J . Phys. Ckem. 1987, 91, 1922-1930
1922
Photodecomposition of Acrolein in Op-N, Mixtures Edward P. Gardner, Paul D. Sperry, and Jack G. Calvert* The National Center for Atmospheric Research,‘ Boulder, Colorado 80307 (Received: October 3, 1986)
The photodecomposition of acrolein in dilute mixtures of synthetic air (24-760 Torr) has been studied with excitation at at 1 atm, 313 or 334 nm. The decomposition of excited acrolein is very inefficient at high air pressures ($d N 6.5 X 313 nm) but increases with decreasing pressures (@d = 8.1 X at 26 Torr). The quantum yields of acrolein loss and the observed products, C2H4, CO, C02, CH20, (HC0)2, and CH,OH, are elucidated by the primary processes I-V: CH2=CHCHO(SI or Tl) C2H4 + CO (I); CH2=CH + HCO (11); CH3CH(S) + CO (111); CH,CH(T) + CO (IV); CH2=CHC0 + H (V). New evidence is given for the mechanism of the reactions of the vinyl radical with 0 2 : CH2=CH + 0 2 (CH2=CH02) OCH2CHO; OCH2CHO CH20 + HCO (4); OCH2CHO + 0 2 (HCO), + H 0 2 ( 5 ) ; the data suggest k S / k 4 6 X cm3 molecule-I. From computer simulations of the sequence of reactions describing acrolein decay, it is estimated that, at low air pressures (26 Torr), @lrI + @Iv > > 6” N dII;in 1 atm of air, $v > > + > &. J values for acrolein photodecomposition in the troposphere are derived from the data.
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for which deactivation of the excited states is especially efficient. Introduction The first studies of acrolein photolysis (145-200 Torr, 313 nm) Carbonyl compounds are formed in the troposphere through were made by Thompson and LinnettI5 and Blacet et a1.I6 They the partial oxidation of the great variety of hydrocarbons which reported CO to be the major gaseous product with C2H4as a minor are emitted by both anthropogenic and natural sources. Oxidation product, and a major portion of the acrolein was polymerized. is initiated largely by HO-radical attack on the hydrocarbons Primary processes 115and 1116were suggested to explain the results. which form one or more molecules of different carbonyl compounds for every molecule of hydrocarbon which is oxidized. CH,=CHCHO kv C2H4 CO (1) However, studies of the photochemistry of the aldehydes and ketones for atmospheric coriditions, Le., small reactant concenCH,=CHCHO + kv CH,=CH + H C O (11) trations but large [O,],are limited to only four of the large number of carbonyl compounds which are generated in the troposphere: f~rmaldehyde,l-~ a ~ e t a l d e h y d e , ~pr~pionaldehyde,~ -~ and aceIn order to understand and to model tropospheric ( I ) Horowitz, A.; Calvert, J. G. Int. J. Chem. Kinet. 1978, 10, 713. (2) Horowitz, A,; Calvert, J. G. Int. J . Chem. Kinet. 1978, 10, 805. chemistry adequately, additional photochemical studies of the wide (3) Moortgat, G. K.; Warneck, P. J . Chem. Phys. 1979, 70, 3639. variety of the more complex carbonyl molecules under tropospheric (4) Moortgat, G. K.; Seiler, W.; Warneck, P. J. Chem. Phys. 1983, 78, conditions are required. 1185. Acrolein has been observed in the t r o p o ~ p h e r e ’at ~ .concen~~ (5) Horowitz, A,; Kershner, C. J.; Calvert, J. G. J . Phys. Chem. 1982.86, 3094. trations which range up to 13 ppbv, but its typical levels are much (6) Horowitz, A.; Calvert, J. G. J . Phys. Chem. 1982, 86, 3105. lower and amount to about 15% of the formaldehyde observed (7) Meyrahn, H.; Moortgat, G. K.; Warneck, P. “15th Informal Conferin urban air masses.I4 Obviously acrolein is an atmospheric ence on Photochemistry”, 1982, Stanford, CA. carbonyl compound of some significance, and accordingly its (8) Meyrahn, H. Ph.D. Thesis, Fachbereich Chemie der Johannes Gutropospheric photochemistry merits study. tenberg-Universitat, Mainz, Germany, 1984. Previous investigators of acrolein photochemistry report a va(9) Shepson, P. B.; Heicklen, J. J . Photochem. 