Spin‐catalysis phenomena

On the grounds of results from quantum chemical calculations, we introduce spin ‐ catalysis as a new concept, and classify different types of chemical reactions according to this concept. We propose a general definition of spin ‐ catalysis as phenomena in which chemical reactions are promoted by substances which assist in overcoming spin‐prohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst. A number of known phenomena fall into this definition and can be classified according to two main categories: (1) spin‐orbit coupling induced and (2) paramagnetic‐exchange induced spin‐catalysis. Other types of spin‐catalysis are also discussed: (3) processes with participation of substances which assist in (a) photochemical and (b) thermal or electrochemical generation of active particles by energy and—by electron transfer [radicals, diradicals, O 2 ( 1 Δ g ), etc.], reacting further without spin‐prohibition: (4) processes induced by an external magnetic field. Processes (3) and (4) are quite general and well known; their inclusion in the spin‐catalysis classification does not introduce any new findings for the chemical kinetics, but the general features, which unite them with the important catalytic processes of the first two types, serve as a useful guide in catalysis theory. Few models of spin‐catalytic processes have been simulated for the purpose of illuminating the principles of spin‐catalysis; the cis ‐ trans isomerization of ethylene catalyzed by molecular oxygen, the external heavy atom effect in ethylene photochemistry, and some others. © 1996 John Wiley & Sons, Inc.