A.
3038
Acknowledgment. One of us (S.A.R.) thanks the Salters’ Company for a Scholarship.
References and Notes (1) J. R. Miller and J. E. Willard, J. Phys. Chem., 76, 2641 (1972). (2) J. R. Miller, Chem. Phys. Lett., 22, 180 (1973). (3) J. Kroh and C. Stradowski, Int. J. Radiat. Phys. Chem., 5, 243 (1973). (4) G. V. Buxton, F. C. R. Cattell, and F. S. Dainton, J. Chem. SOC.,faraday Trans. 1, 71, 115 (1975). (5) G. V. Buxton and K. G. Kemsley. J. Chem. SOC., Faraday Trans. 1, 71, 568 (1975). (6) F. Keiffer, C. Meyer, and J. Regnault, Chem. Phys. Lett., 11, 359 (1971). (7) J. Moan, Ber. Bunsenges. Phys. Chem., 75,668 (1971). ( 8 ) B. G. Ershov and F. Keiffer, Chem. Phys. Lett., 25, 576 (1974). (9) M. Aubailly, M. Bazin, and R. Santus. Chem. Phys. Lett., 31, 340 (1975). (10) J. R. Miller, J. Chem. Phys., 56, 5173 (1972). (11) E. J. Marshall, M. J. Pilling, and S. A. Rice, J. Chem. Soc.,faraday Trans. 2, 71, 1555 (1975). (12) G. Gamow. Z.Phys., 51, 204(1928). (13) D. I. Blokhintsev, “QuantumMechanics”,D. Reidel Publishing Co., Dordrecht, Holland, 1964, p 337. (14) F. S. Dainton, M. J. Pilling. and S. A. Rice, J. Chem. SOC.Faraday Trans. 2, 71, 1311 (1975). (15) B. Brocklehurst. Chem. Phys., 2, 6 (1972). (16) M. J. Pilling and S . A. Rice, J. Chem. SOC.,faraday Trans. 2, 71, 1563 (1975). (17) M. van Smoluchowski, Z.Phys. Chem., 92, 129 (1917). (18) M. Tachiya and A. Mozumder, Chem. Phys. Lett., 28, 87 (1974). (19) F. Keiffer, C. Lapersonne-Meyer, and J. Regnault, ht. J. Radiat. Phys. Chem., 6, 79 (1974). (20) J. F. Wyatt, I. H. Hillier, V. R. Saunders, J. A. Connor, and M. Barber, J. Chem. Phys., 54,5311 (1971). (21) S. A. Rice, D.Phil. Thesis, Oxford University, 1975.
Discussion J. JORTNER. During an American Chemical Society meeting a t San Francisco in 1949 physical chemists were trying to understand
M.Koulkes-Pujo, L. Gilles, and J. Sutton
thermal electron transfer (ET) processes between simple ions in solutions. J. Franck, quoting Libby, pointed out that electron tunneling a t fixed nuclear configuration cannot be implied in this context. Such an approach violates the Franck-Condon principle and implies, in fact, that the nuclei follow the motion of the electron. The Franck-Condon principle has to be applied to ET processes as well as to optical excitation processes. Indeed, thermal ET can be considered as an optical process in the limit of zero photon energy. The electron tunneling concept was recently revived by biochemists in an attempt to rationalize ET in biological systems. I believe that this approach is wrong, violating the Franck-Condon principle. The reactivity of the solvated electron as well as the quasifree electron in solutions and glasses has to be accounted for in terms of a theory which incorporates explicitly nuclear configurational changes.
J. W. HUNT.In your tunneling model, I am interested in the history of the electron yields, and how your analysis of J. R. Miller’s data is extrapolated back to -100 psec. I believe that certain scavengers reduce the yield of electrons a t this time. This could be explained by formation of the encounter pair as suggested by Czapski, or a fast initial rate of e- decay. I wonder if you can discuss your present thoughts on this process. S. A. RICE. If the electron can be trapped about an encounter radius away from the scavenger molecule then Czapski and Peled’s treatment is relevant and a further reduction in the electron yield occurs over the reduction due to dry electron scavenging, since such reactive pairs would react rapidly. Electrons trapped further away from scavengers would decay by electron tunneling. If the electron enters an unrelaxed trap the rate of tunneling from this will be faster than from the extrapolated long time decay. To assess what fraction of electrons decay by “dry” electron and encounter pair formation it is necessary to determine whether or not the electron trap relaxes and over what time scale.
