J . Phys. Chem. 1993, 97, 353G3538
3530
The Ethylperoxy Radical: Its Ultraviolet Spectrum, Self-Reaction, and Reaction with H02, Each Studied as a Function of Temperature Frederick F. Fenter,' Velery Catoire, Robert Lesclaux, and Phillip D. Lightfoot' Laboratoire de Photophysique et Photochimie Moltculaire, Vniversitt de Bordeaux I, 33405 Talence Ctdex, France Received: September 9, 1992; In Final Form: December 23, 1992
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The ultraviolet spectrum of the ethylperoxy radical (C2H502) and the reactions C2H5O2 C2H5O2 products (1) and C2H5O2 H02 C2H502H 0 2 ( 5 ) have been studied using the flash photolysis/UV absorption technique. The spectrum was taken between the wavelengths of 210 and 290 nm and at the temperatures of 298 and 600 K. The room temperature spectrum is found to be in good agreement with previous determinations, with a maximum cross section umax= (4.89 f 0.60) X lo-'* cm2 molecule-' at 240 nm. The temperature dependence of the broadness of the spectrum as well as the value of umaxis analyzed by fitting the data to a Gaussian function that predicts the temperature behavior of broad, structureless UV absorptions. Our results on the C2H5O2 self-reaction are also in good agreement with previous studies, with kl/cm3 molecule-' s-I = (6.7 f 0.6) X exp((60 f 40)/TJ for the temperature range 248-460 K. At higher temperatures, we observe non-second-order kinetic behavior which can be attributed to the thermal decomposition of the ethoxy radical, a product of reaction 1. Our results for the reaction C2H5O2 HO2 are significantly different from the only previous determination of its temperature dependence, especially at and below room temperature, with ks/cm3 molecule-' s-I = (1.6 f 0.4) X lO-I3expi( 1260 f 130)/7')over the temperature range of 248-480 K; our room temperature rate constant is about a factor of 2 greater than the currently accepted value of k5, with k5(298)/cm3 molecule-' s-I = (1.10 f 0.21) X lo-". This result holds implications for the understanding of the reactivity of R02 species with H02, which is important for the chemical modeling of the troposphere.
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Introduction The importance of the reactions of alkylperoxy radicals (RO2) in combustion and in atmospheric chemistry is well established, where these radicals act as intermediates in theoxidation pathway of hydrocarbons.l.2 A comprehensive understanding of the chemistry of these systems must take into account the presence of R 0 2species and the reactions they undergo. To cite a specific case relevant to this study, the concentration of alkylperoxy radicals has an important influence on the oxidative properties of the troposphere. A recent study on chemical kinetic uncertainties in models of tropospheric chemistry identified the C2H502 + H 0 2reaction (chosen torepresent theensemble of non-methane hydrocarbon oxidation) as one of ten reactions whose precise rate-constant determination is essential if the reliability of troposphere chemical models is to be improved.3 The current stateofalkylperoxy radical kinetics, including a careful assessment of the implications of RO2 reactions on atmospheric chemistry, has been recently detailed in an extensive re vie^.^ As for most alkylperoxy radicals, the self-reaction of C2H502 is thought to proceed via association, with subsequent rearrangement leading to product^.^-^ Several studies have been carried out on the ethylperoxy radical self-reaction, both to determine the rate constant7-I3 and to identify the product branching ratios.8,11,14,15 Thereisgoodagreement that,ofthe three pathways generally observed for R02 self-reactions,only those labeled below as (la) and (lb) are important for reaction 1:
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C 2 H 5 0 2 C 2 H 5 0 2 2 C 2 H 5 0+ 0,
(la)
C2H,0H + CH3CH0
+ 0,
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C 2 H 5 0 0 C 2 H 5 0,
(1 b) (IC)
* Author lo whom correspondence should be addressed. ' Current address: IC1 Explosives Canada, Group Technical Center. 801
Boulevard Richelieu. McMasterville, QuCbec, Canada J3G IT9.
