Magnetic field effects in chemical kinetics and related phenomena

Abstract

Although the interest in experimental evidence of magnetic field effects (MFEs) on the kinetics of chemical reactions, which might be characterized by the term magnetokinetics , has a long tradition, an impressive evolution of the field took place only after the discovery and understanding of nuclear and electronic spin polarization phenomena during chemical reactions (CIDNP, CIDEP) in the late 1960s. The so-called radical pair mechanism lying at the heart of these phenomena turned out to be a most valuable key for systematically tracing out MFEs on chemical yields and kinetics. Nevertheless one should be aware that other mechanisms, too, with pairs of triplets, triplet-doublet pairs, or individual triplets, which originated at about the same time and were initially developed for explaining magnetic phenomena on luminescence in organic solids, also have their implication on chemical, particularly on photochemical, kinetics. Phenomenologically, the basic mechanisms of magnetic-field-dependent reaction mechanisms may become apparent in fields and systems as different as the gas phase, the solid and liquid states, interfaces, and microheterogeneous systems such as micelles and in billogical systems. In all of these applications they have specific experimental and theoretical characteristics. Also, the techniques applied to study magnetokinetic phenomena span a large variety, ranging from magnetic resonance detection of spin polarization (CIDNP, CIDEP, ODMR) through simple detection of magneticfield-dependent reaction yields and magnetic isotope effects (MIE) to reaction-yield-detected magnetic resonance (RYDMR). Thus the field of magnetokinetic chemical and related physical phenomena appears as a tree with several roots and many branches. Although each of these branches has been reviewed from time to time (cf. Table l), most of the treatments have been rather specialized, and it is not easy to provide oneself with a broad and general view of the scope, objectives, and achievements of the field. Thus we have found it worthwhile to write this survey, developing the different aspects from a fairly general point of view (cf. section 11), and to review, as comprehensively as possible, the original experimental (section IV) and theoretical (section V) work published since the early 1970s, providing whenever possible a systematic compilation in the form of tables. Furthermore, in section I11 an outline of the various experimental techniques applied in the field is given. Of course, the goals of completeness and compactness were not attainable without compromise. Thus the large field of chemically induced spin polarization phenomena would have been beyond the scope of this review. We have, however, attempted to include those theoretical papers in the field that have a general bearing on the understanding of magnetokinetic effects in general. We felt that, especially where photochemistry is concerned, the borderline between truly chemical and purely physical phenomena should not be defined too formally, since from the mechanistic and theoretical point of view they may be closely related. In order to account for this we included what has been termed related phenomena in the title of this review. Of course. the problem of delimitation cannot be solved without arbitrariness. The more photophysical aspects are mainly to be found in the sections on gas-phase and solid-state phenomena. In the solid state our attention has been mainly directed on work with organic molecular crystals. Only some representative references on inorganic solids and semiconductors are given. We hope that this review may provide a welcome guide to the present body of literature on magnetokinetics, that it may help those working in the field to assess the achievements of current original work, and that it may be a useful framework of orientation for those who want to get into it or get an impression of the present scope of magnetokinetics.