1982, 19, 215. (IO) Gardner, E. P.; Wijayaratne, R. D.; Calvert, J. G. J . Phys. Chem. riety of products and propose many different reaction pathways 1984, 88, 5069. which provide some insight into the complex mechanism of its (1 1 ) Meyrahn, H.; Pauly, J.; Schneider, W.; Warneck, P. J . Atmos. Chem. oxygen-free photolysis.15-22 It is well established, for example, 1986, 4, 277. that the excitation of acrolein at wavelengths within its first (12) Altshuller, A. P.; McPherson, S. P. J . Air Pollut. Control. Assoc. 1963, 13, 109. absorption band (Figure l), 240 < X < 387 nm, populates the (13) Kato, T.; Hanai, Y . ,paper presented at 24th Lecture Meeting, Japan first excited n,r*, singlet state (S1).23924 The weak absorption Society for Analytical Chemistry 1975, Saporo, Japan, Oct 1-5 (AFTIC No. band with its onset near 412 nm has been assigned to the transition 81207). of the ground-state singlet to the first n,r* triplet state (TI). The (14) Graedel, T. E. Chemical Compounds in the Atmosphere; Academic: radiative decay of excited acrolein is relatively unimportant in New York, 1978. (15) Thompson, H. W.; Linnett, J. W. J . Chem. SOC.1935, 1452. both fluid solutions of acrolein and in the gas p h a ~ e ;q+~ < ~ 5. ~ ~ (16) Blacet, F. E.; Fielding, G. H.; Roof, J. G. J . Am. Chem. SOC.1937, X 10” (25 OC, 100 Torr). El-Sayed’s2’ suggestion, formulated 59, 2315. to explain the photochemistry of N-heterocycles, may account for (17) Harrison, A. G.; Lossing, F. B. Can. J . Chem. 1959, 37, 1696. the dominance of nonradiative decay paths for excited acrolein. (18) Osborne, A. D.; Pitts, Jr., J. N.; Darley, E. R. Int. J . Air Water He showed that the S(n,a*)-T(r,**) spin-orbit interaction is Pollut. 1962, 6, 1 . (19) Coomber, J. W.; Pitts, J. N., Jr. J . A m . Chem. SOC.1969, 91, 547. approximately lo3times that of the S(n,r*)-T(n,r*) interaction. (20) Umstead, M. E.; Shortridge, R. G.; Lin, M. C. J . Phys. Chem. 1978, Based upon this evidence Hansen and Lee28 have proposed a 82, 1455. similar mechanism for the unsaturated carbonyl compounds; thus (21) Fujimoto, G. T.; Umstead, M. E.; Lin, M. C. J . Chem. Phys. 1985, the spin-orbit interaction between the first excited singlet S,(n,r*) 82, 3042. and the second excited triplet T2(7r,r*) states may facilitate an (22) Shinohara, H.; Nishi, N. J . Chem. Phys. 1982, 77, 234. (23) Brand, J. C. D.; Williamson, D. G. Discuss. Faraday SOC.1963, 35, efficient SI to T, intersystem crossing, followed by other radia184. So + M. tionless processes, e.g., T2 T I ; TI + M (24) Aves, A. C. P.; Christoffersen, J.; Hollas, J. M. Mol. Phys. 1971, 20, In their review of aldehyde photochemistry, Lee and Lewis29 625. have concluded that the photodecomposition of excited acrolein (25) Becker, R. S.; Inuzuka, K.; King, J. J. Chem. Phys. 1970,52, 5164. is very inefficient. However this conclusion is based upon quantum (26) Hansen, D. A. Ph.D. Thesis, University of California, Irvine, 1973. (27) El-Sayed, M. A. J . Chem. Phys. 1963, 38, 2834. Acc. Chem. Res. yield data derived from studies involving high pressures of acrolein
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‘The National Center for Atmospheric Research is sponsored by the National Science Foundation.
