T-S Mixing

If we estimate the time required for S-T mixing and recombination for a radical pair to be 100 ns( and the lifetime of the plvaloyl radical at 31°C to be v6 ps(i5., we can estimate the rate constant for the exit of t-butyl/plvaloyl radicals from HDTCl micelles to be on the order of 10 -10 sec . This Is nicely In line with exit rates of small phosphorescent probe molecules from similar micelle systems. 

To represent the "physical concept" of S-T mixing, Ward (133) has made use of a pictorial vector model to account for the Ag effect in the first term of eq. 10 and the hyperfine effects of the last two terms in eq. 10 when applied to C1DNP processes. 

At very weak fields, it is sometimes possible to observe an increase in the rate of S-T mixing, which in Fig. 8.6 corresponds to an increase in geminate product yield. This is known as a low-field effect (LFE) and has been a topic of interest recently due to the interest in the effect of environmental electromagnetic fields on human health and... 

Fig. 5-2. Phase pattern of CIDEP spectra through the S-T. mixing (7 < OJ) for the following simple example (1) radical a in a radical pair has one large HF interaction with A, > 0 J and /j = 1/2. (2) radical b in the pair has no HF interaction. (3) The pair is generated from (b) a T-precursor and (c) an S-precursor, respectively. The number of circles schematically shows the population of each sub-level and the black circle indicates the transfer of the population through the S-T. mixing.

From the formulas given, it is evident that S-T mixing can be brought about neither by the Zeeman interaction nor by the secular terms of the hyperfine interaction. However, the nonsecular terms H, rc of the hyperfine interaction,... 

Anomalous (in the sense of radical pair theory) polarization phases and magnetic field dependence were reported for substrates containing 19F nuclei, and were explained by cross-relaxation [67], Azumi and co-workers [68] investigated the photolysis of benzaldehyde. The polarizations could be accounted for by S-T0 mixing at high fields and by S-T mixing at low fields. At a field of 325 mT, however, the authors could not reconcile the CIDNP phase with the predictions of the radical pair mechanism from additional DNP experiments, they concluded that cross-relaxation with... 

Chemically induced dynamic electron polarisation (CIDER) [6, 32], a related phenomenon to CIDNP is the non-Boltzmann distribution of electron spin states. This section will examine how electron polarisation arises from the 5 — To and S T mixing mechanisms (assuming 7(r) < 0) and show how these polarisations can be readily understood within the framework of quantum mechanics. 

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