COMMUNICATIONS TO THE EDITOR
3019
adequately described by a single process with a relaxation time 3-6 X sec and an associated volume change of 6-8 cm3/mole of monomer. The dependence of the absorption on pH is consistent with the notion that the process is the helix-coil transition. (5) E. J. Cohn and J. T. Edsall, “Proteins, Amino Acids and Peptides as Ions and Dipolar Ions,” Reinhold Publishing Corp., New York, N. Y., 1943. (6) G. Schwarz, J . Mol. Biol., 11, 64 (1965). (7) D. W. Davidson and R. H. Cole, J . Chem. Phys., 19, 1484 (1951).
DEPARTMENT OF CHEMISTRY OF WASHINGTON UNIVERSITY SEATTLE, WASHINGTON 98105
R. C. PARKER K. APPLEGATE L. J. SLUTSKY
RECEIVED JUNE 16. 1966
Geminate Recombination in Photochemistry:
A First-Order Process‘ ao
1.0
2.0
3.0
4.0
5.0
Ln If) Wc/rrc)
Figure 1. Ultrasonic absorption as a function of frequency in 0.156 M (moles of monomer/liter) poly-blysine solutions in 0.6 M NaCl a t 35.8” and various values of the pH. Solid linei are least-square fits t o the single relaxation equation. T h e classical value of a/j2 is 13.95 x 10-36 db sec2 cm-1, in good agreement with t h e value for water interpolated from the d a t a of Herzfeld (see ref 3, p 358).
tion equilibrium. There is no absolute assurance that similar phenomena are not involved in our results; indeed the volume change caIcuIated is rather close to that estimated by Cohn and Edsal16for the solvation of a singly ionized group. However, the dependence of C on the pH, much more pronounced than that observed by Burke, Hammes, and Lewis, and in particular the virtually negligible excess absorption at low pH would argue against such an interpretation. The observed variation of C with fH is consistent with the notion that absorption due to perturbation of the helix-coil equilibrium is being observed. Our data do not show a significant increase in the relaxation time when f~ is increased as predicted by Schwarz.8 Howtver, we are restricted to rather low values of f H by solubility limitations and such behavior cannot be ruled out. When attempts were made to analyze our data in terms of a Davidson-Cole’ distribution of relaxation times, somewhat surprisingly, values of p close to 1 resulted indicating a satisfactory fit in terms of a single relaxation. I n summary, the excess acoustic absorption in polylysine solution may be
Sir: The dependence of G values and quantum yields on solute concentration in radiation chemistry2P3and photo~hemistry~-~ has been theoretically interpreted by the use of diffusion kinetics. A recent critical review of the pertinent literature in radiation chemistry led the author’ to induce an alternative model: “excited water,” either an electronically excited state or the H30-OH radical pair which undergoes geminate recombination by a first-order process, is precursor of intraspur hydrogen in the radiolysis of water. Woodss recently published very important results in the photochemistry of iron(I1) in 1 M sulfuric acid. The dependence of @(Fexxl)and @(As“I) on arsenic acid concentration is identical both with the dependence of intraspur hydrogen G values on solute concentration’ and with the dependence of geminate recombination on solute concentration deduced by Noyes5 in that solute concentrations must exceed 0.01 M for significant effects. However, Woods0 concluded that the effect of arsenic acid was not due to inhibition of geminate (1) Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corp. (2) A. Samuel and J. L. Magee, J . Chem. Phys., 21, 1080 (1953). ( 3 ) A. Kuaaermann in “The Chemical and Bioloeical Actions of _Radiations,” Vol. 5, M. Haissinsky, Ed., Academic h e s s Inc., New York, N. Y., 1961. (4) J. C. Roy, R. R. Williams, and W. H. Hamill, J . A m . Chem. SOC., 7 6 , 3274 (1954). ( 5 ) R. M. Noyes, ibid., 77, 2042 (1955). (6) L. J. Monchik, J . Chem. Phys., 24, 381 (1956). (7) T. J. Sworski, J . Am. Chem. SOC.,8 6 , 5034 (1964); Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 263. (8) R. Woods, J . Phys. Chem., 70, 1446 (1966).
Votume 70,Number 9 September 1966
COMMUNICATIONS TO THE EDITOR
3020
recombination since @(Fe"I) and @(As"') are not linearly related to the square root of arsenic acid concentration. The dependence of A@(Fe'I') and @(As1") on arsenic acid concentration is quantitatively given by
could be attributed to inhibition of secondary recombination while decrease of intraspur hydrogen could be attributed to inhibition of primary recombination.
