J. Phys. Chem. 1993,97, 11712-11723
11712
Kinetic and Mechanistic Study of the Self Reaction of CHjOCHzOz Radicals at Room Temperature Michael E. Jenkin' and Gamy D. Hayman Chemical Kinetics Section, AEA Consultancy Services, BSSI, Hanvell Laboratory, Oxfordshire OX1I O R A , U.K.
Timothy J. Wallington,' Michael D. Hurley, and James C. Ball Research StafJ SRL-3083, Ford Motor Company, P.O.Box 2053, Dearborn, Michigan 48121 Ole John Nielsen' and Thomas Ellermann Section for Chemical Reactivity, RiS0 National Laboratory. DK-4000 Roskilde, Denmark Received: June 24, 1993; In Final Form: August 23, 19938
The UV absorption spectrum and kinetics of the self reaction of CH3OCH202 at 298 K have been studied using both the modulated photolysis of C12/CHsOCH3/02/N2 mixtures and the pulse radiolysis of SFs/CH3OCHp/ 0 2 mixtures. The spectrum, characterized in the range 200-290 nm, is in good agreement with the single published determinatiom8 The observed second-order removal kinetics of CH30CH202, ks,a, were found to be sensitive to both the variation of total pressure (1 7-760 Torr) and the composition of the reaction mixtures: 2CH3OCH202 2CH3OCH20 0 2 (sa); CH3OCHO CH3OCH20H 0 2 (5b). The kinetic studies and a detailed product investigation using long path length FTIR spectroscopy (T= 295 K, C12/CHoOCHs/ 0 2 / N 2 system) provide evidence to support a mechanism involving the rapid thermal decomposition of CH3OCH2O by H atom ejection occurring in competition with thereaction with 0 2 : CHsOCHtO (+M) CH30CHO H (+M) (6); CH3OCH20 0 2 CH30CHO HO2 (4). The complications in the measured values of ksoh in the present studies, and those reported previously,8 are believed to occur as a direct result of formation of H atoms from reaction 6. Accordingly, a pressure-independent value of k5 = (2.1 f 0.3) X 1@l2 cm3 molecule-' s-I is derived for the elementary rate coefficient at 298 K, with identical values of the branching ratio a = ksa/ks = 0.7 f 0.1 determined independently from the FTIR product studies and the modulated photolysis experiments. As part of this work, the rate coefficient for the reaction of C1 atoms with CH3OCHzCl was found to be (2.9 f 0.2) X 10-l' cm3 molecule-l s-l.
-
-
+
+
+
-
The role played by organic peroxy radicals (ROz) in the tropospheric oxidation of volatile organic compounds is well documented.'-3 The reactions of R0z with NO, N02, and HOz are key processes in the mechanisms describing the generation of secondary oxidants such as 0 3 , and a variety of carbonyl compounds, peroxides, and organic nitrogen oxide species. The self reactions of RO2, although of limited importance under tropospheric conditions, are usually a major complication in laboratory studiesand often needed to be characterizedthoroughly before reliable data can be obtained on the tropospherically important reactions. In recent years, significant progress has been made in defining structure-reactivity relationships for both the kinetics and branching ratios of reactions of ROz and the influence of a variety of substituent functional groups on their reactivity can be assessed. There is, however, only limited information available on the effect of an "alkoxy" group adjacent to a peroxy radical center. The simplest a-alkoxy alkyl peroxy radical (which are derived from the oxidation of ethers in general) is the methoxy methyl peroxy radical (CH~OCH~OZ), formed in the atmosphere from the OH radical initiated oxidation of dimethyl ether:
-
+ CH30CH3 CHjOCH2 + 0 2 (+M)
+
CH30CH2 H 2 0
+
-
+
1. Introduction
OH
+
(1)
CH3OCH202 (+M) (2) In their FTIR product study, Japar et al.s established that methyl formate (CHsOCHO) is produced almost quantitatively Abstract published in Aduance ACS Abstracts, October 1. 1993.
