1404
J. Phys. Chem. 1984,88, 1404-1408
the kinetics, and the k3values obtained by exponential fits to the four high-pressure points can be taken directly as representing collisional rates k,. The linear fit to these four points, passing through the origin, gives k, = 1.7 X cm3 molecule-' s-l. With the collisional rate known, eq 5 can be used to estimate T, to fit the data at low pressure. The dashed line in Figure 2 shows the calculated curve for T~ = 0.5 s, and the adequate agreement with the data leads to the estimate T, = 0.5 s. Discussion. As the scatter of the points in Figure 4 indicates, this is not a very precise experiment for obtaining the collisional relaxation rate, compared with the much easier continuous-irracm3 molecule-' s-l diation method.I2 The value of 1.0 X from the rate-process analysis is in good agreement with that obtained using a rate-process analysis from the continuous-irradiation experiment2f3or the repetitively pulsed experiment.8 However, the cascade analysis is probably more realistic, and the value of 1.7 X cm3 molecule-' s-' obtained from it may be a better estimate of the collisional relaxation rate. This extraordinarily high rate indicates collisional relaxation of the excited bromobenzene ion at a rate comparable to the ion-neutral orbiting collision rate (-2 X lo4 cm3 molecule-' s-'). As has been noted, rapid charge transfer is a likely mechanism for this.'* (12) Kim, M. S.; Dunbar, R. C. Chem. Phys. Lett. 1979,60, 247.
The present two-pulse experiment is uniquely valuable for determining the slow noncollisional component of the ion relaxation kinetics. The rate-process and cascade relaxation models give the same result for radiative relaxation kinetics, and so it can be stated unambiguously that the lifetime of photoexcited bromobenzene ions is approximately 0.5 s, where we mean the lifetime to refer to the relaxation from an internal energy of 2.4 eV down to an energy 50.4 eV. This is close to the lifetime calculated from the IR absorption data on neutral bromobenzene8J3and thus supports our assumption that the process involved is infrared fluorescence from a vibrationally excited ground-state ion. The radiative relaxation rate observed in the present very direct measurement agrees with that from the slightly less direct, but still time-resolved, technique of repetitively pulsing the laser.8 The kinetic analyses in the continuous-irradiation experiment^^*^ are disappointing in their apparent inability to separate out reliable values for the radiative relaxation rate.
Acknowledgment. The support of the National Science Foundation and of the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. Registry No. Bromobenzene ion, 55450-33-4. (13) Dunbar, R. C. Spectrochim. Acta, Purr A 1975, 31A, 797.
A Laser Flash Photolysis Study of Paraquat Reduction by Photogenerated Aromatic Ketyl Radicals and Carbonyl Triplets' S. Baral-Tosh, S. K. Chattopadhyay, and P. K. Das* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: June 27, 1983)
Results of a kinetic study based on 337.1-nm laser flash photolysis are presented for electron-transfer reactions with paraquat (PQz+) as the oxidant for the triplets of a number of substituted benzophenones and acetophenones and the corresponding ketyl radicals. The ketyl radicals were generated from the ketone triplets by laser flash photolysis in the presence of excess of p-methoxyphenol. The electron-transfer rate constants (2 X lo8-8 X lo9 M-' s-l ) as well as the fractions (0.5-1.0) of the triplet quenching events that result in net reduction of PQ2+correlate well with the electron-releasing nature of the substituents in the ketones. Quantitative data concerning the transient absorption spectra of substituted benzophenone triplets and diarylhydroxymethyl radicals are also reported.
