Some EPR spectra of small organic free radicals

The EPR spectrum of the ethyl radical presented in Fig. 12.2a is readily interpreted, and the results are of interest in respect to the distribution of unpaired spin density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and /8 protons. The two coupling constants are = 22.38 gauss and = 26.87 gauss, and imply extensive delocaliza- 

EPR spectra have been widely used in the study of free-radical reactions. The method has been used both to characterize in detail the structure of radicals prepared by known methods and to detect radical intermediates in the study of reaction mechanisms. An interesting example of the direct study of a radical involves the cyclopropylmethyl radical. Much chemical experience has indicated that this radical 

Analysis of the EPR spectrum of the nitroxide radical can usually provide information about the structure of the original radical. 

The radical was generated by photolytic decomposition of di-t-butyl peroxide in methylcyclopropane, a process that leads to selective abstraction of a methyl hydrogen from methylcyclopropane  

Below -140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above -140 C, however, the spectrum of a second species was seen, and above -100°C, this was the only spectrum observed. This spectrum could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to -140°C. The conclusion is that the ring-opening of the cyclopropylmethyl radical is a very facile process, so that its lifetime above — 100°C is very short. Any cyclopropylmethyl radical that might be in equilibrium with the 3-butenyl radical is present in an amount too small to detect. 

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Fig. 12.2. Some EPR spectra of small organic free radicals (a) Spectrum of the ethyl radical. [From R. W. Fessenden and R. H. Schuler, J. Chem. Phys. 33, 935 (1960) 39, 2147 (1963). Reproduced by permission of the American Institute of Physics.] (b) Spectrum of the benzene radical anion. [From J. R. Bolton, Mol. Phys. 6,219 (1963). Reproduced by permission of Taylor and Francis, Ltd.]...

Some EPR spectra of small organic free radicals... 

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