1470
J. Phys. Chem. 1995,99, 1470-1477
Kinetics of the Reactions CHJO
+ NO, CH30 + NO3, and CH302 + No3
Veronique DaZle,* Gerard Laverdet, Georges Le Bras, and Gilles Poulet Luboratoire de Combustion et Systkmes RCactifs, C.N.R.S. and UniversitC d'Orltans, 45071 OrlCans Cedex, France Received: August 22, 1994@
A discharge flow reactor coupled to laser-induced fluorescence (LIF) and mass spectrometry has been used to study the kinetics of the reactions of CH3O with NO and NO3. The value of the rate constant for the reaction CH30 NO products at 298 K and at 1 Torr of He has been found to be k = (4.8 f 0.4) x lo-', cm3 molecule-' s-l, in good agreement with the literature data. From the kinetic analysis of CH30 by LIF, the reaction CH3O NO3 CH3O2 NO2 (1) has been found to be followed by the reaction CH302 NO3 CH30 NO2 0 2 (2). Modeling calculations gave for the rate constants of these two reactions, at 298 K, kl = (1.8 f 0.5) x lo-', cm3 molecule-' SKI and kz = (1.2 f 0.6) x cm3 molecule-' s-I. Comparison with recent literature data is given, and the atmospheric implication for nighttime chemistry is briefly discussed.
-
+
-
+
+ +
-
+
+
Introduction
Methoxy radicals, CH30, are known to be important intermediates in the oxidation mechanism of hydrocarbons in either combustion processes or atmospheric photochemistry. In the OH-initiated oxidation of methane in the natural atmosphere, CH3O radicals are readily converted to formaldehyde and hydroperoxy radicals: CH30 0 2 H2CO HO2. However, under heavy polluted conditions, CH30 radicals may also react with NO and NO2. Many papers have been published' on the reaction of CH3O with 0 2 , due to its importance in the atmosphere. More recently, the kinetics of the reactions of CH30 with NO and NO2 have also been Whereas in the earlier studies CH30 radicals were observed indirectly, the direct detection of CH3O was first made by laser magnetic resonance.' Then, the improvement of spectroscopic techniques allowed for sensitive detection of CH30 by laserinduced fluorescence (LIF).*-I2 This method has been widely used in the most recent kinetic studies of CH30 reaction^^-^^'^^'^ and has been also applied to the detection of CH30 as a product of the reaction between CH3O2 and NO.'5 A fast reaction of CH30 with the nitrate radicals, NO3, has also been postulated from experiments on the CH3O2 NO, reaction.I6 Under laboratory conditions, this reaction between CH3O and NO3 is a secondary step of the CH302 NO3 reaction, forming peroxy radicals:
+
-
+
+ +
+ NO, - CH,O, + NO, CH30, + NO, CH,O + NO, + 0, CH30
+
(1) (2)
The production of CH3O in reaction 2 has been suggested by analogy with the mechanism of the reaction between HOz and NO3, which has been studied in detail (e.g. refs 17 and 18). The major path for this reaction leads to the formation of OH radicals. Similarly, OH has been shown to react rapidly with NO3: OH HO,
+ NO, - HO, + NO2
+ NO, - OH + NO2 + 0,
These reactions between peroxy and nitrate radicals have been considered to participate in the nighttime oxidation of volatile @
Abstract published in Advance ACS Abstracts, January 1, 1995.