0022-3654/93/2097-11712$04.00/0
from the C1atom initiated oxidation of CH3OCH3 in the presence of NO. By analogy with other organic peroxy radicals,)*' this was attributed to the following reaction sequence:
+
C H 3 0 C H 2 0 2 NO CH3OCH2O
+0 2
-
+ NO, CH3OCHO + HO2 CH30CH20
(3)
(4) The observed product, CHaOCHO, was thus believed to be generated from the reaction of the methoxy methoxy radical intermediate (CH3OCH20) with 0 2 , i.e. analogousto the reaction commonly observed for simple alkoxy radicals possessing C-H bonds a! to the radical center.6 In the absence of NO, CH3OCHO has also been detected as a major product' using m I R spectroscopy,although the yield was not quantified. Under these conditions, CH3OCHzOz is removed by its self reaction (q5), which is believed to have the following major channel^:^^^
-
+
-+
2CH30CH202
+ C H 3 0 C H 2 0 H+ 0,
2CH30CH20 0,
(sa)
CH30CH0 (5b) Consequently, CH30CHO may be formed either directly from channel 5b or from the subsequent chemistry of CHaOCH20 produced in channel Sa. In the single published kinetic study of CH3OCH202, Dagaut et a1.8 generated the radical by flash photolysis of Clz in the presence of CHaOCH3 and 02, with detection by UV absorption spectroscopy. The UV spectrum of CH3OCH202 was found to be typical of an organic peroxy radical (Le. broad and unstructured), with a maximum absorption cross section of 4.06 X cmz molecule-' at 230 nm. In contrast, derived values of k , 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11713
Self Reaction of CH3OCH202 Radicals displayed unusual dependenceson both temperature and pressure. At 298 K, ks apparently varied between 7 X cm3molecule-' s-1 at 25 Torr and 2.4 X cm3 molecule-' s-I at 800 Torr, an unexpected result, since all other R02 radical self reactions studied to date have pressure-independent rate coefficients over comparable In the absence of mechanistic information, it was not clear whether this was due to a pressure dependence of the elementary rate coefficient or as a result of unforeseen secondary chemistry. In this paper, a detailed investigation of the kinetics and products of reaction 5 at temperatures near 298 K is presented. Although under some conditions the observedkinetics were found tobesensitive tovariationof pressure, theapparent ratecoefficient (ksob)could also be varied by changing the composition of the reaction mixtures. As a result of this work, we areable to conclude that the elementary rate coefficient (k5)is insensitive tovariation of pressure. Evidence is presented which supports a mechanism involving the rapid thermal decomposition of the CH3OCH20 radical (formed in reaction 5a) by H atom ejection:
+
CH3OCHZO (+M) CH3OCHO H (+M) (6) The complications in the measured rate coefficient ksob are believed to occur as a direct result of the formation of H atoms from reaction 6. +
2. Experimental Section
The kinetic experiments were carried out using both the molecular modulation (AEA CS, Harwell) and pulse radiolysis (Rim National Laboratory) techniques. The product studies were performed using long path length Fourier transform infrared (FTIR) spectroscopy (Ford Motor Company). Complete descriptions of these experimental systems have been given pre~iously,7.9-~2 so they are only discussed briefly in the present paper2.1. Molecular Modulation Experiments. Experiments were performed at 298 Kin a temperatureregulated, cylindrical quartz reaction vessel (volume 1.2 dm3; length 120 cm). CH3OCH2Oz was generated by the intermittent photolysis of C12 (2.5-55 X lOI5 molecule ~ m - in ~ )the presence of CH3OCH3 (1.2-2.7 X 10'7 molecule cm-3) and 0 2 (6.1-770 X lo1, molecule ~ m - ~ ) :
c1, + hv-.CI+ c1
-
C1+ CH30CH3 CH30CH,
+ 0, (+M)
-
(7)
+
(8)
CH30CH,02 (+M)
(2)
CH30CH2 HC1
Experiments were carried out as a function of total pressure over the range 17-760 Torr, with N2 providing the balance of the gas pressure. Photolysis of Cl2 was achieved using up to five fluorescent black lights (Sankyo Denki FL405), emitting over the wavelength range 310400 nm. Reaction mixtures were monitored using a collimatedbeam from a deuterium lamp, which wasfocusedontotheslit ofa 0.75-mmonochromator(Spex 1700), followed by detection on a photomultiplier (EMI, 9783B). The modulated absorption signals due to the production and removal of CH3WH202 (typically l e 3 )were accumulated and averaged in the manner described previo~sly.'.~ Kinetic measurements were made using flowing gas mixtures to minimizethe accumulationof reaction products. The residence time in the vessel was ca. 15 s. The flows of the constituent gases were either regulated using mass flow controllers (MKS, Type 261) or monitored using calibrated rotameters. The total pressure in the reaction vessel was measured using a capacitance manometer (MKSBaratron, Model 170M-6B), allowingcalculation ofthepartialpreasuresofthecomponentgases. Theconcentration of C12 was also determined directly by conventional absorption spectroscopy (u330nm= 2.56 X cm2 molec~le-').~~ N2 (Air Products, high purity), 0 2 (BOC, breathing grade), CHpOCH3 (Aldrich), and Clz (Union Carbide, 5% in high-purity Nz)were used as received.
