J. Phys. Chem. 1984, 88, 5660-5665
5660
uncertainty in AfHo(C2H5). The mechanisms of pyrolysis are complex, but they have been repeatedly studied. Taking kl,- from this work, ke1,-from ref 41 or 42, and other thermodynamic properties of CH3and C& from ref 12, one may calculate that AfH02ss(CH3)equals 146.9 f 1.641or 146.5 f 1.842 kJ mol-’. Here the rate coefficients are pressure dependent and the temperatures do not overlap. Nevertheless, the results agree well with most of the recent values: 145.7 f 0.8,43146.0 f 0.5,” 146.9 f 0.6,45 and 144 f 346kJ mol-’. The change in ArHO(C2H5)is sufficient to change the predicted thermochemistry for some reactions from endothermic to exothermic. For transfer of the radicals X CH3X + C2H5 CH3 + C2H5X (16) +
the following enthalpies of reaction (in kJ mol-’) have been ~alculated:~~””*~’ H, 17.9 f 1.7; CH3, 6.8 f 1.7; I, 4.2 f 0.5;4 Br, 2.3 i 2.2; NH2, 2.9 f 1.8; C1, -2.7 f 1.8; OCH3, -5.0 f 1.8; OH, -5.8 f 1.7. According to the final number, the C-0 bond in ethanol is about 6 kJ mol-’ stronger than that in methanol.48 (41) Pacey, P. D.; Wimalasena, J. H. J. Phys. Chem. 1980,84,2221-2225. (42) Macpherson, M. T.; filling, M. J.; Smith, M. J. C. Chem. Phys. Lett. 1983, 94, 430-433. (43) Stull, D. R.; Prophet, H. “JANAF ThermochemicalTablRs”, 2nd ed.; National Bureau of Standards: Washington, DC, 1971. (44) McCulloh, K. E.; Dibeler, V. H. J. Chem. Phys. 1976,64,4445-4450. (45) Baghal-Vayjooee, M. H.; Colussi, A. J.; Benson, S. W. J. Am. Chem. SOC.1978,100,3214-3215; Int. J. Chem. Kinet. 1979,II. 147-154. Heneghan, S. P.; Knoot, D. A,; Benson, S. W. Int. J . Chem. Kinet. 1981, 13, 677-69 1. (46) Traeger, J. C.; McLoughlin, R. G. J. Am. Chem. SOC.1981, 103, 3641-3652. (47) Pedley, J. B.; Rylance, J. “Sussex-N.P.L. Computer Analyzed ThermochemicalData: Organic and Organometallic Compounds;”University of Sussex: Sussex, U.K., 1977.
The negative numbers would have been positive with the thermochemistry of ref 5.
Conclusion Study of the establishment of the steady state in the pyrolysis of ethane has been shown to provide consistent values of the rate constants for chain initiation and for three reactions of ethyl radicals. Minor reactions and heat transfer have been taken into account but were found to have little effect. The rate constant for initiation, combined with a literature value for the reverse process, provides confirmation for the established thermochemistry of the methyl radical. The rate constant for the termination reaction, combined with literature data on the reverse process, indicates that the heat of formation of the ethyl radical is larger than the value most frequently quotede5 This method provides a new approach to the study of radical kinetics and thermochemistry, with a precision and sensitivity competitive with the best alternative techniques.
Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for a grant in support of this work. J.H.W. thanks the donors of the Dorothy J. Killam Trust for a scholarship. Registry No. C2Hs, 74-84-0; C2HS,2025-56-1; CH,,2229-07-4. Supplementary Material Available: Appendices describing the derivation of the rate expressions and presenting tables of data (13 pages). Ordering information is given on any current masthead page. (48) It is shown in ref 49 that this trend continues to isopropyl and tertbutyl alcohols. (49) Arnold, D. R.; Nicholas, A. M. Can. J . Chem., in press.
