A PULSERADIOLYSIS STUDY OF SOMECo(II1) COMPLEXES where E and fiz are the energy and the mass of the particle. The value of Ar is 4.6 X 10-8E1/2for helium ions and 6.5 X 10-8E1/2for deuterons where E is in MeV. Since neighboring spurs are most important in determining the extent of reaction, it is suggested that a better approximation of the interspur distance, d, would be d = [{e/(-dE/dx))'+ (A"']''' and a better Z given by Z =
-1 1
Eo
d-1dE (111) Eo a The calculations were performed for lo-' M ethanol7X M nitrous oxide solutions using the values of the constants and parameters given by Schwarz and using e = 62.5 eV per spur. This is not the average spur energy but the single spur energy giving results closest to the calculated yields for y-ray irradiations which were averaged over all spur sizes. The results using both eq I11 (lower LET) and eq I1 are given in Table V and it may be seen that the hydrogen atom yield is computed quite precisely by this mechanism. It is interesting that the observed molecular yields and hydrated electron yields tend to be higher than the
1941
calculated yields. This effect was noted earlier and attributed to correlation between the initial positions of the radical pairs.12 There would be a tendency for the hydrogen atom and hydroxyl radical formed from one water molecule to be closer together than those from different water molecules so that more dissociation actually takes place than is accounted for by the diffusion model for y-ray reactions. This correlation would tend to fade into the background for particle tracks where the over-all radical density is higher thus leading to an apparent increase in total decomposition yield for particle tracks. Perhydroxyl Radical Yields. The yield of perhydroxyl radicals also increases with LET (Table V) . This effect is not accounted for in our diffusion model as the reaction
OH
+ HzOz
+
HOz HzO (9) is not fast enough to produce the Kupperman has suggested that the radical is produced by a small yield of oxygen atoms reacting with OH.23 4
(21) J. L. Magee, Ann. Rev. Phys. Chem., 12, 389 (1961). (22) H. A. Schwara, J. Phys. Chern., 66, 255 (1962). (23) A . Kuppermann in "Radiation Research 1966," North-Holland Publishing Do., Amsterdam, 1967, p 212.
A Pulse Radiolysis Study of Some Cobalt(II1) Complexes. Occurrence of Electronically Excited Cobalt(I1) Products. by William L. WaltzIband Ralph G. Pearson Department of Chemistry, Northwestern University, Evanston, Illinois 60601, and the Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received October 3, 1968)
The rate constants for the reaction of the hydrated electron with some cobalt(II1) complexes in aqueous solutions have been measured and are in the range of 101o-lO1lM-l sec-'. In conjunction with results of others' work, it is suggested that the initial products of these reactions are electronically excited cobalt(I1) complexes of low-spin electronic configuration. Direct evidence for the presence of low-spin Co(bipy)++formed by the reaction of the hydrated electron and Co(bipy)g3+in aqueous, alcoholic media is given. The absorption spectrum of low-spin C0(bipy)3~+ between 400 and 615 mp has been recorded and the kinetics of the disappearance of this species has been studied. The rate constant, kobsd, associated with the disappearance of low-spin Co(bipy)Pat 578 mp is expressed as kobsd = ko 4-kl[Co(bip~)3~+1 -f k~[&],where ko 5 X lo3sec-', k~ = 8 X lo8 M-' sec-l, and k~ = 106-1010M-' sec1 for quenching by various compounds such as oxygen. Possible quenching mechanisms are discussed. Less extensive evidence is given for the formation of low-spin Co(bipy)32+ by the reaction of hydrogen atom with Co(bipy)g3+.