2.2. F'ulse RadiolysisExperiments. CH3OCH202 radicals were generated by the irradiation of SF6/02/CHpOCH3 mixtures in a l-dm3stainless steel reactor with a single 30-ns pulse of 2-MeV electrons from a Febetron 705B field emission accelerator as follows:
SF,
+ --
+ 2-MeV e-
+ CH30CH3 CH30CH2+ 0, (+M) F
F
products
+
CH30CH2 HF
(9) (10)
CH30CH202(+M) (2) Transient absorptions due to CH3OCH202 (typically 10-l) were monitored using the output of a pulsed 150-W Xenon arc lamp, coupled to an internally mounted White optical arrangement allowing variation of the path length in the range 40-160 cm. Spectral features were analyzed using a l-m grating spectrograph (Hilger & Watts), with the light intensity monitored by a photomultiplier (Hamamatsu) and digitized using a transient recorder (Biomation 8100). Measurements of the UV spectrum of CH3OCH202 were made by observing the maximum in the transient UV absorption at short times (1040 ps) after the radiation pulse. Using longer time scales (0.440ms), the subsequent decay of the absorption was monitored to obtain the kinetic information. Gas mixtures were prepared by adding one component at a time and measuring the corresponding partial pressure using an absolute membrane nanometer (MKS Baratron 170) with a resolution of Torr. Experiments were carried out at 298 K over the pressure range 75-750 Torr, with the following reagent ~; concentrations: 02,8.6 X lo1' molecule ~ m - CH3OCH3,2& 22 X lo1, molecule ~ m -SF6, ~ ; varied to make up the desired total pressure. Ultra high purity SFs (99.996, Gerling and Holtz), 02 (L'Air Liquide), and CH3OCH3 (99.9%,Union Carbide Industrial Gases) were used as received. 2.3. Long Path Length FIlR Experiments. All experiments were performed in a 140-dm3&rex reactor, surrounded by 22 fluorescent black lamps (GE F15T8-BL). CH3OCH202radicals were generated by the photolysis of C12 in the presence of CH3OCH3 in O2/N2 mixtures (reaction sequence (7), (8), and (2)) at total pressures in the range 10-700 Torr at 295 i 2 K. The loss of CH3OCH3 and the formation of products were monitored by FI'IR spectroscopy using an infrared path length of 26 m and a resolution of 0.25 cm-'. Infrared spectra were derived from 32-128 madded interferograms. Reference spectra were ob tained by expanding known volumes of the reference material intothelong pathlength cell at appropriatepressuresofairdiluent. Productswere identified and quantified by fitting referencespectra of the pure compoundsobtained at the appropriate total pressures to the observed product spectra using integrated absorption features over the following wavelength ranges (in cm-I): CH3W H O , 1000-1 100;methoxy methyl hydroperoxide(CHpOCH2OOH), 775-850 and 1700-1750; chloromethyl methyl ether (CH30CH2C1), 650-850 and 1200-1300; HCHO, 17W1800, HCOOH, 1050-1 150; and CO, 2050-2250. CH30CHzOOH was prepared by the ozonolysis of vinyl chloride (99.9% purity) in methanol (>99.9% purity) at 195 K14 and was purified by vacuum distillation. No observable impurities were detected by FTIR spectroscopicanalysisof the purified hydroperoxidesample. The other species were obtained from commercial sources at the highest available purity. The initial CH3OCH3 concentrations usedwerein therange (9.1-10.8) X 1014moleculecm-3.Further details regarding the experimental conditions are given in section 3.2. 3. Results
3.1. Self Reaction Kinetics and UV Absorption Specbum of CH&CHzO~. Molecular Modulation Experiments. CH3OCH202 radicals produced by the modulated photolysis of C12/ CHpOCH3/02 mixtures were detected by UV absorption spectroscopy in the wavelength range 200-290nm. Typical modulated
Jenkin et al.
