ions was studied by Margulis et al.156 These authors used solvent mixtures of water or ethanol with glycerol to obtain very highly viscous solvents. Varying the temperature and the solvent composition, they showed that the MFEs are determined by the value of T/q. MFEs on the free radical yield (determined by flash spectroscopy) or on the permanent bleaching reaction (determined by continuous photolysis) became detectable only at T/q values smaller than 10 K/cP. At room temperature this corresponds to viscosities larger than 20 cP. In the systems investigated by Margulis et (cf. Table 8). The magnetic field causes an increase of free radical yield, which results from a suppression of tripletsinglet transitions by a magnetic field. The MFD curves are of the case 2 type, which is taken as evidence for a contribution of the relaxation mechanism. Remarkably, Margulis et al. also found MFEs on the second-order recombination rate constant in the case of riboflavin semiquinone radicals and benzophenone ketyl radicals. The second-order bulk recombination rate is slowed down by the magnetic field, demonstrating that F pairs behave rather like geminate triplet pairs, which has also been confirmed in many CIDNP investigations. MFEs in the photochemistry of quinoline and isoquinoline derivatives have been reported by Hata et al.318-320 Photochemical hydrogen abstractions by the ring nitrogen from the solvent ethanol are believed to be the primary reactions in the photoreactions of 1isoquinolinecarbonitrile (7) and 4-methylquinoline-2carbonitrile (9). For the reaction of 7 (eq 46) the

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Magnetic field effects in chemical kinetics and related phenomena

Data Reduction And Error Analysis For The Physical Sciences

Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a (28)Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure (28)Si material (residual (29)Si concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T(2) up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T(2) of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits.

Electron spin coherence exceeding seconds in high-purity silicon

Alzheimer's Disease Amyloid-β Binds Copper and Zinc to Generate an Allosterically Ordered Membrane-penetrating Structure Containing Superoxide Dismutase-like Subunits

Electron paramagnetic resonance (EPR) spectroscopy provides a variety of tools to study structures and structural changes of large biomolecules or complexes thereof. In order to unravel secondary structure elements, domain arrangements or complex formation, continuous wave and pulsed EPR methods capable of measuring the magnetic dipole coupling between two unpaired electrons can be used to obtain long-range distance constraints on the nanometer scale. Such methods yield reliably and precisely distances of up to 80 A, can be applied to biomolecules in aqueous buffer solutions or membranes, and are not size limited. They can be applied either at cryogenic or physiological temperatures and down to amounts of a few nanomoles. Spin centers may be metal ions, metal clusters, cofactor radicals, amino acid radicals, or spin labels. In this review, we discuss the advantages and limitations of the different EPR spectroscopic methods, briefly describe their theoretical background, and summarize important biological applications. The main focus of this article will be on pulsed EPR methods like pulsed electron-electron double resonance (PELDOR) and their applications to spin-labeled biosystems.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S003358350700460X

Long-range distance determinations in biomacromolecules by EPR spectroscopy.

Studies on the effects of chemically induced dynamic nuclear and electron polarizations (CIDNP and CIDEP), and magnetic effects in radical reactions, have given rise to a new rapidly-progressing field of chemical physics. It came into being about ten years ago and has been attracting the ever-growing attention of researchers in related areas. The present book is a fairly all-embracing review of the state of affairs in this field. The book presents the physical background (both theoretical and experimental) of CIDNP and CIDEP, of the effects of an external magnetic field and magnetic nuclear moment (magnetic isotope effects) on radical reactions in solutions. Great attention has been paid to the application of chemical spin polarization and magnetic effects to solving various problems of chemical kinetics, structural chemistry, molecular physics, magnetobiology, and radiospectroscopy. The book will be useful for physicists, chemists and biologists employing CIDNP, CIDEP and magnetic effects in their investigations, as well as for researchers in related fields of chemical physics. The book can be also recommended for postgraduates and senior undergraduate students.

