QUINONEPHOTOREDUCTION
Quinone Photoreduction.
3245
11. The Mechanism of Photoreduction of
9,lO-Phenanthrenequinoneand 2,t-Butyl-9,lO-Anthraquinonein Ethanol by J. F. Brennan and J. Beutel Photo PrOdUCts Department,Experimental Statwn Laboratory, E. I . du Pont de Nemours & Company, Wilmington, Delaware 19898 (Received February 7,1060)
The photoreduction of 9,lO-phenanthrenequinone(PQ)and 2,t-butyl-9,1O-anthraquinone(A&) in alcohol is examined. The rate constants are determined by flash photolysis and by fitting of optical density us. time data to the mathematical model given in our foregoing paper.' Concentration profiles and quantum yields are calculated. We have investigated the simultaneous photoreduction of 9,lO-phenanthrene quinone (PQ) and 2,t-butyl9,lO-anthraquinone (A&) in ethanol solution, using the previously described mathematical model' to obtain the rate constants for this process. The assumed mechanism is
-% 'PQ A& -% 'A& PQ
'PQ +PQ 'AQ +AQ .--f
'AQ
--f
3AQ
3AQ +- A&
+ PQ
+-
2PQ
+ RHz +PQH. + R H . 'AQ + RHz +AQH, + R H . PQH. + A& +AQH. + PQ A Q 8 . + PQ + P Q H * + A& PQ + R H PQH. + R A& + R H . --+ AQH. + R 2PQH. +PQ + PQH2 2AQH. +A& +- AQHz 2RH. +R + RHZ 'PQ
+-
+ A& AQHz + PQ
AQH.
aAQ
+ A& +2AQ
+ PQ PQHz + A&
+AQHz --t
+ PQH- +PQH2 + AQ
AQH and PQH are the respective semiquinone radicals; R H . is the hydroxyethyl radical (CH3CHOH),
(R14)
was omitted because there is no evidence of its occurrence. Similarly, the chain propagation steps
+ RH2 PQH. + RH2
AQH.
'PQ
3PQ --+ PQ
PQHz
and R is the product of the alcohol oxidation (acetaldehyde). The alcohol concentration is assumed to be constant. The rate constants for semiquinone disproportionation (klo and kls) were separately measured by flash photolysis, and the rate constant for the chain transfer step R9 was estimated from experiments a t various PQ concentrations. The reaction
---t
+ RH. PQH2 + R H .
AQH2
---+
(R16) (R8)
were omitted because there was no evidence of the existence of these reactions from our flash photolysis experiments.
Experimental Conditions Materials. Benzene solutions of PQ (Eastman No. 1269) and A& (Aldrich No. 10,823-5) were washed with an aqueous solution of disodium EDTA to remove trace metal impurities. After evaporation of benzene, the quinones were recrystallized from methanol. Reagent grade absolute ethanol was used without further purification. Extinction Coeficients. Extinction coefficients for quinone and hydroquinone at 436 nm were determined directly in the photoreduction apparatus, using reduced light intensity. I n the case of the quinones, the spectrophotometer cell was opened to the air to prevent photoreduction during the determination. For the AQH2 a known quantity of AQ was chemically reduced with excess stannous chloride. Photolysis. Interference and colored filters were used to isolate the 436-nm line of a 9OW Phillips medium(1) J. Beutel, R. J. Ruszkay, and J. F. Brennan, J. Phys.Chem., 73, 3240 (1969).
Volume 73, Number 10 October 1969
J. F. BRENNAN AND J, BEUTEL
3246 pressure Hg arc. This light was roughly collimated and passed through the sample to a photoelectric detector and the signal displayed as per cent transmission us. time on a Honeywell chart recorder. The samples were degassed by the freeze-pump-thaw technique at ca. 1 p pressure and stirred during photolysis with a 7-mm Teflon-enclosed magnetic stirring bar. Actinometry. Intensity of photolyzing light was determined using conventional ferrioxalate actinometry. Lamp intensity was stable to f1%.