Here we follow the practice of labeling the fractionof ethylperoxy radicals that react via the radical propagation step (la) as a (Le. kla/k1)I6and the ratio of that channel to the terminating step (lb) as @ (i.e. k,,/klb).4 Under atmospheric conditions (298 K, 760 Torr of air), the C2H5O radical produced in channel la reacts on the tens-ofmicrosecond timescale with oxygen to produce HO2:I7-l9
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C 2 H 5 0 0,
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H 0 2 + CH,CHO
(2) Since, at ambient temperatures, the rate constant for the reaction of H 0 2with C2H5O2 is about 2 orders of magnitude greater than that for the self-reaction ( 1),4 the observed second-order rate constant for the loss of C2H502 ( k o b s ) can be related to the k l by the equation: kobs = kl(1 + CY).'^ However, at temperatures approaching 500 K, C2H5O decomposition becomes imwhich introduces the complication of CH3O2 formation:
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+ CH, + C H 2 0 + M CH, + 0, + M -.,C H 3 0 2+ M
C2H50 M
(3) (4)
Although the previous work on the kinetics of the C2H5O2 self-reaction is in fair agreement in terms of the magnitude of the room-temperature rate constant, there is poorer consensus concerning the nature of the temperature dependence. Anastasi and co-workers have twice reported a distinct positive temperature dependence,n*lI several other groups have reported a temperatureindependent rate constant for the reaction,l0~I2and a recent study by Bauer et al. affirms non-Arrhenius behavior at temperatures below 250 K.I3 These previous studies have been limited to a temperature range which avoids the complication of ethoxy radical decomposition. Far less work has been carried out on the reaction between C2H5O2 and HO2. The mechanism for R 0 2+ H02 reactions has been proposed to proceed through an association complex which subsequently decomposes via a four-centered i ~ ~ t e r m e d i a t e . ~ . ~ ~ Of the two direct rate-constant determinations which have been published to date,'0.25 only one of these includes a study of the
0022-3654/93/2097-3530$04.00/00 1993 American Chemical Society
The Ethylperoxy Radical
The Journal of'Physica1 Chemistry, Vol. 97, No. 14, 1993 3531
temperature dependence.2s In addition, some work has been done on end-product a n a l y s i ~ ,most ~ ~ . recently ~~ by Wallington et al., who have determined that the reaction leads solely, within experimental error, to the formation of ethyl hydroperoxide:2h
C,H,02
+ HO,
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C,H,OOH
+ 0,
TABLE I: CzH~02Absorption Cross Sections at 298 and 600 K wavelength (nm)
a( 2 9 8 p h
a( 600)','
210 220 230 240 250 260 270 280 290
2.21 f 0.25d 3.56 f 0.41 4.49 f 0.55 4.89 f 0.60 4.48 f 0.50 3.48 0.38 2.31 f 0.26 1.32f0.15 0.55 f 0.09
1.74 f 0.20d 2.77 f 0.20 3.58 f 0.16 3.77 f 0.19 3.38 0.22 2.86 f 0.28 1.85 f 0.22 1.16 f 0.18 0.65 f 0.10
The previous studies found k5 to have about the same magnitude and temperature dependence as the rate constant for the reaction of the methylperoxy radical with H02.4,25 Until very recently, the kinetics of R02 HO2 reactions, and in particular the temperature dependences of the rate constants, remained largely unexplored. It is therefore not surprising that the apparent " Unitsarecm?molecule I. AIlvaluesaremultipliedby I O i u , Values similarity between CH3O2 and C2H5O2 reactivity toward H 0 2 derived from seven absolute cross section measurements at 240 nm and has guided tropospheric modelers to adopt the CH302 + HO2 threedeterminationsof the relativespectrum. Valuesderived fromseven kinetic parameters for R 0 2+ HO2 reactions in g e r ~ e r a l . ~There '.~~ absolute cross-section measurements at 240 nm and two determinations of the relative spectrum. Errors represent l u and include a Student's is now a developing understanding that the variations are larger 1. than previously t h o ~ g h t . ~ ~ . ~ ~ To date, very little information has been available on the ometrically (within several microseconds) into ethylperoxy temperature dependence of ultraviolet absorption spectra of peroxy radi~als.