Epr and the History of the Thianthrene Cation Radical

Abstract

This chapter is concerned with one radical species only, the thianthrene cation radical, Th + . I shall recount the history of the discovery of the structure of Th + not only because it illustrates how EPR was used successfully after initial uncertainties and errors, but also because Th + , with its structure known, was able to play an important role later in studies of reactions of aromatic cation radicals with nucleophiles. I shall not go into those studies, however.The use of EPR spectroscopy to elucidate the structure of aromatic cation radicals began blossoming in the mid-to-late 1950s. It is interesting to note that some of the earliest publications on the subject appeared in the Journal of Chemical Physics, illustrating that the quest to apply EPR to radical structures was still mainly in the hands of physical chemists (1). Perhaps the reason for that was that physical chemists had the ability to build their own spectrometers, or perhaps that organic chemists were not yet acquainted with the powerful tool. It was known then that cation radicals could be formed by treating aromatic compounds with Lewis acids, such as antimony pentachloride, or by dissolving them in concentrated sulfuric acid. Possibly the first to report on EPR and this phenomenon were Hirshon, Gardner and Fraenkel (HGF), who found paramagnetic resonance absorptions with a number of compounds, among which were thiophenol, diphenyl disulfide, and thianthrene (2). It had been shown much earlier by Stenhouse (3) that thiophenol, in forming a purple solution in H2SO4, was oxidized to diphenyl disulfide. This behavior was described in careful detail, subsequently, by Fries and Volk (4), who showed that the diphenyl disulfide was next converted into thianthrene. Fries and Volk deduced that the purple color of solutions of thiophenol and diphenyl disulfide in concentrated H 2 S0 4 was, in fact, caused by the thianthrene, the "magnificent purple" color of whose H 2 S0 4 solution had been already reported by Stenhouse (3). HGF were aware of these early works, and surmised, therefore, that the paramagnetic spectra of the thio compounds in H 2 S0 4 were caused by similar species. The paramagnetic spectra themselves were not given and the report ends with "we are attempting to ascertain what molecular species are responsible for the unusual paramagnetic resonance spectra that are observed in the thio compounds". As far as I am aware, nothing further was reported by HGF, however. Wertz and Vivo were shortly, thereafter, the first to report the characteristic five-line EPR spectrum that was to become so familiar later on (5). Wertz and Vivo obtained the five-line spectrum with solutions of thiophenol, diphenyl disulfide, and thianthrene in H 2 S0 4 . Recognizing that this 202 Foundations of Modern EPR Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 10/02/15. For personal use only.