R. K. Bajpai, A. K. Gupta, and M. Gopala Rao
1288
second case) which is averaged out by exchange a t pH >12. Under the latter conditions all of the other hyperfine constants and the g factor are identical within experimental error to those observed at pH < l o . In the last example19 the line broadening began a t pH 11 so that the forward rate constant in that case must be closer to l o 9 than to 1010 M - l sec-l. Detailed studies of the line broadening in these cases have not as yet been carried out. A number of reasons for lower than normal forward rates are given by Eigen.11 Among these are hydrogen bridge structures which prevent an easy transfer of the proton to OH- and interference by other nearby negatively charged groups as in nitrilotriacetate (NH" dissociation). Excellent examples of both of these effects for radicals have been observed (case e). The great stability of the electron adducts to o-nitrophenol,3 maleate,20 and phthalate21 toward dissociation has been noted. Each of these radicals has a hydrogen bridge structure, such as I for the
]
[o-cfH\ c---c
