Esr Studies of
(14) (15) (16) (17) (18) (19)
2959
Alkali Metal Salts of TCNE
(b) R. Livingston, A. Zeldes, and E. H, Taylor, Discuss. faraday SOC.,19,166 (1955). "Handbook of Chemistry and Physics," 51st ed., Chemical Rubber Publishing Co.. Cleveland, Ohio, 1970. See ref 8 and references therein for a complete description of the relationship between @-protonsplitting and dihedral angle. (a) W. C. Lin and C. A. McDowell, Mol. Phys., 4, 333 (1961); (b) R. S. Mangaracina, Radial. Res., 32, 27 (1967); ( c ) ,T. 5. Melo, lnt. J. Radiat. Biol,. 23. 247 11973). W. M. Garrison, 'H. 'A. Sokol, and W. Bennett-Corniea, Radial. Res., 53,3715 (1973). (a) F. Patten and W, Gordy, Radial. Res., 14, 573 (1961); (b) W. Snipes and J. Schmidt, ibid., 29, 194, (1966); ( c ) E. G. Liming, ibid., 39,252 (1969). Protonation is likely under these conditions and could occur either
at the. peptide linkage to form -C(OH)-NHform C(OH)2.
or carboxyl group to
(20) P. Neta and R. W. Fessenden, J. Phys. Chem., 75, 738 (1971). (21) R. C. Drew and W. Gordy, Radial. Res., 18, 552 (1963). (22) G. Saxebol, T. B. Melo, and T. Henrikson, Radiat. Res.. 51, 31 (1972). (23) I. Miyagawa, Y. Kurita, and W. Gordy, J. Chem. Phys., 33, 1599 (1960). (24) J . Bolton in "Radical Ions," E. T. Kaiser, Ed., Interscience, New York, N. Y.. 1968. (25) M. Sirnic and E. Hayon, J. Phys. Chem., 77, 996 (1973). (26) A. Meybeck and J. Meybeck, Photochem. Photobiol., 16, 359 (1972). (27) M. A. J. Rodgcrs, H. A. Sokol, and W. M. Garrison, J. Amer. Chem. SOC.,90,795 (1968).
Studies of the Effect of Ion Pairing on Some Si Electron Spin Reactions Involving the Tetracyanoethylene Anion Radical M. T. Watts,
M. L. Lu, R. C. Chen, and M. P. Eastman"
Department of Chemistry, The University of Texas a i E / Paso, €1 Paso, Texas 79968 (Received July 16. 7973) Publication costs assisted by The Ufliversity of Texas at E/ Paso
The role of ion pairing in electron transfer, radical-radical dimerization, and solubility reactions involving the tetracyanoethylene anion radical (TCNE-) have been investigated. Studies of the electron transfer rate between TCNE- and neutral TCNE in dimethoxyethane (DME) yield k(15") = 2.6 X lo8 M - l sec-l and E , = 5.2 f 0.1 kcal/mol. Similar studies in acetonitrile (MeCN) yield k(l5") = 3.2 X lo9 M - l sec-I and E , = 2.3 rt 0.1 kcal/mol. The rate in DME is considered to be that for the loose ion pair while that in MeCN is for the free ion. Complexation of the cation portion of the ion pair, which forms in DME, by the crown ethers dibenzo-18-crown-6 and perhydrodibenzo-18-crown-6does not alter the reaction rate or the activation energy for the electron transfer process. Esr studies indicate that the dimerization of K+TCNE - in tetrahydrofuran and methyltetrahydrofuran is inhibited or prevented by the addition of crown ethers. The spectral parameters for the crown ether complexed K+TCNE- ion pair are g , , = 2.00256, g , = 2.00288, All = 5.56 f 0.1 G, and A L = -0.32 rt 0.03 6. The solubility of alkali metal salts of TCNE in benzene are enhanced by crown ethers. The dissolved species are shown to exist as tight ion pairs with properties very dependent on the nature of the crown ethers. Structures for the crown ether complexed ion pair are discussed.
Introduction
A number of simple reactions involving the alkali metal salts of tetracyanoethylene (TCNE) can be conveniently studied in solution by means of esr spectroscopy. These reactions include electron transfer,1.2 Heisenberg spin ex~ h a n g e , and ~ . ~radical-radical d i m e r i ~ a t i o n . ~Ion . ~ ,pair~ ing has been considered to be a n important factor in all of these reactions but details about ion pairing in solutions containing the TCNE- anion radical have been lacking.' The effects of various counterions on the electron transfer rate between the TCNE- anion radical and its neutral molecule in dimethoxyethane (DME) and in tetrahydrofuran (THF) have recently been reported.2 The rate constant at room temperature for the transfer reaction was in all cases studied. In general, the about lo8 sec-l M results indicated that ion pairing was important in determining the nature of the electron transfer process. In these studies no account was taken of the fact that alkali metal salts of TCNE- are known to dimerize in THF.3.5,6
It is possible that under some conditions the reaction between TCNE and a radical dimer could be a line-broadening process. In addition, it should be noted that neutral TCNE forms T complexes with DME, THF, and a variety of other solvents.8 When T complexes form in a solvent, the electron transfer reaction should be considered to be between a free ion or radical ion pair and a solvent complexed neutral molecule. Heisenberg spin exchange has been studied for KTCNE in DME and in THF.3 This work has indicated that KTCNE forms ion pairs in both solvents and dimerizes in THF. Recently, the effect of crown ethers, a class of macrocyclic polyethers which complex strongly alkali metal ions, on the Heisenberg spin-exchange rate for K+TCNEion pairs in DME has been reported.4 These studies have shown that KTCNE exists as a loose ion pair in DME and that the K+ ion in the ion pair is complexed in a 1:l ratio by the cyclic polyether dibenzo-18-crown-6 (DBC) . The equilibrium constant a t 15" for this complexation was determined to be 4 X lo3M-l. The Journal of Physical Chemistry, Voi. 77, No. 25, 1973
2960
The dimerization of alkali metal salts of TCNE in ethereal solvents has been investigated by esr and optical technique^.^,^.^ The esr studies of Heisenberg exchange in THF solutions of KTCNE suggest that ion pairs are involved in the formation of the dimer. In general, solutions in which the ion pairs are present are characterized by a yellow color while the color of the dimer is pinkish.6 Frozen solutions of NaTCNE and KTCNE have a color characteristic of the dimer and exhibit either no esr signal or a 39 very weak structureless The purposes of this paper are to investigate the role of ion pairing in electron transfer reactions involving 'FCNE-, to discover the extent tQwhich ion pairing i s important in the dimerization of TCKE salts, and to explore new reactions involving TCNE- and its ion pairs. In these reactions the effects of crown ethers on ion pair reactivity will be investigated.
