Communications to the Editor
542
Acknowledgments. The financial support of NSF Grant No. GP-38053X and Environmental Protection Agency Grant No. 800649 are gratefully acknowledged. The contents do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. References a n d Notes (1) G. Boocock and R. J. Cvetanovic, Can. J. Chem., 39,2436 (1961). (2)G. R. H. Jones and R. J. Cvetanovic, Can. J. Cbem., 39,2444(1961). (3)I. Mani and M. C. Sauer, Jr., Advan. Cbem. Ser., No. 82, 142 (1968). (4)E. Grovenstein, Jr., and A. J. Mosher, J. Amer. Cbem. SOC., 92, 3810 (1970). (5) R. A. Bonanno, P. Kim, J. H. Lee, and R. 8. Timmons, J. Cbem. fbys.. 57, 1377 (1972). (6)R. Atkinson and J. N. Pitts, Jr., J. fbys. Chem., 78, 1780 (1974). (7)R. Atkinson and J. N. Pitts, Jr., J. fbys. Cbem., in press. (8) R. Atkinson and R. J. Cvetanovic, J. Cbem. fhys., 55, 659 (1971). (9)S. W. Charles, J. T. Pearson, and E. Whittle, Trans. Faraday Soc.. 59, 1156 (1963).
Department of Chemistry and Statewide Air PollutionResearch Center University of California Riverside, California 92502
R. Atklnson J. N. Pitts, Jr.’
Received September 9, 7974
Trace Monitoring of Ketene Produced in Gas Phase Reactions’ Publication costs assisted by the Office of Naval Research
Sir: Quantitative analysis of trace amounts of ketene present in a gas sample has presented a great deal of practical difficulty to chemical kineticists and others, because of its well-known chemical reactivity2 toward itself, water, and a variety of other substances (including gas chromatographic column material). In view of the fact that ketene is produced in gas phase ozone-olefin reactions,3 observed in smog chamber reactions,* known to be a pyrolytic product of cycl~butanone,~ postulated to be a photolytic product of cyclobutanones,6 a pyrolysis product of acetic acid7 and acetic anhydride, and widely used as reaction intermediates in large-scale industrial processes, its quantitative monitoring in trace or greater amounts is not only desirable for the laboratory studies but also needed for ambient air quality measurements. There has been one recent report of gas chromatographic analysis of ketene and methyl ketene,8 and its comparison with our method will be given later. We wish to report briefly a simple, convenient, and sensitive analytical method which can be adapted for a variety of applications. The key to the successful analysis involves the quantitative conversion of ketene to methyl acetate by an excess amount of “dried” methanol in the gas phase and the assay of methyl acetate by a hydrogen flame ionization detector after gas chromatographic separation. A nanomole quantity can be readily detected. For convenience, ketene was generated by photolysis of a cyclobutanone-methanol mixture a t 305 nm and room temperature in a manner used earlier.9 The conversion efficiency for ketene was measured by comparing the yield of methyl acetate with the yield of ethylene, since ketene and The Journal of Physical Chemistry, Vol. 79, No. 5, 1975
TABLE I: Ratio of Methyl Acetate (A) to Ethylene (B) from Photolysis of Cyclobutanone (CB) in the Presence of Methanol (M)= CB, Torr
M, Torr
Time, min
A/B
10.1 10.0 10.0 10.4 10.0 10.0 10.0 10.5 10.1 10.1 10.1
5 .O 30 0.66 i 0.03 5 .O 30 0.67 i 0.03 10.0 60 0.75 i 0.03 10.2 60 0.77 f 0.03 47.5 60 0.99 i 0.04 80.0 60 1.04 i 0.04 80.0 60 0.96 f 0.04 51.0 15 1.00 i 0.04 50.0 30 0.94 i 0.04 50.0 60 1.00 i 0.04 50 .O 180 0.93 i 0.04 a A cylindrical Suprasil cell (3.5 cm i.d., 7 . 5 cm optical path) with two flat end windows was used. A roughly collimated beam of 1.5 cm diameter was passed through the center axis of the cell.
ethylene should be produced in equal amounts.6 The extent of photolysis was less than 0.5%. The experimental results are shown in Table I. The conversion efficiency of ketene was only 66% a t 5 Torr methanol pressure and 76% at 10 Torr methanol but it was nearly quantitative above 50 Torr methanol. No obvious photolysis time dependence was observed indicating that the time scale of the conversion is certainly less than 15 min. We believe that in the low methanol pressure regime the conversion efficiency could be low because of the importance of the wall reaction of ketene andlor the reaction of ketene with cyclobutanone which compete with the desired reaction. Although no rate constants are available in the literature for the reaction of ketene with methanol either in solution or in the gas phase, the reaction probably occurs with relative high collision efficiency; it could be if the wall reaction is the main competing process. Two dimethylsulfolane columns (DMS) were used at room temperature: A = 20 ft, 0.125 inch 0.d. and B = 22 ft, 0.25 inch 0.d. Hydrocarbons were eluted through columns A and B, and methyl acetate was eluted only from column A by switching out column B since it had much longer retention time. The advantages of our method over the low temperature separation method developed by Laufera are as follows: (1) simultaneous separation of variety of hydrocarbons as well as others from the ketene peak (methyl acetate) poses no great difficulty; (2) a quantitative assay of ketene can be checked readily by calibration with methyl acetate; (3) ketene can be converted to methyl acetate without a serious worry of its loss by dimerization (applies probably to methyl ketene as well); and (4) a gc column such as DMS column is extremely stable and requires no special handling as with a Haloport F column used by Laufer.8 No obvious serious disadvantage of our method is seen. Since the suitable experimental conditions required for the quantitative conversion in each applicable system must be determined specifically, particularly in the absence of the rate data, we shall not pursue detail any further. However, it is clear that this convenient method with appropriate modifications will be very useful in determining routinely trace or greater amount of ketene by the widely used and relatively inexpensive gas chromatographic instrumentation.