0022-3654/87/2091- 1922$01.50/0
1968, I , 8. (28) Hansen, D. A,; Lee, E. K. C. J . Am. Chem. SOC.1973, 95, 7900 (29) Lee, E. K. C ; Lewis, R. S. Adu. Photochem. 1980, 12, 1
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987 1923
Photodecomposition of Acrolein in 02-N2
WAVELENGTH (nanometers) Figure 1. The ultraviolet absorption spectrum of acrolein vapor (25 "C).
Presumably the initial products of I and 11, ethylene and vinyl radicals, respectively, could copolymerize with the acrolein to leave the dominant C O product observed. Harrison and Lossing17 found that Hg(,PI) excited at 253.7 nm sensitized the decomposition of acrolein at low pressures. Process Ia was the major decomposition mode; process IIa and a new process IIIa were suggested to occur also but at much lower efficiencies. CH,=CHCHO
CH,=CHCHO
+ Hg(,PI) -,C2H4+ CO + Hg(ISo) (Ia) + Hg(,P,) -,CH2=CH + HCO + Hg(ISo) (IIa)
CHz=CHCHO
+ Hg(,P,)
-+
CH,=CHCO
+ H + Hg('S0) (IIIa)
The most extensive study of direct acrolein photolysis was carried out by Coomber and PittsI9 at 313 nm (5-63 Torr, 35-200 "C). They studied the effect of several added gases: the addition of CO, caused little change in product quantum yields; added piperylene (an efficient triplet quencher) suppressed strongly CO and C,H,; and N O addition gave a slight increase of the C2H4 quantum yield. They interpreted their results using a vibrationally relaxed, excited acrolein singlet (Sl0),vibrationally excited singlet (SI*),and triplet (T,) reactions:
SI* (or T I )
-
S1* (or SIo)
-
+ C2H4 CH2=CH + HCO
SI*
CO
4
(Ia) (IIa)
Ti
In recent years, Umstead et aL20 Fujimoto et a1.,21and Shinohara and Nishi2, have studied the photochemistry of acrolein excited into the second excited singlet (S2,a,7r*)state. Umstead et aLzoobserved in their studies (200 nm) that both methyl ketene (CH,CH=C=O) and acrolein produce CO with similar, nonstatistical, vibrational distributions. In explanation of this they proposed a previously unexpected primary process (IVa) which CH2=CHCH0
+ hv -, (CH,CH=C=O)
-,
CH3CH
+ CO
(IVa)
was subsequently supported by further work of Fujimoto et aLZ1 Vinyl and H C O radical generation was postulated from indirect evidence in earlier studies with acrolein excitation within the first absorption band.16*17J9 Shinohara and NishiZ2spectroscopically observed vinyl and HCO radicals in their investigation of acrolein photochemistry at 193 nm. This result was unexpected since Dykstram and Davidson and Nitzche31have shown from theoretical studies that partial bond reversal occurs on excitation of ground-state acrolein to the S2(a,r*)state of the acrolein molecule. More specifically,the double bond of the ground state is weakened, while the single bond next to the carbonyl is strengthened, hin(30) Dykstra, C. E. J . Am. Chem. SOC.1976, 98, 7182. (31) Davidson, E. R.; Nitzche, L. E. J . Am. Chem. SOC.1979, 101,6524.