~O.~l Concentration ranges for the self-reaction experradicals. Two groups have published results on the observed iments in molecule ~ m were - ~ [Cl,] = (2-6) x 10'6, [c&] = temperature dependence of the HO2 radical, emphasizing the = (2-6) X lola; the balance to 1 atm broadening of the absorption profile at elevated t e m p e r a t ~ r e s . ~ I - ~ ~(1.5-8) X 1016, and [O,] of N2. Initial chlorine-atom concentrations were (5-20) X 1013 In addition, two studies have included their observations on the molecule ~ m - ~ . temperature dependence of the methylperoxy radical spectrum.33s34The Gaussian nature of the broad ultraviolet absorption For the study of reaction 5,HO2 was produced via the reaction of the hydroperoxy radical has been treated in theoretical detail of C1 atoms with methanol by Langhoff and Jaffe, whocarried out a large basis set calculation C1+ CH,OH HCI CH,OH (9) of the HO2 excited electronic states and who produced a calculated spectrum as a function of temperature.35 More recently, the CH,OH 0, HO, + CH,O "reflection" method developed by Condon)(' has been extended Reactions 9 and 10 are likewise rapid,j9 and, as for the selfto derive simple expressions for the flattening of the Gaussian reaction experiments, the concentrations of methanol and ethane profiles at elevated temperature^.^.^^.^^ In a practical sense, were always sufficient to ensure stoichiometric conversion of understanding the behavior of the ultraviolet cross sections at chlorine atoms into H02 or C2H502. Concentration ranges in high temperatures would be extremely useful, for example, in the molecule cm-3 were [Cl,] = (3-9) X loi6, [CZHS] = (1.5-10) X interpretation of experiments conducted in the thermal regime 10l6, [CH30H] = (1.5-6) X loi6,and [O,] = (3-6) X loi8;the relevant to combustion. There is also theoretical interest in balance to 1 atm of N2.Initial chlorine-atom concentrations determining to what extent, as a function of temperature, the were (8-20) X loi3. shape and magnitude of the broad, featureless absorptions of The ethylperoxy radical spectrum was calibrated relative to polyatomic species can be treated by "reflection" methods. the methylperoxy radical s p e c t r ~ mby ~ ,replacing ~~ ethane with For this study we carried out experiments on the C2H502selfmethane under otherwise identical experimental conditions, with reaction, the C2Hs02 HO2 reaction, and the C2H502 ultraviolet the one exception that the partial pressure of methane was absorption spectrum, each as a function of temperature. These increased, relative to ethane, by a factor of about 100 to experiments were done with the flash-photolysis technique, with compensate for its slower rate of its reaction with chlorine at0ms.3~ ultraviolet absorption as the means of monitoring the radical The room temperature CzH5O2 spectrum was thus determined concentrations. using the value u ~ ~ o ( C H = ~ O4.55 ~ ) X 10-I8cm2 molecule-l.4 Oxygen, nitrogen, synthetic air, methane, ethane (AGA Gaz Experimental Details spkiaux, purity >99.995%), methanol (Merck spectroscopic The details of the flash photolysis/UV absorption technique grade, purity >99.7%), and chlorine (AGA Gaz sptciaux, 5% in as applied in this laboratory have been described in a recent N2,purity >99.9%) were each used without further purification. p~blication.