0022-3654195J2099-1470$09.00/0
organic compounds, which could produce relatively high levels of HO, (OH, HO2) radicals in polluted tropospheric regions.I9 A first kinetic study of reaction 2 has been performed using molecular modulation technique and absorption spectroscopy.20 Computer simulation was necessary to extract the rate constant k2. This study was followed by several direct investigations of reactions 1 and 2 carried out simultaneously in three different l a b o r a t o r i e ~ . ~These ~ - ~ ~ studies are now completed, and two of them have been published r e ~ e n t l y . ' ~ , ' ~ In the present work, the reactions of NO3 with CH3O and CH3O2 have been studied using the discharge-flow technique with LIF detection for CH30 and mass spectrometry for other species. The reaction between CH30 and NO has also been investigated in order to validate the new experimental setup used. Experimental Section
A double discharge fast flow reactor has been equipped with a double detection system including a LIF cell and a quadrupole mass spectrometer (Figure 1). All parts of the Pyrex flow tube (2.54 cm i.d. and 80 cm length) and the central injector (1.09 cm id.) were coated with halocarbon wax to reduce heterogeneous loss of atoms and radicals. In the moveable central injector, CH3O or CH3O2 radicals were produced from the same precursor, CH3 radicals, which then reacted with either NO, or 0 2 , respectively. CH3 radicals were formed from the reaction of an excess of CH4 over fluorine atoms produced in a microwave discharge of F2 highly diluted in the helium carrier gas. The total flow rates in the main flow tube and the injector were 6 x lo3 and 2 x lo3 cm3 s-l, respectively (these flow rates are given at the pressure in the main flow tube). The mean pressure in the reaction zone of the central injector was 0.05 Torr higher than the pressure (typically 1 Torr) in the main tube. In the discharge tube, which contained an alumina cylinder, a total dissociation of F2 was obtained. No measurable impurities were produced, which is not always the case when CF4 is used as the precursor of F atoms. The fast CH3 NO2 reaction is a clean source of methoxy radicals:24
+
CH,
+ NO, - CH,O + NO ( k = 2.3 x lo-" cm3 molecule-' s - ~ ) ~
(The rate constants are given at 298 K unless specified.)
0 1995 American Chemical Society
CH30
+ NO, CH30 f N03, and CH302 + NO3 EXCIMER L A S E R
DYE L A S E R I
I
J. Phys. Chem., Vol. 99, No. 5, 1995 1471
#
I-\
1 JOULEWETER 2 PHOTOHULTIPLIER 3 MULTICHANNEL ANALYSER 4 CHART RECORDER 5 ns SUPPLY 6 LIQUID NITROGEN TRAP 7 PRESSURE TRANSDUCER
I
Figure 1. Discharge-flow reactor coupled to laser induced fluorescence and mass spectrometry (reactants are given for the study of the reactions of No3 with CH30 and CH302).
+
This source was preferred to the F CH30H reaction, which appeared to show complications likely arising from the existence of two competitive channels forming either CH3O or CHzOH, as shown recently.25,26 CH3O thus produced could then react with the other reactant (NO or NO3) which was introduced into the reactor through a side-arm tube. At the end of the main flow tube, CH3O radicals were detected by LIF in a stainless steel cell. CH3O was excited in the 3: band of the .&(2A1) %(2E) electronic transition at 298.3 nm.* The v3 vibration is the C - 0 stretch, and the ql orbital occupied upon excitation is antibonding with respect to the C-0 bond. The fluorescence was excited by a XeCl(308 nm) excimer (Lambda Physik, EMG 101 MSC) pumped dye laser (Lambda Physik, FL,3002). The pulse repetition rate of the excimer laser was fixed at 10 Hz. The exciting radiation was generated by frequency doubling the output from the dye laser. Rhodamine B (Lambda Physik) was used as the dye medium. The laser bandwidth was ca. 0.3 cm-I, and the pulse energy was ca. 1.0 mJ at 300 nm. The flow reactor, the laser beam, and the photomultiplier axis were orthogonal. The laser beam was focused with a concave mirror (f = 50 cm) and collimated using an iris before entering the baffle arm mounted on the main body of the black anodized cell. The beam passed through 6 mm holes in the baffles, which were also black anodized. The undispersed fluorescence was collected at a right angle and focused on a Hamamatsu R2560 photomultiplier tube using two quartz plano-convex lenses. A filter combination was used (Schott U G l l and UG3 with a 310-390 nm band-pass with a maximum transmission at 350 nm). The photomultiplier was operated in the pulse counting mode, and pulses were preamplified before discrimination and counting (Stanford Research Systems SR 445 and SR 430). The delays with respect to the laser firing and the period of integration were variable and were typically set at 0.8 and 6 ps, respectively. The detection system of this laser excited fluorescence gave a detection limit of ca. 5 x lo8 molecule cmP3 (for S / N = 1). The measured fluorescence lifetime of CH30 (1.8 ps) was in good agreement with previous measurements ranging from 1.5 ps8 to 2.1 ps.10 The absolute concentration of CH30 radicals in the reactor was obtained from the computer simulation of the chemical system of the CH30 source taking place in the central injector. The reactions together with their rate constant values at 298 K',5,25-27-31 are listed in Table 1, and the FACSIMILE program was used.32 For the association reactions, the rate constants