Kinetics Study of Selective Solvation of Electrons in Water-Dimethyl Sulfoxide Mixtures A.
M. Koulkes-Pujo,
C.N.R.S.,91 190 Gif Sur Yvette, France
L. Gilles, and J. Sutton Cen-Saclay, 9 1190 Gif Sur Yvette, France
(Received September 10, 1975)
Publication costs assisted by Cen-Saclay
Dimethyl sulfoxide (DMSO) is known to solvate with difficulty anions including the electron and hence the addition of DMSO to a medium increases the anionic reaction rates. We attempted to study this effect in mixtures of H20-DMSO for specific reactions of the solvated electron using the pulse radiolysis technique. To begin with we chose different solutes among those known as good scavengers of electrons, neutral ones, NzO, and positive and negative ionic ones (NOS- and H+). The experiments were carried out with a Febetron delivering 10-nsec pulses of 1.8-MeV electrons. The rate of disappearance of the solvated electron formed by the pulse was measured by direct observation of the decay absorption a t 870 or 600 nm. The ultrapure products were used as supplied except for DMSO which was distilled before use. We first of all determined the value of the rate constant kea,- + DMSO as a function of the DMSO concentration. The reaction is pseudo-first order and the results obtained were in good agreement with those of Cooper, Walker, Gillis and K1assen.l The mixture H20-DMSO has a viscosity and a density greater than water. The viscosity is maximum for a concentration of 0.3 mole fraction.2 It is known The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
that for diffusion rate-controlled reactions, the diffusion coefficient is related to the reciprocal of the viscosity of the medium, by the Stokes-Einstein law ( D = kT/Gror). So the rate constant of the solvated electron with a solute must vary as 1/q on addition of DMSO, if this parameter alone is taken into account. With all the solutes investigated, the disappearance of
Solvated Electrons in DMSO-H*O Mixtures
3039
the electron remained pseudo-first order, so the rate conproton by DMSO. We suggest that an effect of structure stant was calculated by subtracting from the total rate of breaking of water either by DMSO or C104- may be indisappearance the rate of disappearance of the electron volved. This property was already suggested by MacDonald with DMSO, then dividing by the concentration of the soet a1.6 who studied the influence of DMSO on the temperalute. The results are the following. ture of maximum density of water. They concluded that this compound destabilizes the structure of water over the (1) NzO. The rate constant is not always varying with mole fracentire concentration range studied (0.1-3 X the concentration of DMSO in the mixture, as the reciprocal of the viscosity of the medium (theoretical c u r ~ e ) . ~ tion). However, Le Narvor et aL7 arrived a t an opposite conclusion in their work on the ir spectrum of water conThree zones may be distinguished. taining low concentrations of DMSO, where they found no Up to 50% volume of DMSO (0.21 mole fraction) the exchange in comparison with pure water. Our own results perimental and the theoretical curves are identical, seem to support the conclusion of MacDonald et al. but we suggesting that the electron is solvated only by water. also suspect the effect of C104- ions which are known as efAbove 90% of DMSO (0.68 mole fraction), the experimental fective structure breakers. I t would be interesting to look at rate constant increases too rapidly to be measured. The efthe ir spectrum of water-DMSO mixtures in presence of fect of viscosity cannot only be taken into account and we suggest that the electron formed under these conditions is In conclusion, these previous results show that the rate extremely reactive and is different from the hydrated elecconstant of a scavenger with solvated electrons is markedly tron, this conclusion being borne out by the displacement affected by the presence of DMSO. In spite of the complexof the absorption spectrum maximum from 720 to above ity of these solvent mixtures, it seems possible to obtain 1500 nm. Between 66 and 90% DMSO, the measured rate from them more informations than from the constituents constant seems to be lower than that indicated by the theotaken separately. Particularly the effect of strength of hyretical curve. Around 66% DMSO, the solution is known to drogen bonding on the energy degradation of a slow elecbe especially strongly structured by hydrogen bonding and tron seems interesting to follow up. under these conditions, one may expect that the electron is “heavier” and is thus less mobile and also less reactive than References and Notes the hydrated electron. (1) T. K. Cooper, D. C. Walker, H. A. Gillis, and N. V. Klassen, Can. J. The existence of three zones as a function of the concenCbem., 51, 2195 (1973). tration of DMSO has already been seen particularly by (2) J. M. G. Cowie, and P. M. Toporowski, Can. J. Cbem., 39, 2241 (1961). Morel4 who studied the ionic conductivity of C1- in such (3) A. M. Koulkes-Pujo. L. Gilles, and J. Sutton, J. Cbem. SOC., Cbem. Commun., 912 (1974). media. (4) J. P. Morel, Bull. SOC.Cbim. Fr., 1408 (1967). (2) NUN&. This case is completely different from the ( 5 ) J. Courtot-Coupez, and M. Le Demezet, Bull. SOC. Cbim. Fr., 1033 (1969). previous one. The experimental curve is always below the (6) D. D. MacDonald, M. D. Smith, and J. B. Hyne, Can. J. Cbem., 49, 2817 theoretical one, contrary to what happens in the case of (1971). NzO. For low concentrations of DMSO, the rate constant of (7) A. Le Narvor, E. Gentric, and P. Saumagne, Can. J. Cbem., 49, 1933 (1971). eaq- NzO was correctly correlated to the viscosity of the medium. In the case of the reaction of two anions, a second Discussion phenomena seems to be superimposed on the effect of visL. M. DORFMAN.Two brief questions with regard to the rate cosity. constant. (1) Is the heat of mixing known for the system and do For concentrations of DMSO equal to or greater than values correlate with your “structure-breaking” proposal? (2) Does 0.33 mole fraction, the rate of disappearance of the solvatthe value of the rate constant come back down to lower values a t 0.9 or 0.95 mole fraction of DMSO, or in pure DMSO? ed electron is the same as in absence of NO3-. We relate this effect to the desolvation of both the anions and to the A. M. KOULKES-PUJO.(1)Yes, they are known; after discussion increased repulsive coulombic interaction between Noswith Dr. Firestone, he will look a t this point to see if his correlaand the electron. If this hypothesis is confirmed, this may tions may be applied to this case. (2) No, in pure DMSO and around 0.9 mole fraction, NzO reacts too rapidly with electrons have implications in the radioprotective properties of and the rate constant could not be measured. DMSO. (3) HC104. In general, cations are more solvated in G. R. FREEMAN.You mentioned that the rapid increase in rate constant for reaction between electrons and NzO a t mole fractions DMSO than in water. The case of the proton in HC104 greater than 0.5 of DMSO might be due to dry electrons. In hydroseemed interesting to investigate because HC104 is comcarbons the rate constant of reaction of quasi-free electrons with pletely dissociated in these m i x t u r e ~ Moreover, .~ this case nitrous oxide is smaller than that of solvated electrons. The inseems simple because we can avoid complex formation with creased rate constant might be attributable to an increasing elecdifferent cations in DMSO and which could have been used tron mobility without involving reaction from the quasi-free state. as good electron scavengers. Although this work is still in Your value of the rate constant for reaction of electrons with acid in water, 1.9 X 1O1O M-l sec-l, is lower than literature values, progress, we find that, for small concentrations of DMSO -2.2 X 1O1O M-l sec-’. Your curve would extrapolate to near the (60.1 mole fraction) and HC104 lop3 M , the rate constant latter value. Is the maximum real? h , , - + ~ +is increased from 1.9 X 1O’O to 2.2 x 1010 M-1 sec-’. The difference is not very large but it is significant if A. M. KOULKES-PUJO. Yes, we think that the difference is significant; moreover, the viscosity effect is taken into account. we take into account the effect of viscosity. We have very recently obtained results from a greater concentration of N. KESTNER. How does the optical spectrum of DMSO-H20 HC104 (5.6 X lop3 M ) . They confirm the shape of the curve mixtures change as a function of DMSO concentration? Does that but there is a shift of the maximum to 0.1 mole fraction spectrum indicate the same strong preference of the electron for coordination by water in accord with your kinetic data? DMSO and the rate becomes equal to 2.4 X 1O1O M-l sec-l. As the curve passes through a maximum a t low concentraN. KLASSEN. The A,, of the e,- in H20-DMSO solutions tions of DMSO, it seems difficult to correlate the increase changes smoothly between A,, in pure HzO to A,, in pure in rate constant with an increase in the solvation of the DMSO. This smooth change occurs a t low DMSO concentrations.
