1583
NOTES
lo4 M-l cm-1, derived from our own measurements and being in reasonable agreement with those which can be deduced from ref 1) yields the concentrations listed in Table I. The fact that K , as defined in column 4 of Table I, is independent of the concentration supports the assumptions underlying the calculation. Together with the uv absorption spectra we measured the equivalent conductance A of the solutions, the values being listed in Table I. In addition to the spectral measurements the overall concentration of terphenyl mononegative ions, ct, is known from the amount of terphenyl molecule introduced into the sample cell (assuming complete reduction) and the controlled dilution of the ion solution. For all relevant measurements the value for ct coincided within 2001, with the value determined from uv absorption. According to Ostwald’s dilution law, linear extrapolation of l / A against Act towards ct = 0 yields AOand K. The values obtained are A. = 91 i2-l f 8 mole-’ cmz M. and K = 1.2 & 0.3 X The results of our spectral measurements are incompatible with the “simple” dissociation scheme (1). They can, however, be interpreted straightforwardly within the dissociation scheme (2), which is the one originally proposed by Szwarc, Smid, el aLa This interpretation is supported by the fact that the values of K are essentially independent of concentration as shown by Table I. We combined spectroscopic and conductance data by calculating
3.3 X
The values so obtained are listed in Table I. They are close to the value of A, obtained from the Ostwald plot. By definition the dissociation constant determined from uv measurement is identical with the dissociation constant, obtained by the Ostwald extrapolation method. Experimentally, we observe the values of both dissociation constants to compare very well. As the spectroscopic and the conductance method for determining K are independent of one another, we think that the agreement is a significant one.
The Use of Sulfur Hexafluoride to Determine G(e-D,o) and Relative Reaction Rate Constants in DzO by K.-D. Asmus and J. H. Fendler Radiation Research Laboratories. Mellon Institute. Carnegie-Mellon University. Pittsburgh, Pennsylvanta (Receked October 1 7 , 1 9 6 8 )
15613
Several recent reports have indicated that the yield of hydrated electrons in irradiated DzO is higher than in
HzO.l-* The ratio of G (e-H,o) / G (e-D20) is generally found to be 0.90-0.95.1-3 The values for the absolute yield of hydrated electrons in D20, however, showed a considerable scatter; the observed G (e-D,o) apparently depended on the particular system employed, and the discrepancies presumably were due to the fact that most of these determinations were based on indirect meth0ds.l-4 Differences have also been observed for absolute and relative reaction rate constants involving the hydrated electron in HzO and D20.2J The rate constant for the reaction of the electron with D@+, for example, has been determined to be 1.71 X 1OO ’ M-l sec-1,8*5which is lower by a factor of 1.4 than that for the reaction of e-HzO H80+. However, k(ea,-+e,,-) < (1.3 f 0.2) X 10’0 M-’ sec-l was observed to be identical in HzO and Da0.6 We have demonstrated recently that SFe is a speciJic electron scavenger in water ( k ( e - n z O + ~=~ B1.65 ) X10’” M-1 sec-1) producing high yields of easily detectable fluoride ions in this reaction.’ The buffered aqueous SF, system is not hampered by any secondary reactions during the radi~lysis.~Therefore, this system seemed to be a convenient one for determining unequivocally the yield of hydrated electrons in DzOand for obtaining relative rate constants for reactions of e-DaO with SFB and other solutes from competitive studies. The preparation of SF6solutions in D2O was identical with that previously reported.7 The concentration of SFswas (1-2) X M , assuming that its solubility is the same in DzO as in HzO. DzOwas purified by distillation from KMn04 followed by passage of the D2O vapor in a stream of oxygen through a silica tube at 600’. Dosimetery in DzO was based on the Fricke The concentration of fluoride ion in the irradiated samples was determined by using a fluoride ion activity electrode; calibrations were obtained by using standard NaF solutions in DzO having an ionic strength corresponding to that in the irradiated sample^.^ The pD was determined by adding 0.4 unit to the actual reading on a pH meter.8 The results are summarized in Table I. The fluoride M K2HP04) neutral ion yield in a buffered solution was found to be G(F-) = 18.4 f 0.3 from the slope of a linear yield dose plot, extending from 0.1% to more than 10% conversion of SFs. In HzO solutions we have established the relationship G (F-) / G (eaq-) = 6.’ Therefore, the yield of hydrated electrons in
+
(1) E . Hayon, J . Phys. Chem., 69, 2628 (1965). (2) Z. D. Draganib, 0. I. Mibib, and M. T. Nedanovib, ibid., 7 2 , 511 (1968),and references cited therein.