-
TABLE 1: Chemical System and Rate Constants Used in the Numerical Model of the C&O Source reaction k298K' reference
+ +
+
F Ch-CH3 HF F CH3 CH2 HF M CH3 CH3 M-C2& CH3 NO2 CH3O NO CH3 NO2 M CH3N02 M CH3O NO CH2O HNO CH30 NO M C H 3 0 N 0 M CH3O NO2 M CH30N02 M CH30 NO2 CH2O HONO CH30 CH3O products CH3O wall products CH3O CH3 CHq CH2O CH3O CH3 M-CH30CH3 M F CH30 CH2O HF F CH2O HCO HF F+C~H~-CZH~+HF
+
+ + + + - + + + + + + + + + + + + + + + + + - + + + + + + + - + 4
-
-+
a
8.0 x 7.2 x 4.7 x 2.3 x 2.0 x 4.0 x 5.0 x 9.2 x 2.0 7.0 x 3ob 4.0 x 2.0 x
lo-" IO-" lo-" lo-" 10-'2c
10-13
lo-" lo-" 3.0 x lo-''' 6.6 x lo-" 8.4 x
1 5 27 5 5 28 28 5 5 25 this work 29 29 25
30 31
Rate constant units are cm3 molecule-' s-I. Units in SKI.Values
at 1 Torr of He.
have been calculated at a pressure of 1 Torr of He, which is nearly the pressure in the central injector zone where CH3O was produced. Among all the reactions given in Table 1, many of them are negligible under the conditions used. The initial concentration of F atoms was needed as input data, together with the concentration of CHq and N02. Absolute measurement of F atom concentration was made by chemical titration via the very fast reaction with C12, which was introduced through the extemal tube of the injector:
F
+ C1, - FC1+ C1
(k = 1.6 x lo-'' cm3molecule-' s-1)33 The C12 consumption was measured from the mass spectrometric calibration at the peak d e = 70. Although a total dissociation of F2 in the discharge was observed (at d e = 38), some loss of F atoms occurred in the central tube, which was 80 cm long. The chlorine titration gave apparent dissociation efficiency of F2 ranging between 0.50 and 0.70. It was also necessary to use a large excess of C& and NO2 in the production of CH30 in order to keep as negligible as possible the secondary reactions such as the F CH3O reaction, which proceeds at the collision frequency.25 The excess of NO2 over CH3 had to be important enough to reduce the self-combination of CH3, which possesses a rate constant of 4.7 x lo-" cm3
+
Daele et al.
1472 J. Phys. Chem., Vol. 99, No. 5, 1995
+ HNO,
+ HF
molecule-' s-l in 1 Torr of He at 298 K.27 On the other hand, the excess of NO2 had to be limited since CH3O reacts with NO;?:
F
CH30
The necessary excess of HN03 was checked by mass spectrometry (at d e = 63). HNO3 was flowed from a bubbler containing a liquid mixture H2SOmN03 (2:l) maintained at 273 K, and a capillary restriction in the F;? discharge tube prevented any diffusion of HNO3 back to the discharge zone. HNO3 was calibrated from a known flow of gaseous HNO3 prepared in the l a b ~ r a t o r y . ~ ~ Since NO3 was used in excess in the kinetic studies of its reactions with CH3O and CH3O2, accurate measurement of its concentration was needed. Similarly to previous work,35the fast reaction between NO3 and 2,3-dimethyl-2-butene, which was introduced through the central tube, was used for the titration:
+ NO, -products
( k = I. 12 x
cm3 molecule-' s - ' ) ~
Finally, taking into account both the experimental observations and modeling calculations, the optimized production of CH30 radicals was obtained for the following conditions of initial concentrations (in molecule cmP3) in the main flow tube: [F] = 5 x lo", [C&] = 5 x lOI3, [NO;?]= 5 x 10l2. The computer simulation, using the initial concentrations of reactants in the central injector, led to a fixed reaction time of 0.01 s, which corresponded to a distance of 22 cm (distance between the downstream ends of internal and extemal tubes). This reaction time was higher than the time corresponding to the maximum of the CH30 concentration (ca. 0.005 s), but it made the concentration of residual CH3 negligible (lower than 5 x lo9 molecule ~ m - ~ )Otherwise, . the subsequent kinetics of CH3O with NO3 could have been erroneous since a fast reaction exists between CH3 and No3 ( k = 3.5 x 10-l' cm3 molecule-' s-').I3 Under these conditions, the CH3O concentration entering the main zone of the reactor was typically ca. 2 x 10" molecule ~ m - ~In. the reactor, CH30 concentration was observed to decrease in the absence of any other reactant. Using the most recent value for the rate constant of the gas phase recombination of CH30 ( k = 7 x 10-l2 cm3 molecule-' S - I ) , ~this ~ decay could only be assigned to a wall loss of CH30. This was supported by the fact that changing the CH3O initial concentrations had no effect on this decay rate. The mean value measured for this heterogeneous loss of CH30 was (26 f 5) S-'.
