V. F. Tarasov

Electron spin exchange in micellized radical pairs. I. 13C low field chemically induced dynamic nuclear polarization (CIDNP) and 13C radio frequency stimulated nuclear polarization (SNP)

Abstract Electron spin exchange (ESE) in micellized radical pairs (RPs) has been considered. It is shown that the ST_ mechanism and ESE-induced paramagnetic relaxation provide observable low-field CIDNP, high-field SNP and CIDEP spectra. The best simulation of the spectra for 13 C-labelled RPs was achieved for the following ESE potential parameters: an exchange constant in the RP at the recombination radius of about −7 × 10 9 rad s −1 and a parameter λ of the exchange potential decrease of about 0.5 A.

High-frequency tunable EPR spectroscopy of Tm3+ ions in synthetic forsterite

The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from −0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr3+ ion Δs= 66.7 GHz and between spin sublevels of the Cr3+-Cr3+ dimer Δd1 = 71.5 GHz and Δd2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr3+ ions with an Mg2+ vacancy between them replacing three Mg2+ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters Jz= 0.47 GHz and Jt = 0.79 GHz.

Electron spin exchange in micellized radical pairs. III. 13C low-field ratio frequency stimulated nuclear polarization spectroscopy (LF SNP)

Abstract LF (low-field) SNP spectra are reported from the carbonyl carbon in dibenzyl ketone photolyzed in an aqueous micellar solution with sodium dodecyl sulfate. Spectra obtained using resonant radiofrequencies of 0.33 and 1.53 GHz are affected by adiabatic transitions within the region of ST_level-crossing of the radical pair by flip-flop transitions in the corresponding weak magnetic fields. Numerical simulation indicates that all the observable features of the spectra are accounted for by a combination of these transitions and spin relaxation induced by the electron spin exchange interaction. Unexpectedly, no additional broadening caused by the electron dipole-dipole or other anisotropic interactions is evident in them.

Electron spin exchange in micellized radical pairs. II. Magnetic field and magnetic isotope effects in multinuclear pairs

Abstract External magnetic field dependences of geminate reaction yields (MARY) are obtained for some micellized RPs. The dependences may be divided into three types. Type I corresponds to pairs with prevalence of HFI and ST_ mechanisms of intersystem crossing, whereas type II refers to a paramagnetic relaxation mechanism. Type III incorporates features of the first two types. The effects of the spin exchange interaction and the nature of the relaxation are discussed.

Electron spin polarization in an excited triplet-radical pair system: Generation and decay of the state

A computational model to simulate electron spin polarization in the three-spin-1/2 system composed of the molecular excited triplet state of (tetraphenylporphinato)zinc(II) (ZnTPP) and the doublet ground state of the 3-(N-nitronyl-notroxide) pyridine (3-NOPy) stable radical is proposed. The model is based on numerical solutions of the stochastic Liouville equation for the diffusively rotating system where the magnetic dipolar, isotropic Heisenberg exchange, and anisotropic Zeeman electron spin interactions are taken into account in a full measure, whereas the intersystem crossing processes between the singlet and triplet states of ZnTPP are considered in terms of kinetic equations for the relevant spin density matrices. Additional longitudinal and transversal paramagnetic relaxation caused by relative rotation motions of the ZnTPP and 3-NOPy moieties is taken into consideration in the form of the generalized relaxation operator.

Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy

AbstractDivalent and trivalent chromium ions Cr2+ and Cr3+ replacing magnesium ions at octahedral positions in Mg2SiO4: Cr and Mg2SiO4: Cr: Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65–230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed-valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is

Investigation of chemical and spin dynamics in micellized radical pairs by time-resolved stimulated nuclear polarization. Theory and experiment

P_{O_2 }

EPR study of the effect of partial pressure of oxygen in the growth atmosphere on concentration of chromium centers in synthetic forsterite

= 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr2+ ions are distributed over the M1 and M2 positions in a ratio of approximately 2: 1. A change in the oxygen partial pressure