Electron Spin Resonance

Symons, in the last review of ESR in the AnnualReview of Physical Chemistry (1), has already noted that the vast applications ofESR in the fields of chemistry, physics, and biology render it virtually impossible to attempt anything like a full coverage of the literature. I have, therefore, also adopted the approach of selecting topics that are primarily of interest to me, but have found that space and time limitations have made it necessary to reduce even these. The main coverage given may be roughly described as the broad overlap region between the ESR of liquids and spin-relaxation phenomena. However, other topics directly or indirectly related (sometimes perhaps only by the author's imagination) are also included. I have emphasized more recent developments and some recent theoretical developments, including the new theoretical and experimental results on ESR spectra for slow tumbling radicals, rotational reorientation in liquids and solids, spin relaxation theory and line­ width studies, ion pairs, the new phenomenon of chemically induced electron polarization (CIDEP), and the (probably) related old one of Heisenberg spin exchange, saturation and double resonance studies, liquid crystals, and finally gas phase ESR. The most recent detailed review of ESR relaxation in liquids has been given by Hudson & Luckhurst, who covered the literature through 1 967 (2). The literature surveyed here covers primarily the period 1969 through 1 97 1 . Virtually no coverage has been given to the important applications in structural studies and hyperfine interactions, triplet states and triplet dynamics, radicals in (and adsorbed on) solids, and biological studies. Part of the problem of dealing with the vast and growing l iterature in magnetic resonance may be alleviated by the appearance of a new quarterly journal Magnetic Resonance Review (3), which will survey this literature. In particular, Bolton has already covered the literature of 1970 for ESR in solution (4) and it will continue to be done. Recent international activities in ESR include the second symposium on ESR spectroscopy at the University of Georgia, December 7-9, 1970 (5). NATO Summer School Lectures on ESR Relaxation in L iquids held in Norway in 1 971 are to appear in book form (6) and are frequently referred to in this review. A most important publication has been the book by Abragam & Bleaney (7), which provides very extensive coverage of ESR of transition ions and