Radical pair mechanism

Discovered in 1967 by Bargon et al.; Ward and Lowler, CIDNP (chemically induced dynamic nuclear polarization), often pronounced like "kidnip", is an NMR technique that is used to study chemical reactions that involve radicals. Related to CIDEP insofar as the radical-pair mechanism serves as an explanation to both phenomena.

The effect is detected by NMR spectroscopy, usually using ¹H NMR spectrum, as enhanced absorption or emission signals ("negative peaks"). The effect arises when unpaired electrons (radicals) are generated during a chemical reaction. Because the magnetic moment of an electron is more than 600x that of a proton, the spins of many protons are polarized beyond the usual thermal Boltzmann distribution.

The CIDNP experiment is conducted within the NMR tube. The radicals are produced by thermal or , usually from colligation and diffusion, or disproportionation of radical pairs.

The generation of CIDNP in a typical photochemical system (target + photosensitizer, flavin in this example) is a cyclic process shown schematically in Figure 1. The chain of reactions is initiated by a blue light photon, which excites the flavin mononucleotide (FMN) photosensitizer to the singlet excited state. The fluorescence quantum yield of this state is rather low, and approximately half of the molecules undergo intersystem crossing into the long-lived triplet state. Triplet FMN has a remarkable electron affinity. If a molecule with a low ionization potential (e.g. phenols, polyaromatics) is present in the system, the diffusion-limited electron transfer reaction forms a spin-correlated triplet electron transfer state – a radical pair. The actual kinetics are rather complicated and may involve multiple (de)protonations and hence exhibit pH dependence.

...
Wikipedia