The external field also affects spin conservation properties (see the discussion of this problem in Sect. 2.6). Alternating fields are able to affect spin dynamics in the RPs with a definite isotope composition as well. How an external magnetic field influences the contribution of the hfi to the RP recombination depends on the spin-spin interactions (Heisenberg exchange and dipole-dipole interaction) between unpaired electrons. For that reason this chapter also includes a brief discussion of the role played by the interradical interaction in the RP spin dynamics. This problem may be of great significance for reactions that proceed through biradical states or for reactions in restricted spaces like micelles. At high constant magnetic field intensities, the mechanisms of RP singlet-triplet transitions caused by the difference in Larmor frequencies of two radicals in a pair (the so-called Δg-mechanism) and by paramagnetic relaxation originating from anisotropy of the radicals g-tensors start operating.