Zero-field CIDNP

CIDNP and CIDEP Studies.—Two mechanisms have been proposed to account for the observation of electron spin polarization in radical reactions (CIDEP), the first being termed the radical pair mechanism, in which the polarization results from the mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals, and the second the triplet mechanism, in which the polarization originates in the triplet as a result of spin-selective intersystem crossing from the photoexcited singlet state, as is known to occur in several systems from ODMR measurements (see above). It has recently been pointed out 480 that, for the latter mechanism, unequal population of the triplet sub-levels will depend upon zero-field D and E terms and Zeeman terms, but also... 

Only Tg-S mixing will be considered now since it is responsible for most CIDNP spectra, certainly at zero and at high magnetic fields. 

For the chemical application of CIDNP it is important to be aware of the fact that in high magnetic fields the nonequilibrium population of nuclear spin sublevels is formed as a result of transitions solely between singlet (S) triplet zero (Tq) states or a radical pair (Figure 2). These transitions occur due to interaction of an electron spin with spins of magnetic nuclei in the radical pair and they are also dependent on the difference in g-factors of radicals in the radical pair. 

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