5347
J. Phys. Chem. 1986, 90. 5347-5352 TABLE II: Ratio of Average Logarithmic Energy Loss to Reactivity Integral (cu-/l) in the Rare Gas-Ethane System
phase and temp solid at 11-20 K' solid at 77 K gas at 298 K
Ar
0.39 0.037 0.1 l b
Kr 0.21
0.030 0.054c
Xe 0.052 0.014 0.035e
OTemperatures of Ar, Kr, and Xe are 18, 11, and 20 K, respectively. bQuoted from ref l l c and 13. CEstimatedfrom the results of Ar by use of an elastic collision model. The amd/I values thus determined are summarized in Table 11. The a,,/I in the gas phase was quoted from previous papers.1'cJ33'4 It is seen that the values at 77 K are slightly smaller than those in the gas phase.I5 If the reactivity integral in the Seewald, D.; Wolfgang, R. J . Chem. Phys. 1967, 47, 143. The values for Kr and Xe in the gas phase were estimated from the values of aAr/Iby use of an elastic collision model. (15) Since the rare gas-ethane mixtures at 77 K were made by rapid cooling of the gaseous mixture from room temperature to 77 K, rare gases and ethane are probably mixed well in the solid phase. If mixing of the two components is incomplete, an apparent cud/Ibecomes larger than that in the gas phase. (13) (14)
solid phase is approximately the same as that in the gas phase, the energy loss of hot T atoms in the solid phase at 77 K may be less effective than in the gas phase. The &,,,&/I a t 11-20 K is much higher than that in the gas phase. Since the rare gasethane mixtures at 11-20 K were made by slow cooling of the samples, there is some ambiguity for mixing of the two components. Thus we cannot come to a definitive conclusion as to at 11-20 K represent their true whether the high values of amOd/I values at ultralow temperatures or are due to incomplete mixing of rare gases and ethane.
Acknowledgment. We thank Prof. Hiroyuki Yoshida and Dr. Masuo Nakagawa of the Research Reactor Institute, Kyoto University, for their valuable advice. This work was carried out under the Visiting Researchers Program of Research Reactor Institute, Kyoto University. This was done under the Collabollation Program between Japan Atomic Energy Research Institute and Nagoya University. This work was partially supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, and Culture. Registry No. T, 10028-17-8;C2H6,74-84-0; C2D6,1632-99-1;Ar, 7440-37-1;Kr, 7439-90-9; Xe, 7440-63-3;HT, 14885-60-0;DT, 1488561-1; C2H$T, 5505-49-7;CZDST, 40619-17-8.
Interaction of Formate and Oxalate Ions with Radiation-Generated Radicals in Aqueous Solution. Methylviologen as a Mechanlstic Probet Quinto G. Mulazzani,* Mila D'Angelantonio, Istituto di Fotochimica e Radiazioni d'Alta Energia, Consiglio Nazionale delle Ricerche, 401 26 Bologna, Italy
Margherita Venturi, Istituto di Scienze Chimiche, Facolta' di Farmacia, Universita' di Bologna, 401 27 Bologna, Italy
Morton Z. Hoffman,* Department of Chemistry, Boston University, Boston, Massachusetts 0221 5
and Michael A. J. Rodgers Center for Fast Kinetics Research, University of Texas, Austin, Texas 78712 (Received: January 21, 1986; In Final Form: May 15, 1986)
The reduction of methylviologen (MV2+)to MV" by species arising from the interaction of radiation-generated radicals with formate and oxalate ions in aqueous solution has been studied by using the techniques of continuous and pulse radiolysis. C02'-, arising from the reaction of H and OH with HCOY, generates MV" rapidly ( k 1 X 1O'O M-' s-l at zero ionic strength) and quantitatively;k(C0:- + MV2+)decreases with increasing ionic strength, and G(MV'+) increases with increasing [HCOT]. The reaction of eaq-with oxalate ions produces species capable of reducing MV2+,possibly via the intermediacy of C02'-. The oxidation of MV" by the radicals resulting from the reaction of OH and oxalate ions, in competition with their rapid ( k = 2 X lo6 s-l) conversion into C02'-, results in a decrease of G(MV'+) with increasing radiation dose. The relevance of these observations to the use of oxalate ions as a sacrificial electron donor in photochemical model systems for energy conversion is discussed. N
Oxalate ions (C20," and C204H-) have been used as sacrificial electron donors in model systems for the photogeneration of dihydrogen from water.'q2 Oxalate ions have also been involved in a luminescent system triggered by the chemical or electrochemical oxidation of Ru(bpy),2+ to Ru(bpy),'+ (bpy = 2,2'-bi~yridine).~ It was suggestedr3 that the products originating from the oxidation of oxalate ions (reaction 1) undergo rapid decarboxylation, yielding the C02' Dedicated to Prof. Leon Dorfman on the occasion of his retirement.
