2446
Hisao Murai and Kinichi Obi
References and Notes (1)C. U. Pittman, Jr., and S. P. McManus in "Reactive Intermediates in Organic Chemistry", S. P. McManus, Ed., Academic Press, New York, N.Y., 1973. (2)G. A. Olah and J. A. Olah in "Carbonium ions", VoI. 11, G. Olah and P. v. R. Schleyer, Ed., Wiley-lnterscience, New York, N.Y., 1970, pp 715-
782. (3) G. A. Olah and C. U.Pittman, Jr., Adv. Phys. Org. Chem., 4 (1966). (4)G. A. Olah, R . J. Spear, P. W. Westerman, and J. M. Denis, J. Am. Chem. SOC.,96,5855 (1974). (5) C. U. Pittman, Jr., and G. A. Olah, J. Am. Chem. SOC.,67, 5632 (1965). (6)P. J. Stang, Prog. Phys. Org. Chem., 10, 276 (1973). (7)P. J. Stang and T. E. Dueber, J. Am. Chem. SOC.,95,2683 (1973). (8)2 . Rappoport, T. Bassler, and M. Hanack, J. Am. Chem. SOC..92,4985 (1970).
(9)R. W. Taft. R. H. Martin, and F. W. Lampe, J. Am. Chem. SOC., 87, 2490 (1965). (IO)L. D. Kispert, C. U. Pittman, Jr., D. L. Allison, T. B. Patterson, Jr.. C. W. Gilbert, Jr., C. F. Hains, and J. Prather, J. Am. Chem. SOC.,94, 5979 (1972). (11)L. D. Kispert. E. Engelman, C. Dyas, and C. U. Pittman, Jr.. J. Am. Chem. Soc., 93,6948 (1971). (12)C. U. Pittman, Jr., C. Dyas, C. Engelman, and L. D. Kispert, J. Chem. SOC.,Faraday Trans. 2,68, 345 (1972). (13)C.U. Pittman, Jr.. T. B. Patterson, Jr., and L. D. KisDert. J. Org. Chem., 38, 471 (1973). (14)c. U. Pittman, Jr. A. Kress, T. B. Patterson, P. Walton, and L. D. Kispert, J. Org. Chem., 39, 373 (1974). (15)C. U. Pittman, Jr.. A. Kress, and L. D. Kispert, J. Org. Chem., 39, 378 1197A\ ,.-. . I .
(16)C. U. Pittman, Jr., L. D. Kispert, and T. B. Patterson, Jr., J. Phys. Chem., 77, 494 (1973).
Photochemistry of Higher Excited Triplet States of Benzaldehyde, Acetophenone, and Benzophenone at 77 K Hisao Mural and Klnichi Obi* Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo, Japan (Received February 4, 1975; Revised Manuscript Received August 1, 1975)
The photochemistry of benzaldehyde, acetophenone, and benzophenone has been investigated a t 77 K under high-intensity irradiation. The reactions take place through the higher excited triplet state formed by a biphotonic process. Benzaldehyde in the higher excited triplet state decomposes to benzoyl and atomic hydrogen or is converted into a ketyl radical. Onthe other hand, acetophenone and benzophenone undergo only ketyl radical formation. The lowest triplet state of these carbonyl compounds shows no hydrogen abstraction reaction at 77 K.
