E. G. Bagryanskaya

Inclusion Complexes of Nitroxides of Pyrrolidine and Imidazoline Series with Cucurbit[7]uril

Inclusion of nitroxides into the cavity of molecular nanocontainers may prevent or significantly retard nitroxide reactions with cellular reductans and increase nitroxide lifetime. This paper is aimed at studying possible applications of cucurbit[7]uril (CB7) as a molecular nanocontainer for pH-sensitive nitroxides probes. Inclusion complexes of CB7 with different nitroxides--3-aminomethyl-proxyl (AMP), 3-hydroxymethyl-proxyl (HMP), 4-amino-2,2,5,5-tetramethyl-2,5-dihydroimidazole-1-oxyl (ATI), and 2,2,4,5,5-pentamethyl-2,5-dihydroimidazole-1-oxyl (MTI)--were studied. It was found that reversible formation of inclusion complexes AMP@CB7, AMPH(+)@CB7, HMP@CB7, ATIH(+)@CB7, and MTIH(+)@CB7 is accompanied by a decrease of hyperfine interaction (HFI) constant on nitrogen of the nitroxide group and by a 5-7-fold increase of rotational correlation time monitored by electron paramagnetic resonance (EPR). The binding constants of nitroxide@CB7 complexes were determined, and the influence of alkali metal ions and pH on the equilibrium between free and encapsulated nitroxides was studied. The EPR spectra of CB7 mixtures with protonable nitroxides were found to be more sensitive to pH changes than the spectra of pure nitroxides, and the apparent pK of these mixtures was found to increase with CB7 concentration. The nitroxide@CB7 complexes showed higher resistance to chemical reduction with ascorbic acid compared to free nitroxides. Thus, CB7 can be used for the improvement of functional properties of nitroxides.

Electron-nuclear spin transitions in free phosphonyl radicals detected via CIDNP

Electron-nuclear spin transitions in short-lived phosphonyl radicals have been investigated experimentally by nuclear magnetic resonance detection of nuclear polarization in diamagnetic reaction products in low magnetic fields (15–80 mT) for31P-centered radicals formed in laser photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide and 2,4,6-trimethylbenzoylphosphonic acid dimethyl ester. A theoretical model on the basis of the numerical solution of the kinetic equation for the density matrix of a radical with one nonzero hyperfine coupling constant has been employed to study the main peculiarities of this effect and to account for the data quantitatively.

Electron spin relaxation by spin-rotation interaction in benzoyl and other acyl type radicals

In various studies of the spin dynamics in radical pairs, benzoyl-type radicals have been one of the two paramagnetic pair species. Their electron spin relaxation has been assumed to be slow enough to be neglected in the data analysis. This assumption is checked by measuring the electron spin relaxation in a sequence of three acyl radicals (benzoyl, 2,4,6-trimethylbenzoyl and hexahydrobenzoyl) by time-resolved electron paramagnetic resonance spectroscopy. In contrast to the assumed slow relaxation, rather short spin-lattice relaxation times (100–400 ns) are found for benzoyl and 2,4,6-trimethylbenzoyl radicals from the decay of the integral initial electron polarization to thermal equilibrium at different temperatures and viscosities. The relaxation is induced by a spin-rotation coupling arising from two different types of radical movements: overall rotation of the whole radical and hindered internal rotation of the CO group. The predominant second contribution depends on the barrier of the internal rotation. The obtained results are well explained in the frame of Bull’s theory when using a modified rotational correlation time τJ. The size of the spin-rotation coupling due to the internal CO group rotation in benzoyl radicals is estimated to be |Cα|=1510 MHz.

Chemically induced spin polarization of ion-radicals in low magnetic fields studied by double switched magnetic field CIDNP

The n double switched external magnetic field CIDNP technique has been used to study spin dynamics and chemically induced n spin polarization of ion-radicals in low magnetic fields. It is shown that degenerate electron exchange n leads to decay of CIDNP oscillations. The analysis of the phase of the oscillations provides information n about electron spin polarization in low magnetic fields. n It is concluded n that the exchange interaction n in the radical pairs under study is positive.

Influence of nitroxides on CIDNP of micellised radical pairs and short-lived biradicals

The effects of stable nitroxide radicals on stimulated nuclear polarization (SNP) and chemically induced dynamic nuclear polarization (CIDNP) in short-lived consecutive biradicals and radical pairs in homogeneous solutions as well as spin-correlated radical pairs in micelles were studied in high and low magnetic fields. It is shown that experimentally observed effects of nitroxide additions on CIDNP and SNP can be well simulated taking into account only the increase in the rates of relaxation in the paramagnetic species constituting radical pairs or biradicals. Effects of coherent spin evolution in three-spin systems under study seem to be of negligible importance.

The investigation of consecutive micellised radical pairs by the method of stimulated nuclear polarization

The main features of stimulated nuclear polarization (SNP) formation in consecutive micellised radical pairs (RP) have been studied both experimentally and theoretically. The photolyses of dibenzyl ketone (DBK), α-methyldibenzyl ketone (MDBK), and 2,2′,4,4′-tetramethyldimethyl acetone dicarboxylate (PEST) in aqueous solutions of alkylsulfate micelles have been used as model photoreactions. The rate constants of relaxation and RP escape from micelles have been obtained. SNP spectra of consecutive micellised RPs have been compared to the spectra of consecutive biradicals with similar structure of radical centers and chain length equal to the inner diameter of the micelle. It has been shown that the significant difference in the SNP spectra of micellised RPs and biradicals is determined by the difference in their molecular dynamics and rate of reencounters.