C202-/C204HC204*-/C204H*
-+
C2O4'-/C2O4H*+ red
(1)
+ CO,*-/C02 + C02'- + H+
(2)
+ ox
C02
-+
radical (reaction 2) which, being a strong reducing agent (@'(CO2/CO2*-) = -2.0 V)," generates a second equivalent of the (1) Krasna, A. I. Photochem. Photobiol. 1980, 31, 75-82. (2) Pina, F.; Mulazzani, Q.G.; Venturi, M.; Ciano, M.; Balzani, V. Inorg. Chem. 1985, 24, 848-851. (3) Rubinstein, I.; Bard, A. J. J . Am. Chem. Sor. 1981, 103, 512-516.
0022-3654/86/2090-5347$01.50/00 1986 American Chemical Society
5348 The Journal of Physical Chemistry, Vol. 90, No. 21. 1986
reduced form of the electron relay2 or, by reacting with Ru(b~y),~',the luminescent excited state * R ~ ( b p y ) ~ According ~+.~ to early report^,^ C204'- and C204H' are also produced via oxidation of oxalate ions by radiolytically generated OH radicals (reaction 3). One report6 suggested that the radical products of reaction 3 decay via bimolecular processes rather than via reaction 2. Cz042-/Cz04H-
+ OH
-
C204*-/C204H*
+ OH-
(3)
Because of these conflicting reports on the decarboxylation of ~ ,the ~ redox CzO4'- and C2O4H' and our continuing i n t e r e ~ t in processes involving the components of model systems for the conversion and storage of solar energy, we have investigated the interaction between the products of reaction 3 and their successors and methylviologen (1,l'-dimethyl-4,4'-bipyridiniumion, MVz+), in the hope that details concerning the kinetics of reaction 2 could be obtained. In this study, the formation of easily detectable MV'+,* the reduced form of MVZ+,resulting from reaction 4 was examined as a function of concentrations of MV2+ and oxalate ions and pH. For comparison, reaction 4 from the radiolysis of the MV2+/HC02- system9 was reinvestigated.
C02'-
+ MV2+
-
CO,
+ MV'+
(4)
Experimental Section Materials. K2C204 and HC0,Na (Merck) and 2-propanol (Baker) were used as received. Methylviologen dichloride (Aldrich) was recrystallized from water by the addition of acetone, collected on a sintered-glass filter, and dried over CaC12 in a vacuum desiccator. Distilled water was further purified by distillation from acidic dichromate and alkaline permanganate and was then fractionated in an all-silica apparatus. The pH of the solutions was adjusted with H2S04or NaOH (Merck, Suprapur). Nitrous oxide was purified by passage through a column of NaOH pellets and by trap-to-trap distillation. Solutions were saturated with purified N 2 0 or were degassed by standard vacuum line techniques. Procedures. Solutions containing MV2+were freshly prepared and were stable under all concentration and pH conditions employed in this study. Continuous radiolyses were carried out at room temperature on IO-mL samples contained in silica or Pyrex vessels that were provided with silica optical cells on a side arm; absorption spectra were recorded with a Perkin-Elmer Model 555 or Lambda 5 spectrophotometer. The radiation source was a 60Co-Gammacell (AEC., Ltd.) with a dose rate of -8 Gy min-]. The absorbed radiation dose was determined with the Fricke chemical dosimeter by taking G(Fe3+) = 15.5, where G(X) = number of molecules of species X formed or destroyed per 100 eV of energy absorbed by the solution. Pulse radiolyses with optical absorption detection were performed at room temperature by using the 12-MeV linear accelerator of the FRAE Institute, CNR, Bologna.lo Preliminary experiments were performed a t CFKR, University of Texas." The solutions were protected from the analyzing light by means (4) Breitenkamp, M.; Henglein, A,; Lilie, J. Ber. Bunsenges. Phys. Chem. 1976, 80, 973-979.