11714 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
A
" ]/ 0.0
0.0
I
I
I
I
I 400
200
0
'
I
I 600
I 800
Ressw (Torr)
02
00
11
04
06
Time (s)
Figure 1. Absorption waveforms obtained at 220 nm (O), 240 nm (B), and 260 nm (e) from the modulated photolysis of CI~/CHIOCH~/ O2/N2 at 298 K and 760 Torr ([02]/[C12] = 850).
Figure 2. Values of the apparent rate coefficient, k w , measured as a function of pressure at 298 K using a C12/CH,OCH,/02/N2 system in the present study (0)and in thestudyofDagaut et al.*(A) (seediscussion in text). 401
absorption waveforms are shown in Figure 1. Although experiments were carried out under a variety of conditions (as discussed below), the observed behavior of CH3OCH202 was always well described by a second-order kinetic analysis, indicativeof removal via the self reaction:
-
-
+ C H 3 0 C H 0 + C H 3 0 C H 2 0 H+ 0,
2CH30CH20,
2 C H 3 0 C H z 0 0,
(sa) (5b)
Using the methods of analysis described in previous paper^,^^^ values of kkb/a were derived for each set of conditions (kSob is the observed or apparent second-order rate coefficient). Provided the radical production rate (=2k,[Cl2]) is known, both kSoband acanbedetermined. Toenablethis, thefirst-orderdecay constant for C12 (k7) was measured in independent experiments in which static mixtures comprised of Cl2, H2, and 0 2 were photolysed and the decay of C12 was monitored. A value of k7 = (4.4 0.1) X s-I for five black lights was obtained. In an initial series of experiments, the effect of varying pressure in the range 17-760 Torr on the observed behavior of CH3OCHzO2 (monitored at 240 nm) was investigated. For these experiments, [ 0 2 ] was kept constant at 3 X lOI7 molecule ~ m - with ~ , [C12] in thenarrow range2.5-3.5 X lOl5mole~ulecm-~. At low pressures, kS& decreased significantly, in qualitative agreement with the observations of Dagaut et aL8 As shown in Figure 2, however, the absolute values of kSob and the shape of the apparent falloff measured in the present study are significantly different from those observed in the study of Dagaut et a1.* Although the 0 2 concentration used in both studies is similar, the concentrations of C12 used by Dagaut et al. were up to an order of magnitude greater. The effect of variation of both [Clz] and [02] was investigated, therefore, in an extensive series of experiments performed at a fixed pressure (760 Torr), and it was found that the value of kkb was influenced by varying the concentration of either Cl2 or 0 2 . Figure 3 shows the dependence of ksob on the concentrationratio [02J/[CI2]. At high-concentration ratios (ca. lOOO), values of ksoh = 3.5 X 10-12 cm3 molecule-I s-1 were obtained, whereas at low ratios (ca. l), values of ksob< 10-l2cm3 molecule-' s-I were observed. Clearly, therefore, the sensitivity of ksobto the experimental conditions arises from complications in the secondary chemistry and not simply from a complex dependence of the elementary rate coefficient on pressure, as postulated previously.8 More specifically, the dependence on the
*
1 0.1
I 1
I
I
I00
IO
I loa,
I loo00
[O, 11I C12 I
Figure 3. Values of the apparent rate coefficient, k w ,measured as a function of the concentration ratio [Oz]/[C12] at 760 Torr and 298 K. Line is curve computed on the basis of the linear regression displayed in Figure 5.