dering the rupture of the acrolein (S2) molecule to form vinyl radicals. In view of this problem the authors proposed that the observed dissociation occurs from the first excited Sl(n,a*) surface to which the S2 state degrades. Prior to the present study, the photodecomposition of acrolein in oxygen mixtures has been studied only once. In a rather limited study Osborne et irradiated relatively high pressures of acrolein (18-25 Torr) in O2 (25-500 Torr). They observed no effect of O2 on the quantum yields of C2H4and CO and a very inefficient photodecomposition of acrolein. The nature of the primary processes in acrolein and the effect of 0 2 / N 2mixtures on these remained obscure because, at their conditions, the interactions of the primary products with acrolein are favored. It is impossible to derive useful information on the primary photodecomposition modes of acrolein in the troposphere from the diverse and somewhat conflicting published information which was obtained in experiments under very different conditions. In particular, a quantitative evaluation of the effect of oxygen on the primary photochemical processes has not been performed. The set of experiments reported here was designed to simulate the conditions commonly encountered in the troposphere. Small concentrations of acrolein vapor (about loi6 molecules/cm3) in varied amounts of air (up to 1 atm pressure) were photolyzed at 25 "C by using 313 nm in most experiments and 334 nm in a few others. The natureof the primary processes has been clarified and their quantum efficiencies have been determined; these results were used to estimate the first-order photochemical rate coefficient for acrolein decay in the troposphere.
Experimental Section Materials. Acrolein (Aldrich Chemical Co.) containing 3% water and 200 ppm of hydroquinone was purified by repeated distillations on the vacuum line and by gas chromatography. The purified acrolein was stored in a blackened, glass storage bulb. Analysis by FID-gas chromatography (Porapak P/Q) established the minimum purity of the acrolein used in these experiments to be 99.9%. Given in Figure 1 is the near-ultraviolet spectrum of the reactant acrolein as measured with a Varian-Cary 219 grating spectrometer. Oxygen (Union Carbide Corp.) was 99.99%, ultrahigh purity, hydrocarbon-free grade; nitrogen (Union Carbide Corp.) was 99.998%, extra dry grade. Argon and krypton, 99.995% purity (Matheson), were obtained in 1-L glass bulbs with a break-seal. Carbon monoxide (99.995%) and carbon dioxide (99.98%) were research purity grade (Linde). Azomethane was synthesized by the method of Renaud and Leitch32and purified by distillation on the vacuum line. Glyoxal was obtained as the trimeric dihydrate (Sigma Chemical Co.) and thermally decomposed to yield the monomer; this was distilled on the vacuum line and dried over P205. Reagent grade formaldehyde and glacial acetic acid were obtained from Eastman Kodak Co. Water (distilled in glass, HPLC grade) and methanol (HPLC grade) were from Budick and Jackson Co. The additional materials used as standards in the analysis (e.g., Cl-C3 hydrocarbons and oxygen containing species) were obtained from Baker (analyzed reagent grade). Column chromatographic materials, Porapak P/Q (80/100mesh) and molecular sieve 13X (SO/lOO mesh), were products of Alltech Associates, Inc. Photolysis Equipment. The vacuum system and large reaction cell, as well as the associated optical system used to generate the excitation beam and to monitor its light output, have been described previously.I0 In eight experiments a grating monochromator (Jarrell-Ash, Model 82-400), with 2-mm entrance and exit slits, was employed to isolate the desired wavelengths of light (313 or 334 nm). In six other experiments a band of wavelengths (10 nm half-width) centered at 313 or 334 nm was generated using interference filters together with the Hg arc (Osram, HBO 500). Although a higher intensity of light was achieved with the filter systems, there was no significant difference in the quantum yields of the products obtained with the two light isolation methods employed. (32) Renaud, R.; Leitch, L. C . Can. J . Chem. 1954, 32, 546.
1924
The Journal of Physical Chemistry, Vol. 91, No. 7 , 1987
Gardner et al.