+ 0.297/[AsV] (I) 0.083) + 0.493/[AsV] (11)
(9) J. Jortner, M.Ottolenghi, and G. Stein, J . Phys. Chem., 68, 247 (Ig6*). (10) F. S. Dainton and S. R. Logan, Proc. Roy. Soc. (London), AZ87,
l/A@(E'e"I) = (0.645 rt 0.068) 1/@(As1I1)= (1.007
f
with constants and standard deviations determined by the method of least squares. A@(FeI") is the increase in @(Fe"') induced by arsenic acid. Equations I and I1 are consistent with a reaction mechanisrn based on the following assumptions: (1) light absorption yields "excited ferrous ion" which disappears by a first-order process with rate constant ; arsenic acid reacts only with Fel'* with of 1 / ~ (2) rate constant of k to yield arsenic(1V) which oxidizes iron(I1); and (3) the quantum yield for the process which yields iron(II1) in the absence of arsenic acid is reduced by a factor equal to the fraction of Fe"* which reacts with arsenic acid. This reaction mechanism yields eq I' and 11' l/A@(FP1) = (1
+ l/rk[Asv ])/1.777@(Fe1'*)
~ / A @ ( A S ~= I I )(1
(1')
+ l/rk~AsV])/~(Fe"*) (11')
Equations I' and I" become identical with the equations of Woodss with the following substitutions: rk = E ~ K / Iand E ~ @(Fe"*) = Az' = ( A z - A1)/1.777. Thus, the two alternative models give the same dependence of @(FeI'I) and @(As'") on arsenic acid concentrat ion. Equations I and I1 yield values for @(FeT1*)of 0.87 and 0.99, respectively, equal within standard deviations and consistent with @(FeI'*) = 1. They also yield values of 2.04 and 2.17, respectively, for rh. Assuming reaction of arsenic acid with Fe"* is diffusion controlled, r is to sec. There is, therefore, similarity between HzO* and Fe"*. Fel'* may be an electronically excited state or may be either the FelI1-eaq-, FeIII-H, or Fe"'-H30 radical pair which undergoes geminate recombination by a firstorder process. Other recent studiesg-12 in the photochemistry of aqueous solutions indicate that inhibition of geminate recombination is sensibly complete with solute concentrations about 0.01 M . From this apparent contradiction, we may conclude that secondary recombination is inhibited by 0.01 M solute while reaction of solute with Fe"* inhibits primary recombination. If this be true, it is of great significance in the radiation chemistry of water since decrease of interspur hydrogen The Journal of Physical Chemwtry
281 (1965). (11) F. S.Dainton and P. Fowles, ibid., AZ87, 312 (1965). (12) P. L. Airy and F. S. Dainton, ibid., A292, 340 (1966).
CHEMISTRY DIVISION RIDGEN.4TIONAL LABORATORY OAKRIDGE,TENNESSEE 37831 RECE~VED JUNE 28, 1966
T. J. SWORSKI
OAK
Reactivity of Electron-Donor-Acceptor Complexes.
111. Hydrogen Exchange
between Acetylene and Organic Electron-Donor-Acceptor Complexes
we have studied the reacSir: In previous papers*r2 - tivity of -electron-donor-acceptor (EDA) complexes. It was found that the reactivity of such compounds as phthalocyanines and aromatic hydrocarbons increased remarkably when they were brought into contact with sodium, an electron donor, by forming EDA complexes. Accordingly, the exchange reaction of hydrogen between acetylene (or molecular hydrogen) and the complexes takes place at room temperatures, while it does not proceed in the absence of electron donor even a t 200". I n this report phenothiazine is employed as an organic electron donor, and the reactivity of the EDA complexes formed with various organic electron acceptors is studied. Phenothiazine was purified by repeated recrystallization and sublimation. A film was evaporated on the surface of a glass vessel under vacuum, to which various electron acceptors such as 2,3-dicyanoquinone, tetracyanoquinodimethane (TCNQ), pyromellitic dianhydride, 1,3,5-trinitrobenzene. p-chloranil, and 2,3dicyano-5,6-dichloroquinone were sublimed, which resulted in deep green, red-violet, green-black, gray-green, dark green, and deep green complexes, respectively. The hydrogen exchange reaction between the complexes and acetylene was studied in the temperature range between 25 and 140" under an acetylene pressure (1) M. Ichikawa, M. Soma, T . Onishi, and K Tamaru, J . Phys. Chem., 70, 2069 (1966). (2) M. Ichikawa, M. Soma, T . Onishi, and K. Tamaru, J . Catalysis, accepted for publication.