Charge-Transfer Complexation between Methylviologen and Sacrificial Electron Donors EDTA, Triethanolamine, and Cysteine’ D. R. Prasad and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 02215 (Received: February 10, 1984)
Methylviologendication (MV”) forms charge-transfer (CT) complexes with EDTA, triethanolamine (TEOA), and cysteine (Cys) in aqueous solution. The complexes of EDTA and TEOA show enhanced tail absorption from 320 nm well into the visible region; the Cys complex shows a discrete absorption band in the 350-380-nm region. By use of the Benesi-Hildebrand and other treatments for the variations of the absorbances of the complexes with the concentrations of the components, values of KW and tAfor the complexes are obtained as functions of pH. Values of Keq range from 0.29 M-’ for the Cys complex at pH 3.5 to 18 M-’ for the EDTA complex at pH 11.2; Keq values are significantly higher in alkaline solution where the deprotonated donors are better reducing agents. The emission at 525 nm observed in these and other MVZfsystems is believed to arise from a highly luminescent impurity, not a CT complex. The ramifications of CT complex formation on the photochemistry of systems containing MV2+ and sacrificial electron donors are discussed.
Introduction By far, the most widely studied system for the photoreduction of water and the generation of H2 has been the one involving methylviologen (l,l’-dimethyl-4,4’-bipyridiniumdication; MV2+).2*3 Its photosensitized reduction to MV+., mediated by the excited states of R ~ ( b p y ) ~and ~ +other sensitizers, leads to the generation of Hz in the presence of colloidal metals4 or ( 1 ) Presented in part at the 186th National Meeting of the American Chemical Society, Washington, D.C., Aug-Sept, 1983; Abstract PHYS 127. (2) Koryakin, B. V.; Dzhabiev, T. S.; Shilov, A. E. Dokl. Akad. Nauk. SSSR 1977, 233, 620-622. (3) Kalyanasundaram, K. Coord. Chem. Reu. 1982,46, 159-244. (4) “Photogeneration of Hydrogen”; Harriman, A., West, M. A., Eds.; Academic Press: London, 1982.
hydr~genase.~For this procedure to succeed, a sacrificial electron-transfer agent must be present at sufficiently high concentration in order to reduce competitively the oxidized sensitizer (R~(bpy),~+ for, example) before it can react with MV+. and annihilate the energy carrier. The most popular sacrificial reductant has been EDTA;3v4*6 others used include triethanolamine,’ mercaptoethanol,8 and ~ y s t e i n e . ~EDTA is also effective in ( 5 ) Yu, L.; Wolin, M. J. J . Bacteriol. 1969, 98, 51-55.
(6) Harriman, A,; Mills, A. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 2 1 1 1-2 124. (7) Chan, S.-F.;Chou, M.; Creutz, C.; Matsubara, T.; Sutin, N . J. Am. Chem. SOC.1981, 103, 369-379. (8) Krasna, A. I. Photochem. Photobiol. 1980, 31, 75-82. (9) Krasna, A. I. Photochem. Photobiol. 1979, 29, 267-276.
0022-365418412088-5660$01.50/00 1984 American Chemical Society
Complexation between MV2+ and EDTA, TEOA, and Cys
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5661 0.90
0.75
0.60
0.45
0 30
0 15
0.00 0.00 ..
280
3iO
360
400
440
460
520
560
600
Wavelength (nm) Figure 1. Absorption spectra (1-cm cell) of aqueous solutions: curve A, 0.050 M Cys at pH 11.O; curve B, 0.020 M MVZt at ambient pH; curves C, D, E, hnd F, 0.020 M MVZt and 0.10 M Cys at pH 3.5,9.0, 10.0, and 11.0, respectively. Inset: Absorbance (1-cm cell) at 350 nm as a function of pH for 0.020 M MVZt in 0.10 M Cys.