i
0.20
TIME (min)
Figure 1. Optical density (at 436 nm) vs. time for a series of AQ and PQ solutions in ethanol. The points give the experimentally observed values, the curves are computed from the model. The initial conditions are: 0, J O = 6.10 X 10-7 einstein/l. sec, [PQ] = 2.60 X 10-4M, [A&] = 7.2 X 10-8; 0,Io = 2.55 X 10-7 einstein/l. sec, [PQ] = 2.65 X 10-4 M, [AQ] = 9.50 x 10-4 M; 0, IO = 6.10 X 10-7 einstein/l. sec, [PQ] = 2.60 X 10b4M, [A&] = 7.5 X lOw4M; 0 , l o = 2.30 X 10-7 einstein/l. see, [PQ] = 2.72 X lO-4M, [A&] = 7.95 x 1 0 - 3 ~ .
Flash Photolysis. Solutions M of the quinones in ethanol were degassed by bubbling argon through the cell for ca. 30 min and sealing the cell. These were flash photolyzed using a xenon flash lamp. The analyzing beam was provided by a tungsten lamp powered by a 6-V storage battery and fed through a Bausch and Lomb monochromator. Signals were detected with a Dumont 6292 photomultiplier tube and displayed on an oscilloscope. The flash lamp had a 6-psec half-width and an energy of 160 electrical joules. Extinction coefficients of the semiquinones were estimated from the initial optical density achieved by the flash and the total change in quinone concentration. The Journal of Physical Chemistry
TIME (min)
Figure 2. The computed behavior of [8AQ] and [*PQ]us. time. The dashed curve is [IAQ], the solid curve is [IPQ]. The point a t which the OD us. time curves (Figure 1) are a t a minimum is indicated by X. The corresponding initial conditions are: (a) IO = 6.10 X lo-' einstein/l. sec, [PQ] = 2.60 X 10-4M, [A&] = 7.2 X 10-aM; (b)Io = 2.55 X 10-7 einstein/l. sec, [PQ] = 2.65 x 10-4M, [A&] = 9.50 X lO-*M; (0) IO = 6.10 einstein/l. sec, [PQ] = 2.60 X 10-4M, [A&] = 7.5 X lO-4M; (d) 10= 2.30 einstein/l. see, [PQ] = 2.72 x lO-4M, [A&] = 7.95 X 1 0 - w .
Calculations and Results The fit between the experimental and the calculated optical density (OD) us. time curves is shown in Figure 1. The rate constants from which the calculated curves are obtained are given in Table I. The extinction coefficients (at 436 nm) for the absorbing species were taken to be €(PQ) = 1200 1. mol-' cm-l
€(A&) = 8 1. mol-l cm-l E(AQH)~= 2600 1. mol-' em-l The values of kl through ks as well as kll through IC15 are
3247
QUINONE PHOTOREDUCTION I
I
I
I
I
I
1.8
1.4 K W
I-J
z
\
-1
1.0
0
I
0.6
0.2 10
20
30 40
50 60 70 80
90
TIME (min)
Figure 3. The computed behavior of [AQH.] and [PQH.] us. time. The dashed curves are [AQH.] X lo6 M , the solid curves are [PQH-] X 10' M. X designates the point a t which the corresponding OD us. time curves (Figure 1) are at a minimum; a, b, c, d designate the same initial conditions as in Figure 2. TIME bin)
Figure 5. The computed behavior of [PQ] us. time. X designates the points at which the corresponding OD us. time curves are at a minimum; a, b, c, d correspond to the same initial conditions as in Figure 2.
Table I: Calculated Rate Constants of the Photoreduction of 9,lO-Phenanthrenequinoneand 2,t-Butyl-9,lO-Anthraquinone Reaction no.
R1 R2 R3 R4 R5 R6 R7 R9b R10= R11 @
los.
TIME (min)
Figure 4. The computed behavior of AQHt us. time. X designates the points a t which the corresponding OD us. time curves are a t a minimum; a, b, c, d designate the same initial conditions as in Figure 2.
Rate constant
x
105
1.0 x 1.0 x 4.5 x 8.0 X 1.0 x 8.0 x 2.0 x
106 105 106 lo-' loa 104 109 106
1.0
1.0 x 109
1.0
x
Reaction no.