Introduction The hydrogen abstraction reaction of the n,r* triplet state of benzophenone1 has been extensively studied by a number of workers. It has been established that the following reaction occurs at room temperature Ph&O*(TI)
+ RH
--t
PhzCOH
+R
where RH is the solvent molecule. At low temperature, only a few studies have been made on the photochemical reactions of benzophenone. Sharp and coworkers2 have observed the EPR signal of the ketyl radical at 123 K. Kuwata and Hirota have observed a broad singlet EPR spectrum in pure crystals and in ethanol and EPA solutions of benzophenone at 77 K by irradiating the samples with ultraviolet light for several hours.3 Farmer, Gardner, and McDowell have suggested that diphenylketyl radicals are observed at 77 K by EPR mea~urernent.~ On the other hand, Godfrey, Hipern, and Porter5 have proposed that the ketyl radical was formed in the flash photolysis of benzophenone in isopentane solution at temperatures higher than but not below -100 K. The formation of the ketyl radical a t 77 K is, therefore, questionable. In general, aromatic molecules in rigid media can undergo the photosensitization reaction by the higher excited triplet state through a biphotonic process. Detailed studies of the energy-transfer process from the higher triplet state The Journal of Physical Chemistry, Voi. 79, No. 22, 1975
have been carried out for naphthalene, naphthalene derivatives, and aromatic Since these molecules have fairly long triplet lifetimes in rigid media, it is easy to make the triplet concentration high by light irradiation. On the other hand, since the lifetimes of the triplet states of aromatic carbonyl compounds are very short at 77 K compared with other aromatic molecules, no study has been reported on the biphotonic process of these compounds. In order to observe the biphotonic process of these molecules, it is necessary to use a high-intensity lamp as a light source. In this work, the biphotonic processes of benzaldehyde, acetophenone, and benzophenone have been investigated at 77 K using an extra-high-intensity mercury lamp. Experimental Section The EPR spectrometer used in this experiment was a conventional X-band type (JEOL JES-3BS-X) operated with 100-kHz modulation. In order to cool the sample, a quartz dewar or a variable-temperature dewar was inserted into a TEol1 cylindrical cavity. A 3- or 4-mm 0.d. quartz tube was used for the EPR measurements. Mn2+ doped in MgO powder was used as a standard for the intensity and the hyperfine splitting of the spectra. The optical absorption spectra were measured a t 77 K using a Hitachi EPS-3T spectrometer. In order to prevent the rise of sample temperature, the absorption cell was im-
2447
Photochemistry of Benzaldehyde, Acetophenone, and Benzophenone mersed in liquid nitrogen. The emission spectra were measured with a Hitachi 139 monochromator. The excitation was carried out a t 365 nm using a high-pressure mercury lamp combined with an interference filter. The irradiation source used for the photolysis was an aircooled high-intensity high-pressure mercury lamp (Toshiba SH-1200-L). A Halio glass filter, which transmitted light of wavelength longer than 300 nm, was used in this work. The incident-light intensity was varied by using calibrated wire screen filters. In order to eliminate the effects of infrared irradiation from the lamp, a cylindrical quartz cell, 200 mm in length and 50 mm in diameter, containing distilled water was placed between the lamp and dewar as an infrared filter, Since this filter had no effect on the results, the influence of the infrared light is negligible. Methanol, ethanol, and EPA (ether-isopentane-ethanol, 5:5:2) were used as solvents. Methanol was Merck spectroscopic grade. Benzaldehyde, acetophenone, and the other solvents were reagent grade and were used without further purification. The guaranteed reagent, benzophenone, was recrystallized twice from a methanol solution and dried in a vacuum system. The concentration of benzaldehyde and acetophenone was 0.2-1.0 vol YO,and that of benzophenone was less than M. The solutions were thoroughly degassed with a high-vacuum system (10-5-10-6 Torr) by multiple freeze-pump-thaw cycles.
