Radical-Triplet Pair Mechanism

Electron polarisation obtained by the Radical-Triplet Pair Mechanism (RTPM) [30] is related to the RPM in that diffusive encounters are still required, but differs in that it involves the interaction of a photoexcited triplet state (S = 1) with a doublet state (S = 1 /2) radical. At high concentrations, the production of a photoexcited triplet state interacts with the doublet to form quartet and doublet states, which in the Zeeman basis can be expressed as  

Assuming J 0 where the doublet states are lower in energy than the quartet states. 

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In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

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... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

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. 

This is the main reason why a new type of CIDEP is produced through the radical triplet pair mechanism (RTPM). Many variations of CIDEP due to the RTPM have been observed. For example, net absorptive CIDEP was observed in the T-D quenching for positive J values [10]. Here, the level-crossings between IG3/2) - -D-i/2) Q n)produce such... 

Scheme 6 Photo-Arbuzov rearrangement of arylethylphosphites via a long-lived triplet proximate radical pair mechanism. Reprinted with permission from [22]. Copyright 2001 American Chemical Society...

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. 

Photolysis of diazomethane in carbon-tetrachloride in the presence of benzophenone yields 1,1,1,2-tetrachloroethane showing an enhanced absorption due to the triplet carbene. The direct photolysis of diazomethane proceeds via singlet methylene CIDNP-studies of the photolysis of methyl-diazoacetate, for which a radical pair mechanism was suggested, were recently challenged 2). 

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. 

In 1956, Doering et al. reported that methylene (CH2) inserted into the C H bonds of pentane, 2,3-dimethylbutane, and cyclohexene with no discrimination (other than statistical) between chemically different sites CH2 was classed as the most indiscriminate reagent known in organic chemistry. Doering and Kirmse also demonstrated that the C—H insertion reactions of CH2 in solution were direct, single barrier concerted processes with transition states that could be represented as 27 (Fig. 7.12). In particular, they did not proceed via initial H abstraction to give radical pair intermediates that subsequently recombined. (Triplet carbene C H insertions, however, do follow abstraction-recombination, radical pair mechanisms, as demonstrated in classic experiments of Closs and Closs and Roth (see Chapter 9 in this volume). 

The insight of photoinitiation is complicated. Even when CT absorption is observed, the initiation process may not start from a charge transferred state or form ion-radicals. An alternative mechanism is triplet excitation via charge transfer absorption. Namely, when the CT excited level is higher than the triplet level, a considerable amount of the CT excitation would be converted to the triplet state. The TMPD+-naphthalene pair fits in this case (20). Conversely, the contribution of CT might be predominant even when the CT interaction in the ground state is not observed. As shown in Eqs. (14) and (16), charge transfer interaction will not take part in photoexcitation but occurs in the excited state. Possible reaction mechanismus may be explained as follows. 

The geometric isomerization of olefins via photochemical electron transfer is well known28,29 and can be divided into two categories (a) isomerization via the radical cation, in which case the olefin is the donor in the presence of an excited electron acceptor (b) isomerization via the radical ion pair, which leads to the triplet-excited olefin, and in this mechanism the olefin is the acceptor. This subject is not discussed in this chapter because of space limitations. However, several reviews30 can be consulted in this regard. 

Deviations from Boltzmann intensities were first noted in 1963 [72] they are known as CIDEP [73,74]. These effects can be caused by two different mechanisms, each giving rise to CIDEP spectra with different characteristic intensity patterns. CIDEP effects can have two sources they may be transferred to the intermediates from their immediate precursor, typically a triplet state (triplet mechanism, TM), or generated early during their existence (radical pair mechanism, RPM) [75]. 

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