repairifig the electron hole formed when semiconductor photosensitizers, such as CdS,I0 interact with MV2+. Now, MV2+forms chargetransfer complexes with a wide range of anions (halides,"J2 ~ulfides,'~ tetraphenylb~rate,'~ ben~ilate,'~.'~ hexa~yanoferrate(II),'~J~and carboxylic acid^'^*'^), amines,20 phenols, hydroquinor~es,'~ and Such complexation often gives rise to new absorption bands or increased tail absorption to the red of those of the components alone.23 From the dependence of the new absorbances on the concentrations of the components, it is possible to evaluate the equilibrium constant for the complexation reaction and the molar absorptivity of the complex as a function of ~ a v e l e n g t h . ~ ~ In a recent c o m m ~ n i c a t i o nwe , ~ ~reported that MV2+ forms photoactive charge-transfer complexes with EDTA and triethanolamine (TEOA) in aqueous solution. At the concentrations of these species normally used in the water photoreduction systems, complexation is not insignificant and could play an important role in the overall photochemical mechanism; we have shown that complexation in the R U ( ~ ~ ~ ) , ~ + / M V ~ + / system E D T Aleads to values of the quantum yield of MV+. formation that depend on the extent of ion-paired aggregation among the components.26 In this paper, we report the details of our spectrophotometric examination of the complexes formed between MV2+ and EDTA, TEOA, and cysteine (Cys). (IO) Duonghong, D.; Ramsden, J.; Griitzel, M. J . Am. Chem. SOC. 1982, 104, 2977-2985. (1 1) Ebbesen, T. W.; Levey, G.; Patterson, L. K. Nature (London) 1982, 298, 545-548. (12) Ebbesen, T. W.; Ferraudi, G . J . Phys. Chem. 1983,87, 3717-3721. (13) White, B. G. Trans. Faraday SOC.1969, 65, 2000-2015. (14) Sullivan, B. P.; Dressick, W. J.; Meyer, T. J. J. Phys. Chem. 1982, 86. - - , 1473-1 - - 418. (15) Barnett, J. R.; Hopkins, A. S.; Ledwith, A. J. Chem. SOC.,Perkin Trans. 2 1973, 80-84. (16) Deronzier, A,; Esposito, F. Noun J . Chim. 1983, 7, 15-19. (17) Nakahara, A,; Wang, J. H. J . Phys. Chem. 1963.67, 496-498. (181 Oliveira. L. A. A,: Haim. A. J . Am. Chem. SOC. 1982. 104. 3363-3366. (19) Jones, G., II., personal communication. (20) Poulos, A. T.; Kelley, C. K.; Shone, R. J. Phys. Chem. 1981, 85, 823-828. (21) Deronzier, A. J. Chem. SOC.,Chem. Commun. 1982, 329-331. (22) Poulos, A. T.; Kelley, C. K. J . Chem. Soc., Faraday Trans. 1 1983, 79, 55-64. (23) "Organic Charge-Transfer Complexes"; Foster, R., Ed.; Academic Press: London, 1969. (24) Benesi, H. A,; Hildebrand, J. H. J . Am. Chem. SOC.1949, 71, 77111-771-17 -. -- - . - . .
(25) Hoffman, M. Z.; Prasad, D. R.; Jones, G., 11; Malba, V. J . Am. Chem. SOC.1983, 105, 6360-6362. (26) Mandal, K.; Hoffman, M. Z. J . Phys. Chem. 1984, 88, 185-188.
0.05
0.10
0.15
0.20
0.25
[ Donor 1, d
0.30
0.35
(
0
Figure 2. Variation of absorbance at 350 nm as a function of [donor] for solutions containing 0.020 M MV2+and EDTA at pH 10.0 (m), TEOA at pH 10.0 (O), and Cys at pH 9.0 (A).
