Implications for Spin-Relaxation in Linked Radical Pairs

4 IMPLICATIONS FOR SPIN-RELAXATION IN LINKED RADICAL PAIRS 

Theoretically, Ti relaxation of individual radicals a and b is related to a k contribution in the radical pair by  

FIGURE 10.5 Contributions to in the theoretical simulation of spin relaxation in the radical pairs from DCA-POZ and DCA-PSZ evaluated under the assumption of 0 (data points). The full simulations are represented by the curves denoted / -POZ/DQtot.A 0.45 and fc-PSZ/DQtot, respectively. The contribution from the esdi mechanism (k-esdi )=6E-7) corresponds to an effective translational diffusion constant of D = 6 x 10 cm s. The curves indicated as / -POZ/DQ.A represent the contributions of the ahfi mechanism, the indicating the factor by which the theoretical anisotropy parameter A is reduced. The curve indicated hy k-PSZ/DQ.g denotes the contribution due to the g tensor anisotropy in the PSZ radical. The constant values c POZ and c PSZ represent the field-independent contributions to k. For details of the calculation cf. Ref. 23. 

FromA = 3.4x 10 s, weobtainTc= 1.38 x 10 sandrjr = 13.8cPA. Assuming an effective radius between 2.5 and 3.0 A that seems geometrically reasonable, the effective viscosity would range between 0.50 and 0.89 cR For 1,2-dichloroethane, the solvent used in the experiments rj = 0.464 cP at 25 °C. Since, however, the radical is linked to the complex by a tetramethylene chain one might well expect some increase of the effective viscosity for rotational diffusion. 

To assess the spin-relaxational contribution /Cr ahf of anisotropic hyperfine coupling we only take the contribution from the nitrogen nuclei into account. This approach is justified on the basis of the theoretical DPT calculations showing that the contribution of anisotropic coupling to the H spins is at least by one order of magnitude smaller. Details of the calculations of for the radical pairs are given in Ref. 23. In Fig. 10.5, 

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