Micellar magnetic fields, radical pair

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. 

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]. 

Radical Pair Reactions in Micellar Solution in the Presence and Absence of Magnetic Fields... 

In 1981, the author s group found large MFEs and MIEs on the radical pair lifetime (Trp) and the escape radical yield (Fe) under magnetic fields of 0 - 70 mT with an ns-laser photolysis at room temperature for the photoreduction reactions of benzophenone (BP), benzophenone-sodium dodecyl sulfate (SDS) solutions [1]. Here, micellar molecules act as hydrogen donors (RH). The scheme of such photoreduction reactions of the benzophenone isotopes (XCO) can be represented by the following reaction scheme ... 

Table 7-1. Magnetic field and magnetic isotope effects on the radical pair lifetime (Xrp) observed for the photoreduction reactions of benzophenone isotopes in SDS micellar solutions at room temperature under magnetic fields less than 70 mT [1] and under the fields of 0.1 - 1.34 T. (Reproduced from Ref. [2b] by permission from The Japanese Chemical Society)...

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. 

Evans, C.H., and J.C. Scaiano Photochemical Generation of Radical Cations from a-Terthienyl and Related Thiophenes Kinetic Behavior and Magnetic Field Effects on Radical-Ion Pairs in Micellar Solution. J. Am. Chem. Soc. 112, 2694-2701 (1990). 

C. H. Evans and J. C. Scaiano, Photochemical generation of radical cations from alpha-terthienyl and related thiophenes kinetic behavior and magnetic field effects on radical ion pairs in micellar solution, J. Am. Chem. Soc., 1990,112, 2694. 

In this article, we emphasize on the distance dependence of magnetic field effect (MFE) on donor-acceptor (D-A) pair inside confined environment of A0T/H20/n-heptane reverse micellar (RMs) system. For this study N, N-dimethyl aniline (DMA) is used as an electron donor while the proton-ated form of Acr is treated as an electron acceptor. We report the occurrence of an associated excited state proton transfer with the photoinduced electron transfer between Acr and DMA forming corresponding radical pair (RP) and radical ion pairs (RIP). The fate of these reaction products has been tested in the presence of an external magnetic field ( 0.08T) by varying the size of the RMs. The MFE between Acr and DMA has been compared with the results of the interactions between Acr and TEA. We accentuate the importance of the localization of D and A inside the RMs, and the intervening distance between the pair to be the critical component for observing substantial MFE. 

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