Organic Radicals in Solution

BY B. J. TABNER

1 Introduction

My report for Volume 11 A covers the literature published between June 1985 and May 1987. This is double the period covered in my previous report.

It has become apparent during the preparation of this report that the study of organic radicals in solution by e.s.r. spectroscopy continues to retain a considerable interest. This is not only due to the continual development of new areas of study but also reflects recent advances in instrument technology (for e.g., the development of the spin echo technique). Consequently e.s.r. is now being applied to study new problems, particularly those involving transient species. In addition to these new areas, e.s.r. spectroscopy will always play an important role in the investigation of radicals present as reaction intermediates and in the study of the kinetics of their reactions. There is also a wealth of structural information that can be obtained from a study of e.s.r. spectra. With several new techniques available for the analysis of complex hyperfine patterns, particularly ENDOR spectroscopy, it is now possible to examine systems of increasing size and complexity. Thus, with the unpaired electron able to act as a probe, information can be obtained on the structure of larger and more complex systems. The hyperfine data obtained in such investigations not only reveals information on the unpaired electron distribution throughout the π-system but is also available to obtain information on such features as substituent effects and conformational preferences.

Readers interested in ENDOR spectroscopy and its methodology will find a useful review in Volume 10 B of these reports. This latter review covers several recent developments as well as instrumental aspects and advanced techniques. An important nucleus in magnetic resonance is 13C as it provides a means of obtaining information on those positions in a molecule to which no proton is attached. Its hyperfine splitting constant is often quite sensitive to structural changes. Normally 13C-enriched samples are required but deuteriation provides a means of studying 13C ENDOR when the nucleus is present in natural abundance.

It is pleasing to see that the text by Wertz and Bolton, originally published in 1972, has now been reprinted, and also that collections of papers presented at several conferences have now been published. Since my last report the plenary lectures and papers presented at the 22nd AMPERE Congress held at Zurich, and at the Faraday Discussion on 'Radicals in Condensed Phases' have both appeared. In a new innovation the papers presented at the 18th International Conference on ESR in Organic and Bio-organic Systems, held at Leeds, have been published in a special issue of the Faraday Transactions.

Several reviews have appeared which readers interested in organic radicals will find valuable. Symons and Cox have compared the power of µSR and ESR in probing structures of radicals. Due to the greater zero-point energy of muonium (relative to hydrogen) large hyperfine isotope effects are observed in µSR. There appears to be the possibility of preparing some novel species, such as muonated radical cations and radical anions. Another technique that has developed greatly over the last few years is that of ESR imaging. In two reviews Ohno describes the technique and some of its applications. This technique has applications to the study of diffusion of organic radicals in polymers and to the study of the distribution of radicals after the mixing of two flowing reagents.

The application of e.s.r. spectroscopy to the study of small strained radicals has been reviewed by Ingold and Walton. The use of the technique to study the rearrangement reactions of such radicals, and to provide information on their configuration, is described.

The growing interest in identifying radicals very shortly after their formation and following their subsequent reactions is the subject of a review, by McLauchlan, of flash photolysis e.s.r. One of the main continuous-wave techniques for obtaining e. s. r. spectra of transient radicals produced by flash-photolysis involves time-integration. A variant of this method (MISTI), involving switching on the microwave field after radical creation, provides a method for enhancing signals. A further development in the study of radicals very shortly after their creation, and their subsequent recombination reactions, involves the study of the 1: Organic Radicals in Solution effects of magnetic and microwave fields. The problems encountered as a result of non-uniform concentration distributions following photochemical radical generation have also been discussed.

The post-experimental treatment of spectroscopic data continues to be aimed at resolution enhancement (so that information can be obtained on long-range small hyperfine couplings) employing the Fourier transform method, and at the extraction of hyperfine splitting constants in complex or poorly resolved spectra with the aid ofmicrocomputers.

2 Carbon-centred Radicals

As on previous occasions I have divided this, the first part of my report, into two sections. In the first section I shall cover 'simple' alkyl radicals (including cyclic alkyl radicals) and in the second section I shall cover delocalized radicals. Once again the investigation of alkyl radicals by e.s.r. spectroscopy covers a wide range of interests. The study of the structure and conformation of these radicals continues, as does that of their formation by addition and...