Spin polarization and magnetic effects in radical reactions

1. Introduction.- 2. Theory of Spin Exchange.- 2.1 Exchange Interaction Between Two Paramagnetic Particles.- Dependence of Interaction Energy on Spin.- Spin-Hamiltonian of Exchange Interaction.- The Nature of Exchange Interaction. The Two-Electron System.- Interaction Between Many-Electron Systems. Delocalization and Spin Polarization.- Anisotropy of Exchange Interaction.- Semi empirical Estimates of Exchange Integral.- 2.2 Qualitative Description of Spin Exchange Process.- Spin Exchange.- Spin Exchange in a Two-Electron System.- Spin Density Matrix.- Spin Motion Due to Exchange Interaction.- 2.3 Formal Kinetics of Spin Exchange in Magnetically Dilute Solutions.- Binary Collision Approximation.- Rate of Bimolecular Spin Exchange Process.- Kinetics of Spin Exchange for Particles with Spins S = 1/2.- Kinetic Equations for Free Radicals Showing Hyperfine Structure (HFS) in Their ESR Spectra.- 2.4 Spectroscopic Manifestations of Spin Exchange.- Spectral Diffusion Due to Spin Exchange.- Shape of ESR Spectra.- Slow Exchange.- Fast Exchange.- Other Methods of Measuring the Rate of Spin Exchange.- Effect of Saturation.- Electron-Electron Double Resonance (ELDOR).- Electron Spin Echo.- 2.5 Theory of Sudden Collisions.- 2.5.1 Model of Sudden Collisions.- Basic Assumptions.- Equations for the Estimation of xc.- 2.5.2 Kinetic Equations.- Steady-State Equation for the Operator of Collision Efficiency.- 2.5.3 Spin Exchange Between Radicals.- Model System.- Spin Exchange Between Radicals in the Presence of Hyperfine Interaction.- 2.5.4 Spin Exchange Between Radicals with Anisotropic Interaction.- Rotation of Partners at the Moment of Their Contact.- Orientation Relaxation of Particles in Intervals Between Reencounters.- 2.5.5 Spin Exchange of Radicals with Other Paramagnetic Particles.- Spin Exchange Between Radicals and Spins SB with Long Times of Paramagnetic Relaxation.- Shape of ESR Spectra.- The Influence of the Relaxation of Spin SB on the Rate of Exchange.- Anisotropy of Spin Exchange.- 2.5.6 Spin Exchange in Biradicals.- The Model.- Kinetic Equations.- The ESR Spectrum of Nitroxide Biradicals.- 2.6 The Model of Diffusive Passage.- The Model.- An Equation for the Operator of Collision Efficiency.- The Kinetic Equations.- The Spin Exchange Between Two Kinds of Particles with Spins SA = SB = 1/2.- Comparison of Results Obtained from Models of Sudden Collisions and Those of Diffusive Passage.- 3. Experimental Measurement of Spin Exchange Rate.- 3.1 Experimental Measurement of Spin Exchange Rate Constants from ESR Spectra.- 3.1.1 Exchange Between Identical Particles with S = 1/2.- Doublet Spectrum with a Zero Initial Width.- Complex Spectra.- Determination of Rate Constants from Exchange Broadening of Complex Spectra.- Determination of Rate Constants from Exchange Shift of the Lines of Complex Spectra.- Determination of Rate Constants from Exchange Narrowing of Complex Spectra.- Peculiarities of Exchange Narrowing in Concentrated Solutions.- Comparison of Spin Exchange Rate Constants Measured by Different Techniques.- 3.1.2 Exchange Between Different Particles.- 3.2 Other Methods of Measuring the Spin Exchange Rate Constants.- 3.2.1 Continuous Wave Saturation Method.- 3.2.2 Electron-Electron Double Resonance.- 3.2.3 Electron Spin Echo.- 3.3 Separation of Exchange and Dipole-Dipole Contributions to Concentration Broadening of ESR Lines.- Theoretical Estimation of Dipole Broadening.- Experimental Estimation of Dipole-Dipole Contribution at Low Viscosity.- Separation of the Cases of Exchange and Dipole-Dipole Broadening According to Viscosity and Temperature Dependences.- 3.4 Separation of the Cases of Weak and Strong Exchange.- 4. Spin Exchange in Chemistry and Biology.- 4.1 Study of Diffusion Collisions in Solutions.- Hydrodynamic Model of Diffusion Collisions.- Potentialities of Spin Exchange.- 4.1.1 Collisions in Single-Component Solvents.- Spin Exchange Between Radicals.- Spin Exchange Between Free Radicals and Paramagnetic Complexes.- 4.1.2 Collisions in Complex Systems.- Binary Solvents.- Liquids Near Critical Points.- Polymeric and Heterogeneous Systems.- Collisions of Macroradicals.- 4.1.3 Study of Intramolecular Collisions of Paramagnetic Fragments.- 4.2 Estimation of the Energy of Exchange Interaction During Collisions.- 4.2.1 Free Radicals.- 4.2.2 Spin Exchange with Metal Complexes.- 4.2.3 Discussion of Results.- 4.3 Collisions of Charged Particles.- 4.3.1 Experimental Results.- Spin Exchange in Solutions of Anion Radical (S03)2N02-.- Spin Exchange Involving Charged Nitroxide Radicals.- Spin Exchange Between Anion Radicals in Organic Solvents.- 4.3.2 Comparison of Experiments on Spin Exchange with the Diffusion Theory of Collisions of Charged Particles in Liquid.- 4.3.3 Study of Collisions Between Aquo Complexes of Transition Metals.- Role of Counterions in Collisions of Aquo Complexes of Vanadyl.- An Example of the Process with Weak Influence of Electrostatic Repulsion.- 4.4 Biological Applications of Spin Exchange.- Paramagnetics Used as Spin Probes.- 4.4.1 Measurement of Binary Collision Rate of Molecules in Biological Systems.- 4.4.2 Study of Location of Paramagnetic Centers in Biopolymers.- Quantitative Interpretation of the Values of Kg for Spin Exchange Processes Involving Paramagnetic Centers of Biopolymers.- 4.4.3 Development of Individual ESR Spectra of Spin Labels via Spin Exchange.- 4.4.4 Spin Exchange Titration.- Kinetics of Reactions Involving Oxygen Molecules.- Coordination of Paramagnetic Ions by Macromolecules.- 4.4.5 Other Applications.- 4.5 Conclusion.- List of Frequently Used Notations and Abbreviations.- References.