Experimental Section Methylviologen dichloride (Aldrich or BDH) was recrystallized several times from methanol and dried at 70 "C under vacuum for more than 24 h. EDTA (disodium salt; Baker Analyzed Reagent) was used without further purification. TEOA (Aldrich) was subjected to vacuum distillation and crystallization as the HC1 salt. Cys (hydrochloride monohydrate; Fisher) was recrystallized frdm water and dried under vacuum. Distilled water was passed through a Millipore purification train and was used in the preparation of all solutions. All spectral measurements, unless otherwise indicated, were performed at ambient temperature (-22 "C). When the temperature dependence of the equilibrium constant was evaluated, the temperature of the solutions was controlled to hO.1 "C. UV-vis absorption spectra of solutions contained in 1-cm cuvettes were recorded on a Cary 210 spectrophotometer. Emission and excitation spectra were recorded on Perkin-Elmer MPF 2A and 44A spectrofluorometers, the former equipped with a thermostated cell compartment and the latter with a differential corrected spectrum unit. Results Absorption Studies. Aqueous solutions of MV2+and EDTA or TEOA give rise to enhanced absorption from the near-UV into the visible spectral region; the absorption spectra have already been published.25 Mixtures of MV2+ and Cys yield a new absorption band in the 350-380-nm region; Figure 1 shows the dependence of the MV2+/Cys complex absorption on the composition of the solution. In contrast, the individual components of these systems at the same concentrations do not absorb appreciably above 320 nm. The magnitude of the absorption is a direct function of the concentrations of the components. Figure 2 shows the dependence of the absorption on [donor] at fixed [MV2+] (0.020 M) and pH. Although the MV2+/EDTA system shows a saturation effect at high [EDTA] in alkaline solution wherein the absorbance of the solution no longer increases upon further increase of [EDTA], such an effect is not observed in neutral or acidic EDTA solutions or for TEOA, even to the extent of neat alkaline solvent. No saturation effect was seen for Cys up to 0.2 M, the highest concentration that could be employed in alkaline solution without the spontaneous thermal formation of MV+-. Inasmuch as the donors undergo acid-base equilibration (pK,(EDTA): 0.0, 1.5, 2.0, 2.8, 6.1, 10.2; pK,(TEOA): 7.8; pK,(Cys): 1.9, 8.2, 10.3),27it is not surprising that the spectral absorptions due to the complexation of these species with MV2+ are a function of pH (see ref 25 for EDTA and TEOA and the (27) "Critical Stability Constants"; Martell, A. E., Smith, R. M., Eds.; Plenum Press: New York, 1974; for EDTA, Vol. 1, p 204; for TEOA, Vol. 2, p 118; for Cys, Vol. 1, p 47.
5662
Prasad and Hoffman
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
8 M
8
\o
-F
C V
> 3
n ! /
0.00
J
300
400
500
600
700
800
Wavelength (nm) Figure 3. Excitation and emission spectrum (corrected) of a solution = 395 containing 0.020 M MV2+ and 0.10 M EDTA at pH 10.0 (A,,, nm, Ae- = 525 nm). The short-wavelength tail of the emission spectrum is Rayleigh scattering of the exciting light, and the small peak at 445 nm is a Raman line of the solvent. Inset: Relative emission intensity (525 nm) as a function of pH.
inset in Figure 1 for Cys). The changes in absorption that are observed reflect the pK, values of the donors. In the case of Cys, the spectral titration curve does not reach a plateau after the deprotonation of the sulfhydryl group. Such an ill-defined saturation effect has also been observed in the titration of free Cys in basic solution;z8Cys is converted by base to glycine, glutamic acid, and alanine.29 It is entirely possible that some of the absorbance in the MV2+/Cys system at high pH is due to a contribution from complexes with these decomposition products. The thermal reduction of MVz+ in alkaline solution in the absence of any other reductant is a known p h e n ~ m e n o nexcept ; ~ ~ for the concentrated Cys system, this reaction is not observed in the other systems up to pH 12. Changes in the pH did not change the shape of the absorption profiles to any great extent for the MVz+/EDTA and MV2+/ TEOA systems; however, in the case of the MVz+/Cys system, the tail absorption in acidic medium develops a peak that shifts slightly to the red with increasing pH (Figure 1). The absorption spectrum of the MVZ+/EDTAsystem at pH 10 was examined as a function of temperature at 10,22, 35, and 45 "C. As the temperature is increased, there is a slight diminution of the absorbance at fixed [MV2+] and [EDTA]. Emission Studies. The reports that charge-transfer complexes produce a luminescence between MVZ+and thioureazz or CdS10,31 with A, = 525 nm led us to examine this phenomenon in the systems considered here. Indeed, aerated or deaerated solutions of MVz+ with EDTA, TEOA, or Cys yield virtually identical excitation and emission spectra; Figure 3 shows corrected spectra obtained for the MV2+/EDTA system at pH 10. The excitation and emission maxima are identical with those reported for the MVz+/thiourea system2zand are the same for MVZ+/TEOAand MV2+/Cys mixtures. In fact, virtually identical emission and excitation spectra are observed for solutions containing 0.020 M MVz+ and 0.1 M oxalate, benzilate, formate, or chloride. The emission from the MV2+/EDTA system at 525 nm was examined as a function of pH (inset of Figure 3). Interestingly, the emission is more intense in acidic solution (pH (5) despite the fact that the absorption at 395 nm, where no discrete band (28) Gorin, G.J. Am. Chem. SOC.1956, 78, 767-770. (29) Wieland, G. In "Glutathione"; Colowick, S., Lazarow, A,, Racker, E., Schwarz, D. R., Stadtman, E., Waelsch, H., Eds.; Academic Press: New York, 1954; p 5 5 . (30) Calderbank, A.; Charlton, D. F.; Farrington, J. A,; James, R. J . Chem. Soc., Perkin Trans. 1 1972, 138-142. (31) Gratzel, M. In 'Energy Resources Through Photochemistry and Catalysis"; Gratzel, M., Ed.; Academic Press: New York, 1983; p 71.