Introduction Because of the current interest in solar energy research, kinetics and mechanisms of photoinduced redox reactions have received considerable attention. Specifically, viologens2-12 have been commonly used as facile oxidants for a wide variety of photoexcited species, both organic and inorganic. The long-lived one-elec(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2486 from the Notre Dame Radiation Laboratory. (2) (a) Keller, P.; Moradpour, A. J . Am. Chem. SOC. 1980, 102,
7193-7196. (b) Amouyal, E.; Zidler, B.; Keller, P.; Moradpour, A. Chem. Phys. Lett. 1980, 74, 314-317. (3) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. Chem. Phys. Lett. 1979, 61,522-525; J . Am. Chem. SOC.1979, 101,4815-4824. (4) Rodgers, M. A. J.; Becker, J. C. J . Phys. Chem. 1980,84,2762-2768. ( 5 ) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helu. Chim. Acta 1978, 61, 2720-2730. (6) Kiwi, J.; Gratzel, M. J . Am. Chem. SOC.1979,101, 7214-7217. (7) Kalyanasundaram, K.; Gratzel, M. Angew. Chem., Int. Ed. Engl. 1979, 18, 701-702. (8)Krasna, A. I. Photochem. Photobiol. 1980, 31, 75-82; 1979, 29, 267-276. (9) Okura, I.; Kim-Thuan, N. J. Chem. SOC.,Chem. Commun. 1980,84. (10) Davidson, R. S.; Bonneau, R.; Fornier de Violet, P.; Joussot-Dubien, J. Chem. Phys. Lett. 1981, 78, 475-478. (11) Das, P. K. J . Chem. SOC.,Faraday Trans. 1 1983, 79,1135. (12) Takuma, K.; Shuto, Y.; Matsuo, T. Chem. Lett. 1978, 983-986.
0022-3654/84/2088-1404$01.50/0
tron-reduction products from viologens have been shown to function as intermediaries in photocleavage of water and in photogalvanic action. Pertinent to the present work, the reduction of paraquat (1,l'-dimethyl-4,4'-bipyridinium dication, PQz+ by photogenerated ketyl radicals has been found13 to be useful in photocurrent generation. In a preliminary paper14 from this lalboratory, one of us reported on the photoreduction of paraquat mediated by several aromatic carbonyl triplets. We are now presenting the results of a relatively complete laser flash photolysis study concerning electron transfer to paraquat from the triplets of several substituted benzophenones and acetophenones and the corresponding ketyl radicals. Quantitative aspects of the electron transfer in terms of rate constants and yields of primary photoproducts have been stressed. In addition, data are presented for the extinction coefficients of transient absorption of a number of substituted benzophenone triplets and corresponding diarylhydroxymethyl radicals.
Experimental Section The benzophenones and acetophenones, purchased from Aldrich, Eastman, or Pfaltz & Bauer, were purified by recrystalli(13) (a) Chandrasekaran, K.; Whitten, D. G. J . Am. Chem. SOC.1980, 102,5119-5120. (b) Ibid. 1981, 103,7270-7275. (14) Das, P. K. Tetrahedron Lett. 1981, 22, 1307-1310.
0 1984 American Chemical Society
Laser Flash Photolysis Study of Paraquat Reduction
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1405
+
TABLE I: Rate Constants for Electron Transfer to Paraquat from Aromatic Carbonyl Triplets and Ketyl Radicals and Efficiency of Electron Transfer in Triplet QuenchingC
zation from ethanol, aqueous ethanol, or benzene n-hexane mixtures or by vacuum distillation. 1-Methylnaphthalene (Eastman) was distilled under vacuum. Acetonitrile (Aldrich, Gold Label) was used as received, and water was treated by passage through a Millipore Milli-Q system. Paraquat (Eastman) was reprecipitated from a concentrated solution in aqueous methanol by adding a large volume of acetone. Because of uncertainty of the water content, a stock solution (0.3-0.5 M) of paraquat was made in water, and the exact concentration was determined by measuring the optical density of a solution made M-' cm-' by appropriate dilution from it and using 2.0 X for maximum extinction coefficient of PQ2+at 256 nm in water. This concentrated solution was used in units of pL/mL for experiments where the kinetics of reactions of PQ2+with carbonyl triplets and ketyl radicals were observed at varying [PQ2+]. The laser flash photolysis setup is described e1~ewhere.l~For all of the experiments, laser pulses (337.1 nm, -8 ns, 2-3 mJ/pulse) from a Molectron UV-400 system were used for excitation. Solutions kept in rectangular quartz cells with 1-3-mm path lengths along the analyzing light were flash photolyzed with laser pulses intersecting the analyzing beam within the cell at an angle of -2OO. Deaeration of solutions was carried out by bubbling oxygen-free argon (15-20 min).