Results (I ) EPR Measurements. Benzaldehyde. Benzaldehyde in methanol, ethanol, and EPA glassy solvents was irradiated with light of wavelength longer than 300 nm a t 77 K. The typical observed EPR spectra are shown in Figure la. These spectra are due to the solvent and other types of radicals. By raising the temperature of the sample, the solvent radical spectrum could be selectively reduced. In case of ethanol, for instance, the ethanol radical spectrum completely disappeared at 119 K and an asymmetric spectrum remained as shown in Figure lb. If the measurements were carried out under high sensitivity, the satellite spectrum of the natural isotope of carbon (13C) was observed on both sides of the central asymmetric absorption. By further increasing the temperature to 127 K, the central asymmetric absorption spectrum also disappeared and only a broad spectrum with a little proton hyperfine structure remained as shown in Figure IC.In the cases of the methanol and EPA solvents, similar results were also obtained. Thus, it is concluded that the spectrum shown in Figurelb consists of a fairly sharp asymmetric spectrum and a broad one with hyperfine structure. The latter is assigned to the a-hydroxylbenzyl radical for reasons which will be discussed later. The g value a t the peak of the asymmetric spectrum was 2.0009 f 0.0004 and the value for the isotopic carbon-13 hyperfine coupling constant was -360 MHz. Bennett and Milelo have observed the EPR spectrum of the benzoyl radical. They reported the principal values of the g tensor and fairly large isotopic hyperfine coupling constants for carbon-13. The g value obtained in this work is close to the isotropic value (2.0008) of the principal g tensor reported by Bennett and Mile. The value for the isotopic carbon-13 hyperfine coupling constant (-360 MHz) is also close to the average value (367 MHz) obtained by them and to the value in the liquid phase (359 MHz) measured by Krusic and Rettig.ll Therefore, the asymmetric spectrum is assigned to the benzoyl radical. The relative rates of radical formation were estimated by
v spectra of the radicals obtained in the photolysis (A 1300 nm) of benzaldehyde in ethanol: (a) after light irradiation at 77 K; (b) raising temperatureto 119 K; (c)raising temperature to 127 K . Figure 1. EPR
A
H-
30 G
spectra of the radicals obtained in the photolysis (A 2300 nm) of acetophenone in ethanol: (a)after light irradiation at 77 K; (b) raising temperature to 113 K. Flgure 2. EPR
integrating the absorption spectra: a-hydroxylbenzyl radicals are formed 1.9 times faster than benzoyl radicals in glassy ethanol a t 77 K. Acetophenone. In the photolysis of acetophenone in glassy ethanol a t 77 K, complex EPR spectra were obtained; the results are shown in Figure 2a. By raising the temperature, the solvent radical spectrum disappeared and a symmetric EPR spectrum with hyperfine structure remained. Figure 2b shows the spectrum obtained a t 113 K. The same spectrum was also obtained in glassy methanol. The spectrum is assigned to the methylphenylketyl (l-hydroxyl-1-phenylethyl) radical. In case of acetophenone, no benzoyl radical was observed. Benzophenone. When benzophenone in methanol, ethanol, and EPA glasses was irradiated at 77 K, the EPR spectra of mainly the solvent radicals were observed. By raising the temperature of the glass, the solvent radical spectrum disappeared and a symmetric singlet EPR signal (AHms, N 15 G)remained. As an example of this behavior, the results obtained in ethanol at 143 K are shown in Figure 3. The spectrum is assigned to the diphenylketyl (1-hydroxyl-1,lThe Journal of Physical Chemistry, Vol. 79. No. 22, 1975
Hisao Murai and Kinichi Obi H-
30 G
Figure 3. EPR spectrum of the radicals obtained in the photolysis (A 1 3 0 0 nm) of benzophenone in ethanol at 77 K, followed by raising temperature to 143 K.