TABLE I: Summary of Experimental Conditions and Quantum Yield Results from Acrolein Photodecomposition in Simulated Air Mixtures
reactant concn, molecule/cm3 [O,] X [N2] X [C3H40]X
quantum yields of products
10-16
10-18
10-18
temp, 'C
1M 2F 3F 4M 5F 6F 7M 8F 9M 1OM 11M
1.16 1.16 1.14 1.15 1.16 1.16 1.15 1.15 1.16 1.16 1.16
0.157 0.166 0.275 0.499 0.721 1.511 2.334 2.204 3.276 4.288 5.168
0.664 0.654 1.250 2.057 2.890 6.028 9.326 9.412 13.80 17.18 20.66
22.6 24.2 24.0 25.9 24.7 22.8 25.1 25.8 26.1 25.4 22.6
0.0834 0.0786 0.0681 0.0334 0.0257 0.01 37 0.0091 0.0088 0.0073 0.0069 0.0065
12M 13F 14M
1.16 1.16 1.16
0.155 0.154 2.785
0.6 12 0.616 11.92
26.7 26.5 25.0
0.0224 0.0201 0.0061
run no.a
C3H40
C2H4
(loss)
Excitation
3
Amax
C02 = 313 nm
0.0523 0.0521 0.0230 0.0121 0.0084 0.0035 0.0023 0.0024 0.0019 0.001 8 0.0018
0.0106 0.0101 0.0057 0.0035 0.0027 0.0020 0.0017 0.001 7 0.0018 0.0018 0.0019
= 334 nm 0.0038 0.0007 0.0038 0.0008 0.0014 0.0011
CO
CH20
0.0714 0.0674 0.0420 0.0244 0.0188 0.0102 0.0079 0.0082 0.0059 0.0053
0.0149 0.0141 0.0079 0.0058 0.0045 0.0013 0.0009 0.0010 0.0006 0.0005 0.0004
0.0112 0.0114 0.0047
0.008 0.0082 0.0005
(HC0)2 0.0056 0.0031 0.0020 0.0021 0.0024
CH30H
Z/Ab
0.0032 0.0042 0.0038 0.0014 0.0006 0.0006 0.0005 0.0004 0.0004 0.0003 0.0004
0.81 0.90 0.82 0.59 0.61 0.62 0.57 0.79 0.57 0.56
0.0014 0.0014 0.0003
0.43 0.49 0.69
Excitation, A,
0.0016
'The letters F and M indicate that an interference filter or a monochromator, respectively, was used to isolate the light in this experiment. bThe fraction of the carbon atoms in the acrolein loss which is accounted for in the product analysis obtained in the given experiment. Two chemical actinometers were used: pure azomethane (aN2 = 1.0, 25 "C) and acetone (aco= 1.0, >125 "C). In actinometer experiments the concentration of the reactant gas was adjusted so that the fraction of light absorbed was approximately equal to that measured in the acrolein-air mixtures. The absorbed light intensities were low, about 2 X 1O'O and 3 X 10" quanta cmW3 s-I with the monochromator and filter systems, respectively. Photolysis Procedures. Before each experiment the reaction cell was heated (100 "C), evacuated, cooled (25 "C), and filled with a synthetic air mixture free of acrolein. The mixture was then circulated by an in-line glass pump (Teflon rotor, magnetically driven), sampled, and then analyzed by gas chromatography for possible impurities. If none were detected, which was commonly the case, the cell was reevacuated, the lamp ignited and stabilized, the incident intensity determined by the photomultiplier, the shutter closed, and the cell filled with an accurately measured pressure of acrolein, usually near 0.36 Torr; alternatively, a calibrated acrolein/argon mixture, 9.94: 1, or acrolein/krypton mixture, 31.4:1, was employed. This step was followed by the addition of the desired amount of oxygen and, subsequently, nitrogen in the ratio of 1:4, and the resulting mixture was mixed thoroughly for 30 min with the circulating pump. A control sample of the unphotolyzed mixture was then trapped in a calibrated volume (attached to the cell) for analysis following the experiment and preceding the analysis of the photolyzed mixture. Photolysis was initiated for a desired time period which allowed only a few percent conversion of the acrolein. The light intensity was monitored throughout the run so that an integrated measure of the light intensity transmitted through the cell during the experiment could be obtained. Product Analysis. Chromatographic separation and quantification of the products were performed with the analytical system described previously;I0 it incorporated a microprocessor-controlled sample handling system to transfer and load an aliquot of the reaction mixture onto the GC column. Analysis was accomplished using a Varian (Model 2700) gas chromatograph