Chemically induced dynamic radical pair mechanism

CIDEP (Chemically Induced Dynamic Electron Polarization) Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscope... 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

In 1963, Fessenden and Schuler [1] found during irradiation of liquid methane (CFLt and CD4) at 98 K with 2.8 MeV electron that the low-field line for both hydrogen and deuterium atoms appeared inverted (emissive signals) and that the central deuterium atom line was very weak. Although the cause of such anomalous ESR spectra was not clear at that time, similar anomalous signals have been observed in many reactions and have been called " Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP should be due to non-equilibrium electron spin state population in radicals and could also be explained later by the radical pair mechanism as CIDNP. 

The most striking evidence for a possible radical pair mechanism in the case of vinylarenes is the observation of chemically induced dynamic nuclear polarization (CIDNP). CIDNP was observed in the product of the stoichiometric hydrogenation of 1,1-diphenylethene (156) and in both carbonylation and saturation products of styrene (164). Scheme 11 shows the proposed mechanism of the reaction of styrene with HCo(CO)4. 

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