ratio [02]/ [Clz] indicates a direct competition between reactions of a short-lived intermediate with 0 2 and with Cl2. In explaining this effect, and identifying the key intermediate (referred to as "X" in the following discussion), several criteria must be fulfilled: (a) CH3OCH202 displayed second-order kinetic behavior under all experimentalconditions. Consequently,X cannot be CH3OCH202 itself. (b) The observed production efficiency of CH3OCH202 from Clz photolysis was independent of experimental conditions (i.e. the initial production of CH3OCHzOz was not inhibited). This indicates that X must be a short-lived species generated from reaction of CH3OCHzO2, i.e. probably as a result of reaction channel 5a. (c) Either the reaction of X with 02leads to further removal of CHoOCH202, or the reaction of X with C12 leads to regeneration of CHpOCH202, or both. (d) For the observed pressure effect to be consistent with the observed [02]/[C12] effect, the reaction of X with 0 2 must become less competitive as the pressure is lowered, indicating that it is an associationreaction. Sinceat lowvaluesof [02]/[C12],apressure effect can even be observed at high pressures (e.g. the data of Dagaut et al. displayed in Figure 2). the reaction of X with 0 2 must be well below its high-pressure limit, indicating that X is a small species. A plausible explanation for all the experimental observations may be forwarded, if it is assumed that CH3OCHzO formed in reaction channel 5a decomposes by ejection of an H atom in
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11715
Self Reaction of CH3OCH202 Radicals
t
' -1.0
I 4.8
'
I
6.6
4.4
' 4.2
I 0.0 f
' 0.2
I
'
0.4
0.6
I 0.8
' 1.0
Figure 5. Values of the apparent rate coefficient, k- plotted as a function of the parameterf(see discussion in text): ( 0 )data obtained at 760 Torr; (0) data obtained at 25 Torr. Line is linear regression of 760-Torr data. 1
I
i
6'o
I
+
1
t
I
I
C"3"
Figure 4. Schematic representation of the C1 atom initiated oxidation mechanism of C H J O C H ~at high pressure.
200
I
I
I
220
240
260
d, I
I
280
300
Wavelength (nm)
--
Figure6. Absorption spectra measured for CH3OCH202 at 298 K ( 0 ) molecular modulation experiments with [Oz]/[Cl,] = 1.1 (lower line is
preference to reacting with 0 2 : 2CH30CH20,
2CH,OCH,O
+ 0,
(sa)
CH3OCHO + H (+M) (6) Other than H, it is difficult to identify a species which fulfills all the criteria outlined above for the key intermediate 'X". At high total pressures and [02], the association reaction of H with 0 2 is favored, leading to further removal of CHsOCH202 as follows: CH,OCH,O (+M)
H
HO,
--
+ 0, (+M)
HO, (+M)
+ CH,0CH202
(1 1)
products
(12) As the total pressure is lowered, or the [02]/[C12] ratio is decreased, the pressure-independent reaction of H with C12 becomes more competitive, leading to production of C1 and regeneration of CH~OCHIOI:
H + C1, C1+ CH,0CH3
+
-+
HCl
+ C1
CH30CH,
(13)
+ HCl
(8)
CH,OCH, 0 2 (+M) CH3OCH20, (+M) (2) According to this mechanism (shown schematically in Figure 4) and assuming reaction 12 is exclusive for H02, ksot.,,should vary between (1 u)k5 when reaction 11 dominates and (1 - u)ks when reaction 13 dominates, where a = k5./k5. Furthermore, the complete dependence of kkb on [02] and [Clz] should be given by the expression
+
k5, = (1 + f W 5 where the parameter f (k11[021 - k13[Cl~I)/(k11[0~1 + k13[C12]), taking values between -1.0 and +1.0. Figure 5 shows
fifth-order polynomial fit to data); (0) molecular modulation experiments with [02]/[Clz] = 850 (upper line obtained by adding H 0 2 spectrum to lower line); (A)pulse radiolysis experiments; ( 0 )data of Dagaut et al.