Results Rate Constants for Electron Transfer. Laser flash photolysis of acetophenones and benzophenones in the presence of 0.2-2 mM PQ2+in 1:9 water:acetonitrile (v/v) showed the growth of spectral absorption (A, 395 and 610 nm)16 due to the paraquat radical ion, PQ+., on a microsecond time scale. The growth followed clean first-order kinetics provided the transient absorbance changes (AOD) were maintained low (C0.05) by attenuating or defocusing the laser beam and by keeping the substrate concentrations small (2-10 mM). That the formation of PQ+. is triplet mediated is shown by the fact that, in the case of benzophenones, the triplets monitored at 520-540 nm decayed with rate constants comparable with those observed for the growth of PQ+. under similar conditions. The first-order rate constants (kobsd)for the growth of spectral absorption of PQ+- at 610-615 nm were measured as a function of PQ2+concentrations and the bimolecular rate constants (kqT)for electron transfer were obtained from linear plots (correlation coefficients 20.99) based on eq 1, where T~ denotes kobd = TT-' + kqT[PQ2+] (1) lifetime of the carbonyl triplet in the absence of a quencher. In order to minimize the second-order component of the triplet decay (due to triplet-triplet annihilation), measurements of kobsd were done under attenuated laser intensities (0.5-1 mJ/pulse). The rate constants (kqK)for electron transfer from aromatic ketyl radicals to PQ2+were determined by generating the radicals by photoreduction of the corresponding carbonyl triplets by an excess of p-methoxyphenol (0.5-0.1 M). The sequence of the processes involved is shown in eq 2-7. Benzophenone and ace-
>CEO
hv +
-
*Si
+
Ti*
(2)
rT-l
Ti* PQ2+
Ti*
TI* + + p-CH,O-C,H4-OH
+
>CEO [>C=O]+*
+
>C-OH
>C-OH
-
(3)
+ PQ+* (4) + p-CH3O-C,jH4-O.
products
(5) (6)
(15) (a) Das, P. K.; Encinas, M. V.; Small, Jr., R. D.; Scaiano, J. C. J . Am. Chem. SOC.1979, 101, 6965-6970. (b) Miedlar, K.; Das, P.K. Ibid. 1982, 104, 7462-7469. (16) Farrington, J. A.; Ebert, M.; Land, E. J.; Fletcher, K. Biochem. Biophys. Acta 1973, 314, 312-381. Kosower, E. M.; Cotter, J. L. J. Am. Chem. SOC.1964,86, 5524-5527.
substituent benzophenones: none (1) 4.4'-dimethoxy- (2) 4-methoxy- (3) 4-methyl- (4) 4-fluoro- ( 5 ) 4-chloro- (6) 4,4'-dichloro- (7) 4-(trifluoromethy1)-(8) acetophenones: none (9) 4-methoxy- (10) 4-methyl- ( 1 1) 4-flUOIO- (12) 4-chloro- (1 3) 4-cyano- (14)
*
10-9k T a M-1 ;-l'
10-'k K a qb
M.1 8 1 '
2.7 7.8 5.6
0.65 1.o
1.4 5.1
1.0
3.3
0.80
3.3 2.8
1.3 2.8 2.1 1.7
0.55
1.2
0.73 0.75
0.73
0.43
0.55
0.25
0.63
4.3
1.1
5.4 4.6 4.0
5.2 8.0 4.1
3.1 3.5 1.5
3.6
0.5 1
a 515%. Obtained by using eq 9 (see text); k 1 5 % . 1:9 water:acetonitrile (v/v).