diphenylmethyl) radical. In the case of benzophenone, the characteristic asymmetric spectrum of the benzoyl radiLight Intensity callo was not observed. If benzophenone labeled with carFlgure 4. Relative rate of the radical formation vs. relative light intenbon-13 a t the carbonyl group (91.9 atom % 13C) was irradisity: (a)benzaldehyde; (b) acetophenone; (c) benzophenone. ated at 77 K, only a trace of benzoyl with carbon-13 hyperfine structure was observed. Therefore, it can be concluded that the formation of benzoyl radical is negligibly small. 10 ,-\ ; Identification of Ketyl Radicals. In order to identify the )I ',r-EMISSION I I C radicals remaining after raising the temperature of the samples, the EPR measurements were carried out simultaneously with light irradiation at -140 K. The spectra obtained were the same as those for the radicals mentioned above. Since the lowest triplet state (3n,a*) of the carbonyl x 6 compounds studied could abstract atomic hydrogen from the solvent molecule at this temperature, the radicals were 0 I I I identified as the ketyl radicals (a-hydroxylbenzyl, methylphenylketyl, and diphenylketyl) in the cases of benzaideWavelength ( nrn I hyde, acetophenone, and benzophenone, respectively. Flgure 5. Absorption and emission spectra obtained in the photolysis Light Intensity Dependence. The relative rate of radical of benzophenone in EPA at 77 K. formation in the photolysis was estimated by measuring the maximum height of the spectra. Figure 4 shows the desured in glassy EPA at 77 K. The excitation of the intermependence of the rate of radical formation at 77 K on the diates was accomplished at 365 nm by using a high-preslight intensity ( I O ) ;it appeared to be proportional to 10". sure mercury lamp. The results are shown in Figure 5. A The results show that the values of n are equal to 1.5, 1.7, good mirror-image relationship was observed between the and 1.4 in the cases of benzaldehyde, acetophenone, and absorption and emission spectra. Therefore, the emission benzophenone, respectively. If the low-intensity light spectrum was thought to result from the intermediates. source was used, no radical spectrum was observed after irIn the cases of benzaldehyde and acetophenone, similar radiation for 2 0 4 0 min. Therefore, the radical formation is results were observed as in the case of benzophenone. Howtaking place through a two-photon process which will be ever, the absorption spectra of the ketyl radicals formed discussed later. from benzaldehyde and acetophenone were not ascertained (2) Optical Absorption and Emission Spectra. Benzobecause of their overlap with the broad bands of the interphenone in glassy EPA was irradiated at 77 K and the abmediates. sorption spectra were measured. The results are shown in Figure 5. The spectra show the characteristic bands of the Discussion ketyl radicals12J3at 530 and 559 nm and a very broad band that overlaps with the bands of the ketyl radicals. Benzaldehyde, acetophenone, and benzophenone are excited to the h , a * state by light of wavelength longer than Since the ethanol (CH3CHOH) radical has a very broad 300 nm. It is well-known that the h , a * state of these comband with a peak at 517 nm,14 the broad band observed in this experiment could be attributed to the ethanol radical. pounds transfers to the 3n,n* state by intersystem crossing Since EPA was used as the solvent, the CH~CHZOCHCH~ with a quantum yield of almost unity15 and that the 3n,n* state abstracts a hydrogen atom from the solvent molecule radical is also responsible for the broad band. This radical at room temperature. Since no radical was observed after seems to have an absorption band in the same region as irradiating the solutes in rigid glassy solvents at 77 K for CH3CHOH. Another broad band (A, -350 nm) was also 20-40 min with the low-intensity mercury lamp, it was conobserved in the region between 300 and 400 nm. This band cluded that the hydrogen abstraction reaction by the 3n,7r* was too strong to be identified as the second electronic state of these compounds does not take place or was very transition of the ketyl radicals. slow at 77 K. This conclusion agreed with the results of After melting the solution by raising the temperature, Godfrey and coworker^.^ the bands in the visible region disappeared immediately. Compared with naphthalene, naphthol, etc., of which the On the other hand, the band with a peak at 350 nm rephotosensitization reactions through the biphotonic promained after melting the solution under vacuum and was cess have been r e p ~ r t e d , " ~ the lifetimes of the lowest tripfairly stable at room temperature. However, this band dislet states of the aromatic carbonyl compounds are quite appeared gradually on exposure to air. It was, therefore, short;l5 they are 1.5, 2.3, and 4.7 msec for benzaldehyde, concluded that diphenylketyl radicals and some intermediacetophenone, and benzophenone, respectively, at 77 K. ates are produced at 77 K. Therefore, the biphotonic process is expected to occur only The emission spectrum of the intermediates was mea-
L\w
'\
I
The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