+
For the study of the CH302 NO3 reaction, using CH3O2 and NO3 radicals as initial reactants, CH3O2 was produced by introducing 0 2 instead of NO;? through the central injector: CH,
+ 0, + M -CH30, + M
k = 1.3 x cm3 molecule-' s-l at 298 K and 1 Torr of He.' A high concentration of 0 2 was necessary (typically 9 x 1015 molecule ~ m - for ~ ) nearly completion of this reaction in the injector. Among the secondary reactions (given in Table 3), the reaction between CH3O2 and CH3 was taken into account in the computer simulation of the CH30Z source since it produced some residual CH30: CH30,
+ CH, - CH30 + CH,O
( k = 4.5 x IO-'' cm3 molecule-' s-')'~ The reaction distance in the injector was the same as that for the CH30 production, and initial concentrations of F (as measured from Cl;?titration) and C€& were in the ranges (0.71.7) x 10l2and (3.9-7.8) x lOI3 molecule ~ m - respectively. ~, Unlike CH30, the decay rate of CH302 in the absence of any other reactant was very low under these experimental conditions. The CH302 radicals were titrated using their reaction with NO:
CH30, -t NO
-
CH,O
+ NO,
( k = 7.7 x IO-';? cm3 molecule-' s-')' NO2 was calibrated by mass spectrometry (at d e = 46). Finally, the nitrate radical was prepared in the fixed sidearm tube, where the dissociation of F2 in an alumina discharge tube produced F atoms, which then reacted with HN03:
+
NO,
( k = 2.7 x lo-" cm3 molecule-' s - ' ) ~ ~
NO,
-
+ (CH,),C=C(CH,),
products
( k = 4.5 x IO-'' cm3 molecule-' s-1)35 The consumption of molecular reactant was measured, before and after each kinetic run, by mass spectrometry (at d e = 84). The dissociation of F2 had to be complete in order to avoid the complication due to the reaction between FZand 2,3-dimethyl2-butene. This titration gave concentrations of NO3 which were in excellent agreement with those of F atoms deduced from chlorine titration experiments. This indicated that the conversion of F into NO3 was quantitative. Besides, it was shown that any loss (homogeneous or heterogeneous) of NO3 did not exist significantly in the reactor. The purity of reactants used was as follows: He (Alphagaz, 99.9995%) was used as the diluent passed through a N2 trap before entering the reactor; F2 (Ucar, "laser" quality, 5% mixture in He) and CHq (Ucar, 99.9%) were used without further purification; NO2 (Alphagaz, 99%), NO (Alphagaz, 99.9%), and 2,3-dimethyl-2-butene (Aldrich, 99%) were purified by trapto-trap distillation. Results and Discussion
+
1. Reaction CH30 NO. In order to validate this experimental system, the kinetics of the reaction between CH30 and NO has been investigated at room temperature, under pseudo-frst-order conditions (excess of NO). At a total pressure of 1 Torr of He, with a flow velocity of 1200-1300 cm s-l, the CH30 decay observed from the LIF signal was measured for the following ranges of initial concentrations: [NO] = (0.311.0) x lOI3 and [CH30] = (0.9-2.8) x 10" molecule ~ m - ~ . CH30 concentrations were measured as described in the previous section. The first-order rate constant was corrected for the axial diffusion of