The Journal of Physical Chemistry, Vol. 83, No. 19, 7979
Communications to the Editor
cage than reactions between pyrazyl and alcohol radicals so that the radical pair is removed but the cage is not changed by diffusion. In any of the possibilities described above, the rate of the pyrazyl radical stabilization as observed by EPR represents that of a hydrogen abstraction reaction. In conclusion, activation energies for pyrazyl stabilization and phosphorescence quenching agree with each other and are 2.1-2.3 kcal mol-l. This value shows the activation energy for the hydrogen abstraction reaction by
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the triplet pyrazine. The effect of the solvent cage is negligible in this case.
References and Notes (1) D. V. Bent, E. Hayon, and P. N. Moorthy, Chem. fhys. Lett., 27,
544 (1974). (2) S.J. Formosinho, J. Chem. Soc., Faraday Trans. 2,72, 1332 (1976). (3) H. Murai, M. Jinguji, and K. Obi, J . fhys. Chem., 82, 36 (1976). (4) R. Livingston and W. R. Ware, J . Chem. fhys., 39, 2593 (1963). (5) M. H. Cohen and D. Turnbull, J . Chem. fhys., 31, 1164 (1959). (6) 0.Haida, H. Suga, and S.Seki, f r o c . Jpn. Acad., 48, 663 (1974).
COMMUNICATIONS TO THE EDIT0 Comments on ”Reduction of Cobalticytochrome c by Dithionite” Publication costs assisted by the National Science Foundation
Sir: It was reported recently1 that the reduction of cobalticytochrome c by dithionite is biphasic. From semilogarithmic plots of (OD, - OD,)/(OD, - OD,) vs. time the authors1 claim to have extracted first-order rate constants hZobsdand kgobsdfor the fast and slow portions of the reactions, respectively. The interpretation given by the authors’ to account for the biphasic behavior is encompassed in eq 1-4. On the basis of this scheme, the
soz- + cocyt c+ sz0:-
+ cocyt e+
Sz04- + Cocytc+
-so2 + -szo4- + ka k3
k4
COcyt c
(2)
be found for the observed biphasic behavior. Perhaps, as suggested by the authors,l there may be two protein species or Cocytc+ exists in two conformations. Finally, it must be noted that, because of the incorrect mechanistic assignment, any conclusions regarding the agreement between the theory of vibronically coupled electron tunneling3 and the experimentally determined kinetic parameters must be regarded with suspicion.
References and Notes (1) Chien, J. C. W.; Dickinson, L. C. J . Biol. Chem. 1978, 253, 6965-6972. (2) Lambeth, D. 0.; Palmer, G.J. Biol. Chem., 1973, 248, 6095-6103. (3) Hopfiekl, J. J. froc. Nafl. Acad. Sci. U . S . A . ,I 9 7 4 71, 3640-3644. Department of Chemistry State University of New York Stony Brook, New York 1 1794
Albert Halm
Received December 27, 1978
COcyt c
(3)
2SOz + Cocytc
(4) authors concluded that k 2 = ~ z o b a d ( [ S 2 0 4 2 ~and ] Kh3 ~)~1~2 = h20bSd/, TS402-1. . Comment on “Picosecond Time-Resolved +
I
The purpose of this communication is to point out that the proposed interpretation of the observed biphasic behavior cannot be correct, and in fact that the interpretation is internally inconsistent. For the proposed mechanism to be consistent with a fast and a slow firstorder disappearance of Cocytc+, it is necessary that the equilibrium between SOz- and Sz04- be established slowly compared to the rates of reactions 2 and 3 and that [SO,-], < [Cocytc+],. However, under these circumstances, the concentration of SOz- is depleted as the reaction proceeds and therefore the disappearance of cOcytc+ corresponding to the rapid phase cannot proceed according to an exponential decay as claimed.’ The same argument can be viewed in an alternate way. In order for h2 to be calculated as hZobad( [S202-]K1)-1/2, it is necessary that SOL be always in equilibrium with S204’-. However, under these circumstances, it is predicted that the disappearance of cOcyt c+ will be first order in Cocytc+ and not biphasic, as observed. Using the known2 rate constants for the forward and reverse reactions in eq 1,we can see that the rates of the two reactions reported in the paper under consideration2 are sufficiently slow for SO2- to be in equilibrium with Sz042-at all times. Therefore, another explanation must 0022-3654/79/2063-2553$0 1.OO/O
Spectroscopic Study of Solvated Electron Formatlon from the Photoexcited P-Naphtholate Ion” Publication costs assisted by Argonne National Laboratory
Sir: Recently, Matsuzaki, Kobayashi, and Nagakural (MKN) reported a rate of solvation of the electron of 1.8 X 10’O s-l (55 ps risetime) in 6.2 M aqueous NaOH. We feel this value is erroneous. In Figure 1 we show experimental data from a picosecond pulse radiolysis experiment.2 It is clear that the growth of the absorption of the hydrated electron in NaOH is virtually identical with that in water where the solvation time is in the 1-5 ps ~ a n g e .In~ Figure 1 of ref 4 we show that we can easily resolve the solvation time of the electron in ethanol (18 ps? 23 ps5). From the data shown it is clear we could observe a solvation time as short as 15 ps for the electron in 6.2 M NaOH if such a solvation time existed. It is clear then that the solvation time for the electron in 6.2 M NaOH is less than 15 ps. We do not presume to know the reasons for the disparity between our experimental results and theirs but we suspect they are not looking at the solvated electron. In Figure 2 we show the spectrum of the solvated electron in 6.2 M 0 1979 American
Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
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Communications to the Editor
the electron. This same circumstance may be present in water-NaOH mixtures. In conclusion, we feel that it is unlikely the electron has a solvation time of 55 ps in 6.2 M NaOH. The reason for their results are not clear; however, they may be due to observing a different species than the hydrated electron.