equipped with flame ionization (FID) and thermal conductivity (TCD) detectors. Two columns were used: (1) Porapak P/Q (mixture) SO/lOO mesh, 70 "C isothermal; and (2) molecular sieve 13X, 80/100 mesh, 50 and 75 "C isothermal; both columns were 3.0 m in length and 2.6 mm i.d., and the flow of helium carrier gas was 57 mL/min. Reactant aliquots were loaded sequentially onto the Porapak P/Q to analyze CH4, C 0 2 , C2H2,C2H4, C H 2 0 , H 2 0 , CH30H, HCOCHO, and residual reactant CH2==CHCH0. The molecular sieve column provided an excellent separation of N2, 02.CO, CH,, and C2H4. The FID detector was employed to quantify all reactants and products except H 2 0 , CO, C 0 2 , N2, and 02.These were determined by switching the column effluent to the TCD detector, accomplished automatically without altering
TABLE 11: Thermochemical Data for Selected Species of Possible Importance in the Photodissociation of Acrolein AHr'(298 K),
species CH,=CHCHO CH,=CHCO CH,=CH2 CH3CH(singlet) CH,CH(triplet, ground state) CHZ=CH HCO
co
kcal mol-' -18.32" 16.6b 12.50' 91d 82' 72 f 3J 9.0' -26.42'
"Derived from the measured enthalpy of formation of liquid acrolein from K h a r a ~ c hand ~ ~ Stull et al.34and the heat of vaporization of liquid acrolein listed by Reid et al.35 *Estimated by assuming D(CH2=CHCO-H) = D(HC0-H) = 87 kcal/mol. CFrom B e n ~ o n . ~ ~ dCalculated from the data of Umstead et aI.*O and an assumed singlet-triplet energy gap of 9 kcal/mol as estimated by Border and Davidson." eUmstead et al.;,O estimate is assumed to refer to the CH3CH species in the ground (triplet) state. fEstimate of Sharma et al.38with error limits set to include the estimates of B e n ~ o n . ~ ~ the chromatographic conditions. Usually analyses of 3-5 separate aliquots of the photolyzed reaction mixture were made for each experiment; the average of these was used to calculate the product quantum yields (reported in Table I). Product GC peaks were identified routinely by their retention times with confirmation by mass spectrometry (CEC, Model 21-104) of a captured sample of the column effluent.
Results The photolysis conditions and the quantum yields for acrolein photodecomposition in simulated air mixtures are summarized in Table I. It is clear that the dominant products are CO and C2H,. Other identified products for which quantum yields were determined are CH20, glyoxal (HCOCHO), C 0 2 , and CH30H. Minor products which were identified unambiguously but the amounts of which were not quantitatively determined are Ha,CH4, C2H2,and CH3C0,H. The run numbers in Table I are followed by either the letter M or F which designate whether the monochromator or an interference filter, respectively, were used to isolate the initiating light in the given experiment. Where runs under similar conditions were made (e.g., runs 1M and 2F, and 12M and 13F) but the alternative methods of light isolation were employed, no significant difference can be seen in the quantum yields of the products obtained. Discussion Thermochemistry of the Possible Primary Processes in Acrolein Photochemistry. The enthalpies of formation of the various species
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987 1925
Photodecomposition of Acrolein in 02-N2
-CH,=CH+HCO
-CHz=CHCO+ H -CHsCH (S)+CO -CH3CH(T)+CO
* 0O I
1
\
-20
product channels for reaction. summarized in Table 11 may be used to evaluate the potential for occurrence of the several different photodecomposition modes previously suggested for acrolein excited at wavelengths -
5
10.00
5.00
I-
z
3 0
2.50
0
4
8
12
16
20
24
[MI, molec cmdX IO-'' Figure 8. Plot of the quantum yields of the various primary processes in acrolein photolysis at 313 nm (log scale) vs. the concentration of air: CHpCH+ CO (111 and IV), triangles; CO + C,H, (I), squares; CH2= CHCO + H (V), diamonds; CH2=CH + HCO (II), plus signs; estimated by computer simulation.