!
0
I
I
I
I
I
10
20
30
40
50
I 60
IAEDTA], M-' Figure 4. Benesi-Hildebrand plots for solutions containing 0.020 M MV2+ and EDTA at pH 8.0 (B) and 11.2 ( 0 ) (A = 350 nm).
has ever been observed, is less intense. When normalized to constant absorbance, the emission intensity decreases linearly from pH 4 to 6 and then reaches a plateau for further increase in pH. The quantum yield for emission of a solution containing 0.020 M MV2+ and 0.10 M EDTA at pH 4.6, relative to quinine sulfate,3zwas 0.04. In the absence of additional solutes, 0.020 M MVZ+exhibits emission and excitation spectra virtually identical with those obtained for the mixtures. The actual emission intensity of MV2+ solutions, which can be extremely weak or moderately strong under identical experimental conditions, depends upon the past history of the solution (time since preparation, pH) and the solid MVZ+ (peculiarities of individual commercial samples, exposure to light and air, recrystallization medium) from which the solution is prepared.
Discussion The equilibrium constant (K,) for the formation of a 1:l complex between MVz+ and a donor species in solution, and the molar absorptivity of the complex at a wavelength of interest (ex), can be evaluated most easily by use of the Benesi-Hildebrand treatment34for which [MVZ+]o/AAx= l/(ehl) l/(Kqexl[D]o), where AAx is the change in solution absorbance. A plot of [MVZ+Io/AAxvs. l/[D], is predicted to be linear; for I = 1 cm, e, = l/intercept and K, = intercept/slope. Other methods of analysis of these absorbance data, based upon algebraic rearrangement of the fundamental equations in order to minimize extrapolation errors and identify multiple equilibria, are available from the literature (Foster:33 AA,/[D], = K,e,I. [MVz+]o - K,,AAx; [MV2']o[D]o/AA, [D]o/(exl) l/(KeqeAl); Scatchard? A A x / ( [ M V z + ] ~ [ D l ~=) Ke,ql K,AAx/[MVZ+]o). In the case where the absorbance of one of the free components, such as MVZ+,is not negligible, the absorbance change of the solution upon complex formation is due to the increased absorbance of the complex and the diminished absorbance of the free MV2+. Under such conditions, the Nash treatment36is appropriate to use. Here, a plot of 1/ [EDTA], vs. (1 - (A,/Arnv))-l, where A, is the total absorbance of the system at the wavelength of interest and A, is the absorbance of MVz+ in the absence of EDTA, is linear with an intercept equal to -Kq and a slope equal to Kq - (Kqex/,emv)(emv is the molar absorptivity of MVZ+at the wavelength of interest). Complexation Equilibria. Figure 4 shows the Benesi-Hildebrand treatmentz4of data at 350 nm for the MV2+/EDTA system
+
+
(32) Demas, J. N.; Crosby, G . A. J. Phys. Chem. 1971, 75, 991-1024. (33) Foster, R.; Hammick, D. L.; Wardley, A. A. J. Chem. SOC.1953, 38 17-3 820. (34) Scott, R. L. Reel. Trav. Chim. Pays-Bas 1956, 75, 187-189. (35) Scatchard, G. Ann. N.Y. Acad. Sci. 1949, 51, 660-672. (36) Nash, C. P. J . Phys. Chem. 1960,64, 950-953.