543
Communications to the Editor
References and Notes (1) This research has been supported by the Office of Naval Research Contract No. N00014-69A-0200-9005. (2) See, for example, R. N. Lacey in “The Chemistry of Alkenes,” Vol. 1, S. Patai, Ed., Interscience, New York, N.Y., 1964, p 1161. 13) . , R. Atkinson, B. J. Finlayson, and J. N. Pltts, Jr., J. Amer. Chem. Soc., 95, 7592 (1973). (4) J. M. McAfee, A. M. Winer, and J. N. Pitts, Jr. in “Symposium on Chemical Kinetic Data for the Lower and Upper Atmosphere,” Warrenton, Va., Sept 1974; to be published in ht. J. Chem. Kinet. (5)M. N. Das, F. Kern, T. D. Coyle, and W. D. Walters, J. Amer. Chem. Soc., 78, 6271 (1954), analyzed as acetic acid after reaction with liquid water. (6) (a) S. W. Benson and G. B. Kistiakowsky, J. Amer. Chem. Soc., 64, 80 (1942); (b) T. H. MaGee, J. Phys. Chem., 72, 1621 (1968), ketene peak was eluted from Porapack Cl column but not quatitatively. (7) P. G. Blake and G. E. Jackson, J. Chem. SOC., 13, 94 (1969). (8) A. H. Laufer, J. Chromafogr. Sci., 8, 677 (1970). (9) (a) N. E. Lee and E. K. C. Lee, J. Chem. Phys., 50, 2094 (1969); (b) H. A. J. Carless, J. Metcalfe, and E. K. C. Lee, J. Amer. Chem. Soc., 94, 7221 (1972).
5.0-
4.0-
3.0-
2.0-
George M. Breuer Fred J. Grleman
Department of Chemistry University of California Imine, California 92664
1 .o
Edward K. C. Lee”
1.0
I 2.0
I 3.0
I 4.0
I
I
5 . 0
6.0
1 I 7 . 0 x 1 o - ~ h l
[O?l
Received October 2 1. 1974
Figure 1. Stern-Volmer plots showing the dependence of &=, on the oxygen concentration: 0 ,thioindigo (I);0, 6,6’-diethoxythioindigo (11) in benzene.
Evidence for the Intermediacy of the Triplet State in the Direct Photoisomerization of Thioindigo Dyes Publication costs assisted by the
US.Army Research Office
--
Sir: We have recently observed the quenching of the photocis isomerization (but not of the fluoreschemical trans cence or cis trans isomerization!) of thioindigo (I) and 6,6’-diethoxythioindigo (11) by oxygen in benzene solutions, indicating the presence of a triplet intermediate in these reactions. Stern-Volmer plots1 of the data resulted in straight lines with slopes of 947 and 420, respectively, as shown in Figure 1. By assuming k, = 7.0 X lo9 for the diffusion-controlled energy transfer from the dye triplet to oxygen in benzene,2 the following triplet lifetimes can be TABLE I: Comparison of Isomerization Quantum Yield Ratios
1.13 1.27
I I1
3 .OO 2.13
2.66 1.68
2.63 1.72
calculated: 135 nsec (for I) and 60 nsec (for 11). Chemical evidence for the presence of singlet oxygen (the expected product of the quenching process) was obtained by observing the thioindigo-sensitized oxidation of 1,3-diphenylisobenzofurane to o-dibenzoylbenzene in an air-saturated solution, a reaction specific for singlet oxygen that had first been reported by Scheffer and O ~ c h i . ~ Plots of the photostationary state [trans]/[cis] ratios vs. [ 0 2 ] yield straight lines with values of 1018 (I) and 440 (11) for the slopdintercept ratios, in satisfactory agreement with the values obtained above for the Stern-Volmer slopes. This is consistent with our observation that +c-t is not affected by oxygen and indicates that the triplet intermediate that is being quenched can only be reached from the trans side and is not a common intermediate for both isomerization processes.4 The effect of oxygen on the &.t/&+c ratios was also determined (from photostationary state measurements) for the sensitized cis-trans isomerization of these two compounds, using tin(1V) tetraphenyltetrahydroporphyrin as the “high-energy” sensitizer for I5 and 1,2;5,6-dibenzanthracene for 11, and compared with the quenching of the direct isomerization, as shown in Table I. The excellent agreement between the quantum yield ratios in the last two columns proves that &+ for the sensitized reaction is also independent of the oxygen concentration. The results demonstrate that the direct trans cis isomerization of these two dyes takes place through a relatively long-lived transoid triplet intermediate. On the other
-
TABLE 11: Quantum Yields, Rate Constants, and Lifetimes (Direct Isomerization)” ~~~
~~
~
~~
~
1O7k,, Dye
--
$rb
-
418,
bt-c
@t- tC
4c-
t
sec-’
107kisc, sec-’
Ts,
7t 7
nsec
nsec
135 1.7 13.4 0.12 0.45 5.2 0.9 60 4.4 113 0.57 0.45d a Solvent benzene, solutions degassed. b These values are ca. 40% higher than reported previously,6 due to an upward revision in the @e value for Rhodamine B used as the reference substance.7 CQuantum yield of the radiationless deactivation of trans molecules via the twisted triplet. This value is higher than previously reported (0.37)6due to improved experimental conditions and application of the Lamola-Hammond correction for the reverse reaction.8 I
0.71
I1
0.04
0.23 1.02
0.11 0.45
The Journal of Physical chemistry, Vol. 79, No. 5, 1975