R12 R13 R15 R17 RIBa R19 R20 R2 1 R22
Rate constant
1.0 x 1.0 x 1.5 X 1.0 x 6.0 X 8 x 5 x 5 x 1.0 x
109 105 106
108 lo6 109 10-8
106 102
The flash photolysis values are klo 1.5 X lo9and k18 = 4.5 b The approximate experimental value is 2 x 104. -1
x
approximate in that only the relative efficiencies of semiquinone formation and the observed quantum yield of quinone disappearance of 0.6 were considered in deriving these values. I n fact, only the ratios
Volume 73, Number 10 October 1969
J. F. BRENNAN AND J. BEUTEL
3248
x
that the efficiency of AQH. formation must be lower than that of PQH. formation. In the flash photolysis experiments the addition of triethanolamine to an AQ-alcohol solution showed no decrease in signal intensity at 435 nm but a marked decrease in the rate of disappearance of the observed intermediate. The addition of toluenesulfonic acid, on the other hand, caused the complete disappearance of the signal at 435 nm. In the case of PQ flash phot~lysis,~ the PQH. signal was observed at 385.5 nm and the addition of acid led to no change in either rate or signal intensity, while the addition of base led to a new signal at 545 nm, which decayed more slowly than the previously observed signal. We estimate from these data that AQH is approximately 90% dissociated4 in pure ethanol (AQH- Ft AQ. - H+) while PQH. remains undissociated in the same environment. This accounts for the apparently low value of kls and may also explain the low values of IC16 and kn. We are implying that the equilibrium
-
+
I
TIME (min)
AQH.
Figure 6. Computed instantaneous quantum yield of [AQHz] and [PQHa] formation us. time. The initial conditions are 10 = 6.10 x 10-7 einstein/l. sec., [PQ] = 2.60 X 10-4 M , [A&] = 7.2 X M.
1
I
I
I
I
1
1
0,7
0.1
1
1
i
1 IO
20
30
40 50 60 TIME (min)
70 80 90
Figure 7. The computed instantaneous quantum yield for total hydroquinone formation us. time. The initial conditions in a and b correspond to a, and b in Figure 2. X designates the minimum of the corresponding OD us. time curves (Figure 1).
may be considered validly accurate. We assumed nearly 100% efficiency in the formation of the quinone triplets, 88% efficiency for PQH. formation, and 71% efficiency for AQH. formation. Other workers have found approximately 100% efficiency for the formation of semiquinones,2 but the somewhat lower efficiencies, which we find it necessary to assume, may be due to small errors in actinometry. It seems clear, though, The Journal of Physical Chemistry
A&*-
+ H+
is maintained during the entire course of the reaction. AQH. is made by reactions R15 ( k l ~= 6 X lo6 1. mol-' sec-l) and R17 (k17 = 100 1. mol-' sec-l). During the first phase of the reaction, while only PQ is reduced, AQH. is consumed by reaction R7 (IC7 = 100 1. mol-' sec-'); in the intermediate phase when both AQ and PQ are reduced, it is consumed by reactions ~ 8 X R22 (klz = 100 1. mol-' sec-') and R18 ( k = lo6 1. mol-' sec-l), and during the final phase, when only AQ is reduced, AQH. is consumed by reaction R18 alone. The rates of these processes, a t the concentrations of AQH. here encountered (-lo-' M), are very much lower than the reported6 rates for establishing equilibrium with acid-base pairs of this type. The value of lCz2 may be as high as 10*without appreciably affecting the curve shapes. The remaining rate constants are as accurate as the fit shown in Figure 1 permits (between 5 and 10%). The behavior in time of the separate species is shown in Figures 2 through 5. Figure 6 shows the separate instantaneous quantum yield for AQ and PQ disappearance, respectively, and Figure 7 shows the sum of these two curves. The calculated decrease in quantum yield can be attributed to the lower efficiency of the primary processes of A& reduction. The calculated set of rate constants may be considered unique; no other set can be found which simul-
(2) B.Atkinson and M. Di, Trana. Faraday rSoo., 54,1331 (1958). (3) This work was done by Dr. L. C. Fischer of our laboratory. (4) (a) J. N. Pitts, H. W. Johnson, and T. Kuwana, J . Phys. Chem., 66, 2456 (1962); (b) T. E. Gaugh and M. C. R. Simmons, Trans. Faraday Soc., 62,269 (1966). (5) M.Eigen, Ber. Bunsenges. Phys. Chem., 67,763 (1903).