( 5 ) Draganic, I. G.; Gal, 0. Radial. Res. Reu. 1971. 3, 167-207 and references therein. (6) Getoff, N.; Schworer, F.; Markovic, V. M.; Sehested, K.; Nielsen, S. 0. J. Phys. Chem. 1971, 75, 749-755. (7) (a) Venturi, M.; Mulazzani, Q. G.; Hoffman, M. 2. Radial. Phys. Chem. 1984, 23, 229-236. (b) Venturi, M.; Mulazzani, Q. G.; Hoffman, M. 2. J. Phys. Chem. 1984.88, 912-918. (c) Mulazzani, Q. G.; Venturi, M.; Hoffman, M. 2. J. Phys. Chem. 1985, 89, 722-728. (d) Prasad, D. R.; Mandal, K.; Hoffman, M. 2. Coord. Chem. Reu. 1985, 64, 175-190. (e) Prasad, D. R.; Hoffman, M. 2.; Mulazzani, Q . G.; Rodgers, M. A. .I. J. Am. Chem. SOC.,in press. (8) Watanabe, T.; Honda, K. J. Phys. Chem. 1982.86, 2617-2619. (9) Farrington, J. A.; Ebert, M.; Land, E. J.; Fletcher, K. Biochim. Biophys. Acta 1973, 314, 372-381. (10) Hutton, A.; Roff, G.; Martelli, A. Quad. Area Ric. Emilia-Romagna 1974, 5 , 67-74. (11) Foyt, D. C. Comput. Chem. 1981, 5, 49-54.
Mulazzani et al. of a shutter and appropriate cut-off filters. The radiation dose per pulse was monitored by means of a charge collector placed behind the radiation cell and calibrated with a 0.1 M KSCN aqueous solution saturated with 0, using Gc = 2.15 X lo4 at 500 nm1.I2 Analyses. The concentration of MV" generated by the radiation was determined spectrophotometrically, taking 6 = 1.37 X lo4 M-I cm-' at 600 nmS8 Generation of Reducing Radicals. The radiolysis of aqueous solutions generates ea;, O H radicals, H atoms, and molecular H2 and H202according to eq 5, where the numbers in parentheses H20 eaq- (2.8), O H (2.Q H (Oh), H2 (0.451, H 2 0 2 (0.8) (5) -+
represent the G values for the species in the absence of scavenging by solutes. In the presence of solutes that scavenge these species and at high concentrations of scavengers where reaction within the spurs occur to an increased extent, different G values may result. In N,O-saturated solutions (25 mM), ea; is converted to OH according to reaction 6; in acidic solution, eaq-is converted to H atoms according to reaction 7. eaq- + N,O O H OH- N, (6)
-
+
k6 = 8.7 X lo9 M-I eaq-
+ H+
-
+
(ref 13)
S-'
H
(7)
k , = 2.2 X l o i o M-' S-I (ref 13) HCOY reacts rapidly with H ( k = 3 X 10' M-' s-' ) l 4 and with OH ( k = 3 X lo9 M-I s-I)l5 according to reaction 8. The pK, of the conjugate acid of C02'- (C0,H') is 1.4.16 HCOY + H / O H C0,'H2/H20 (8)
-
+
The rate constants for the scavenging of e, -, H, and O H by oxalate ions are k(C204H-+ ea;) = 3.2 X 1O9,I3k(C204" + ea;) = 1.7-4.8 X 107,13k(Cz0:H ) = 1.6 X lo6 l 4 (a value of > [C02'-]. Under these conditions, the formation of MV'+, monitored at 600 nm, follows strictly first-order kinetics with kohd proportional to [MV2+] (Figure 1). The values of the slopes of these plots, which equal k4, decrease with increasing [HCO,] (Figure 1, inset). The value of k4 also decreases when C2042- (0.01-0.5 M) is present in 0.05 M HCO,-; the conditions were such that practically all H and OH reacted with HC02-. The values of k4 from C2042-fHC02-reproduced, within experimental error, the values obtained for HCO, alone, from