The Journal of Physical Chemisfry, Vol. 82, No. 6, 1978 749
Communications to the Editor
relation ( n = 2.4, A", = 7.6 kcal/moP8). Taking, thus, w d = 0.13 eV yields W d / h p = 1.6 X deg-', a value higher for indole and than those of both coefficients Co (0.4 X 0.55 X for phenol). Addition of the quencher was then expected to increase the temperature coefficients in both cases, and the increase was indeed observed experimentally (Figure 2, curves c and d). Again a common asymptotic value was reached by both coefficients at higher quencher concentration, whose value [(C,), = 1.7 X deg-l] agrees with that of W d / h p estimated above.
'r\ I
0.25
0.50
His
0.50
1.0
CClr
Molar Concentratlon of Ouencher
Figure 2. Influence of the quencher concentration on the fluorescence temperature coefficlent (C,) for (a) indole histidine/water, pH 5; (b) phenol histidinelwater, pH 5; (c) indole CCi,/dioxane; (d) phenol CCi,/dioxane.
+
+
+ +
N AHv/n,14 where AHv is the heat of vaporization (10 kcal/mol for water18) and n = 2.4 for associated liquids: wd 0.17 ev. At 30 " C (kp= 8 eV deg), one then has W d j h p = 2.1 X deg-' a value greater than that of Cofor phenol (0.85 X but smaller than that of Co for indole (3.0 X Addition of the quencher was therefore expected to increase the temperature coefficient of the phenol solution and to decrease that of the indole solution. Results presented in Figure 2 (curves a and b) show that both effects were observed, and that the two experimental curves have the predicted common asymptote at high quencher concentrations ([Q] > 0.4 M), whose ordinate [(C,), N 2 X lo-' deg-l] is furthermore in excellent agreement with the estimated value of W d / h P . (b) Dioxane Solutions. Using C C 4 as the quencher, similar experiments were performed in dioxane, where the estimated values of w d are 0.125 eV from the thermal dependence of vi~cosity'~ and 0.138 eV from Eyring's
Conclusion This study shows that dynamic quenching may either increase or decrease the thermal dependence of fluorescence in solution, according to the properties of both solvent and solute. It also shows that the temperature coefficient is a useful parameter that permits quantification of this phenomenon and may also give information on the activation energy for diffusion.
References and Notes J. B. Birks in "Organic Molecular Photophysics", Vol. 1, J. 8. Birks,
Ed., Wiley, London, 1973, pp 32-41. M. S. Walker, T. W. Bednar, and R. Lumry in "Molecular Luminescence", E. C. Lim, Ed., W. A. Benjamin, New York, N.Y., 1969, pp 135-152. M. S. Walker, T. W. Bednar, R. Lumry, and F. Humphries, Photochem. Phofobiol., 14, 147 (1971). E. P. Klrby and R. F. Steiner, J . Phys. Chem., 74, 4480 (1970). G. Laustriat and D. Gerard in "Excited States of Biological Molecules", J. B. Birks, Ed., Wiley, London, 1976, pp 388-399. D. Gerard, G. Laustriat, and H. Lami, Biochim. Biophys. Acta, 263, 482 (1972). C. F. Chen, Anal. Leff., 1, 35 (1967). J. Eisinger and G. Navon, J . Chem. Phys., 50, 2069 (1969). G. Pfeffer, H. Lami, G. Laustriat, and A. Coche, Colloque International d'Electronique NuclBaire, OCDE, Paris, 1963. N. Glasser, private communication. K. K. Turoverov and 8. V. Shchelchkov, Blofizika, 15, 600 (1970). R. Voltz, J. Kleln, F. Heisel, H. Laml, G. Laustriat, and A. Coche, J . Chlm. Phys., 63, 1259 (1966). A. M. North, "The Collision Theory of Chemical Reactions in Liquids", Wlley, New York, N.Y., 1964, pp 19-36. S.Glasstone, K. Laidler, and H. Eyring, "Theory of Rate Processes", McGraw-Hill, New York, N.Y., 1941, Chapter 9. M. Shinitzky and R. Goidman, f u r . J . Biochem., 3, 139 (1967). D. Gerard, Thesis, University of Strasbourg, 1975. "American Institute of Physics Handbook", 2nd ed, McGraw-Hill, New York, N.Y., 1963, p 11-181. "Physikalisch-ChemlscheTabellen", 5th ed, W. A. Roth and K. Scheel, Ed., Berlin, 1931, pp 1483-1490.