~ Two sets of apparatus were used in this study. The first, the low-temperature apparatus of ref 5,was operated between Results and Discussion the temperatures of 248 and 350 K; the second, the high temperature apparatus, was used over the range of 295-600 K. I. The Ethylperoxy Radical Ultraviolet Absorption Spectrum. In every experiment, the radicals were generated by argonThe room temperature spectrum for the ethylperoxy radical as flashlamp photolysis of chlorine; the chlorine atoms produced in measured in this laboratory is presented in Table I and in Figures the flash reacted with radical precursor molecules present in the 1 and 2. In Figure 1, it is it is seen that this work yields values cell in large excess. For the study of the ethylperoxy-radical for the ultraviolet cross sections that are slightly larger than the self-reaction and spectrum, the gas mixture that passed into the previously reported values,7,9.10.12.13,42 but that this spectrum is in cell included ethane, oxygen, and nitrogen, in addition to chlorine: good agreement with most of the previously reported spectra and in particular with the two most recently p u b l i ~ h e d . ~ ~ . ~ ~ Cl, 2C1 (Anash > 280 nm) (6) It is interesting to note that the C2H502 spectrum recommended C1+ C2H, C2H, HC1 in the recent review article4on peroxy radicals, taken to be identical (7) to that of Bauer et aI.,l3 has a peak intensity about 5% less than C,H, + 0, C,H,02 that for the recommended CH3O2 spectrum. Our spectrum lies nearly 5% above the recommended CH3O2 spectrum. Several Reaction 7 is rapid,j9 and sufficient ethane was added to ensure other laboratories that have measured the two spectra with that a stoichiometric conversion of chlorine atoms into ethyl identical techniques have also observed that ethylperoxy absorbs radicals took place. Likewise, oxygen concentrations were always slightly more strongly than CH302.4.'0,12.42 sufficiently large to convert ethyl radicals quickly and stoichi-
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Fenter et al.
3532 The Journal of Physical Chemistry, Vol. 97, No. 14, 1993
TABLE 11: Gaussian Fitting Parameters for the C2H~02and CH102 Spectra
"I
experimental value"
calculated value
experimental value"
calculated value
~(298)' a(O)
50.7 f 1.7
48.7 f 1.8
rr(H T)"
46.6 f 1.4 4.93 f 0.06 3.80 f 0.04 238.7 f 0.4 238.9 f 0.3
50.7h 51.2 44.5 4.93h 4.59
48.7h 49.2 41.0 4.64h 4.23
~,,,~(298)(
u,,,(H
T)"
X,,,(298)'
X,,,(H 300
250 wavelength (nm)
2'0 0
Figure 1. Comparison of all reported UV spectra for the C?Hs02 radical: open squares, Adachi et al. (ref 7); open circles, Munk et al. (ref 9); open triangles, Cattell et al. (ref IO); crosses, Wallington et al. (ref 12); dashed line, Bauer et al. (ref 13); dots, Maricq and Wallington (ref 42); solid line, Gaussian fit to this work (data points not shown). 5,
500
250 Wavelength (nm)
300
Figure 2. U V spectrum of the CzHsOz radical at 298 and 600 K. The lines represent optimized Gaussian fits to the experimental data. See the discussion for details.
We include the C2H5O2 spectra in Figure 2 with the following important caveat: the spectrum a t 600 K is measured relative to the CH302 spectrum at the same temperature (just as it is a t 298 K), and the temperature dependence of CH302 is assumed to follow the empirical relationship derived in ref 33. In that work u24o(CH,O2) at room temperature was taken to be the average of several determinations from different laboratories; the temperature dependence of the spectrum was then determined by a series of experiments conducted a t 662 K, in which qa(CH302) was calibrated against that for ozone at the same wavelength. A rather large uncertainty (2u = 14%) is associated with this determination. The motivation behind the measurement of the 600 K spectrum was to quantify the amount of HO2 produced via reaction 8b: This has been shown to be an important additional channel for the reaction of C2H5 and 0 2 at high temperatures and at low press~res.~3.~4 In their study of reaction 8, Wagner et al.44were able to describe experimental results spanning a wide range in temperature and pressure using a kinetic model in which HO2 is formed only from the decomposition of the excited C2H5O2 intermediate. They conclude that direct H-atom abstraction by 02 begins to become important at temperatures in excess of 1000 K. In our study, initial H02 production would manifest itself as a distortion of the absorption profile at low wavelengths where HO2 absorbs strongly. The excellent Gaussian fit to the 600 K spectrum in Figure 2 provides evidence that the branching ratio ksb/(ksa+ k 8 b ) is small (
