Literature Cited Arnold, S . J., Kubo, M., Ogryzlo, E. A., Advan. Chem. Ser. 77,133 (1968). Begeman, C. R., Vehicle Emissions, Society of Automotive Engineers, Tech. Progr. Ser. 6, 163 (1964). Broadbent. A. D.. Gleason. W. S.. Pitts. J. N.. Jr.. Whittle. E., Cheh. Comh. 1%8, p.‘ 1315. ’ Calvert, J. G., Pitts, J. N., Jr., “Photochemistry,” pp. 297,368, Wiley, New York, 1966. DASA Reaction Rate Handbook, M. Bortner, Ed., DASA No. 1948.1967. Falick, A., ‘Mahan, B., Myers, R., J. Chem. Phys. 42, 1387 (1965). Foote, C. S . , Accounts Chem. Res. 1, 104 (1968). Kautsky, H., Trans. Faraday SOC.35, 56 (1939). Kawaoka, K., Khan, A. U., Keams, D. R., J. Chem. Phys. 46,1842 (1967). I
,
Khan. A. U.. Pitts. J. N.., Jr.., Smith. E. B.. ENVIRON.SCI. TECHNOL. 1, 657 (1967). Kummler, R., Bortner, M., GE TIS Rept. R67SD20 (May 1967). Kumrder, R., Bortner, M., Baurer, T., ENVIRON. SCI.TECHNOL. 3,248 (1969). McNeal, R. J., Cook, G. R., J. Chern. Phys. 47,5385 (1967). Pitts, J. N., Jr., AAAS Divisional Symposium on Air Pollution, UCLA, June 19, 1967. Pitts, J. N., Jr., Khan, A. U., Smith, E. B., Wayne, R. P., ENVIRON. Scr. TECHNOL. 3,241 (1969). Pullan, H., “Final Report on Integrated Preamplifier Assemblies,” DOD No. 254 AD 357134, Feb. 19,1965 (classified confidential). Snelling, D. R., Chem. Phys. Letters 2, 346 (1968). ~~
Received for review February 19, 1969. Accepted July 29,1969.
Singlet Oxygen in the Environmental Sciences Evidence for the Production of
02(1~~)
by Energy Transfer in the Gas Phase
R. P. Steer, J. L. Sprung, and J. N. Pitts, Jr. Department of Chemistry, University of California, Riverside, Calif. 92502
Evidence for the production of OdlAg) by energy transfer to oxygen from the first excited triplet states of benzene and naphthalene in the gas phase has been obtained. Emission from OdlA,) at 1.27 microns in illuminated oxygen-naphthalene mixtures and the production of 2,3-dimethyl-2-hydroperoxy-1-butene in irradiated benzene-oxygen-tetramethylethylene mixtures have been observed. Application of these results to photochemical air pollution is discussed.