CH30 in helium using D C H ~ O = IH~ 434 cm2 s-' (calculated at T = 298 K and P = 1 Torr). This correction factor ranged from 1.01 to 1.13. A correction was also made to take into account the contribution of the CH30 NO2 reaction, for which k = 1.12 x lo-'* cm3 molecule-' s - ] . ~ Considering the typical NO2 concentration used in the CH3O source, this correction was small (lower than 5 s-'). The pseudo-first-order plot thus obtained is given in Figure 2. The slope yields the value for the rate constant of the CH3O NO reaction at 298 K:
+
+
k = (4.8 f 0.4) x lo-'' cm3 molecule-' s-' The error is twice the standard deviation. The zero intercept gives the value of the heterogeneous loss rate of CH3O: 28 f 11 s-l. This result confirms the value obtained in the direct
CH30
+ NO, CH30 + NO3, and CH3O2 + NO3
J. Phys. Chem., Vol. 99, No. 5, I995 1473 2.0
molecule-' s-l
k = (4.8 f 0.4) x 500
-
h
400
3
1
-
i
/
+ fi'y
M h
1.5
'B
1
A
[NO31 = 6.8 x
1013 molecule
I \\
T
I,
x
1
I
I
I 4
2
0
I 6
I
I
1
8
10
12
0
\x
10
[NO](1013 molecule "3)
-
-
TABLE 2: Literature Data for the Rate Constant of the Reaction CHJO NO Products at 1 Torr of He and 298 K k (lO-lz)a techniqueb reference 5.4' PP-LIF 4 3.8 f 0.7 DF-LJF 5 28
4.9 4.8 4~0.4
DF-LJFMS this work a Rate constant units are cm3 molecule-' SKI. DF = discharge flow; PP = pulsed photolysis; LIF = laser-inducedfluorescence;MS = mass spectrometry. Extrapolated from high-pressure values assuming a third-order regime at 1 Ton. Recommended value. measurement of the wall loss of CH3O in the absence of any other reactant (26 f 5 s-'). In Table 2, the literature values of the rate constant for the CH30 NO reaction are given together with the recommended value.28 There is a good agreement between these values and the present one when the data are compared under similar conditions (1 Torr of He and 298 K). These values are the sum of the rate constants of the two channels which have been considered for this reaction:
+
+ NO + M -C H 3 0 N 0 + M CH30 + NO - H,CO + HNO
CH30
The addition channel would represent between and 20%J of the overall reaction at 1 Torr of helium. The objective of the present investigation was not to go into the detail of the mechanism of this reaction, which would require pressure dependence measurements of the overall rate constant and quantitative information on the products, but to validate the discharge-flow-LIF experiment from this kinetic measurement. 2. Reactions CHJO NO3 and CH302 NOJ. In the first kinetic runs of the reaction CH30 NO3, it was clearly observed that the CH3O temporal profile, as measured from the decay of the LIF signal, did not behave in a pure first-order fashion. A plateau appeared after a typical reaction time of about 20 ms, as shown in Figure 3. This observation confiied the anticipated mechanism, similar to that of the OH NO3 reaction:" the secondary step leads to the re-formation of CH3O from the reaction of NO3 with the peroxy radical initially
+
+
30
40
50
t (ms)
Figure 2. Pseudo-first-order plot of the rate constant for the reaction CH3O + NO products at 298 K and 1 Torr of He.
