Radical pair reactions magnetic field dependence

Figure 2 shows the magnetic field dependence of the ratio of the yields of symmetrical (2) and unsymmetrical (1) products for the reaction of benzyl chloride with Et3GeNa in benzene17. As earlier observed in the reactions of alkyllithiums with dichlorodiphenylmethane9, the field dependence pattern qualitatively reflects two basic mechanisms of radical pair theory — HFI and Ag mechanisms. In this particular case, the cage effects in nonviscous media (benzene) create the necessary prerequisites for the... 

Let us demonstrate how the theoretical modeling of the experimental magnetic field dependence could support the proposed reaction mechanism and prove the involvement of the suggested radical pair-precursor of the main reaction products. According to Scheme 1, the experimentally measured products ratio 2/1 could be expressed as the following ratio of their formation rates ... 

THE MAGNETIC FIELD DEPENDENCE OF RADICAL PAIR REACTIONS... 

Fig. 6-2. Theoretical prediction of the magnetic field dependence on the product yield (7(5)) in the reactions through radical pairs (a) the Ag mechanism (AgM), (b) the HFC mechanism (HFCM), (c) the mixed effect of the AgM and the HFCM, and (d) the LCM. The full curves indicate the magnetic field dependence of cage (escape) products produced from S-(T-) precursors. The broken curves indicate the dependence of escape (cage) products produced from S-(T-) precursors. In this figure, cage products mean those produced from singlet radical pairs. The curves for triplet states are omitted for simplicity, but they show similar dependence as those of escape products. (Reproduced from Ref. [34] by permission from The Chinese Chemical Society)...

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). 

Case (a) corresponds to the reactions where k / r + k holds. In this case, the magnetic field dependence of radical pair decay shows a shallow reversion. The reactions of BP and NQ in SDS micellar solutions (Figs. 12-9A and 9C) belong to this case. 

The observed magnetic field dependence of the rate constants ( o and k ) of radical pair decays shown in Fig. 12-9 can be explained quantitatively by the following procedures (1) We can see from by Eq. (12-24) that ks is represented by the sum of k and k + k. (2) The magnetic field independent kg. values for the reactions in micellar solutions can well be determined with an optical-detected ESR technique, which will be explained in Chapter 14. 

In a related study. Levin et have examined the relaxation of triplet exciplexes through electron transfer pathways. The magnetic field dependence of the radical ion pair recombination rate constants indicated that these reactions were predominantly of triplet radical ion pairs recombining through electron... 

Assuming reaction can only proceed via the singlet channel (to form a stable product) and for the radical pair to be in a singlet state, the magnetic field dependence on product yield can be summarised as follows ... 

Reactions of benzophenone-cyclohexa-dienyl radical pair depend on surfactant, magnetic field and l3C content... 

The magnetic isotope effect (MIE) is one of the most important techniques which have been developed in the course of studies of MFEs on chemical reactions. It is noteworthy that the MIE is a new type of isotope effect This effect comes from the difference in nuclear spin, but not in nuclear mass. According to the HFCM, the S-T conversion of radical pairs depends on the HF interaction between nuclear and electron spins in the component radicals, even in the absence of an external magnetic field. Therefore, it is possible for MIEs to appear in most reactions which show MFEs. 

Silverman s studies on mechanism based MAO inactivation have provided overwhelming support for the role of electron transfer in the MAO catalyzed dealkylation of amines. It must be mentioned however that spectroscopic attempts for detecting the radical ion intermediates have hitherto been unsuccessful. Yasanobu and coworkers could not find EPR spectral evidence for radical intermediates in MAO-catalyzed oxidation of benzylamine [205]. Miller et al. failed to observe the flavin semiquinone or an amine-flavin adduct in rapid-scan-stopped flow spectroscopy [206]. The only time-dependent absorption change observed in this study was the bleaching of the oxidized flavin. Furthermore, no influence of a magnetic field up to 6500 G was observed on the rate of MAO B reduction. The reaction rates of systems with kinetically significant radical pair intermediates are known to be altered... 

---