I. Introduction 603 II. Spin Catalysis in Three Spin Systems 604 A. Spin Wave Functions and Spin States of Triads 604 B. Spin Catalysis: How It Functions 605 C. Two Mechanisms of Spin Catalysis 606 III. Spin Catalysis by Radicals 606 A. Radicals as the Spin Catalysts 606 B. Biradicals and Triradicals 608 IV. Paramagnetic Ions as the Spin Catalysts 608 V. Cis−Trans Isomerization 610 VI. Dynamics of Spin Catalytic Processes 610 A. Static Model and Quantum Beats 611 B. Dynamic Model 611 VII. Conclusion 611 VIII. Acknowledgments 612 IX. References 612

The spin catalysis of chemical reactions is a physical phenomenon of spin transformation of chemically reactive species induced by the exchange interaction of these species with external spin carriers. It manifests itself in the radical pair recombination, cis-trans isomerization of molecules with double bonds, recombination of spin-polarized and spin-aligned hydrogen atoms at cryogenic temperatures, etc. The theory and quantitative kinetics of spin catalysis in three-and four-spin systems are considered. Quantitative hierarchy of spin carriers with respect to their spin catalytic efficiency is discussed.

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