+
20
+
+
Figure 3. Experimental (points) and simulated (curves) decays of CH30 as a function of reaction time (see text).
formed:
+ NO3 - CH302+ NO2 CH302+ NO3 -.CH30 + NO, -I-0, CH30
(1) (2)
Of course, the time of appearance of the plateau was dependent on the initial NO3 concentration and could reach 50 ms for the lowest NO3 concentration used. However, in all cases, a total disappearance of CH3O was never observed. Consequently, a numerical simulation of the reaction system was required to extract the rate constants for reactions 1 and 2, which were interdependent. The detailed mechanism is given in Table 3 with the most recent available kinetic parameters for the reactions involved. The wall loss of CH30 was given the mean experimental value measured in the present conditions (30 s-l). The other input parameters were the initial concentrations of both radicals, which were determined in independent experiments, as described in the Experimental Section. [NO310 was deduced from chemical titration using 2,3-dimethyl-2butene. [CH30]o was calculated from the simulation of the CH30 source, and the LIF signal was calibrated after taking into account the quenching of the fluorescence by HNO3. This very efficient quenching of the CH3O fluorescence was observed in the presence of HNO3 used in large excess over F atoms to produce NO3. Therefore, the CH3O calibration was made in the presence of the same flow of HN03 as used during the kinetic measurement. Finally, the experiments were performed and simulated under the following conditions: T = 298 K, P = 1- 1.2 Torr, flow velocity = 1100-1330 cm s-l, [NO310 = (0.7-8.7) x 1013 molecule ~ m - and ~ , [CH30]0 = (0.3-3.1) x 10" molecule cm-3. From sensitivity analysis, it was shown that most of the secondary reactions listed in Table 3 were negligible except, of course, reaction 2 and also the wall loss reaction of CH30. As shown in Figure 3, the experimental points were fitted by the simulated curve yielding the "best couple" of values for kl and k2. When reaction 2 was not included in the model, the lower curve (b) was obtained. Inversely, when all reactions consuming CH3O radicals were removed from the mechanism, except reaction 1, the upper curve (a) was calculated. This
Daele et al.
1474 J. Phys. Chem., Vol. 99, No. 5, 1995
observation indicated that reaction 1 was the major process consuming CH3O under our experimental conditions. In this data analysis, it was also considered that other channels were thermochemically feasible for both reactions 1 and 2: AHR(kcal mol-’) CH,O
+ NO,
-
+
CH302 NO,
-9.1
CH,OONO,
-31.6
CH,ONO
+ 0,
-36.6
+ HNO, -79.3 CH302+ NO3 -.. CH,O + NO, + 0, -9.1 - HNO, + CH,O, -8.2 -HNO, + CH,O + 0 -19.7 HO, + CH,O + NO, -36.1 -CH30N0, + 0, -49.6 - HONO + CH,O + 0, -66.0 CH,O
reaction
+
+ +
CH3O NO3 CH302 NO2 CH30z No3 CH3O NO2 0 2 CH3O 0 2 CH2O H02 CH3O CH3O products CH3O wall products CH30 CH3 CH4 CHzO CH3O CH3 M CH3OCH3 M CH3O NO CHzO HNO CH30 NO M CH30NO M CH3O NO2 M CH3ON02 M CH30 NO2 CH2O HONO CH302 CH302 CH3O CH3O 0 CHzO CH30H CH302 C&02 CH302 CH302 CH30zCH3 0 2 CH302 CH30 CHzO CH3OOH CH302 C H 3 0 CHi02 CH3OH CH302 wall -products CH302 HOz CH300H + 0 2 CH302 HO2 CH20 H20 0 2 CH3Oz CH3 CH30 CH3O CH302 NO CH30 NO2 CH30z NO2 M CH302NOz M CH3 CH3 M - C C ~ H ~ M CH3 NO3 CH3O NO2 CH3 0 2 M-CH30z M CH3 NO2 CH3O NO CH3 NO2 M CH3N02 M HOz HOz H202 0 2 HOz CH, CH3O OH HOz C H 3 - C h 0 2 HOz NO NO2 OH HO2 NO3 products OH NO3 HOz NO2 NO3 NO2 M - - 2 0 5 M NzOs M-NO3 NO2 M NO3 NO NO2 NO2