each primary processes as a function of the employed [MI. It is seen that, while the processes I11 and/or IV forming CH3CH and CO and process I forming C2H4 and CO are favored at low [MI, these become less important at high [MI as the process V forming CHz==CHCO and H and process I1 forming CH2=CH and HCO acquire dominance. The contributions of the triplet and singlet states to acrolein photodecomposition are compared in Figure 7. For the experimental conditions employed here, the SI T I intersystem crossing is very efficient, 99.5%. Reactions of the triplet state dominate at low pressures, but as this state is quenched more effectively at the higher oxygen or M concentrations, the fraction of products formed from the singlet state becomes higher than that of the triplet. In conclusion, the dependence of the quantum yield of each process on the [MI is summarized in Figure 8. Photolysis of Acrolein-Air Mixtures at 334 nm. Three experiments were carried out at 334 nm in this investigation. The smaller quantum yields observed do not have the precision obtained in the 313-nm experiments. However, they do provide additional pertinent information; see Table 1. Since approximately 6 kcal mol-' less enerm is available at 334 nm than at 313 nm. a somewhat less vibrationally excited singlet is formed upon excitation. This may provide an explanation for the less efficient photodecomposition of acrolein excited at this wavelength. As anticipated from the absorption spectrum of acrolein (Figure 1) in which structure appears in the long wavelength region, be extended at lower the lifetime of the singlet excited energies of excitation, providing more opportunity for it to undergo
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--
1930
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J . Phys. Chem. 1987, 91, 1930-1935
S1 T, intersystem crossing and TI So quenching by collision. The average ratios of the observed product quantum yields at for the only matched experiments 334 and 313 nm, (f$x)334/(&)313, at the lowest pressures of added air (runs 1 , 2 and 12, 13 of Table I) are 0.27, 0.07, 0.07, 0.16, 0.56, and 0.38, where x = acrolein loss, C2H4,COz, CO, CH20, and CH30H, respectively. The same quantum yield ratios for experiments near 400 Torr, obtained by interpolation of the 313-nm data, give 0.76,0.67,0.63,0.68,0.63, 0.67, and 0.75 with x = acrolein loss, C2H4,C 0 2 , CO, C H 2 0 , HCHCHO, and CH30H, respectively. The different ratios for the two pressure regimes may reflect the lifetime differences between SI and Tl states; the decomposition of the excited singlet, the dominant source of products at the higher air pressure, is much more rapid than the decomposition of the triplet, and hence it is much less likely to be quenched than the excited triplet; see Figure 7. Although the 334-nm data are insufficient to perform any detailed analysis of the reactive channel distribution, a comparison with the 313-nm data suggests that at 334 nm decomposition of the excited acrolein triplet processes I, 111, IV, and V is more strongly suppressed than that by process 11. This is unexpected in view of our considerations of the thermochemistry of the various processes. The Apparent First-Order Photolytic Rate Constant for the Decay of Acrolein in the Lower Troposphere. We may estimate the approximate apparent first-order rate constant (J) for the photodecomposition of acrolein in the lower atmosphere from the present quantum yield data, the absorption cross sections (Figure l), and the estimated actinic flux calculated by Demerjian et al. (best estimate surface albedo).53 The wavelength dependence of the quantum yield cannot be well defined from our experiments at only two wavelengths within the absorption band. However, qualitatively useful J estimates can be obtained by assuming that the two data points for & at 313 (0.0066) and 334 nm (0.0044) define a linear relationship with A, extending over the wavelength region of significance to the lower troposphere (380 > h > 290 nm). Following this procedure we estimate J (s-') X lo6 1.7, 1.6, 1.4,0.84, 0.22, and 0.05 for solar zenith angles ( Z ) of 0, 20, (53) Demerjian, K. L.; Schere, K. L.; Peterson, J. T. Adv. Enuiron. Sci. Technol. 1980, 10, 369.