Photochemistry of Benzaldehyde, Acetophenone,and Benzophenone under high-intensity light irradiation. The rate of product formation by this process is approximately dP = azon -
dt where ZO is the incident light flux in the sample. The exponent n has a value between 1 and 2 depending on both the physical characteristics of the host molecule and the experimental condition^.^ The observed values of the exponent were 1.5, 1.7, and 1.4 in the cases of benzaldehyde, acetophenone, and benzophenone, respectively. It was, therefore, concluded that the higher excited triplet states formed through the biphotonic process contribute to the radical formation. Benzaldehyde. The radicals formed through the biphotonic process at 77 K were the benzoyl, a-hydroxylbenzyl, and solvent radicals. The benzoyl radical is thought to be produced directly by the decomposition of the higher excited triplet state of benzaldehyde PhCHO*(Tn)
-
PhCO
+H
(2)
Harrison and Lossing16 proposed the following decomposition mode as the radical formation process in the Hg-photosensitized reaction PhCHO
+ Hg*
-
Ph
+ CHO + Hg
(3)
Since no formyl radical was observed, the decomposition to phenyl and formyl radicals does not occur under our experimental conditions. Two processes are responsible for the production of the a-hydroxylbenzyl radical. The first is a hydrogen abstraction reaction from the solvent molecule by the higher excited triplet state of benzaldehyde PhCHO*(Tn)
+ RH
-
PhCHOH
+R
(4)
The second is the photosensitized decomposition of the solvent molecule by the higher excited triplet state of benzaldehyde PhCHO*(Tn)
+ RH
-
PhCHO
+R +H
(5)
followed by the attachment of atomic hydrogen to benzaldehyde
+
PhCHO H PhCHOH (6) Since these two processes give the same products, it is difficult to distinguish between them. Acetophenone. The radicals produced at 77 K were the methylphenylketyl and solvent radicals. The EPR spectrum of the methylphenylketyl radical shown in Figure 2b is interpreted by assuming the following approximate values for the hyperfine coupling constants: (1) 12 G for the three methyl protons, ( 2 ) 6 G for the ortho and para protons of phenyl, and (3) relatively small values for the meta protons of phenyl and the hydroxy proton as compared with others. Considering the superconjugation of methyl protons, these values are in good agreement with those for the a-hydroxylbenzyl radical as reported by Wi1s0n.l~ Methylphenylketyl would be formed by the same mechanism as the a-hydroxylbenzyl formation in the case of benzaldehyde discussed above. Since neither the benzoyl nor the acetyl radical was observed, the decomposition of acetophenone in the higher excited triplet state does not occur. Benzophenone. The radicals observed were of the same nature as those in case of acetophenone: diphenylketyl and
2449
solvent radicals. The EPR spectrum of diphenylketyl obtained at room temperature shows the proton hyperfine structure.17 Since the molecular motion is suppressed at low temperature, the spectrum is broadened. This results in only a broad singlet as shown in Figure 3. Diphenylketyl would be formed by the same mechanism as discussed above. Since a trace of benzoyl was observed in the photolysis of the labeled ( 13C) benzophenone, the decomposition of benzophenone in the higher excited triplet state is thought to be only a minor mode of the primary processes. Benzaldehyde breaks up into benzoyl and atomic hydrogen which would easily move out from its cage and would, thus, result in decomposition. On the other hand, in the cases of acetophenone and benzophenone, the methyl and phenyl radicals formed in decomposition would not readily move out from their cages at 77 K and, thus, would recombine with the benzoyl radical to form acetophenone and benzophenone. Therefore, decomposition is hardly observed in these compounds. Filipescu and M i n d 8 have reported that in the photolysis of benzophenone in 2-propanol at room temperature a stable intermediate is produced by the reaction of the diphenylketyl and 2-propanol radicals and is assigned as 1(phenylhydroxylmethylene)-4- (dimethylhydroxylmethyl)2,5-cyclohexadiene. This compound has broad absorption and emission spectra with peaks at 325 and 440 nm, respectively. It is fairly stable under vacuum but disappears on exposure to air. Since the fairly stable intermediates observed in our experimental absorption and emission spectra behave in similar ways to that proposed by Filipescu and Minn, these intermediates would be the same kind of compounds as suggested by them. However, the detailed assignments of the intermediates have not been carried out in this study. In conclusion the photochemical reactions of benzaldehyde, acetophenone, and benzophenone take place through the higher excited triplet state formed by the biphotonic process a t 77 K. Two modes of reactions are proposed as follows: (1) ketyl radical formation PhC=O*(Tn)
I X
+ RH
--+
PhCOH
I X
+R
and (2) decomposition to the benzoyl radical PhC=O*(Tn)
I
--
PhC=O
+
X
X where RH is the solvent molecule and X is H, CH3, or phenyl. In the cases of acetophenone and benzophenone, ketyi radical formation is predominant. However, in the case of benzaldehyde, both reactions occur. Acknowledgment. We wish to thank Professor I. Tanaka for his helpful comments and discussions.