I
n view of the recent speculation regarding the possible
importance of singlet oxygen in the chemistry of polluted urban atmospheres (Khan, Pitts, e? al., 1967; Pitts, Khan, et al., 1969; Kummler, Bortner, et al., 1969), we report some preliminary results of experiments in which 041A,) has been produced by energy transfer from the triplet states of some aromatic sensitizers to oxygen in the gas phase. The significant role of singlet oxygen as an oxidant has only recently begun to be appreciated, and whereas extensive work has now been carried out in the liquid phase (Foote, 1968; Gollnick and Schenck, 1967), studies of oxidations involving singlet oxygen in the gas phase are sparse. The first direct observations of the formation of 04lAg) by photosensitization in the gas phase were reported by Snelling (1968), who observed emission at 1.27 microns in irradiated benzene-oxygen mixtures, and by Kummler (1968), who observed similar emission in illuminated benzaldehyde-oxygen mixtures. More recently, Kearns, Khan, et al. (1969) and Wasserman, Kuck, et al. (1969) have also observed the production of OdlA,) in gas phase 02-sensitizer systems using ESR measurements. 946 Environmental Science & Technology
The purpose of this paper is to confirm these results and to report the observation of the formation of oxidation products in an irradiated sensitizer-oxygen-olefin system. Experimental
Emission studies were performed in a flow system shown schematically in Figure 1. Oxygen (Matheson ultrapure grade) was saturated with naphthalene vapor (vapor pressure -100 microns at room temperature) and passed into a cylindrical quartz cell, 12 inches long and 3 inches in diameter, illuminated by two unfiltered Hanovia 673A-10 500-watt medium pressure mercury lamps. The gas mixture was then passed into an integrating sphere consisting of a 3-liter borosilicate glass bulb having a reflective gold inner surface (produced by vacuum evaporation) and a single borosilicate glass window through which the emission was observed. The detection system consisted of a chopper, monochromator (Bausch & Lomb, Model 33-86-25) with near-infrared grating, and liquid nitrogen-cooled germanium photodiode, the signal from which was passed into a Princeton Applied Research INTEGRATING SPHERE
n FLOWMETER
Os
IN
CTO
PUMP
3
MANOMETER
U MANOMETER
igure 1. Flow system for emission studies
Model 120 lock-in amplifier and thence to a recorder. Most experiments were conducted with the monochromator slits fully open- Le., under extremely low resolution. The smallest concentration of 04 ‘A,) detectable by this method was estimated to be approximately 5 X lo-* mole per liter. Oxygen flow rates were measured by capillary flowmeters and lay in the 1 to 5 X lo-‘ mole per second range. Pressures were typically of the order of 5 to 10 Torr and residence times in the reactor were about 1 to 3 seconds. Photooxidation product analysis studies were carried out using conventional high vacuum techniques. The reaction cell was of quartz, 20 cm. long and 4.5 cm. in i.d., and a Hanovia Type SH medium pressure mercury lamp with Vycor filter (50% transmission at 2537 A.) was employed. Benzene and tetramethylethylene (TME) were purified by conventional means. Oxygen (Matheson research grade) was used without further purification. Photooxidation products and residual reactants were analyzed by gas chromatography of the -196’ C. condensable materials using a 3-foot X 1/8-inch, 10% silicone XE-60 on deactwated Chromosorb P (80/100mesh) column at 70” C . Results and Discussion
In the emission studies, reproducible signals were observed at 1.27 microns when the range of wavelengths from 1.0 lo 1.4 microns was scanned, indicating that 0 2 ( lA,) was present in the irradiated naphthalene-oxygen mixture. This signal disappeared when the exciting lamps were turned off or the traps containing the naphthalene were cooled, indicating that the 02(1AO)is probably produced by energy transfer to oxygen from excited naphthalene in the gas phase. This observation is qualitatively in support of the Kautsky (1939) mechanism for the production of singlet oxygen by energy transfer from triplet sensitizer molecules. The essential steps of this mechanism involve the excitation of the sensitizer (Sens) from its ground singlet state (SO)to its first excited singlet state (SI), followed by intersystem crossing to its first excited triplet state (TI). Energy transfer then occurs from the excited triplet sensitizer to oxygen. This reaction scheme may be represented by Sens(So)
Sens(Tl)