1.3
Solvent:
tophenone triplets are known" to react with p-methoxyphenol (eq 5) with rate constants in the limit of diffusion control. At the high concentrations of p-methoxyphenol used, the decay of the carbonyl triplet (predominantly via reaction 5) was complete within a short time (C20 ns) following the laser pulse, and the subsequent oxidation of the resultant ketyl radical by PQ2+ (eq 7) could be followed on a microsecond time scale in terms of the growth of transient absorption due to PQ+. at 610 or 395 nm. In the case of benzophenones, concomitant decay of the ketyl radicals at 320-345 nm was also observed. As with triplet quenching, the rate constants (kqK)for the reaction of the ketyl radicals with PQz+ were obtained from the slopes of the plots of koM (measured from the growth of transient absorption at 610 nm) vs. PQ2+ concentrations. Table I summarizes the kinetic data concerning kqTand kqK. It should be noted that prolonged laser irradiation of solutions containing the carbonyl compounds and PQ2+ does not lead to any permanent blue coloration in the presence or absence of p-methoxyphenol, suggesting that PQz+ is mostly regenerated in the system by back electron transfer from PQ'. to p-methoxyphenoxy radical when the phenol is used and to radical cations (or related products) when the phenol is not added. The decay of PQ+. observed on a relatively long time scale (-100 ps) supports this conclusion. Efficiency of Electron Transfer: Yields of PQ+.. The transient absorption spectra at 350-700 nm were recorded in the case of several carbonyl triplets following their quenching by PQz+. These spectra were practically identical with the spectrum of PQ+. reported in the literature.I6 Parts A and B of Figure 1 show the transient spectra obtained with benzophenone as the substrate in the absence and presence of p-methoxyphenol, respectively. Figure 1B should correspond to an equimolar mixture of PQ+. and p methoxyphenoxy radicals; the phenoxy radical (E,,, -5 X lo3 M-' cm-' a t 403 nm in 1:2 benzene:di-tert-butyl peroxide)'*" appears as a weak shoulder at 400-415 nm in this spectrum. The absence of a prominent band other than those due to PQ+. in Figure 1A suggests that the radical cation of benzophenone (or a secondary species derived from it) has relatively weak absorption in the spectral region under consideration. It is possible that the radical cations from aromatic ketones, particularly the ones containing no methoxy substituents (to stabilize the radical cation),
(17) Das, P. K.; Encinas, M. V.; Scaiano, J. C. J . Am. Chem. SOC.1981, 103,4154-4162. (18) (a) Das, P. K.; Encinas, M. V.; Steenken, S.;Scaiano, J. C. J. Am. Chem. SOC.1981, 103, 4162-4166. (b) Das, P. K.; Battacharyya, S. N. J . Phys. Chem. 1981, 85, 1391-1395.
1406
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
Baral-Tosh et al.
0.00
300 400 500 600 700
800
WAVELENGTH, NM WAVELENGTH, NM
Figure 1. Transient absorption spectra observed at 1.5 ps following the laser flash photolysis of benzophenone solution in 1:9 water:acetonitrile (v/v) containing (A) 2.8 mM PQzt and (B) 2.8 mM PQz++ 0.08 M p-methoxyphenol.
Figure 2. Triplet-triplet absorption spectra of (A) 4-methoxybenzophenone, (B) 4,4'-dimethoxybenzophenone, and (C) 4-(trifluoromethy1)benzophenone in 1:9 water:acetonitrile (v/v) observed in each case at -300 ns following 337.1-nm laser flash.
