Vol. 82 pect that appreciable distortion of C* in the quenching complex would be necessary to facilitate the transition. By the Franck-Condon Principle, formation of the ionic complex could well lead to the necessary strong oscillations, rapidly carrying the system to a crossing point with the ground state, and final internal conversion.20 The electronic energy would not be primarily transformed into relative translational motion of C and M, as indicated in the two-dimensional figure, but into the internal modes of both 11 (the complexed metal ion) and C. It is not likely that the idealized ionic form would be merely a strongly polarized complex, in which nuclear distortion is sufficient to permit internal conversion, in view of the several cases of weak quenching observed here.Such a mechanism affords, in particular, some explanation of the effects of changing solvation (hydration) on the probability of quenching, although a quantitative discussion is difficult. The possibility of formation of the complex evidently depends on the relative free eneigies of C* -M and C--M+, which is to say, on the respective oxidation-reduction potentials, whose values will depend on solvation. In the particular case of Mn++, the relatively low rate may be associated with the special stability of the half-(20) J Franck and R. Livingston, J . Chein P / t > s , 9, 184 (1941) filled shell.21 Franck-Condon restrictions on the charge-transfer process will also influence the rate,22 and, by involving participation of ion-solvent vibrations in the configuration necessary for transition, will lead to entropy as well as energy of activation barriers. Finally, the charge-transfer process may involve movement of heavy particles (H atoms) belonging to molecules in the complexing envelope of the quenching ion.20This mechanism suggests the possible existence of charge-transfer absorption bands in solutions containing quencher and substrate a t high concentrations A search for such bands in the systems studied here has given negative results thus far. A modification of this picture frequently has been discussed, in which the complex decomposes to a pair of one-electron oxidation -reduction products, which then return to the initial state via relatively slow back-reactions. Such a process evidently permits spin-conservation in the fast step, with either diamagnetic or paramagnetic quenchers.Further work is in progress on the quenching of both singlet and triplet states in mixed solvents.(21) C. K. Jgrgensen, Acta Chem. Scand , 10, 1605 (1956). (22) B. Zwolinski, R. J. Marcus and H. Eyring, Ckem. Revs., 65, 157 (1955). WALTHAM, MASS.