COMMUNICATIONS TO THE EDITOR terated and deuterated alcohols (and with some ethers) were determined by a competition method which utilizes the reaction3 of S O 2 with 1,3,5-trimethoxybenzene (TMB) 580 nm, E 5700 M-I cm-l). Since the to yield TMB ,A( Pubilcatlon cost assisted by the Institut fur Strahienchemle rate constant for reaction of SO4-. with TMB is used as a standard, this rate constant was redetermined by pulse irradiating, with 2.8-MeV electrons, argon-saturated Sir: The SO4-- radical has been shown to react with aqueous solutions at pH 7-8 containing 1mM K2S208and benzene derivatives by electron transfer.1-6 A similar 0.01-0,l mM TMB and monitoring the buildup of TMB mechanism has also been proposed for the reaction of S O 2 with alcohol^.^ However, it has recently been s u g g e ~ t e d ~ ~ a~t 580 nm. h(S04-.+ TMB) was found to be (2.4 f 0.5) X lo9 M-l s-l, which is slightly higher than the value3 that this reaction involves H abstraction. In order to reported previously. For determination of the k(S04-. elucidate the reaction mechanism, the rate constants for alcohol) values, the solutions pulse-irradiated typically the reaction of SO4-. in aqueous solution which undeuPulse Radiolysis and Electron Spln Resonance Studies Concerning the Reactlon of SO,-. with Alcohols and Ethers in Aqueous Solution
'-
+. +
0022-3654/78/2082-0749$01 .OO/O
@ 1978 American Chemical Society
750
The Journal of Physical Chemistry, Vol. 82, No. 6, 1978
Comrnunlcations to the Editor
TABLE I: R a t e Constants for t h e R e a c t i o n SO,-. with A l c o h o l s a n d Ethers D e t e r m i n e d a t 20 kH,b M" Substrate
S-I
k D b M-'
t
M-' kH/kD
S-I
2 'Ca
k(OH t
substrate):
S-'
M-1 s-1
9 x lo8 Methanol 3.2 X 1.2 x 2.7 1.1x 1.7 x 109 Ethanol 1.6 x 107 6.7 X l o 6 2.4 7.9 x 2-Propanol 3.2 x 107 1.2 x 10' 2.7 3.2 X 1 0 ' 2.0 x 1 0 9 4.5 x 108 2 - M e thyl-2-pro pan01 4.0 x 105 4.4 x 1 0 4 d 1,4-Dioxane 1.6 x 107 9.2 X l o 6 1.7 1.8 x 109 2.7 x 109 Tetrahydrofuran 1.0 x 108 5.1 X 10' 2.0 a R a t e constants for r e a c t i o n of OH with t h e substrates are i n c l u d e d for comparison. k H ( k D ) are t h e r a t e constants for with u n d e u t e r a t e d (deuterated) substrates. .reaction of SO.; k, = ( k H - npk )/n,, w h e r e n,(np) is t h e n u m b e r of H a t o m s a t C,(Cp). kp is t h e r a t e c o n s t a n t for a b s t r a c t i o n of o n e H a t o m from kp = k(S0;. t 2-methyl-2-propanol). Kp. e Averaged values, t a k e n from r e f 20.
lo6
lo6 lo6
lo6
4.
contained 0.01-0.5 M alcohol, 1 mM K2S208,and 0.1-0.2 mM TMB. As seen in Table I, the rate constants for reaction of SO4-.with the alcohols and ethers containing C-H bonds are a factor of 2-3 larger than those for reaction of SO4-. with corresponding C-deuterated substrates. The reaction of SOP with alcohols and ethers is thus seen to involve the breaking of a C-H (C-D) bond. The kH/kDvalues for the reaction of SO4-. are slightly larger than thoselO determined for the reaction of OH with alcohols which also proceeds by H abstraction. Except for the case of 2-propanol, the k ( S 0 p + alcohol) values determined in the present study are lower than t h 0 ~ e l l - lreported ~ previously. The latter values were measured by monitoring the decay of Sop. As pointed out by Henglein,ll this method is likely to yield values which are too high, due to contribution of second-order decay of SO4-.. Column 5 of Table I contains the partial rate constants k, (rate constant for abstraction of one H atom from C,) for SO2 attack on the alcohols. In calculating these values it was assumed that the partial rate constant k, (rate constant for abstraction of one H atom from C,) is equal to 1/9 k ( S 0 p 2-methyl-2-propanol) for all alcohols and that the contribution of H abstraction from the alcoholic OH group is negligible (see ESR data). The partial rate constants k, increase strongly with increasing alkylation a t C,. This effect is suggested to reflect the electrophilicity6 of SOP which is larger than that1°J4 of OH. A plot of log. (k(S04-. substyate)/k(SO4-. + methanol)) vs. log (k(0H substrate)/k(OH + methanol)) yields a