(3) E. M.Fielden and E. J. Hart, Radiation Res., 3 3 , 426 (1968). (4) F. S. Dainton, A. R. Gibbs, and D. Smithies, Trans. Faraday SOC..6 2 , 3170 (1966). (5) E. J. Hart and E. M. Fielden, J . Phys. Chem., 7 2 , 577 (1968). (6) L. M. Dorfman and I. A. Taub. J . Amer. Chem. SOC.,8 5 , 2370 (1963). (7) K.-D. Asmus and J. H. Fendler. J . Phys. Chem., 7 2 , 4285 (1968). ( 8 ) P.K. Glasoe and F. A. Long, ibid.. 6 4 , 188 (1960). Volume 73,Number 6 May !$BO
1584
NOTES
Table I: The yield of F- and Hydrated Electrons, and Relative Rate Constants for the Reaction of Electrons with Competing Electron Scavengers in HzO and DzO H $0
W-) a35a-I k(s."-+CHPOCEl)/k(e5"-+ E Fd k(eaa-+NOdkeaa-+EFd k(esa-+HaO+ or D~Odkea~-+ EFs)
Da 0
16.6 st: 0.3 18.4f 0.3 2.76 st: 0.05 3.06 f 0.05 0.39 0.39 0.46 0.46 1.57 0.75
D20, G(e-D,o), is equal to 3.06 f 0.05. This value confirms the significant difference in the yield of hydrated electrons between D2O and H20, the latter being 2.76 f 0.05.7 Our G(e-D20) is similar to that of 2.9 determined by Fielden and Hart3 by means of pulse radiolysis but is higher than that reported by others whose systems did not allow direct determinations.'B2 The ratio of G(e-H,o)/(G(e-D,o) obtained in all the various systems, however, corresponds very well with the value of 0.90 determined in the present SFa system. The addition of other electron scavengers to an aqueous SF6 solution results in a decrease of G(F-). Relative rate constants for the reaction of electrons with the competing solutes, as derived from the usual competition kinetics, are listed in the table. No differences were found between H20 and D20 for the rate constant ratios of the reaction of electrons with sF6 relative to that with either a neutral species such as CHsCOCHs or with a charged species such as Nos-. However, the reaction of the electron with DaO+ relative to that with SF6is slower by a factor of 1.9 than the corresponding one in HzO. It is interesting to note that the recombination of DaO+ and OD- 9 is also slower by almost the same factor, 1.7, than the recombination of HsO+ and OH-.10 These results indicate that the observed effects in the two solvents are probably attributable to the solvated protons rather than to any inherent properties of the electron or the solvent. It appears, therefore, that only the transfer of a proton from H@ or D30+ to the negative entity is responsible for the differences in the observed relative rate constants, presumably due to a somewhat higher activation energy of this reaction in D2O. The rate constant for the reaction of SFe with electrons in DzO, calculated from the data given in the table and kps0-+e-,,10),6 is -2.6 X 10'O M-' sec-l. This value is somewhat higher than that obtained in H2O.l However, unwarranted conclusions should not be drawn from this fact until the rate constant for the reaction of electrons with SF6 in D2O from a direct measurement is available. Acknowledgment. This work was supported in part by the U. S. Atomic Energy Comission. (9) G. Ertl and H . Gerischer, Z . Elektrochem. Ber. Bunsenges. Physik. Chem., 66, 560 (1962). (10) M. Eigen and L. de Maeyer, Z . Elektrochem., 59, 986 (1955).
Tliu Journal of Physical Chemielry
Donor-Acceptor Complexes of
2,3-Dichloro-5,6-dicyanobenzoquinone with Tetrahydrofuran, Tetrahydropyran, and l,4-Dioxane by Manjit S. Sambhi Department of Chemistry, University of Malaya, Kuala Lumpur, Malaysia (Received May 6 , 1968)
Donor-acceptor complexes formed between donors of low ionization potentials and acceptors of high electron affinities can dissociate into ion-radicals in polar solvents1p2and on irradiating with light of suitable wavelength.a*4 Ward has shown that solutions of tetracyanoethylene (TCNE) or pyromellitic dianhydride (PMDA) in donor-solvent tetrahydrofuran, which were irradiated, exhibited intense esr spectra which can be attributed to the anion-radicals of TCNE and PMDA, respectively. Ether-anhydride complexes have recently been used to facilitate photochemical formation of cyclobutane derivatives from alkyl olefins6 and as photosensitizers for vinyl polymeri~ation.~JIt was therefore of interest to investigate the donor-acceptor complexes of cyclic ethers with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) , a strong acceptor.8
Experimental Section Materials. 2,3 - Dichloro - 5,6 - dicyanobenzoquinone was recrystallized twice from dry methylene chloride. The mp 212-213' and the infrared spectrum were consistent with literature data.Q Tetrahydrofuran (THF) , tetrahydropyran (THP) , and l14-dioxanewere shaken with potassium hydroxide pellets, passed through an alumina column, refluxed over calcium hydride, fractionally distilled through a 35-cm Vigreux column, and fractionated again from sodium-anthracene complex. The ethers were used immediately after purification. Peroxide tests before and after use were found to be negative. 1,BDichloroethane was partially dried over calcium chloride, refluxed over calcium hydride, and fractionally distilled through a 35-cm Vigreux column. Procedure. The experiments were performed in a (1) R. Foster and T . J. Thomson. Trans. Faraday SOC..5 8 , 860 (1962). (2) J. W. Eastman, G. Engelsma, and M. Calvin, J . Amer. Chem. SOC.,84, 1339 (1962). (3) 0. Lagercrantz and M. Yhland, Acta Chem. Scand., 16, 1043 (1962). (4) R. L.Ward, J. Chem. Phys., 39, 852 (1963). (5) H. D.Scharf and F. Korte, Chem. Ber., 98, 764 (1965). (6) K. Takakura, K. Hayashi, and S. Okamura, J . Polymer Sei., B2, 861 (1964). (7) C. E. H. Bawn, A. Ledwith, and A. Parry, Chem. Commun., 490 (1965). (8) G.Briegleb, Angew. Chem., Int. Ed., 3, 617 (1964). (9) D.Walker and J. D. Hiebert, Chem. Rev., 67, 153 (1967).
