The Radical Pair Mechanism

On the basis of the idea that CIDNP effects are caused by interactions within radical pairs, quantitative formalisms capable of explaining many features of CIDNP were given by Closs and by Kaptein and Oosterhoff . The theory was later refined and modified by various authors, but the new developments did not change the basic concepts. 

Two further assumptions now give the CIDNP effects if combined with the dynamic pair behavior described above. They are  

Pairs may undergo transitions between singlet and triplet states with nuclear spin-dependent probabilities in the time between successive encounters. 

At radical-radical encounters, reaction to products c occurs for singlet state pairs only. Triplet state pairs do not react and separate instead. 

The forces which drive the intersystem crossing are the nuclear spin-dependent hyperfine interactions in the radicals and the electron Zeeman interactions. This becomes evident from the following after pair formation in the magnetic field of a NMR spectrometer, say, the two unpaired electron sfnns precess about the magnetic field axis starting from defined initial phase an es. These iititial phase angles are different for the four possible initial electronic states T Tq, 

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The other mechanism which has been advocated58 is that known as the radical-pair mechanism , in which two cation radicals are thought of as intermediates held in a solvent cage so preserving the intramolecularity of the reaction, viz. 

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. 

If the photo-Fries reaction would occur via a concerted mechanism, the absence of solvent should be of minor importance for the formation of rearranged products. However, conclusive evidence supporting the radical pair mechanism arises from the experiments carried out with phenyl acetate (10) in the vapor phase. The major product in the irradiations of 10 is phenol (13), which accounts for 65% of the photoproducts. Under these conditions, less than 1% of ortho -hydroxyace-tophenone (11) appears to be formed [19,20]. Conversely, when a high cage effect is expected, as in rigid matrixes (i.e., polyethylene), the result is completely different, and phenol is practically absent from the reaction mixtures [29]. In the intermediate situation (liquid solution), both rearranged products and phenol are formed in variable amounts depending on solvent properties. These observations... 

Any CIDNP-based assignment of the sign and relative magnitude of hfcs is valid only if the radical pair mechanism (RPM) is operative they become invalid if an alternative process is the source of the observed effects. The triplet-Overhauser mechanism (TOM) is based on electron nuclear cross-relaxation. For effects induced via the TOM, the signal directions depend on the mechanism of cross-relaxation and the polarization intensities are proportional to the square of the hfc. Thus, they do not contain any information related to the signs of the hfcs. 

CIDEP originates in two independent processes, the triplet mechanism and the radical pair mechanism. The last one arises in spin correlated pairs [60]. The final spectrum gives a direct insight in the working mechanism. 

It is now well established that both CIDEP and CIDNP have their origins in the formation and removal reactions of free radicals. As a result of this, it is now possible to gain information, not normally obtained from magnetic resonance studies, for those photochemical reactions which show CIDEP and CIDNP. An example of this is those photochemical reactions in which the primary radicals react immediately to regenerate the starting compounds. The regenerated compounds may show CIDNP, and this is often the only evidence that this reaction has occurred. In the radical-pair mechanism, spin polarization is caused by the spin-selective reaction. While it is generally not possible to monitor by esr the selective reactivity of the radical pairs as a function of their nuclear spin states, CIDNP has proved to be a valuable tool to probe the small difference in reactivity of the nuclear spin states of the radical pairs. 

The utility of CIDEP in photochemistry was greatly enhanced when it was realized (131) that the radical-pair mechanism is not the exclusive spin polarization mechanism. Initial triplet spin polarization produced by the different intersystem-crossing rates to the excited triplet sublevels can be "transferred" to radicals formed by the photochemical reaction of the polarized triplet. 

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. 

A similar representation of a pictorial vector model for CIDEP processes is more difficult to formulate, but recently Monchick and Adrian (100) have succeded in casting the stochastic Liou-ville model of CIDEP into the form of a "Block-type" equation with diffusion which led to a generalized vector model of the radical-pair mechanism to give a clear qualitative picture of both CIDNP and CIDEP. 

Ag of the primary radical pair is usually very small, and therefore a pure net CIDEP effect due to the radical-pair mechanism... 

On the other hand, the overwhelming success of the radical-pair mechanism is well supported by many experimental investigations (2,18,49,106,119). In a systematic study of the photoreduction of quinones by phenols, Adeleke and Wan (2) have confirmed the effect of the sign of the Ag on the polarization of the semiquinone and the phenoxy radicals as dictated by eq. 10, and Elliot and Wan (49) have obtained relative individual enhancement factors of the esr lines of the durosemiquinone radical formed in the photoreduction of the parent quinone by isopropanol which agree very well with the values predicted by eq. 16. 

High-Field CIDNP. The same S-Tq mixing of the radical-pair mechanism described above applies to high-field CIDNP. A further simplication and a good approximation for the CIDNP processes can be made by assuming that J = 0. The necessary sequence of events required for CIDNP can be represented by the following scheme (88) ... 

Based upon the current theories of CIDEP and CIDNP, we propose that in many photochemical systems the primary photochemical reaction of the excited triplet state contributes to magnetic polarization via the triplet mechanism. The secondary reaction of the polarized primary radicals may transfer their initial polarization to the "secondary radicals" provided that the radical reactions can compete with the radical spin-lattice relaxation process (59,97). On the other hand, secondary reactions of the primary radical pair or the uncorrelated F pair contribute to polarization by the radical-pair mechanism. A general scheme showing the possible and simultaneous operations of both the... 

This chapter is concerned with chemical reactions that occur while the system is still in the paramagnetic world. After an explanation of the radical pair mechanism and a brief treatment of experimental details, three case studies are presented that illustrate the application of CIDNP to transformations of radicals into other radicals and to interconversions of biradicals. 

CIDNP effects are described qualitatively and quantitatively by the radical pair mechanism, which is depicted in Chart 9.1. ... 

A detailed description of CIDEP mechanisms is outside the scope of this chapter. Several monographs and reviews are available that describe the spin physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from singlet-triplet electron spin wave function evolution during the first few nanoseconds of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism (TM) is a net polarization arising from anisotropic intersystem crossing in the molecular excited states. For the polymers under study here, the TM is net E in all cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a later section. Other CIDEP mechanisms, such as the radical-triplet pair mechanism and spin-correlated radical pair mechanism, are excluded from this discussion, as they do not appear in any of the systems presented here. 

The changes of polarizations with the addition of various sensitizers may be regarded as strong evidence for the radical pair mechanisms of CIDNP and provide a nice example of the dependence of the effects on the precursor states. Two other important features of CIDNP also become apparent from the results of the studies outlined above. 

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