straight line with a slope corresponding to 3 , which demonstrates that SO4-. is more sensitive than OH with respect to the changes at C, induced by a-alkylation.lj This leads to an increased selectivity of S O 2 as compared to that1°J6 of OH with respect to H abstraction from C, vs Cp. On the basis of the k, and k, values, in the reaction of SO4-. with ethanol and 2-propanol the yield of H abstraction from C, is >99%. In order to further test the mechanisms of reaction of Sop with alcohols, ESR experiments were carried out. If SO4-- reacts with alcohols by electron transfer, alkoxyl radicals should be formed via deprotonation of the initially produced radical cations. It has been demonstrated17J8 that alkoxyl radicals can be scavenged by the aci anion of CH3N02. On photolysis of 50 mM K2S20ssolutions at pH 10-11 containing 1-2 M methanol, ethanol, 1- or 2propanol, and 10-100 mM CH3N02no spin adducts of the type ROCHzNOB-.were detected. In all cases the radicals detected were of or derived from the a-hydroxyalkyl type and P-hydroxyalkyl radicals were not observed, with the exception of the 2-methyl-2-propanol system where the radical CHz(CH3)2C(OH)was found to be produced. The
+
+
+
2-methyl-2-propoxyl radical is known to undergo P fragmentation to yield methyl radical which subsequently adds to CH2N02-to yield CH3CHzH02-..17On reaction of SO4-with 2-methyl-2-propanol in the presence of CH2NOf, the radical HOC(CH3)zCH2CHzN02-.was the only species observed which was derived from 2-methyl-2-propanol.le For comparison, on production of 2-methyl-2-propoxyl radicals by photolysis of di-tert-butyl peroxide in the presence of CH2N02-intense lines from CH3CH2N02-. were seen. From the signal-to-noise ratio observed for HOC(CH3)2CH2CHzN02-in the 2-methyl-2-propanolK2S2O8-CH2N0~ system and assuming equal lifetimes for this radical and CH3CH2N0pit is concluded that the rate constant for H abstraction from the OH group by SOP is a factor 14 smaller than the measured rate constant for reaction of SO4-. with 2-methyl-2-propanol. The results of the ESR experiments are therefore in support of the results of the kinetic studies described above further demonstrating that the reaction of SO4-. with alcohols proceeds by H abstraction from a C-H bond and not by electron transfer.
References and Notes 0. P Chawla and R. W. Fessenden, J. Phys. Chem., 79,2693 (1975). H. Zemel and R. W. Fessenden, J. Phys. Chem., 79, 1419 (1975). P. O'Neill, S. Steenken, and D. Schulte-Frohlinde, J . Phys. Chem., 79, 2773 (1975). S. Steenken, P. O'Neill, and D. Schulte-Frohllnde, J . Phys. Chem., 81, 26 (1977). K. Sehested, J. Holcman, and E.J. Hart, J . Phys. Chem., 81, 1363 (1977). P. Neta, V. Madhavan, H. Zemel, and R. W. Fessenden, J. Am. Chem. SOC.,99, 163 (1977). A. Ledwith, P. J. Russell, and L. H. Sutcliffe, Chem. Commun., 964 (1971). K. M. Bansal and R. W. Fessenden, Radiat. Res., 67, 1 (1976). P. Maruthamuthu and P. Neta, J . Phys. Chem., 81, 1622 (1977). M. Anbar, D. Meyerstein, and P. Neta, J. Chem. SOC.6, 742 (1966). E. Heckel, A. Hengleln, and G. Beck, Ber. Bunsenges. Phys. Chem., 70, 149 (1968). E. Hayon, A. Treinin, and J. Wilf, J. Am. Chem. SOC.,94, 47 (1972). J. L. Redpath and R. L. Willson, Int. J . Radiat. 6/01.,27, 389 (1975). M. Anbar, D. Meyerstein, and P. Neta, J. Phys. Chem., 70, 2660 (1966). A slmilar effect Is shown by the H2P04.radical (see ref 9). K.-D. Asmus, H.Mockel, and A. Henglein, J. Phys. Chem., 77, 1218 (1973). B. C. Gilbert, R. G. G. Holmes, H. A. H. Laue, and R. 0. C. Norman, J . Chem. SOC., Perkin Trans. 2, 1047 (1976). Y. Kirino and R. W. Fessenden, Mellon Institute of Science, Radlatlon Research Laboratories, Quarterly Report, July 1-Sept 30, 1974, p 10.
In addition, radicals derived from reaction of SO -- with CH2NOpwere
seen (see ref 1 for information on the S20)--CH2N02- system). L. M. Dorfman and 0. E. Adams, Nail. Stand. Ref. Data Ser., Nafl. Bur. Stand., No. 46 (1973). Instiiui fur Strahlenchemie im Max-Planck-Instltut fur Kohlenforschung 0-4330 Mulheim, West Germany
H. Elbenberger
S. Steenken' P. O'Nelll D. Schulte-Frohllnde
Received December 27, 1977