40,60, 78, and 86O, respectively. Therefore, at a solar zenith angle of 40° the lifetime of acrolein is approximately 10 days. This is comparable to that which we estimate for acetone under similar conditions (14 days) and significantly longer than that for acetaldehyde (3 days) and formaldehyde (0.2 days). The quantum yields of acrolein loss observed in this work at 3 13 nm exhibit a marked dependence on the concentration of air ([MI, molecule ~ m - which ~ ) is described reasonably well (for 8 X lo1' < [MI < 2.6 X l O l 9 by the following relation: I / ( & - 0.00400) = 0.086 -k 1.613 x 10-"[M] For 3 13 nm & increases from 0.0065 at 1 atm (sea level) to about 0.014 at 0.26 atm (10 km). Accordingly, if we consider the 3 13-nm data to be representative of acrolein excitation throughout the 290-380-nm region, the lifetime of acrolein is expected to decrease to less than 5 days near the top of the troposphere (Z = 40O). Therefore, if photodissociation were the only loss mechanism for acrolein in the troposphere, we would infer a reasonably long residence time. However, the measured rate constant for HO-radical reaction with acrolein54is reasonably large, k = 1.9 X lo-" cm3 molecule-'s-]. Assuming an average [HO] of lo6 molecule cm3, representative of the lower troposphere near 40° N, we estimate that the lifetime of the acrolein molecule with respect to HO-radical attack is about 0.6 days. Therefore it is clear that the major sink for acrolein in the lower troposphere, and probably at all elevations within the troposphere, is its reaction with H O radicals. Acknowledgment. This work was supported in part by an Interagency Agreement (DW 499303 19-01- 1) between the Environmental Protection Agency and the National Science Foundation. We are grateful to Dr. Sasha Madronich for his calculation of the J values employed in this work. Registry No. C2H4,74-85-1; CO, 630-08-0; CO,, 124-38-9; C H 2 0 , 50-00-0; (HCO),, 107-22-2; CHJOH, 67-56-1; CH,=CH', 2669-89-8; N2, 7727-37-9; 02,7782-44-7; H2, 1333-74-0; CHI, 74-82-8; C2H2, 7486-2; CH3C02H,64-19-7; acrolein, 107-02-8. (54) Atkinson, 1983, 15, 75.
R.; Aschmann, S. M.; Pitts, Jr., J. N. Inr. J . Chem. Kine?.
Density of the Electronic States of Graphlte: Derlvation from Olfferential Capacitance Measurements H. Gerischer,* R. McIntyre,* D. Scherson,+ and W. Storck Fritz-Haber-Institut der Max-Planck-Gesellschaft. 0-1000 Berlin 33, West Germany (Received: March 25, 1986)
Differential capacitance data for stress-annealed pyrolytic graphite in acetonitrile containing tetrapropylammonium tetrafluoroborate have been measured over an energy range of 3 eV. The data are interpreted within a model which has successfully been applied to semiconductor electrolyte contacts. In this model the overall capacitance is considered as a space charge capacitance inside the solid in series with a Helmholtz double-layer capacitance at the interface. In contrast to normal metals, the relative low density of electronic states of the semimetal graphite results in the measured capacitance being dominated by the space charge contribution of the graphite. From a correlation between the local position of the Fermi level and the local density of excess charge, the density of states around the Fermi surface can be calculated from such data. The result is compared with theoretical calculations of the electronic properties of graphite.
Introduction It has been shown in a preceding that the unusually small differential capacitance of stress-annealed pyrolytic graphite compared to metallic conductors2 can be related to the relatively
small density of electronic states at the Fermi surface of this material. A correlation between the local positi-on of the Fermi level and the local density of excess charge, based On electrostatics the density and energy band was used in Order to
Permanent address: Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106.
(1) Gerischer, H. J . Phys. Chem. 1985, 89, 4249. (2) Randin, J. P.; Yeager, E. J. Electrochem. SOC.1971, 118, 711. J . Elecrroanal. Chem. 1972, 36, 251. 1974, 54, 93.
0022-3654/87/2091-1930$01.50/0
0 1987 American Chemical Society