+ + + + + + + -. + + + + + -. + + + + + + + + + + - ++ ++ 2 + + - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - + + + + + + - + + + - + + + + + + + + - + +
0 7
-+
Even if most of these proposed channels are unlikely on a mechanistic basis, the potential existence of non radical carrier steps was considered for reactions 1 and 2. As expected, in all cases, the experimental CH3O profiles could not be fitted, which confirms the proposed mechanism. Fourteen independent kinetic measurements were performed, as summarized in Table 4. The values of both kl and k~ obtained for each simulation are given with their 95% confidence level. The resulting mean value for these rate constants at 298 K, with one standard deviation uncertainty, are
k298KU
reference
kl variable k2 variable 1.9 x 10-15 7.0 x 30b 4.0 x 10-I’ 2.0 x lo-” 4.0 x 5.0 x 9.2 x lO-I3 2.0 x 10-13 1.4 x lo-’’ 2.8 x 4.7 x 10-14 5.0 x 1.0 x lo-” 4b 5.2 x 4.0 x 4.0 x lo-” 7.7 x 10-12 4.5 x 10-14c 4.7 x lo-” 3.5 x lo-’’ 1.3 x 10-14‘ 2.3 x 10-I’ 2.0 x 10-12‘ 1.7 x lo-’* 3.3 x lo-” 6.0 x 8.6 x 4.1 x 2.3 x lo-” 1.5 x 0.044*,‘ 2.6 x lo-”
this work this work 1 25 this work 29 29 28 28 5 5 1 1 1 29 36 22 1 1 29 1 1 27 13 1 5 5 1 29 29 1 1 1 1 1 1
Rate constant units are cm3 molecule-’ s-I. at 1 Torr of He.
Units in
s-l.
Values
TABLE 4: Experimental Conditions and Modeling Results for the Studs of the CH30 NO3 Reaction at 298 K
+
s-l
[No310 [CH30lo (1013)~ ( i o l y
k, = (1.2 f 0.6) x lo-’’ cm3 molecule-’ s-’ A second kinetic treatement was used to measure kl. In the first part of the CH30 decay curve before reaching the plateau (see Figure 3), a simple first-order analysis could be made, leading to the plot given in Figure 4. Similarly to the CH3O NO kinetics, a correction was applied to account for the contribution of the reaction between CH3O and NO,, used as the precursor of CH30. Under our experimental conditions, this contribution was lower than 6 s-l. The slope of the straight line thus obtained (Figure 4) gave for the rate constant of reaction 1
+
k , = (1.4 f 0.2) x lo-’’ cm3 molecule-’ s-’ The intercept, 24 k 6 SKI,is in excellent agreement with that measured in the CH3O NO study, 26 f. 5 s-l, as well as with that measured in the absence of NO3 during this series of experiments, 25 4~8 s-l. Although slightly lower, the above value for kl agrees with the value obtained from the modeling treatment. Then, a direct measurement of k2 was attempted using NO3 and CH302 as initial reactants. CH3O2 was produced in the central double injector in the place of CH3O (see the Experimental Section):
+
+
+
-+
+
k, = (1.8 z t 0.5) x lo-’, cm3 molecule-’
TABLE 3: Chemical System and Rate Constants Used in the Numerical Modeling of the Reactions CH30 NO3 and CHtO2 NO?
0.71 0.30 1.48 2.17 2.13 2.15 2.50 1.82 3.19 2.56 3.30 3.06 3.47 3.00 3.58 2.89 3.85 2.14 4.90 1.90 5.14 2.77 6.19 2.66 6.82 1.75 8.71 2.18 mean values
kl (lO-”)a
0.85 < 1.03 < 1.25 1.03 < 1.41 < 1.93 1.67 < 2.38 < 3.40 1.46 < 1.96 < 2.63 1.11 < 1.38 < 1.72 0.88 < 1.23 < 1.72 1.32 < 1.67 < 2.11 2.70 < 2.84 < 2.99 1.38 < 2.09 < 3.16 1.60 < 1.96 < 2.35 1.09 < 1.45 < 1.86 1.96 < 2.22 < 2.51 1.91 < 2.10 < 2.31 1.45 < 1.76 < 2.13 (1.82 i 0.48) x lo-’?
a Units for concentrations are molecule cm3 molecule-’ SKI.
CH,O,
k2
0.65 0.23 1.57 0.69 0.81 0.72 0.40 0.66 1.25 0.63 0.39 0.57
< 0.88 < 0.97 < 0.88 < 3.31 < 2.79 < 4.96 < 1.31 < 2.46 < 1.44 < 2.57
