Manifestation of spin exchange relaxation in microwave field induced magnetic field effects on recombination of confined radical pairs

Electron paramagnetic resonance line shifts and line shape changes due to heisenberg spin exchange and dipole-dipole interactions of nitroxide free radicals in liquids 8. Further experimental and theoretical efforts to separate the effects of the two interactions

The work in part 6 of this series (J. Phys. Chem. A 2009, 113, 4930), addressing the task of separating the effects of Heisenberg spin exchange (HSE) and dipole-dipole interactions (DD) on electron paramagnetic resonance (EPR) spectra of nitroxide spin probes in solution, is extended experimentally and theoretically. Comprehensive measurements of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDT) in squalane, a viscous alkane, paying special attention to lower temperatures and lower concentrations, were carried out in an attempt to focus on DD, the lesser understood of the two interactions. Theoretically, the analysis has been extended to include the recent comprehensive treatment by Salikhov (Appl. Magn. Reson. 2010, 38, 237). In dilute solutions, both interactions (1) introduce a dispersion component, (2) broaden the lines, and (3) shift the lines. DD introduces a dispersion component proportional to the concentration and of opposite sign to that of HSE. Equations relating the EPR spectral parameters to the rate constants due to HSE and DD have been derived. By employing nonlinear least-squares fitting of theoretical spectra to a simple analytical function and the proposed equations, the contributions of the two interactions to items 1-3 may be quantified and compared with the same parameters obtained by fitting experimental spectra. This comparison supports the theory in its broad predictions; however, at low temperatures, the DD contribution to the experimental dispersion amplitude does not increase linearly with concentration. We are unable to deduce whether this discrepancy is due to inadequate analysis of the experimental data or an incomplete theory. A new key aspect of the more comprehensive theory is that there is enough information in the experimental spectra to find items 1-3 due to both interactions; however, in principle, appeal must be made to a model of molecular diffusion to separate the two. The permanent diffusion model is used to illustrate the separation in this work. In practice, because the effects of DD are dominated by HSE, negligible error is incurred by using the model-independent extreme DD limit of the spectral density functions, which means that DD and HSE may be separated without appealing to a particular model.

Flexible Biradicals in Liquid and Supercritical Carbon Dioxide: The Exchange Interaction, the Chain Dynamics, and a Comparison with Conventional Solvents

X-band time-resolved electron paramagnetic resonance (TREPR) spectra of three flexible biradicals of varying chain length and structure were obtained in liquid and supercritical carbon dioxide (CO2) solutions and compared to conventional solvents. For C16 acyl-alkyl biradical 1a, an average spin exchange interaction between the radical centers, J(avg), was obtained by spectral simulation using a simple model for spin-correlated radical pairs (SCRPs) and a small amount of T2 relaxation from a previously established J modulation mechanism. A large solvent effect on J(avg) was observed for the first time, varying by almost 1 order of magnitude from CO2 (J(avg) = -120 +/- 10 MHz) to heavy mineral oil (-11 +/- 3 MHz) for 1a. For C15 bis(alkyl) biradical 1b, spectra obtained under supercritical conditions are only slightly different from those detected in liquid CO2 but differ from spectra taken in benzene. For C10 acyl-alkyl biradical 2a, more emissive spin polarization due to S-T- mixing is observed in CO2 than in benzene. These results are discussed in terms of solvent properties such as dielectric constant, viscosity, and specific interactions. Both chain dynamics and changes to the equilibrium distribution of end-to-end distances can alter J(avg) and the observed ratio of S-T0 to S-T- mixing; however, faster chain dynamics is concluded to be the most likely cause of the observed effects in these systems.

Time resolved electron spin resonance of spin correlated micelle confined radical pairs: shape of the anti-phase structure

A new approach to the theory of the EPR of spin correlated radical pairs (SCRP) is formulated. It is based on spectral exchange methods as applied to the two-site model and takes into account explicitly both the motion of one quantum (transverse magnetization) and two quantum coherences (polarizations) of SCRPs. This innovation allows for the interpretation of anti phase structure (APS) spectral shape asymmetry and for the transformation of the initially created ST0RPM polarization into the APS spectral pattern. The results of this method as applied to micellized spin correlated radical pairs have been compared with the exact numerical solution of the corresponding stochastic Liouville equation and is commented on.