Complexation between MV2+ and EDTA, TEOA, and Cys TABLE I: Equilibrium Constants and Molar Absorptivities for CT Complexes of MVZ+with EDTA, TEOA, and Cys in Aqueous Solution at 22 OC
donor
DH
EDTA
4.6 8.0 10.0
t k , M-' cm-' (A, nm)
L. M-'
method Benesi-Hildebr and ~~~~
TEOA CYS
11.2
2.5 (350) 10 (350) 42 (320) 1 2 (320) 23 (330) 21 (350) 20 (350) 20 (350) 21 (350) 19 (350) 11 (370) 19 (350)
4.0 10.2
2.5 (350) 18 (350)
3.5
9.0 11.0
25 (350) 88 (360) 104 (374)
1.3 13 8.2 11 14 13 14 14 14 13
1%
~
~
Nash Benesi-Hildebrand Foster Scott Scatchard Nash Benesi-Hildebrand
18
0.60 I .O 0.29 3.9 7.1
as a function of pH with [MV2+Io= 0.020 M. The values of Kq and cA are presented in Table I for that treatment and those from the other methods; the Nash method is required at the shorter wavelengths where the absorbance due to uncomplexed MV2+is no longer negligible. Also included in Table I are Kq and tAvalues for the MV2+/TEOA and MV2+/Cyssystems. It should be noted that some of the values of Kq and eSs0 for MV2+/EDTA and MV2+/TEOAcomplexes reported earlierZ5have been corrected; a larger number of data points and an enhanced statistical treatment was used in this present study. Enhanced absorption due to complex formation can result from random collisional encounters of the species in solution (contact pairs)23or from stronger interactions that involve some change in the electronic radial distribution between the pairs (chargetran~fer).~'As a rule, Kq values for contact pairs are generally significantly less than unity. In contrast, values for charge-transfer (CT) complexes may be as high as several thousand for 12/amine systems;38most values of Kes, as exemplified by organic 1:1 CT complexes, are [MV2+],. Using EDTA as an example, its use in the photochemical model system containing R ~ ( b p y ) , ~as+ the photosensitizer has been mainly at pH 5 where the quantum yield of formation of H, from the interaction of MV+. with colloidal Pt is at a maximum?6 under these conditions, -10% of the MV2+ would be present as the MV2+/EDTA complex. From the generally accepted mechanism3,4,6,7,49-51for the formation of H2, one can see the important ~
~~
~
~~
~
~~
~~
~
~~~
(46) Novakovic, V., unpublished results. (47) McKellar, J. F.; Turner, P. H. Phorochem. Photobiol. 1971, 13, 437-440. (48) Kuczynski, J. P.; Milosavijevic, B. H.; Lappin, A. G.; Thomas, J. K. Chem. Phys. Lett. 1984, 104, 149-152. (49) Kalyanasundaram, K.; Kiwi, I.;Grltzel, M. Helu. Chim. Acta 1978, 61, 2720-2730. (50) Amouyal, E.; Zidler, B. Zsr. J. Chem. 1982, 22, 117-124. (51) McLendon, G. In "Energy Resources Through Photochemistry and Catalysis"; Grltzel, M., Ed.; Academic Press: New York, 1983; p 99.