References and Notes (1) A. Beckett and G. Porter, Trans. Faraday Soc., 59, 2038 (1963). (2)J. H. Sharp, T. Kuwana, A. Osborne, and J. N. Pitts, Jr., Chem. /nd. (London),508 (1962). (3) K. Kuwata and K. Hirota, Bull. Chem. SOC.Jpn., 34, 458 (1961). (4) J. B. Farmer, C. L. Gardner, arld C. A. McDowell. J. Chem. Phys., 34, 1058 (1961). (5) T. S. Godfrey. J. W. Hipern, and G. Porter. Chem. Phys. Lett., 1, 490
(1967). (6) K. S. Bagdasar'yan. V. I. Muromtsev, and Z. A. Sinitsyna. Dokl. Akad. Nauk SSSR, 152,349 (1963).
The Journal of Physical Chemistry. Vol. 79, No. 22, 1075
2450
P. A.
(7) K. S . Bagdasar'yan, Z. A. Sinitsyna, and V. I. Muromtsev, Dokl. Akad. Nauk SSSR, 153,374 (1963). (8) S. Siege1and K. Eisenthal, J. Chem. fhys., 42, 2494 (1965). (9) K. Shimokoshi, Y. Mori, and I. Tanaka, Bull. Chem. SOC. Jim..409 302 (1967). (IO) J. E. Bennett and 6. Mile, Trans. faraday Soc., 67, 1587 (1971). (1 1) P. J. Krusic and T. A. Rettig, J. Am. Chem. Soc., 92, 722 (1970). I. SOC. Jpn.. 45, 1357 (1972). (12) S. Arlmitsu and H. Tsubomura, &/Chem.
Skotnlchi, A. G.Hopkins, and C. W. Brown
(13) E. Hayon, T. Ibata, N. N. Lichtin, and M. Simic, J. fhys. Chem., 76, 2072 (1972). (14) M. C. R. Symons and M. Townsend, J. Chem. fhys.. 25, 1299 (1956). (15) S. L. Murov, "Handbook of Photochemistry", Marcel Dekker, New York, N.Y., 1973. (16) A. G. Harrison and F. P. Lossing, Can. J. Chem., 37, 1696 (1959). (17) R. Wilson, J. Chem. SOC.E, 84 (1968). (18) N. Filipescu and F. L. Minn, J. Am. Chem. Soc., 90, 1544 (1968).
Time Dependence of Quantum Yields for the Photooxidation of Sulfur Dioxide Peggy A. Skotnlcki, Alfred 0. Hopkins, and Chris W. Brown* Department of Chemlstty, University of Rhode Island, Klngston, Rhode Island 0288 1 (Received August 9. 1974; Revised Manuscript Received June 30, 1975)
Quantum yields for the photooxidation of SO2 have been studied in a static system as a function of time using Raman bands of SO2 and so3 to determine the extent of reaction. The quantum yields were found to decrease during the first 5 to 10 hr. It ie suggested that this decrease is due both to the formation of a film on the walls of the reaction cell and to a back reaction.
