Radical pair mechanism, chemically induced

One of the important possible mechanisms of MF action on biological systems is the influence of free radical production. Chemical studies predict that MFs may affect free radical reactions through the radical pair mechanism [201]. A reaction between two free radicals can generate a free radical pair in the triplet state with parallel electron spins. In this state free radicals cannot recombine. However, if one of the electrons overturns its spin, then free radicals can react with one another to form a diamagnetic product. Such electron spin transition may be induced by an alternative MF. 

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... 

The current theories of chemically induced magnetic polarization can therefore be summarized into the two basically different mechanisms the photoexcited triplet mechanism (PTM) responsible for the initial electron polarization and the observed Overhauser effect in nuclear polarization, and the radical-pair mechanism which, to date, accounts for almost the remaining bulk of the known polarization systems. We proceed to describe the simple physical models of these two mechanisms by beginning with the more sophisticated radical-pair theory. 

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. 

From the start, these phenomena were recognized as spin polarizations (deviations of the populations of the nuclear spin states from the Boltzmann distribution) caused by radical reactions. As the first attempts to understand their generation erroneously focussed on Overhauser effects, they were christened "chemically induced d)mamic nuclear polarizations". Although only partially correct, that name has stuck, possibly because its acronym CIDNP (usually pronounced "kidnap") evokes the picture of radical scavenging. However, only 2 years later the now universally accepted quite different explanation, the hitherto unknown radical-pair mechanism, was found, again by two groups independently." ... 

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. 

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

Our research aims at elucidating the chemical nature of the redox centers involved in light-induced charge separation, the mechanisms of reaction and the role of the protein counterpart. During this last year our work has concerned mainly the primary radical pair and the secondary acceptor A,. 

The role of nuclear spins in chemical reactions does not seem to have been noted before the advent of CIDNP. It is easy to see, however, how (i) and (ii) can lead to a sorting mechanism for nuclear spins and therefore to strong polarizations in radical pair products. This is demonstrated in Figure 1, which shows a typical photo-induced radical reaction. 

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