Measurements have been performed on the carbonhyphen;13 hyperfine splitting in the ESR spectra of a series of methylhyphen; and chlorohyphen;substituted semiquinones and of the cyclooctatetraene anion radical. The splittings were observed from carbonhyphen;13 nuclei present in natural abundance (approx. 1), and the carbonhyphen;13 satellites were distinguished from spurious lowhyphen;intensity lines arising from sidehyphen;reaction produced radical impurities by means of careful intensity measurements as well as other techniques. The largest observed carbonhyphen;13 splitting had a magnitude of 1.7 gauss, and in most spectra smaller splittings were found. The carbonhyphen;13 satellites were overlapped by the major lines in the spectra (arising from proton splitting), but after correcting the measured intensities for this overlap, good agreement was found between the predicted and measured intensities in those spectra in which a detailed analysis was possible. In thephyphen;benzosemiquinone ion, a previously observed line of low intensity has been identified as a splitting arising from carbonhyphen;13 nuclei, and has been assigned, on the basis of intensity measurements, to the four CH carbon atoms. No splitting was found from the carbon atoms in the CO bonds of this compound, and it was concluded that the splitting is less than 0.6 gauss. Splitting constants tentatively identified with the methyl carbon atoms of the methylhyphen;substituted compounds were in the neighborhood of 1.5 gauss. The splitting in the cyclooctatetraene anion indicates that the three sigma bonds attached to a carbon atom lie in a single plane. The experimental results are in agreement with the theory of carbonhyphen;13 hyperfine splittings recently developed by Karplus and Fraenkel. Comparison of theory and experiment indicates that these results provide a very sensitive measure of the pihyphen;electron spin densities in the radicals. The valencehyphen;bond theory of the spinhyphen;densities in the methyl substituents is shown to be inadequate in its present form, but the molecularhyphen;orbital theory appears to give good agreement. This latter agreement is based on certain as yet unjustified assumptions, and therefore both theories require rehyphen;examination. The spectrum of the previously observed semiquinone ion formed from 2,5hyphen;dihydroxyquinone was interpreted in order to obtain an estimate, in conjunction with other data, of the hyperfine splitting at the carbonhyphen;13 nucleus of the carbon atom in a CO bond. The analysis also shows that the two CH carbon atoms in this compound have negative pihyphen;electron spin densities.
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