An E.S.R. study of some alkyl substituted benzene anion radicals : an experimental evaluation of the extent of vibronic interaction

Abstract

E.S.R. studies of the proton hyperfine splitting in the anion radicals of benzene, toluene, ethylbenzene, iso-propylbenzene, t-butylbenzene, and p-xylene show that it is strongly dependent upon temperature. In the case of the monoalkylbenzenides there are two contributions to this observed temperature dependence. First, there is a rapid thermal averaging over two near-degenerate states which have different spin-density distributions. The energy separations between these two near-degenerate states have been measured for the above series of monoalkylbenzenides and are reported here. It is also possible to measure experimentally the spin-density distributions for the two states between which the averaging takes place. With this information one can obtain an experimental estimate of the extent of vibronic interaction which occurs in these systems. Such estimates are presented here and compared with those obtained theoretically. The second source of the strong temperature dependence of the proton hfs lies in the temperature dependence of the sigma-pi interaction parameter, Q _H. The sigma-pi interaction parameter has been studied as a function of temperature for all members of the above series and the results reported. Finally, restricted rotation of the iso-propyl group in the anion radical of iso-propylbenzenide was observed and the barrier to rotation has been measured.