An ESR study of free-radical protonation equilibria in strongly acid media

Protonation of ascorbic acid radical, HPO/sub 3//sup -/, and SO/sub 3//sup -/ in strong perchloric acid solution has been studied by means of changes in ESR parameters. The behavior of the ascorbic acid radical shows that the use of published acidity functions does not well describe the observations in that changes in ESR parameters occur over a much wider range of acidity function than expected. Treatment in terms of an equilibrium involving H/sup +/(H/sub 2/O)/sub 4/ works much better. The pK so determined is -0.86 +/- 0.1 in contrast to an earlier value of -0.45. A continuous increase in /sup 31/P hyperfine constant for HPO/sub 3//sup -/ was observed from 650 G at pH 2.4 to 772 G at 61.2% HClO/sub 4/. This change could not be explained by a single protonation to H/sub 2/PO/sub 3/, but if a second step giving H/sub 3/PO/sub 3//sup +/ was also included a reasonable fit to the data could be obtained. Approximate pK values of 0.0 and -1.7 were found for the first and second steps, respectively. No systematic change in the g factor for phosphite radical outside of the error limits +/- 0.00008 was found over the whole range. The radical SO/submore » 3//sup -/ was produced over the range from pH 11.3 to 61.2% HClO/sub 4/ and no change in the g factor larger than +/- 0.00001 was found. It is possible that this radical does protonate to HSO/sub 3/ but that no change in g factor accompanies this transformation.« less