TABLE 11: Transient Absorption Spectral Data' of Substituted Benzophenone Triplets and Corresponding Ketyl Radicalsb
-
form an OH adduct (radical) in aqueous solutions via reaction with water (eq 8). The OH radical adduct of benzophenone is
bolt* c
RYo
J
knownlg to have an absorption maximum at 390 nm (in water); under low resolution, such adducts possibly remain masked under the intense absorption due to PQ+-. The fraction (7)of triplet quenching events that result in net electron transfer was determined on the basis of the assumption that the transient absorbance observed at 610 nm as a result of the quenching was entirely due to PQ+.. The end-of-pulse absorbance (AOD;gN) at 415 nm due to the 1-methylnaphthalene (1 -MN) triplet observed under conditions of complete quenching of a ketone triplet in the presence of excess of 1-MN (-0.04 M) was compared with the plateau absorbance (AODgho,.) at 610 nm due to PQ+. under >90% ketone triplet quenching by PQ2+,the ketone solutions for the two AOD measurements being optically matched a t 337.1 nm. 1 was calculated from eq 9 by using 11.2
X lo3 M-' cm-' for the extinction coefficient of the 1-MN triplet a t 415 nm. This value was obtained by comparison of end-of-pulse AOD's due to the benzophenone triplet (E,,, 6.5 X lo3 M-' cm-' at 520 nm in a ~ e t o n i t r i l e )and ' ~ ~ the 1-MN triplet derived by energy transfer from the former. kobsd in eq 9 is the observed pseudo-first-order rate constant for the growth of PQ+. absorption; at the high PQ2+concentrations (1-6 mM) used, koM was much higher than rTmlso that an uncertainty in rT-I (Le., intercept of the plot of kobsd against [PQ2+]) did not introduce any serious error in the measurement of 7. The maximum extinction coefficient of PQ+. (egg+.) in wet acetonitrile was assumed to be same as that in water (1 1.5 X lo3 M-' cm-1).20 The data concerning 7 for a number of substituted benzophenones and acetophenones are presented in Table I. (19) (a) Bensasson, R. V.; Gramain, J.-C. J . Chem. Soc., Faraday Trans. I 1980,76,1801-1810. (b) Ledger, M. B.; Porter, G. Ibid. 1972,68,539-553. (20) Trudinger, P. A. Anal. Biochem. 1970, 36, 222-225.
triplet
ketyl radical
10-3cmax, Amax, 10-3cmax, nm M-lcm" nm M-'cm-' 315 11.8 328 29.6 520 6.5 545 3.4 4,4'-dimethoxy350 7.8 343 34.9 440 7.1 560 2.4 545 5.2 6 75 3.6 4-methoxy335 5.6 335 36.4 450 4.7 550 3.2 520 5.4 680 3.4 4-methyl315 11.7 330 28.3 525 6.6 545 3.3 4-flUOrO315 1.2.1 323 33.0 520 5.9 545 3.2 4-chloro320 12.8 335 29.7 5 35 7.0 555 3.4 4,4'-dichloro320 12.9 338 41.5 545 9.6 570 4.2 4-(trifluoromethyl)320 12.4 332 30.3 530 5.8 550 4.6 ' Estimated maximum errors: +5 nm for wavelength maxima and ~ 2 0 % for extinction coefficients. Solvent: 1:9 water:acetonitrile. substituent none
Amax,
Transient Absorption Spectra of Substituted Benzophenone Triplets and Ketyl Radicals. Except for 4-methoxy- and 4,4dimethoxy-substituted benzophenones, the triplet-triplet (T-T) absorption spectra of 4-substituted benzophenones listed in Table I are very similar to the T-T spectrum of benzophenone itself and are characterized by two absorption maxima at 3 15-320 and 520-540 nm, respectively. Additional maxima at 540-550 and -680 nm are observed for the triplets of 4-methoxy-substituted benzophenones, the T-T spectra of which are presented in Figure 2 along with the T-T spectrum of 4-(trifluoromethy1)benzophenone (at 300-800 nm). The T-T extinction coefficient data were obtained by comparing end-of-pulse transient absorbances due to the ketone triplets (in the absence of a quencher) with those due to 1-MN triplets generated under complete energy-transfer quenching of the ketone triplets by 1-MN at 0.04-0.05 M. These experiments were done with solutions optically matched at the laser wavelength (337.1 nm). The T-T extinction coefficient of the 1-MN triplet at 415 nm was taken to be 11.2 X lo3 M-' cm-'
Laser Flash Photolysis Study of Paraquat Reduction