References and Notes (1) A. Matsuzaki, T. Kobayashi, and S. Nagakura, J . Phys. Chem., 82, 1201 (1978). (2) C. D. Jonah, Rev. Sci. Instrum., 46, 62 (1975). (3) P. M. Rentzepis and R. P. Jones, J. Chem. Phys., 59, 766 (1973). (4) G. A. Kenney-Wallace and C. D. Jonah, Chem. Phys. Lett., 39, 596 (1976). (5) W. J. Chase and J. W. Hunt, J . Phys. Chem., 79, 2835 (1975). (6) E. J. Hart and M. Anbar, "The Hydrated Electron", Wiiey-Interscience, New York, 1970, p 40. (71 Reference 6. D 49. (8) U. K. Kianning, C . R. Goldschmidt, M. Qttolenghi, and G. Stein, J . Chem. Phys., 59, 1753 (1973). (9) C. D. Jonah and J. R. Miller, unpublished results.
TIME (48ps/div)
Figure 1. Superposition of the growth of absorption of the hydrated electron at 600 nm in water (solid line) and in 6.2 M NaOH (dotted line). The two curves were normalized together since the electron density and energy deposition is higher in the sodium hydroxide solution.
Chemistry Division Argonne National Laboratory Argonne, Illinois 60437
Charles D. Jonah
Received February 5, 1979
Theory of the Structure and Excited State Reactivity of Alkyl Substituted Cyclopropanes. Irnpllcations for Thermal Reactions Publication costs assisted by the University of Connecticut
.o
450
600
750
WAVELENGTH (nm)
Flgure 2. Spectrum of the hydrated electron in 6.2 M NaOH at 1 ns after the pulse.
NaOH. Note that the peak of the spectrum is at 675 nm not a t 760 nm as shown in Figure 1 of MKN. That 675 nm is reasonable for the peak of the spectrum, recall that the peak of the solvated electron is a t 715 nm6 and that the spectrum of the solvated electron shifts toward the blue7 in the presence of high concentrations of salts. Therefore, a peak at 675 nm is consistent with previous experimental work while a peak at 760 nm is not. We do not know the identity of the species giving rise to the spectrum shown by MKN. However, we do note that the spectrum is strikingly similar to that of the spectrum assigned to the P-naphthol singlet by Klanning et al.7 (see Figure l b of ref 7) with a maximum around 760 nm. This or the excited singlet state of the P-naphtholate ion are possible species since the electron is presumed to be formed by the photoejection of an electron from the 6-naphtholate ion in their experiments. MKN have explained the relatively long solvation time of the electron as being due to the high viscosity of the solution. While viscosity is a good predictive parameter for solvation times in linear alcohols, it is not a good parameter in systems with many OH bonds; for instance ethylene glycols and glycerol-water mixturesg have very fast solvation times. This we suspect is due to the fact that the molecule need not be completely rotated, as an alcohol might have to be, to bring an OH bond to interact with 0022-3654/79/2083-2554$0 1.OO/O
Sir: The reactivity of substituted cyclopropanes continues to be of great interest to experimental and theoretical chemists.' The types of transformation available to cyclopropane rings are shown in Figure 1. Pathways a (stereomutation) and b (isomerization) are accessible thermally and have many examples.2 Recently, products via pathway c have been reported by photochemical excitation of alkyl substituted cyclopropane^.^^^ The purpose here is to show that products obtained by pathway c are consistent with a two-step mechanism involving the initial cleavage of a single C-C bond followed by the breaking of the second bond as is shown in eq 1. An interesting and
as yet unexplained feature of both the thermal and excited state reactions is the fact that a more substituted C-C bond, 1, is usually broken rather than a less substituted CH3
I
1
CH3
I
2
bond, 2.5 Potential energy curves for the opening of the CCC bond angle, 8, in cyclopropane and a series of related alkyl substituted cyclopropanes is given in Figure 2a for the ground state and in Figure 2b for the first excited singlet state.'!*
0 1979 American
Chemical Society