+ 02(3Z,-)
+ hv
-+
-+
Sens(Sl)
Sens(S0)
+ 041B,f or lA,)
(1)
(3)
Singlet O2produced in step 3 may initially be formed in either the lAgor lZ0+state (23 and 37 kcal. per mole above the ground state, respectively). Whether OXlA,) is formed directly in step 3 or via the formation of the IZ,+state cannot be elucidated from the present preliminary results, since several possible reactions are known (Wayne, 1963) which could effect conversion of the 12 to the IA state. Work is in progress to clarify-this particular point. In the photooxidation studies, benzene, oxygen, and TME (typically 20, 70, and 2 Torr,respectively) were irradiated for one hour at room temperature. 2,3-Dimethyl-2-hydroperoxy1-butene (I) was isolated in large yields. No significant reaction was observed when benzene was excluded from the reaction mixture or the mixture was stored for prolonged periods in the dark. The hydroperoxide (I) is known to be the only product of the reaction of 02(lA,) with TME in the gas phase (Broadbent et al., 1968; Winer and Bayes, 1966), the reaction being (4)
Thus, the presence of I as the major photooxidation product provides strong evidence for the formation of 02( lA,) in the benzene-oxygen-TME system. Although quantitative determination of the small amounts of hydroperoxide formed is difficult, preliminary results indicate that at least half of the TME consumed during the reaction ends up as I. Benzene and naphthalene were chosen as model sensitizers for the present study, because both are known to be relatively stable to photooxidation and both have appreciable quantum yields of formation of their triplet states (Calvert and Pitts, 1966).The use of sensitizers other than benzene to effect photooxidation in illuminated sensitizer-oxygen-olefin systems is currently under investigation in this laboratory. These studies may be of considerable importance in understanding the chemistry of polluted urban atmospheres. First, because of the relatively high concentrations of aldehydes, ketones, and aromatic hydrocarbons present in polluted urban atmospheres and because of the large extinction coefficients of many of these compounds in the near-ultraviolet region of the spectrum, there exists a good potential source of triplet sensitizers which may produce singlet oxygen. Second, the presence in urban atmospheres of high concentrations of pollutants (such as olefins) which may be rapidly oxidized by singlet oxygen (but not by ground state oxygen) suggests that singlet oxygen may possibly play an important role as an oxidant in photochemical air pollution. Acknowledgment
The authors gratefully acknowledge financial support of this research by Grants AP 00109 and AP 00771, Research Grants Branch, National Air Pollution Control Administration, Consumer Protection and Environmental Health Service, U. s. Public Health Service. They also thank R. P. Wayne, Physical Chemistry Laboratory, Oxford, for constructing the germanium photodiode detector and for helpful discussions. One of us (JLS), acknowledges NIH Training Grant TO1 ES-0084 from the Division of Environmental Health Sciences. Literature Cited
Broadbent, A. D., Gleason, W. S., Pitts, J. N., Jr., Whittle, E,, Chem. Comm. 1968,1315. Calvert, J. G., Pith, J. N., Jr., “Photochemistry,” Wiley, New York, 1966. Foote, C. S., Accounts Chem. Res. 1, 104 (1968). Gollnick, K., Schenck, G. O., “Oxygen as a Dienophile,” “1,4-Cycloaddition Reactions,” J. Hamer, Ed., Academic Press, New York, 1967. Kautsky, H., Trans. Faraday Soc. 35, 216 (1939). Kearns, D. R., Khan, A. U., Duncan, C. K., Maki, A. H., J. Am. Chem. SOC.91, 1039 (1969). Khan, A. U., Pitts, J. N., Jr., Smith, E. B., ENVIRON. SCI. TECHNOL. 1, 656 (1967). Kummler, R. H., private communication to R. P. Wayne, 1968. Scr. Kummler. R. H.. Bortner, M. H., Baurer,. T.,. ENVIRON. TECHNOL. 3, 248 (1969): Pitts, J. N., Jr., Khan, A. U., Smith, E. B., Wayne, R. P., ENVIRON. Scn. TECHNOL. 3, 241 (1969). Snelling, D. R., Chem. Phys. Letters 2, 346 (1968). Wasserman, E., Kuck, V. J., Delavan, W. M., Yager, W. A,, J. Am. Chem. SOC.91, 1040 (1969). Wayne, R. P., Singlet Molecular Oxygen, “Advances in Photochemistry,” Vol. 6, W. A. Noyes, Jr., G. S. Hammond, J. N. Pith, Jr., Eds., Interscience, New York, 1969 (in press). Winer, A. M., Bayes, K. D., J . Phys. Chem. 70, 302 (1966). Receioed for review March 12, 1969. Accepted June 30, 1969. Part III in a series on “Singlet Oxygen in the Environmental S C ~TECHNOL. . 3, Sciences.” Part I I was published in ENVIRON. 241 ( I 969). Volume 3, Number 10, October 1969 947