Prasad and Hoffman locations in the reaction scheme where the presence of MV2+/ EDTA complexes could affect the yield of products: (1) competitive absorption of the incident light, (2) competitive quenching of the excited photosensitizer, and (3) change of the efficiency of cage release of the redox products from the quenching reaction. R ~ ( b p y ) , ~Z% + *R~(bpy)~~+
--
(4)
4
(8)
+ MV2+ R ~ ( b p y ) 3 ~++MV+* Ru(bpy),j+ + MV+. Ru(bpy),'+ + MV2+ R ~ ( b p y ) , ~++ EDTA Ru(bpy),,+ + EDTA',, MV+* + EDTA',, MV2+ + EDTA EDTA+,, EDTA' + H+ EDTA' + MVZ+ MV+. + products Pt MV+- + H+ MV2+ + '/2H2 *Ru(bpy)3,+
(1) (2) (3)
(5)
(6) (7)
Considering a system containing 0.01 M MV2+ and 0.1 M EDTA at pH 5, the absorbance at 450 nm, the wavelength at which Ru(bpy)t+ has a spectral maximum with €450 1.45 X lo4 M-' ~ m - ' due , ~ to the complex would be of the order of 0.0005 (€450 -0.5 M-' cm-I). It is clear that the minimum concentration of Ru(bpy)32+that could be used in order for at least 90% of the light to be absorbed by the photosensitizer is -3 X lo-' M. The usual use of 10-5-104 M Ru(bpy):+ means that competitive light absorption by the MV2+/EDTA complex will not be an important factor. With regard to the quenching of *Ru(bpy);+, it should be noted that kq = 1.2 X lo9 M-' s-l in the absence of EDTA at ambient pH (w = 0.84 M with Na2S04)and (1.0-1.4) X lo9 M-' s-' in the presence of 0.001-0.1 M EDTA at pH 4.7 (w = 0.84 M with N a 2 S 0 4and/or EDTA).52 Thus, the small fraction of MV2+ present as the complex does not have an appreciable effect on the kinetics of scavenging. The efficiency of production of R ~ ( b p y ) ~and ~ + MV'. in quenching reaction 2 is a key parameter limiting the yield of high-energy products. The interaction of *R~(bpy)~,+ and MVZ+ in the absence of ion-pair complexation can be viewed as generating the redox products within a solvent cage (reaction 9); release of the redox products from the solvent cage (reaction 10) competes with geminate-pair back electron transfer (reaction 11). The efficiency of cage release of the redox products (vcr) is given by klOl(k10 + k l ) . *Ru(bpy)?+ + MV2+ [R~(bpy)3~+***MV+*](9)
-
+ MV+* [R~(bpy)3~+*..MV+*]R ~ ( b p y ) , ~++ MV2+ [R~(bpy)3~+..*MV+.] Ru(bpy)$+ +
(10)
(1 1) From flash photolysis experiments, the value of vcr in the absence of EDTA is -0.25 at ambient pH in the presence and absence of modest concentrations (-0.1 M) of simple salts (e.g., J~*~~ NaCI, Na2S04)and at pH 4.7 in 0.1 M acetate b ~ f f e r ; ~values of qcr in acidic or neutral solutions containing EDTA have not been reported. In our recent examination of the quantum yield of formation of MV+- from the continuous photolysis of the R U ( ~ ~ ~ ) , ~ + / M V ~ + /system,52 E D T A we concluded that, for solutions containing 0.02 M MV2+ at pH 4.7 and constant ionic strength, qcrin 0.1 M EDTA is approximately 3 times larger than in 0,001 M EDTA, under these conditions, the fraction of the MV2+ complexed is 10% and -0.1%, respectively. It would appear that vn for complexed MV2+is -30 times larger than for the uncomplexed species. Inasmuch as qcr cannot be larger than unity, a value of the efficiency of cage release of the redox products for uncomplexed MY2+is calculated to be no greater than -0.03. This value is highly disordant with that of -0.25 quoted above --*
-
( 5 2 ) Mandal, K.; Hoffman, M. Z. J . Phys. Chem. in press. (53) Maestri, M.; Sandrini, D. N o w . J . Chim.1981, 5, 637-641. (54) Kalyanasundaram, K.; Neumann-Spallart, M. Chem. Phys. Lett. 1982, 88, 7-12.