3249
PARTIAL MOLALVOLUME OF SEVERAL COMPOUNDS IN 6 M UREA taneously leaves the experimentally measured rate constants unaltered, provides high efficiency of semiquinone formation and a quantum yield of quinone disappearance of 0.6, and fits a range of initial quinone concentrations and illuminations. A remaining limitation is the uniqueness of the mechanism upon which the model is based. Naturally, if a
different mechanism were postulated, then the derived set of rate constants would be different also. The experimental and calculational procedures here described are, we believe, equally applicable to the investigation of any pair of quinones as long as there is a redox potential between the respective hydroquinones and quinones.
The Partial Molal Volume of Several Alcohols, Amino Acids, Carboxylic Acids, and Salts in 6 M Urea at 25'1 by Walter A. Hargraves and Gordon C. Kresheck Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60116 (Received Fehruary 10, 1960)
Density data have been obtained for homologous series of alcohols, amino acids, and carboxylic acids and their sodium salts at various concentrations in 6 M urea at 25". Partial molal volumes of the solutes at infinite dilution were found to be larger in urea solutions than in water for all of the compounds studied. The results have been interpreted i? terms of a separate contribution of the polar head group and nonpolar side chain. The general increase in AVto with increasinglength of the hydrocarbon side chain and consideration of solubility parameters are consistent with a structure-breakingrole of urea in aqueous solution. Electrostriction of ions appeared to be less in 6 M urea than in water for the compounds studied. Finally, it appears as though differences between the partial molal volume of proteins in water and 6 M urea are to be expected on the basis of the solvent effects observed in this study.
A knowledge of the thermodynamic properties of model compounds in aqueous urea solutions is fundamental to an understanding of the urea denaturation of proteins and the effect of urea on water structure. Previous studies have shown that the transfer of nonpolar groups from water to 6 M urea a t 25" is accompanied by a negative free-energy changej2taa positive enthalpy and entropy changeJ4and a decrease in partial molal heat ~ a p a c i t y . ~The results are consistent with the view that urea behaves as a structure breakers in these systems. However, it was recently proposed that urea behaves as a structure makere when added to aqueous solutions. The view was taken that urea and water combine to form more clusters than water alone with the formation of interstices which accommodate nonpolar solutes more readily than in the absence of urea. It would appear that volume measurements would permit a distinction to be made between the above two mechanisms since it has been observed that the transfer of hydrocarbons from a nonpolar solvent to water results in a negative volume change,? which is considered to reflect the ordering of water molecules around the
hydrocarbon molecules, or structure making.*rP It would, therefore, be expected that positive volume changes would accompany the transfer of nonpolar groups from HSO to 6 M urea solution if urea behaves as a structure breaker. The increases in molal volume with increasing temperature for alcohols1o and alkyl(1) (a) This work was supported by a research grant (GM-13623) from the National Institute of General Medical Sciences, U. S. Publio Health Service. (b) Taken in part from the thesis submitted by W. A. Hargraves for the Master of Science degree, Northern Illinois University, 1969. (c) Presented in part before the Division of Biological Chemistry at the 154th National Meeting of the American Chemical Society, Chicago, Ill., Sept 1967. (2) P. L. Whitney and C. Tanford, J. Biol. Chem., 237, PC1735 (1962):Y. Noiaki and C. Tanford, ibid., 238,4074 (1963). (3) D. B. Wetlaufer, S. K. Malik, L. Stoller, and R. L. Coffine, J. Amer. Chem. SOC.,86,508(1964). (4) G. C. Kresheok and L. Benjamin, J . Phys. Chem., 68, 2476 (1964). (5) H.S. Frank and W. Y . Wen, Discussions Faraday SOC., 24, 133 (1957). (6) M.Abu-Hamdiyyah, J.Phys. Chem., 69,2720 (1965). (7) W.L.Masterton, J . Chem. Phys., 22, 1830 (1954). (8) W. Kauzmann, Advan. Protein Chem., 14, 1 (1959). (9) G. NQmethy and H. A. Scheraga, J . Chem. Phys., 36, 3401 (1962). Volume 78,Number 10 October 1969