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1407
7
, -0.6-0.4-0.2 0.0 0.2 0.4 0.6 Q+
WAVELENGTH, NM
Figure 3. Transient absorption spectra of ketyl radicals produced by 337.1-nm flash photolysis of (A) 4-methoxybenzophenone, (B) 4,4'-dimethoxybenzophenone, and (C) 4-(trifluoromethyl)benzophenonein the presence of -0.02 M p-methoxyphenol in 1:9 water:acetonitrile (v/v). Each spectrum was observed at -2 *s following the laser flash. Note that the small band system at 410 nm in each spectrum is due to pmethoxyphenoxy radical.Is
(see above). The spectral data are presented in Table 11. The transient spectra of diarylhydroxymethyl radicals obtained by reduction of the ketone triplets with p-methoxyphenol exhibit two maxima at 325-345 (sharp and intense) and 540-580 nm (weak and broad), respectively. The spectra obtained from 4methoxy-, 4,4'-dimethoxy-, and 4-(trifluoromethy1)benzophenone are shown in Figure 3. Comparison of transient absorbance due to the ketyl radicals with those due to PQ+- produced by reduction of PQz+ (eq 7) enabled us to determine the extinction coefficients of the ketyl radicals. The spectral data obtained in this manner are summaxjzed in Table 11. It is noted that the value 3.4 X IO3 M-' cm-' for cK545 of the diphenylhydroxymethyl radical measured by paraquat reduction in this study is in good agreement with that (3220 M-' cm-') in water determined by pulse radiolysis.21 On the basis of the extinction coefficient data for substituted benzophenone triplets and corresponding ketyl radicals, it is concluded that the efficiency'of electron or hydrogen transfer via reaction 5 in wet acetonitrile is in the range 0.9-1.0. Nearly quantitative photoreduction of the benzophenone triplet by phenols in benzene has been reported in a previous study" from this laboratory. Discussion It is evident from the rate constants in Table I that the aromatic carbonyl triplets are as susceptible to oxidation by paraquat as the ketyl radicals derived from them. For the substituted benzophenones, k > kqK. This resultZ2is important because when photored~ctiod~ of viologens is meant to be carried out via ketoneand alcohol-derived ketyl radicals produced by hydrogen abstraction from alcohols by carbonyl triplets (eq IO), care should >C=O* + RZCHOH >C-OH + RZC-OH (10) +
be exercised to keep [PQ2'] low enough to make the reaction with alcohol dominate over the direct oxidation by PQz+(eq 4). Similar consideration of possible competitive reactions of the intermediate and its precursor with the probe applies when paraquat is ~ ~ e (21) Land, E. J. Proc. R . SOC.London, Ser. A 1968, 305, 457-471. Bensasson, R.; Land, E. J. Photochem. Photobiol. Rev. 1978, 3, 163-191. (22) The rate constant, 2.7 X lo9 M-' s-' , measured by us for the reaction of benzophenone triplet with PQz+in wet acetonitrile differs considerably from that (1.03 X lo' M-I s-I) reported in ref 13a for benzophenone phosphorescence quenching by PQz+in acetonitrile. This discrepancy seems too large to be explaned by solvent effect. Our experiments in aqueous acetonitrile with increasing water content (5-50%) did not show any increasing trend in kST. No attempt was made to study the triplet quenching in neat acetonitrile because of apparent insolubility of PQ2+ (as chloride) in the absence of water.
Figure 4. Hammett plots for (A) acetophenone and (B) benzophenone triplet quenching by PQz+ in 1:9 water:acetonitrile (v/v). The u+ values were taken from ref 25. The numbers identifying the ketones are shown in Table I.
0.60
?
n 0.50
Figure 5. Plot of R T In kqK for diarylhydroxymethyl radicals in 1:9 water:acetonitrile (v/v) vs. half-wave reduction potentials of the corresponding benzophenones (acetate buffer, KC1, 40% ethanol, p H 5.2, 3.5 N calomel electrode reference). The E l l z data were taken from ref 25. For numbers identifying the ketyl radicals, see Table I.
