Spin exchange between paramagnetic particles is one of the few elementary bimolecular processes in solutions that is readily accessible to detailed experimental studies and to strict theoretical description. Due to this fact spin exchange is now widely used in solving various problems of chemistry, physics, and molecular biology. Spin exchange allows one to study in detail elementary acts of collisions of molecules. By use of spin exchange one can obtain quantitative information on collision rates of molecules in solutions, steric hinderance in collisions, electrostatic interactions in collisions of charged particles, and overlap of electron orbitals in collisions. Par ticularly valuable is the possibility of using spin exchange to study col lisions in such complex systems as polymer solutions, multicomponent mix tures, heterogeneous mixtures, and especially biological systems. This monograph contains a comprehensive description of spin exchange in solutions. It consists of four chapters. The introductory chapter contains a brief summary of the physical principles of spin exchange and outlines the range of its main applications. In this chapter a historic background of research in this field is also given. The second chapter contains a com prehensive presentation of the theory of spin exchange including that of its spectroscopic manifestations. The third chapter is devoted to a detailed description of the technique of experimental studies of spin exchange. The last - the fourth - chapter summarizes the results of experimental studies of spin exchange and considers various applications of spin exchange in che mistry, molecular physics, and biology.
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.