+
The Journal of Pbysical Cbemistry, Vol. 79, No. 26, 1975
3040
Leon M. Dorfman and Bradley Bockrath
J. BELLONI.I would note an important point in polar media where we have difficulty getting experimental values of the mobilities of esalv-, the rate constant values of e- reactions give an indication of the mobility. In your case, if I remember, the maximum of esolv- spectrum is far away in the ir, despite the high static dielectric rate constants you observe for the decay in pure
DMSO.
A. M. KOULKES-PUJO.We correlated the high rate constant we observed to the solvation (or nonsolvation) of the electrons; an electron which absorbs in the ir, as you say, is less solvated so more reactive, so I agree with you. But I do not agree completely to a correlation of any anomaly between high dielectric constant and maximum absorptions of the solvated electrons. Other parameters have to be taken into account.
The Solvated Electron as Reducing Species in the Submicrosecond Formation of Reactive Transients. Carbanions in Solution Leon M. Dorfman" and Bradley Bockrath Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received July 23, 1975) Publication costs assisted by the U.S. Energy Research and Development Administration
The solvated electron, generated by pulse radiolysis, has proved to be extremely useful in fast reaction studies as the primary reducing species which forms organic intermediates of interest in other areas of chemistry. Organic free radicals1 have been generated by dissociative attachment to appropriate solutes in a variety of solvents. e,-
+ RC1-
R.
+ C1-
('1
Aromatic radical anions2 have been formed by nondissociative attachment to aromatic compounds. e,-
+A-
Am-
(2)
In the work we report here, carbanions have been generated3 by dissociative attachment to organomercury cornpounds in tetrahydrofuran.
-
+ RzHg
e,-
R-
+ RHg
(3)
In this way, the benzyl carbanion, PhCHZ-, has been formed and studied. It exhibits a uv absorption band with maximum a t 362 nm. Absolute rate constant^^^^ for its formation, as well as for the formation of (PhCHz-,Na+), in the following reactions, have been determined. e,-
+ (PhCH2)zHg
-
PhCH2-
-
+ PhCH2Hg.
(4)
k4 = 2.7 X 1O1O M-l sec-l PhCH2k5
+ Na+
PhCHZ-,Na+
(5)
= 1.5 X 10l1M-l sec-l
e,-
+ Na+
(Na+,e,-)
(6)
k s = 7.9 X 1011M-l sec-l
(Na+,e,-)
+ (PhCH&Hg k7
= 7.9 X
-
PhCH2-,Na+
lo9 M-l
sec-l
The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
+ PhCH2Hg(7)
This is a convenient way of generating benzyl carbanion for fast reaction studies, and much information about its absolute reactivity has been obtained. The method may be extended to other carbanions5 and other solvents. Acknowledgment. This work was supported by the United States Energy Research and Development Administration. References and Notes (1)M. S.Matheson and L. M. Dorfman, J. Chem. Phys., 32, 1870 (1960). (2) L. M. Dorfman, Acc. Chem. Res., 3 , 224 (1970). (3)E. Bockrath and L. M. Dorfman, J. Am. Chem. SOC.,g6,5708(1974). (4)E. Bockrath and L. M. Dorfman, J. Phys. Chem., 77, 1002 (1973). (5)E. Bockrath and L. M. Dorfman, J. Am. Chem. soc., 97,3307(1975).
Discussion T. TUTTLE. While the stoichiometry of the species you identify as Na+,e,- is undoubtedly correct, in view of the very large shift in the optical spectrum attributed to the association, I wonder whether some more substantial interaction between the electron and Na+ is involved as for example in the solvated monomer. €3. BOCKRATH.It would seem that the question of the structure of the 890-nm band cannot be answered solely upon the basis of the magnitude of the shift of the absorption maximum observed upon association.
L. M. DORFMAN.The question to which you are drawing attention. the structure of the ion Dair. involves the degree of interposi~. tion of solvent, Le., to what extent is it a solvent-separated ion pair? J. W. Fletcher's paper will likely deal with this, as will the comparison I shall make with other alkali metals, notably lithium. In our kinetic studies we refer only to the stoichiometry. J. W. FLETCHER.We have attempted to correlate the position of the absorption of Na+.e,- in T H F and other solvents with percent atomic character of the monomer as determined by ESR. This is considered in our paper.