J. Phys. Chem. 1984, 88, 5665-5669
5665
and must be reconciled by further investigation. Irrespective of the absolute value of vn in the presence of EDTA, it is clear that increasing complexation of MV2+ with EDTA causes an increase in the cage release yield of redox products. Reactions 12-15 illustrate the role of EDTA as a static scavenger
We have f o ~ n d as~ well ~ , ~that ~ the quantum yield of MV+formation in the continuous photolysis of the model system in alkaline solution is a strong function of the concentrations of all three components, a phenomenon explained in terms of ion-pair interactions and aggregate formation among the highly charged species that causes variation in qcr. *Ru(bpy),'+ + [MV2+*-EDTA] The fact that the C T complexes absorb light in the visible and [Ru(~~~)~~+...MV+....EDTA] (12) near-Uv spectral regions makes them attractive potential candidates for the photochemical formation of high-energy products [Ru(~~~),~+...MV+....EDTA] in the absence of any photosensitizer. Excitation of a CT complex, R u ( ~ P Y ) ~ ~MV+* + EDTA (13) in which some charge transfer exists even in the ground state, can be seen as producing excited states in which extensive charge [Ru(~~~)~~+...MV+....EDTA] separation has been achieved. Unfortunately, the overall yields R ~ ( b p y ) 3 ~ + [MVZ+.-EDTA] (14) of redox products from CT complexes are generally rather 1ow.11~12,14-16~20~22~25~55,56 We will discuss the photochemistry of [Ru(~~~)~~+...MV+....EDTA] the MV2+/EDTA system, and the dependence of +(MV+-) on R ~ ( b p y ) 3 ~ +MV+. EDTA',, (15) [acceptor] and/or [donor], in detail in a separate p ~ b l i c a t i o n . ~ ~ for R ~ ( b p y ) within ~ ~ + the solvent cage; ion-pair complexation of Acknowledgment. This research was supported by the Office the metal cations and MV+. are not shown for the sake of clarity. of Basic Energy Sciences, Division of Chemical Sciences, U S . Hence, lfcr = ( k 1 3 + klS)/(k13 + k14 + kl.5). Department of Energy. We thank Vincent Malba and Professor Although complexation of MV2+with sacnficial donors becomes G. Jones for many important discussions concerning this work. significant in alkaline solution, the absorbances of the complexes Registry No. MV2+, 4685-14-7; MV2+/EDTA, 87174-65-0; MV2+/ are still too low to compete with a modest concentration of inTEOA, 87174-66-1; MV2+/Cys, 92096-50-9. tensely absorbing photosensitizer. However, now the existence of over half of the MV2+ as a complex will have kinetic conse(55) Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi, J. K.; Rentzepis, quences. At pH 11 in 0.1 M EDTA, kq = 8.4 X lo8 M-l s-l which P. M. J. Am. Chem. SOC.1983, 105, 6167-6168. can be attributed to the increased electrostatic repulsion of sim(56) Gschwind, R.; Haselbach, E. Helu. Chim. Acta 1979, 62, 941-955. ilarly charged R u ( ~ ~ ~ ) , ~ + / Eand D T MVZ+/EDTA A complexes. (57) Prasad, D. R.; Hoffman, M. Z., manuscript in preparation. +
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Chemical Behavior of SO,- and SO,- Radicals in Aqueous Solutions Robert E. Huie* and P. Neta* Chemical Kinetics Division, Center for Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: March 28, 1984)
The chemistry of the radicals SO3-and SO5-has been investigated by using pulse radiolysis with kinetic spectrophotometry. Rate constants for the oxidation by SO< of a variety of organic compounds were measured and equilibrium constants determined for the reactions of SO3- with chlorpromazine and phenol. SO3- was found to be a mild oxidant with a redox potential of E ( S 0 3 - / S 0 3 2 - )= 0.63 V (vs. NHE) at pH >7 and E ( S 0 3 - / H S 0 3 - ) = 0.84 V at pH 3.6. The reaction of SO3- with O2 was shown to produce SO5-. The oxidation of several compounds by SO5-was found to occur more rapidly than their oxidation by SO). E(S05-/HS05-) was estimated to be approximately 1.1 V at pH 7 .
The autoxidation of aqueous solutions of sulfur dioxide (SO,,,,, followed by a subsequent radical chain reaction. HS03-, and S032-)has been studied for well over a century (for The production of S(V) by the Ce(1V) oxidation of S(1V) has references to the literature since 1898, see Westley;' for recent been confirmed by using ESR.7,8 In addition, SO3- has been reviews, see Huie and Peterson2 and Hoffmann and Boyce3). It detected by ESR in the reaction of bisulfite with various peroxides: is known that,trace amounts of transition metals are required for with a horseradish peroxidase-hydrogen peroxide system,I0 and this reaction4 and that it can be initiated by ultraviolet l i ~ h t . ~ , ~ with prostaglandin hydroperoxidase." ESR also has been used to detect SO3-in the radiolysis12and the photolysis13of bisulfite In both cases, compounds which are known to be free radical scavengers were observed to inhibit the reaction. This led to the solutions. suggestion that both the metal ion catalyzed and the light initiated Kinetic studies on SO3-have been carried out by using kinetic reactions involved the primary production of the sulfur(V) radical, or pulse r a d i o l y ~ i s * ~ J ~ J ~ spectrophotometry with flash photoly~is'~J~
so