solutions having the same ionic strength. G(MV'+), determined from the maximum absorption formed at 600 nm from solutions containing 0.1 M HC02- and 0.5 mM MV2+,is independent of pH (2.8-12.5); at a given [HCO;], the yield of MV" is proportional to the radiation dose (Figure 2, inset), with G(MV'+) independent of the dose. However, as is shown in Figure 2, G(MV'+) increases with increasing [HC02-], approaching a plateau value of -7 when [HC02-] > 0.3 M. The continuous irradiation of N20-saturated solutions containing 0.1 M HC02- and 0.1 mM MV2+ at pH 7.0 and 12.5 results in the formation of MV'+ which is stable in the absence of air. As shown in Figure 3, the absorbance at 600 nm increases almost linearly with increasing radiation dose, reaching a maximum that corresponds to an apparent rex+ction of -80% of the MV2+ originally present and then decreasing linearly with increasing dose. G(MV'+) decreases slightly with increasing dose and has a value of 7.2 f 0.2 extrapolated to zero dose. The value of G(-MV'+) calculated from the linear decrease of the absorbance at 600 nm is 2.9 f 0.1 at pH 7.0 and 12.5; the spectrum of MV07a was not detected. Mpn+ f 2-Propanolf N 2 0 System. When a N20-saturated solution containing 0.5 mM MV2+and 0.l M 2-propanol at pH l l .5 is pulsed, MV" is formed with G(MV'+) = 6.3 f 0.1 independent of radiation dose (1.7,5.4, and 16.7 Gy per pulse). Within 0.5-1.5 ms after the pulse, the absorbance developed at 600 nm decreases by 10%; the time required for the completion of this process
decreases with increasing radiation dose, Le., with increasing [MV"]. From the half-life of the reaction and its dependence upon [MV"], the bimolecular rate constant for the process has been evaluated; k E 7 X lo8 M-' s-l . After this process is completed, the absorbance of MV" is infinitely stable. M p +f Oxalate f N 2 0 System. Inasmuch as the pK, values of oxalic acid are 1.23 and 4.19,22this system was investigated at pH 2.8 and 7.6 where the predominant species in solution are C204H- and C2042-,respectively. The pulse irradiation of NzO-saturated solutions containing 0.1 M oxalate and varying amounts of MV2+ at pH 2.8 results in the formation of stable MV". MV" forms according to first-order kinetics with kohd proportional to [MVZ+];at short times, small deviations from first-order behavior are observed when [ MV2+] > 0.5 mM. The bimolecular rate constant for the formation of MV" equals (4.3 f 0.2) X lo9 M-' s-l. The value of G(MV'+) is independent of [MV2+]but decreases with increasing radiation 6 (Figure 4). dose; at zero dose, G(MV'+) The continuous irradiation (60 Gy) of a N20-saturated solution containing 0.1 M oxalate and 0.1 m M MV2+ at pH 2.8 results in the formation of MV" with G(MV'+) = 4.5 (Figure 4); under these conditions, MV'+ is unstable and decays slowly in the time frame of minutes. The pulse irradiation of N20-saturated solutions containing 0.1-0.5 M C2042- and varying amounts of MV2+ at pH 7.6 results
(21) Ross, A. B.; Neta, P. Natl. Stand. Rex Data Ser. (U.S., Natl. Bur. Stand.) 1982, 70, 1-96.