Spin exchange : principles and applications in chemistry and biology

The theory of electron spin-echo signal decay resulting from dipole-dipole interactions between paramagnetic centers in solids

Historical introduction transition metals organic radicals gas phase EPR radicals formed electrochemically radicals formed by radiation applications to biological systems spin-spin interactions time-domain EPR EPR imaging development of EPR instrumentation other EPR detection methods prospects.

Foundations of Modern EPR

Keeping in mind ion-radical pairs in a photosynthesis reaction centre first of all, we calculated free induction and spin echo (ESE) signals for an ensemble of radical pairs which initially start in a singlet state. It was shown that the intensity of signals should oscillate depending on the time interval τ between the start of a pair and a microwave pulse forming free induction (FI) or between the start of a pair and the first of two microwave pulses forming primary ESE signal. ESE phase of spin-correlated pairs does not coincide with the corresponding ESE phase of radical pairs in thermal equilibrium. One should also note an interesting feature of FI: immediately after the microwave pulse free induction signal equals zero, and non-zero free induction signal appears only due to spin evolution. This behaviour formally resembles the situation occurring when the primary ESE is formed: a light pulse which creates spin-correlated radical pairs acts as the first microwave pulse in conventional spin echo experiments. Analysis of FI and ESE in experiments on pulse photolysis or radiolysis may provide useful information about the contribution of spin-correlated radical pairs.

Kev M. Salikhov

Papers

Spin polarization and magnetic effects in radical reactions

Spin exchange : principles and applications in chemistry and biology

The theory of electron spin-echo signal decay resulting from dipole-dipole interactions between paramagnetic centers in solids

Foundations of Modern EPR

Peculiarities of free induction and primary spin echo signals for spin-correlated radical pairs