Electron spin relaxation of radicals in weak magnetic fields

Abstract

The review summarizes the results of studies on specific features of spin relaxation of radicals in liquids in weak magnetic fields.Key words: electron spin relaxation, weak magnetic field, free radicals, electron paramag netic resonance, spin polarization, chemically induced dynamic electron polarization.Many photochemical reactions involve radical stages. The interaction of external constant and alternating mag netic fields with electron and nuclear spins of intermedi ate radicals is the reason for the magnetic field and spin effects in radical chemical reactions. One of the key pa rameters determining the magnitude of the magnetic field effects is the spin relaxation rate of the intermediate radi cals. 1,2 In the case of long lived radical pairs (RPs), e.g., RPs in micellar solutions, 2 radical ion pairs, 3 biradicals, 4 etc., the spin relaxation is one of the main processes de termining the probability of RP recombination. The spin relaxation rate is an important parameter used in calcula tions of the magnetic field effect, 2 EPR spectra, 5 kinetics of chemically induced dynamic electron polarization (CIDEP), 1,6 optically detected EPR spectra, 3 stimulated nuclear polarization (SNP) spectra and kinetics, 7 etc. Usu ally, the magnetic field effects and the CIDEP, SNP, and optically detected EPR spectra are calculated assuming additive contributions of (i) electron relaxation in indi vidual radicals due to the interaction between an electron in a given radical and surrounding nuclei (modulation of anisotropic or isotropic HFI constant) or with the angular momentum of this radical (spin rotational relaxation) and (ii) dipole dipole and exchange relaxations due to the interaction between electrons in the radicals within the RP. Methods for the description of electron relaxation in the interacting radicals are not available as yet.To date, the electron spin dynamics and relaxation of free radicals have been well studied in strong magnetic fields that much exceed typical HFI constants of organic radicals. Various experimental techniques were elaborated and detailed theoretical concepts were proposed, which makes it possible to determine the spin relaxation rates by analyzing experimental data. 8, 9 Recently, there has been increasing literature on the behavior of radicals in weak magnetic fields whose magnitude is at most equal to the HFI constants of the radicals. Spin relaxation in weak magnetic fields has long been poorly studied both experi mentally and theoretically. Experimental studies are scarce because of relatively small number of methods of measur ing the relaxation times of radicals in weak magnetic fields. L Band time resolved EPR investigations of the radicals characterized by high HFI constants (30-70 mT) allow one to obtain information on the spin relaxation of radi cals in weak and zero magnetic fields. 10,11 Because the sensitivity of c.w. EPR in weak magnetic fields is low (~10 15 spins per sample), in this case information on elec tron spin relaxation can be extracted using indirect mag netic r...

Heisenberg spin exchange effects of nitroxide radicals on Overhauser dynamic nuclear polarization in the low field limit at 1.5mT

“…In Eq. (3) the nitrogen Larmor frequency is neglected and while the equations give the same result described by Guiberteau and Grucker [16] the simpler formalism of Fedin et al is used [38]. At high magnetic fields C 2 and C 3 converge to zero.…”

Interaction of Cyclopentano Cucurbit[6]uril with Alkaline Earth Cations and Supramolecular Assemblies with Aid of [ZnCl4]2–

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