Radical pair magnetic field effects

Magnetic Field Effects on the Dynamics of the Radical Pair in a Cgo Clusters-Phenothiazine System... 

Yonemura, H., Kuroda, N., Moribe, S. and Yamada, S. (2006) Photoindiced electron-transfer and magnetic field effects on the dynamics of the radical pair in a Cgo clusters-phenofhiazine system. C. R, Chim., 9, 254-260. 

Haidar, M Misra, A., Banerjee, A. K. and Chowdhury, M. (1999) Magnetic field effect on the micellar (C)-pyrene- radical-pair system. J. Photochem. Photobiol. A, 127, 7-12. 

Fig. 14.13 The oscillating magnetic field effect (OMFE) in the triplet state radical pair reaction.

This magnetic field effect can be used to control to some extent the molecular weight of some polymers formed by free radical initiators. Such initiators undergo homolytic dissociation to form a radical pair A B which attacks the ends of growing polymer chains to add further monomeric units (section 6.2) polymerization ends when the free radicals A and B (or A and A, B and B ) recombine. Assume there are only two radicals (A and B ) in a micelle if one of these escapes from the micelle there can be no... 

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

The ion pair spin multiplicity may be a valuable tool to affect the BET rates and to probe the ion pair dynamics via magnetic field effects [36], Even weak magnetic fields are known to influence relative probabilities of singlet and triplet reactions [34], Chemically induced dynamic nuclear polarization (CIDNP) is a particularly informative technique [12]. Many bond scission reactions and rearrangements in cyclic radical ions have been successfully explored using this approach. Both structural data (spin densities) and approximate kinetic informations are indirectly available from such experiments [12]. 

If we are to use the radical pair theory to explain the effects of micellization on the cage reaction probability as well as the magnetic field effect, it is mandatory that we be able to observe CIDNP in these systems. In addition, since CIDNP is sensitive to events on the time scale of the radical pair lifetime, detailed analysis of the CIDNP can often lead to mechanistic insight to the dynamics of the radical pair. Below we describe one such result. 

Several chapters of this book show how magnetic field effects, as well as CIDEP and CIDNP spectral patterns, can be used to solve chemical problems. It should be noted that the study of how applied magnetic fields perturb chemical reactivity is a topic that is highly relevant to biological processes involving radical pairs, for example, photosynthesis. ... 

MAGNETIC FIELD EFFECTS ON RADICAL PAIRS IN HOMOGENEOUS SOLUTION... 

Magnetic Field Effects upon Chemical Reactions due to the Radical Pair... 

Werner et al. measured solvent, isotope, and magnetic field effects in the geminate recombination of radical ion pairs [23. 26]. They found similar MFEs in reaction (6-21) in acetonitrile (ACN), dimethylformamide (DMF), ethanol, and 2-propanol. Their typical results on the magnetic field dependence of the pyrene triplet yield in acetonitrile are shown in Fig. 6-9. This figure shows that the yield of each reaction decreases with increasing B from 0 T, but that the increase is saturated at 65 mT. Such MFEs can be explained by the HFCM, where the S-T conversion rate in (6-21c) is reduced by magnetic fields. The Bm value of the HFCM can be expressed by Eqs (6-11) and (6-12). 

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