for trapping biradicals produced via intramolecular hydrogen abstraction (Norrish type 11 photoreaction) of certain aromatic carbonyl compounds. As expected, the electron-transfer rate constants for both carbonyl triplets and ketyl radicals follow the electron-releasing capability of the substituents in para positions. The Hammett plots for monosubstituted carbonyl triplets are shown in Figure 4. The slopes (p') of these plots are close to -0.05 for both acetophenones and benzophenones. The correlation between CJ+ and kqK'sis less satisfactory for both methylarylhydroxymethyl and diarylhydroxymethyl radical series. A consideration of the reduction potentials of benzophenone (Ell2= -1.14 V vs. 3.5 N calomel electrode in 40% ethanol; acetate buffer; pH 5.2)25and PQ2+ ( E l l z = -0.44 V in water vs. shows that the free energy change (AG,,) for the reduction of the diphenylhydroxymethyl radical by PQz+ is close to -10 kcal/mol. The linear plot of RT In kqK(in V) against the reduction potential of several substituted benzophenones (Figure 5) gives a slope of -0.27. The small magnitude of the slope relative to a value of -0.5 expected from Marcus theoryZ4and observed3Jsbin several cases at small lAGetl is commensurate with the onset of bending of electrond transfer ~ ~ rate ~ constants * ~ ~from the limit of diffusion control. From the point of view of practical application, the data regarding electron transfer yields ( 7 ) in triplet quenching should (23) (a) Small, Jr., R. D.; Scaiano, J. C. J . Am. Chem. SOC.1977, 99, 7713-7714. (b) Small, Jr., R. D.; Scaiano, J. C. J . Phys. Chem. 1977,81, 828-832; 1977, 81, 2126-2131, 1978, 82, 2662-2664. (24) Marcus, R. A. J . Chem. Phys. 1965, 43, 679-701. Marcus, R. A,; Sutin, N. Inorg. Chem. 1975, 14, 213-219. (25) Siegerman, H. In "Techniques of Chemistry"; Weinberg, N. L., Ed.; Wiley: New York, 1975; Vol. V, Part 11.
J. Phys. Chem. 1984, 88, 1408-1414
1408
be of interest. Evidently, like k T, 7 also shows a correlation with the electron-releasing nature of para substituents in both of the acetophenone and benzophenone series. One possible explanation for 7 being less than unity is that the triplet quenching occurs in part via triplet energy transfer (eq 1l), the triplet energy ( E T ) >C=O*
+ PQ2+
+
>C=O
+ PQ2+*
(11)
of PQz+ being 71.5 kcal/mo1.26 While we cannot rule out the involvement of the energy transfer as a parallel process in the triplet quenching in the case of acetophenone (ET = 74.1 kcal/ mol)27a and its derivatives, it seems unlikely, from energetic considerations, that energy transfer can be important in the case of benzophenone (ET = 69.2 k~al/mol)~'"and its derivatives. For benzophenones, 1 - 7 reflects the ease of back electron transfer in the electron-transfer-derived ion pair in the solvent cage producing the reactants in the ground state (in competition with its dissociation into separated radical ions). Apart from small changes in the location and intensity of absorption maxima, the spectra of the diarylhydroxymethyl (26) Ledwith, A. Ace. Chem. Res. 1972, 5, 133-139. (27) (a) Murov, S. L. "Handbook of Photochemistry"; Marcel-Dekker: New York, 1973. (6)Wiberg, K. G. "Physical Organic Chemistry"; Wiley: New York, 1964.
radicals are very similar to one another. Among the corresponding triplets, however, significant difference is noticed in the spectra of the 4-methoxy-substituted ones relative to the other 4-substituted benzophenones. The absorption at 440-460 and -680 nm appears as well-defined maxima in the case of 4-methoxy- and 4,4'-dimethoxybenzophenones, while only shoulders are observed in these spectral regions of T-T spectra of benzophenone and other 4-substituted benzophenones in Table 11. An earlier study28has recognized the dependence of T-T spectra of substituted acetophenones on the nature of the lowest triplet, 3(n,7r*) or 3(.n,7r*). It appears that the methoxy substitution in the para positions of benzophenone brings about a change in the character of the lowest (n,?r*) triplet by mixing a large amount of (a,r*) character, and this results in intensification of some of the T, T1transitions which are otherwise relatively dipole forbidden. In conclusion, the photoreduction of paraquat via ketyl radicals produced as a result of hydrogen abstraction from alcohols by aromatic carbonyl triplets is complicated by the parallel oxidation of the triplets by paraquat. The kinetic data obtained in the present study should be relevant in designing optimum conditions for photocurrent generation based on these intermediates.