Experimental Section The spectrometer used in these experiments and the techniques used to correct for modulation frequency broadening have been previously d e ~ c r i b e d .Measure~ ments of g value were carried out using a Varian V-4532 dual sample cavity. All g values were determined relative to the signal produced by KTCNE in UME (2.00279 f 0.00002).lo The measurements of electron transfer rates were carried out in the slow exchange limit and all experiments in which the temperature was varied were corrected to a constant concentration of neutral TCNE by means of published densities.11J2 The viscosities for DME and MeCN were taken from the literature.1*.12 In these experiments the overlap of hyperfine lines could be neglected because r f A , < 0.3.3J3Here F is the !ine width and A N is the 14N hyperfine splitting constant for the TCNEradical. In order to obtain a larger filling factor the esr experiments in which benzene and other Iaw dielectric solvents were used were carried out in a sample tube equipped with a 3-mm i.d. Thermal American Spectrosil quarvz side arm. The sample tubes used in these experiments were cleaned in several ways to determine whether ions from the wall of the sample tube were participating in ion pair formation. Such effects have previously been noted by Graceffa and T ~ t t 1 e . l ~ The first cleaning technique employed involved washing the sample tube in water, soaking the tube in dichromate cieaning solution, washing with water, and soaking in an ethanolic sodium or potassium hydroxide solution. The sample tube was then carefully washed with distilled deionized water and dried. In the second technique employed the sample tubes were washed in 50:50 concentrated HZS04-HN03 and were then washed in distilled deionized water and dried. The third technique involved soaking the sample tube in a benzene-crown ether solution, washing with acetone, and drying. The acetonitriie (MeCN) used in these experiments was twice distilled from P4010 and stored under vacuum, while the benzene was dried and stored over sodium ribbon. The TCNE was purchased from Eastman Organic Chemicals and was purified by recrystallization from 1,2dichloroethane followed by two sublimations. The macrocyclic polyethers, dibenzo-18-crown-6 (DBC) and perhydrodibenzo-18-crown-6 (PHBC), were purchased from Aldrich Chemical Co. The DBC was recrystallized from toluene and dried under vacuum (mp 163-164"). The PHBC was crystallized a number of times from n-hexane The Journal of Physicai Chemistry, Voi. 77, No. 25, 1973
M. T.
Watts, M. L. Lu. R. C. Chen, and M. P. Eastman
and dried under vacuum. No attempt was made to separate the isomers of PM C. Samples of isomers A and B of PHBC were obtained from E . I. du Pont de Nemours and Company, Inc. The NaTCNE and KTCNE were prepared as described in the 1 i t e r a t ~ r e . l ~ It was observed that degassed solutions of TCNE in carefully dried DME and MeCN showed noticeable decomposition of the TCNE within several days of preparation. Presumably, this was due to the decomposition of the T complex formed between the solvent and the TCNE. The TCNE- radical, by itself, was stable for long periods of time in both solvents. To minimize the error due to the decomposition of neutral TCNE, line width measurements were begun immediately after adding the neutral TCNE to the radical containing solutions. This addition was accomplished by means of a break-seal. TO avoid systematic errors, the order (with respect to temperature) in which the line widths were determined was randomized and the results a t 15" were checked intermittently throughout the experiments. It is believed that the decomposition of TCNE is not a significant factor in these experiments.
esults and Discussion ( a ) Electron Transfer. The studies on TCNE reported here were carried out in the region of slow exchange where the observed exchange rate constant kobsd is given by the expressionl6 317 1.52 X IO'(T' - Td)f.,, hobcd= _ _ _ _ _ _ ~ - (1)
[TCNEI
Here T and ro are the first derivative line widths in the presence and absence of transfer, and f m is a statistical factor which compensates for the lack of observable linebroadening effects in electron transfer between radicals with the same nuclear spin configuration. When a reaction involves a diffusion step and a reaction step, kobsd i s related to the rate constant for the reaction step, k a c t , and the rate constant for the diffusion step, k d , f f , by the expressionls 1 holw
-
1 ~
hdlff
+,-
1
(3
nacl
For the electron transfer reaction considered kdlff can be estimated from the expression3J8 (3)
Here Q represents the viscosity of the solvent. Figure 1 shows a plot of log hobsd and of log hac,. us. bfT for KTCKE-TCNE in MeCN. A least-squares fit of the data for k,,,. to the Arrhenius equation yields A = 1.8 0.4 X 1011 M - I sec-I and E, = 2.3 f 0.1 kcal/rnol. The value of k,,,. a t 15" is 3.2 X lo9 sec-I. The estimated error in this value is 10-15%. Figure 2 shows a plot of k,,,. for MTCNE-TCNE in E)ME. Were kact N kohsd since k d , f f - l