Most previous inve~tigationsl-~ of the photooxidation of SO2 in its first allowed absorption region (2600-3300 A) were performed at fixed times, since wet chemical analysis made it difficult to do time-dependent studies. The amount of SO3 formed from SO2 was determined by dissolving the SO3 in a suitable solution, and measuring the amount of S042- formed. Cox1 showed that this method could lead to serious errors if SO2 dissolved forming S032-, which could be converted to S042- in the solution. In order to avoid this problem he ran blanks with pure SO2 to determine the amount of S042- formed from S02. Recently, we developed a new method for the analysis of an so3-so2 gas m i ~ t u r e Both .~ SO2 and SO3 have one strong band in their Raman spectra: 1150 cm-l for SO2 and 1068 cm-I for SO3. The spectrum of a gaseous mixture can be measured without removing the gases from the reaction cells. From the intensities of the two bands and calibration data on intensity vs. pressure, the amounts of SO2 and SO3 present can be determined. It takes less than 10 min to measure the Raman spectrum in the region of interest; thus, it is possible to do time dependent studies of the photooxidation of S02, i.e., to remove the reaction cell from the photolysis chamber, measure the Raman spectrum, and return the vessel to the chamber for continued photolyais. Photolysis experiments were carried out in a Rayonet Srinivasan-Griffen photochemical reactor with lamps emitting a band centered at 3000 A. A 35-cm long, 4-cm diameter cylindrical tube (both quartz and Pyrex were used) with a Teflon stopcock at one end was used as the reaction cell. The cell was cleaned prior to each use by soaking in hot chromic acid overnight, rinsing with 3 N aqueous ammonia overnight, and washing with distilled water and deionized water several times. It was then evacuated to Torr, and degassed by heating to -300'C. Finally, a small pressure of oxygen was introduced and subjected to a microwave discharge to oxidize any impurities remaining on the walls. SO2 (Matheson, 99.98%) and oxygen (M. G. ScientifThe Journal of Physical Chemlstry, Vo/. 79, No. 22, 1975
ic, 99.99%) were vacuum distilled several times prior to use and the reaction cell was filled using standard manometric procedures. Quantum yields for SO3 were determined from the amount of SO3 formed and intensity of the light absorbed ( I , ) by S02. The pressure of so3 relative to SO2 was determined from the Raman ~ p e c t r a this ; ~ was converted to absolute pressures using the known initial pressure of SO2 and assuming that for each molecule of SOB formed two molecules of SO2 reacted. The incident intensity was determined by a diethyl ketone actinometer for both quartz and Pyrex cells. Extinction coefficients for both diethyl ketone and SO2 were estimated from published ~ p e c t r a ,and ~ it was assumed that absorptions of both substances followed Beer's law. Raman spectra were measured on a Spex Industries Model 1401 double monochromator, with a CRL Model 52-A argon ion laser (-600 mW at the sample) and photon counting detection. The spectral measurements were made with the 4880-A laser line but, in several experiments, the results were confirmed with the 5145-A laser line to eliminate the possibility of surface films absorbing in the region of Raman scattering. Details of the measurements and calibration data are discussed el~ewhere.~ Cox1 pointed out that the major problem with all of the previous studies of the photolysis of SO2 has been the wide range of quantum yields obtained. In this note we show evidence for these discrepancies. In Figure l the quantum yield for SO3 is plotted vs. time for an initial SO2 pressure of 200 Torr. After 1 hr, -1.5% of the SO2 is converted giving a quantum yield for SO3 of (7.4 f 0.7) X After 3 hr a conversion of -2.4% SO2 is reached with a quantum yield for SO3 of (4.1 f 0.4) X The quantum yield levels off after about 15 hr at -0.5 X lov3. The quantum yield for SO3 vs. time for an initial SO2 pressure of 100 Torr is plotted in Figure 2. The results are practically the same as for the initial SO2 pressure of 200