-
(28) Lutz, H.; Breheret, E.; Lindquist, L. J . Phys. Chem. 1973, 77, 1758-1762.
Photochlorination of Chloroethane and Chloroethane-d, E. Tschuikow-Rbux,* T. Yano,+ and J. Niedzielskif Department of Chemistry, University of Calgary, Calgary, Alberta, Canada TZN 1N4 (Received: June 30, 1983)
The hydrogen/deuterium abstraction from C2H5Cl and C2D5Cl by ground-state chlorine atoms has been investigated between 8 and 94 "C. Results hom the internal competition in chloroethane and chloroethane-d5combined with the results of external competition with CH4 as the reference reaction have yielded rate constant data for the following reactions: CH3CH2Cl+ C1- CH$HC1+ HCl, k,; CH,CH2Cl+ C1- CH,CH,Cl+ HCl, k2p; CD$D,Cl+ C1- CD$DCl+ DC1, k,,D; CD3CDC1 C1- CD2CD2Cl DC1, k2p,D.The temperature dependence of the rate constants (cm3 s-I) is given by kzs = (1.43 f 0.29) X lo-" exp[-(462 f 71)/Tj, kap= (1.35 h 0.28) X lo-'' exp[-(871 72)/Tj, k2s,D= (0.72 k 0.14) X lo-" exp[-(468 & 70)/Tj, and k2p,~ = (0.60 h 0.12) X lo-" exp[-(1156 72)/Tj1. The results confirm the general trend of chlorine atom attack being faster at the substituted carbon atom. Kinetic isotope effects for the abstraction of primary and secondary hydrogen are kH/kD = 5.8 and 2.0 at 298 K, respectively. The magnitude of these relatively weak isotope effects agrees with expectationsbased on other exothermic chlorination reactions and suggests that in the temperature range of the investigation tunneling does not play an important role.
+
+
Introduction Our recent investigations of the vacuum ultraviolet photolysis of gaseous polychloro and chlorofluoro derivatives of ethane14 have revealed that the principal channel of photodecomposition involves the formation of chlorine atoms produced either directly by rapid, consecutive C-C1 bond cleavage reactions or from the dissociation of an excited C12* molecule generated by molecular elimination in the primary process. The chlorine atoms so formed undergo competitive reactions involving hydrogen atom abstraction from the parent molecule and addition to the product olefins. Both processes yield halogenated ethyl radicals, albeit with a different energy content. The quantitative interpretation of the results has been somewhat impeded by the lack of, or uncertainty in, the kinetic data of C1 atom reactions with the halogenated hydrocarbons. In an effort to gain further insight into the subject we Visiting scientist. Present address: Hitotsubashi University, Kunitachi, Tokyo, Japan. *Visiting scientist. On leave from Department of Chemistry, Warsaw University, Warsaw, Poland.
0022-3654/84/2088-1408$01.50/0
have undertaken a systematic study of the photochlorination of various D-labeled chloroethanes. Aside from the intrinsic interest in the kinetics of these systems, the impetus for this work has been enhanced by the recent renewed attention paid to reactions of atomic chlorine in connection with the chemistry of pollutants. The objectives of this work were, firstly, to obtain accurate mechanistic and rate data on the H / D abstraction by chlorine atoms. Secondly, to establish the magnitude of the hydrogen/ deuterium kinetic isotope effect, which, particularly in the case of atomic chlorine reactions, with low activation energies, is of considerable theoretical interest. Finally, with improved analytical procedures we considered it worthwhile to reexamine some of the older data, but at much lower conversion to minimize complications arising from secondary processes. (1) T. Yano and E. Tschuikow-Roux, J . Phys. Chem., 83,2572 (1979). (2) T. Yano and E. Tschuikow-Roux, J. Chem. Phys., 72, 3401 (1980). (3) T. Yano, K.-H. Jung, and E. Tschuikow-Roux, J . Phys. Chem., 84,
2146 (1980). (4) T. Yano and E. Tschuikow-Roux, J . Phys. Chem., 84, 3372 (1980).
0 1984 American Chemical Society