acorresponding plot of ksot.,,vsffor the set of data displayed in Figure 3 (total pressure 760 Torr), with f calculated using recommended values of k11 and k13.I~The linearity of this plot is consistent with the posulated mechanism and also allows determination of both k5 and u from the slope and intercept (errors = f2u):
k, = (2.13 f 0.30) X lo-', cm3 molecule-' s-' u = 0.67 0.1 1
*
Further support for the mechanism presented above comes from the UV spectral measurements of CHpOCH202. Experiments were performed in which the variation of the modulated absorption signal with wavelength was investigatedat [02]/[C12] ratios of both 1.1 and 850. The derived absorption cross sections are presented in Figure 6,along with the data of Dagaut et a1.: obtained using the flash photolysis technique. The spectrum measured with [02]/[C12] = 1.1 is in excellent agreement with the data of Dagaut et al., both in shape and magnitude. With [02]/[C12] = 850, however, the data obtained at the short end of the wavelength range in the present study clearly lie above those measured with [02]/[Cl~]= 1.1. This is consistent with the mechanism postulated above, since at high [02]/[Ch], a contribution to the measured spectrum is made by a steady-state concentration of HO2 formed from reaction 11. At low [02]/ [Clz], theformationofH02isinhibited,sincereadon13competes effectively with reaction 11. As can be seen in Figure 6, the difference between the two sets of data matches the HO2 spectrum
Jenkin et al.
11716 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
extremely closely. Furthermore, these data suggest the steadystate concentration ratio [HOz]/[CH3OCH202] = 0.3 when reaction 1 1 dominates, which according to the postulated mechanism, should be equal to the expression 2aks/klz. Thus, on the basis of the values of ks and a derived above, k12 is the order of 1 6 l 1 cm3 molecule-' s-I, i.e. typical for reactions of larger organic peroxy radicals with H O Z . ~ ? ~ Since the reaction of H with 0 2(reaction 1 1) is stronglypressure depender~t,'~ the apparent falloff in k5ok at lower pressures observed by Dagaut et a1.8 and in the present experiments is also qualitatively explained by the proposed mechanism. However, on the basis of reported values of kl I as a function of pres~ure,'~ the present data suggest that the reduction in kSobwith a decrease in pressure at approximately constant [Oz]/[C12] (Figure 2) is not as great as would be expected from the pressure dependence of the competition between reactions 1 1 and 13 for H. Clearly, the ejection of H from CH3OCH20 (reaction 6) will itself possess a pressure dependence, possibly allowing other reactions to become competitive for CH3OCH20 at low pressures. To provide more information, a series of experiments was performed at 25 Torr to investigate the effect of varying [02]/[C12] on the measured value of kkb. The results are shown as a function of the parameter f i n Figure 5, along with those obtained at 760 Torr. For the 25-Torr experiments, [Clz] was kept constant at (3.0 f 0.5)X 10'5 molecule ~ m - with ~ , [02] varied in the range 1-6 X 1017 molecule ~ m - ~Although . at the lowest [OZ](f = -0.79) the measured value of ksok is in reasonable agreement with those obtained at 760 Torr, the increase in ksob as [OZ](and therefore f) is raised is far more rapid than expected solely on the basis of the competition between reactions 1 1 and 13 for H. As a result, the dependence of ksobonf is no longer linear, with the (1 + a)ks (i.e. maximum) value being approached at values off lo'* molecule ~ m - when both reactions are correspondingly more rapid. The effect of this competition on the data obtained at 760 Torr is not observable, however, because, under conditions when reaction 4 is able to compete with reaction 6, [02] is sufficiently high that H reacts almost exclusively with 0 2 and reactions 4 and 6 are essentially indistinguishable. At lower [02] (
