Yu. D. Tsvetkov

Electron-electron double resonance in electron spin echo: Model biradical systems and the sensitized photolysis of decalin

Abstract Model systems, comprising frozen glassy solutions of stabilized radicals and biradicals of the nitroxyl type, have been used to test the applicability of electron-electron double resonance in electron spin echo (ELDOR ESE) in studies of the spatial distributions of free radicals arranged in groups in solids. The method was used to investigate the spatial distribution of alkyl radicals generated by the sensitized photolysis of glassy naphthalene solutions in decalin at 77 K. and detected radical pairs.

Electron dipole–dipole interaction in ESEEM of nitroxide biradicals

Abstract Fourier transform of the standard primary electron spin echo envelope modulation (ESEEM) of nitroxide biradicals in organic glass shows a peak ascribed to electron–electron dipole–dipole interaction. The dipolar couplings obtained in imidazoline biradical and double spin-labeled peptide Trichogin GA IV are in good agreement with pulsed electron–electron double resonance (PELDOR) data. The intensity of this peak is increased substantially in a new three-pulse relaxation-induced dipolar modulation enhancement (RIDME) experiment. This experiment involves the non-resonant partner spin in the formation of ESEEM via a relaxation-induced change of the local dipolar field experienced by the resonant spin.

Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ‐irradiated 2‐methyltetrahydrofuran glass

Three specifically deuterated 2‐methyltetrahydrofurans (MTHF) have been synthesized to delineate the proton (deuteron) arrangement and hence the molecular arrangement around a trapped excess electron in γ‐irradiated glassy MTHF at 77 K. The electron–deuteron anisotropic hyperfine interaction was detected by its modulation of the two‐pulse electron spin echo envelope. Comparison of experimental and simulated modulation patterns shows that the excess electron is located about 3.7 A below the center of the plane of the ring carbons in MTHF on the side opposite the methyl group and that three or four MTHF molecules constitute the first solvation shell. A second moment analysis of the EPR linewidths of the excess electron in the variously deuterated MTHF’s independently supports the same structural conclusions. The shortest electron–proton distance is then ∼3.1 A, which compares moderately well with previous, less accurate ENDOR results. This structure seems somewhat more expanded than that indicated by theore...

Superslow rotations of nitroxide radicals studied by pulse EPR spectroscopy

Abstract The rotation-induced saturation transfer in EPR spectra has been studied in pulse experiments using an electron spin-echo spectrometer supplied with a magnetic field modulation coil. After the action of a saturating pulse (40 nsec width, 5 G amplitude) a rectangular pulse of current is applied to the modulation coil which shifts the resonant magnetic field by 0 to 65 G. This method makes it possible to study the time evolution of the longitudinal magnetization at various parts of an EPR spectrum, the magnetization being detected utilizing a 90–180° pulse sequence. The range of rotation times that can be studied has a lower limit of several microseconds and an upper limit of the order of magnitude of the radical spin-lattice relaxation time T 1 . The saturation transfer in an EPR spectrum for nitroxide radicals in dibutyl phthalate has been investigated at temperatures below 210 K. The effects observed have been analyzed theoretically in detail. Theory coincides with experiment only for the model of rotation by large-angle jumps. The model of continuous diffusion contradicts experimental results. Rotation correlation times can be easily determined from experimental data. The temperature dependence of the rotation time slows down sharply in the vicinity of the glass transition point.

Parameters of quadrupole coupling of 14N nuclei in chlorophyll a cations determined by the electron spin echo method

Modulation effects induced by interaction of an unpaired electron with 14N nuclei of pyrrole rings in the electron spin echo from chlorophyll a radical cations have been investigated. The modulation frequencies have been determined by Fourier analysis of the echo signal decay. The modulation frequencies approximate to the quadrupole resonance frequencies of 14N nuclei.

Echo-induced EPR spectra of nitroxides in organic glasses : model of orientational molecular motions near equilibrium position

Abstract The shapes of echo-induced EPR spectra (i.e of the field dependence of ESE signal intensity) for 15 N nitroxides in organic glasses are shown to be in good agreement with those computed for the model where magnetic relaxation is caused by stochastic orientational motion of a molecule near its equilibrium position.

Ratio analysis of electron spin echo modulation envelopes in disordered matrices and application to the structure of solvated electrons in 2-methyltetrahydrofuran glass

A new analysis for two pulse and three pulse electron spin echo modulation data is presented which is independent of the echo decay function. The experimental ratios of maxima to minima of the echo modulation curve are compared with simulated ones to obtain the number of interacting nuclei, their distances, and their isotropic coupling by a least squares fit. From the best simulation, the empirical echo decay function can be determined and the original experimental data can be simulated. This new method is applied to new three pulse and old two pulse [L. Kevan et al., J. Chem. Phys. 63, 409 (1975)] electron spin echo data on solvated electrons in 2‐methyltetrahydrofuran (MTHF) glass at 77 K. This analysis together with previous second moment data gives a revised model for electron solvation in this matrix based on a statistical orientation of the MTHF ring carbon planes toward the electron. In this statistical configuration model three equivalent MTHF molecules form the first solvation shell with their ri...

Electron spin echo of spin-polarised radical pairs in intact and quinone-reconstituted plant photosystem I reaction centers

Abstract Light-induced spin-polarised P700+A1− pairs in intact and quinone-reconstituted photosystem I reaction centres were studied by electron spin echo (ESE) spectroscopy. The observed strong ESE envelope modulation was attributed to magnetic dipolar and exchange interactions in the pairs. The values of these interactions were derived from Fourier-transformed time traces and appeared to be D = −1.71 ± 0.05 G and J = 0.010 ± 0.015 G, respectively. A magnetic field effect on the radical pair lifetime induced by microwave pumping was observed. The reconstituted 2,3-dibromo-1,4-naphthoquinone was shown to be located in the same (A1) site as the native phylloquinone.

Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ‐irradiated 10M sodium hydroxide alkaline ice glass

Electron spin echo signals associated with trapped electrons in aqueous matrices show a modulation due to hyperfine interaction with the surrounding magnetic nuclei. The three pulse electron spin echo modulation gives information about the next nearest neighbor deuterons surrounding the electron. Analysis of the phase change in the three pulse modulation gives an effective deuteron interaction distance of 3.6 A. The two pulse spin echo modulation depends on both the nearest and next nearest neighbor deuterons. Simulations of this modulation support a structural model in which the number of equivalent nearest neighbor deuterons to the electron is 6 at a distance of 2.1 A and with an isotropic coupling constant of 0.9 MHz (5.8 MHz for protons). The orientation of the water molecules in the first solvation shell is not uniquely determined and two possible models are proposed. In model I there are three first solvation shell water molecules with their molecular dipoles oriented toward the electron so that the...

The effect of microwave pulse duration on the distance distribution function between spin labels obtained by PELDOR data analysis

A method is proposed for obtaining a distance distribution function between spin labels in spin-labeled molecules on the basis of the data of pulsed electron-electron double resonance with regard to the finite duration of microwave (mw) pulses. It is shown that taking into account the finite duration of mw pulses makes it possible to extend the range of the studied distances between spin labels to the region of short distance in which the magnitude of the dipole-dipole interaction becomes comparable to or exceeds the spectrum widths of exciting mw pulses. With frozen glassy solutions of biradicals as model systems with a length between spins of less than 2 nm, the shape for the distance distribution function between labels was obtained, and the value and sign of the exchange interaction were estimated. It is demonstrated that the analysis of the dipole-dipole interaction neglecting the duration of mw pulses can lead to substantial distortions in the shape of the distance distribution function.