(22) Weast, R. C. In Handbook of Chemistry and Physics, 56th ed.;CRC: Cleveland, OH, 1975-1976.
-
E
:
1.0
e 0
1
0.5 I
0
100
200 Dose, G
300
IO
Figure 3. Absorbance at 600 nm as a function of radiation dose from the continuous radiolysis of N20-saturated solutions containing 0.1 m M MV2+ at natural pH. [HCOT] = 0.1 M (A),[C2042-] = 0.1 M (0); optical path = 1.0 cm.
-
5350 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 ,
,
,
,
Mulazzani et al.
I
C, 0;- 0.1 M
a
ln
;; ;o 20
-2
e
'j \
Y 10
c
v."
2
-5
0
2
4 Time P I
0.3 [MV2*] ,mM i
,
,
l
70
"0.0
#
,
140
,
210
,
,
,
t
350
280
DOSE, G v Figure 4. G(MV'+) as a function of radiation dose from the radiolysis of NzO-saturated solutions containing oxalate ions and 0.1 mM MV2+. pH 2.8, continuous (A)and pulse (A);pH 7.6: continuous ( 4 ) and pulse (0). Solid lines represent calculated values from the computer analysis as described in the text.
t
u:l 1 %t g0.0
0.3
0.7
1.0
T I NE, SECONDS
1.3
1.7
2.0
1Os
Figure 5. Formation of absorbance at 600 nm from the pulse radiolysis of NzO-saturatedsolution containing 0.1 M Cz042-and 0.1 mM MV2+ at pH 7.6. Dose/pulse = 4.6 Gy; optical path = 2.0 cm. Solid line represents the calculated curve from the computer analysis as described in the text.
in the formation of stable MV". In general, the profile of the rise of the absorbance at 600 nm shows an induction period (Figure 5 ) . Good single-exponential plots are, however, obtained from these curves by neglecting the initial part corresponding to the induction period (Figure 6, inset). The observed rate constant does not increase linearly with increasing [MV2+]but reaches or approaches a plateau value that depends on [C2042-](Figure 6 ) . G(MV'+) decreases with increasing radiation dose and, for the same dose, increases slightly when [C2042-]is increased from 0.1 to 0.5 M. The G(MV'+) values obtained in the presence of 0.1 M C20d2-and 0.1 mM MV2+ as a function of dose are shown in Figure 4. The continuous irradiation of a N20-saturated solution containing 0.1 M C2O4*-and 0.1 m M MV2+ at pH 7.6 also results in the formation of stable MV'+. As shown in Figure 3, the absorbance a t 600 nm does not increase linearly with increasing radiation dose and remains well below the curve obtained in the presence of HC02-. As shown in Figure 4, G(MV'+) decreases with increasing dose. M p + / C 2 0 4 2 - / A rSystem. From the pulse radiolysis of an Ar-purged solution containing 0.1 M C2042- at pH 7.6, the rate constant for the reaction of eaq- with Cz042-is found to be k =
6
8
0.5
0
0.4 0.8 [MV"] , mM
2
Figure 6. Observed rate constant (kobsd)for the formation of MV'+ as a function of [MV2+]and [C202-] from the pulse radiolysis of N20saturated solutions at pH 7.6. Dose/pulse = 5 Gy. Inset: typical first-order plot of the formation of absorption at 600 nm from the pulse radiolysis of N,O-saturated solution containing 0.1 M C20d2-and 0.1 mM MVZ+at pH 7.6. Dose/pulse = 5 Gy.
(3.1 f 0.2) X lo7 M-' s-l; values ranging between lo7 s-'), a good fit of the formation kinetics of MV'+ is obtained at low doses (
