7966
An Electron Spin Resonance Study of Counterion Effects on the Rate Constant of Disproportionation of the Cyclooctatetraene Anion Hans van Willigen Contributionf r o m the Department of Chemistry, Uniuersity of Massachusetts-Boston, Boston, Massachusetts 02116. Receiued M a y 5, 1972
Abstract: An esr study has been made of the counterion effect on the electron transfer reaction 2COT- .-,COT + COT*-. A nonequilibrium COT- concentration could be created using the photoelectron transfer reaction COT2- COT hv -,2COT-. The results show that k increases strongly with increasing counterion radius. Similar trends have previously been found in electron transfer reactions involving transition metal complexes. They are attributed to the role of the counterion as the bridging group between two anions in the activated complex.
+
+
y subjecting a solution of A2-, A-, and A, where A is a n aromatic molecule, involved in the disproportionation equilibrium 2A-
+A2- + A
to near-uv irradiation, one can induce an increase in Aconcentration by photooxidation of A*-. This provides a convenient way to study the kinetics of the disproportionation or electron transfer reaction involving two mononegative ions. Studies of the kinetics of disproportionation reactions are of some interest since they may contribute to our understanding of electron transfer mechanisms, which continue to be the subject of extensive investigations especially in inorganic chemistry. Here we report the results of an esr investigation of the counterion effects on the kinetics of disproportionation of the cyclooctatetraene (COT) radical anion in methyltetrahydrofuran (MTHF). It is found that the counterion effects parallel, at least qualitatively, trends observed in electron transfer reactions between inorganic ions, for instance MnOlz-
+ MnOa- 5=+ Mn04- + MnOi2-
Experimental Section The solvents, methyltetrahydrofuran (MTHF) and 1,2-dimethoxyethane (DME). were dried over calcium hydride, vacuum distilled, and stored over Na-K alloy before use. COT (Chemical Procurement Laboratories) was used without further purification. Lithium tetraphenylboron was prepared according to the method reported by Bhattacharyya, et Samples were prepared according to standard procedures5 in an all-glass evacuated apparatus. Unless stated otherwise the initial COT concentration was between and 5 X 10-1 mol; the solutions were reduced with Li. 5 X Na, K, Rb, or Cs until [COT] = [COT*-]. In the temperature range of interest the [COT-] concentrations were negligibly small in the absence of light (the disproportionation equilibrium constant is of the order of lo8or larger). Esr spectra were recorded with a Varian V4502 spectrometer with a 9 in. magnet, equipped with a variable-temperature accessory. Temperatures were measured with a copper-constantan thermocouple and could be maintained constant within i1 '. ( I ) A. G. Evans and J. C. Evans, J . Chem. SOC.E,271 (1966). ( 2 ) H. Taube, "Electron Transfer Reactions of Complex Ions in Solution," Academic Press, New York, N. Y., 1970. (3) J. C. Sheppard and A . C. Wahl, J . Amer. Chem. SOC.,79, 1010 (1957). (4) D. N. Bhattacharyya, C. L. Lee, J. Smid, and M. Szwarc, J . P h y s . Chem., 69,608 (1965). ( 5 ) D. E. Paul, Ph.D. Thesis, Washington University, 1954.
Special flat quartz cells fitting in the low temperature insert of the cavity were used in the experiments. To minimize concentration gradients of the photoproducts across the cells the path length was kept small (either 1 mm or 0.1 mm). The kinetic data did not vary significantly with cell path length. A Philips SP 500 high-pressure Hg arc was used as light source for the itz situ radiation experiments. The light was focussed on the optical window of the cavity with glass lenses and a CuSOl solution was used as heat filter. Long-term irradiation of COT-COT-COT2- solutions did not result in noticeable decomposition.
Results Characterization of the Spectra. During in situ irradiation of the solutions the Li, Na, and K samples gave rise to photosignals which could be from 5 X to as much as 1OOOX the background signal in the absence of light in the temperature range from -60 to - 130". For the R b system the maximum photosignal (around - 130") was about 3 X the background signal. Irradiation of the Cs system did not result in a detectable signal. The Li reduced samples gave rise to a nine-line spectrum with a proton hyperfine splitting constant (hfsc) of 3.17 G and a peak-to-peak line width of 0.06 G. Over the temperature range studied n o sign of Li hfs was found. Hence we were dealing with free COTanions or loose ion pairs in which the spin density at the Li nuclei is zero. The esr photosignals of the Na reduced samples could be assigned to the COT-.Na- ion pair. The N a hfsc was 0.96 G and showed no temperature dependence; the peak-to-peak line width in the spectra was 0.40 G. The K reduced samples gave rise to nine broad lines (1.5 G line width) with indications of unresolved K hfs. The spectrum is assigned t o the COT-.K+ ion pair. The radical ions created by irradiation of the R b system did not give rise to a resolved esr spectrum. The presence of two R b isotopes with nuclear spin probably accounts for this fact. The hyperfine data derived from the spectra agree Strauss quite well with those reported by Strauss, et cites substantially larger line widths, however. It seems likely that the s1ov.w electron transfer rates between anions and dianions in the temperature range of interest here accounts for this fact. It has been (6) H. L. Strauss. T. J . Katr, a n d G. I
