J. Phys. Chem. 1995, 99, 12829-12834
A Temperature-Dependent Kinetics Study of the CH302 Ionization Mass Spectrometry
12829
+ NO Reaction Using Chemical
Peter W. VillaltaJ L. Gregory Huey,t and Carleton J. Howard* Aeronomy Laboratory, NOM, Environmental Research Laboratories, Boulder, Colorado 80303 Received: April 12, 1995; In Final Form: June 15, 1 9 9 P
+
The rate constant of the CH3O2 NO gas-phase reaction was measured over the temperature range 199-429 K using chemical ionization mass spectrometry detection of the CH3O2 reactant. The temperature-dependent expression for the rate constant was determined to be k(T) = (2.8 f 0.5) x lo-', exp((285 f 60)/T) cm3 molecule-' s-I. The 298 K rate constant. k = (7.5 f 1.3) x lo-', cm3 molecule-' s-I, agrees well with previous results.
Introduction The methylperoxy radical, CH3O2, is an important atmospheric species formed by the oxidation of methane in the atmosphere.
+ OH - CH, + H,O CH, + O('D) - CH, + OH CH, + C1- CH, + HC1 M CH, + 0, -.CH302 CH,
(1) (2)
(3) (4)
The ozone budget in the troposphere is linked to the reaction of CH3O2 and NO through the following reaction sequence.
+
-
+ NO, NO, + hv (1 < 380 nm) -NO + 0 M 0 + 0, - 0, CH302 NO
CH30
(5)
(6) (7)
CH3O2 is also consumed in the troposphere by reactions 8-10 shown below. M + NO, CH,O,NO, CH,O, + HO, - CH,OOH + 0, CH302+ RO, - products
which agree within their experimental uncertainties. The currently accepted ~ a l u e ' ~of, ' ~k298K = 7.7 x lo-', cm3 molecule-' s-I is the average of these. In addition, the laserinduced fluorescence studiesl03' found that the reaction of CH3O2 and NO leads predominately to the production of NO2 and CH3O. While the room temperature rate constant and the products of the CH3O2 and NO reaction are well-known, conflicting results have been observed in the two studies which investigated the temperature dependence of the rate constant. Using laserinduced fluorescence detection of NO,, Ravishankara et al.Io measured the rate constant over the range 240-339 K and concluded it was independent of temperature. In contrast, the study by Simonaitis and Heicklen5 using W absorption detection of CH3O2 found a significant temperature dependence over the temperature range 218-365 K and reported k(T) = (2.1 f 1) x lo-', exp((380 f 250)/T) cm3 molecule-' s-'. In the present study, our primary objective is to use chemical ionization mass spectrometry with a variable temperature flow tube reactor to measure the rate constant for reaction 5 over a wide temperature range. A secondary objective of the study is to develop a general detection method for peroxy radicals using chemical ionization mass spectrometry with positive ions.
'
Experimental Section Apparatus. A schematic of the chemical ionization mass
spectrometer is shown in Figure 1. It consists of a neutral flow tube reactor coupled to an ion flow tube/quadrupole mass, spectrometer commonly referred to as a flowing afterg10w.I~ (9) The flowing afterglow part of our instrument has been described previou~ly,'~ and therefore the remainder of the apparatus will (10) be the focus of the description given here. Both the flowing afterglow t e c h n i q ~ eand ' ~ flow tube kinetic measurements using If the production of 0 3 in the troposphere is to be correctly chemical ionization mass spe~trometry'~,'~ have been described evaluated, the rate constants for reactions 5 and 8-10 must be previously. accurately known over the temperature range from approxiThe neutral flow reactor is a 120 cm long x 2.54 cm i.d. mately 215 to 300 K. While the rate constant for reaction 5 is Pyrex tube. It runs parallel to the ion flow tube and joins the known with relative certainty at room temperature, its variation flowing afterglow through a right angle Pyrex valve 50 cm from over the temperature range of the troposphere is not. the downstream end of the ion flow tube. This valve is throttled Molecular modulation spectroscopy,2 UV a b ~ o r p t i o n , ~ - ~ to control a neutral flow tube pressure (1-10 Torr) which is photoionization mass ~pectrometry,~ electron impact mass significantly greater than the ion flow tube pressure (0.40-0.80 spectr~metry,~-~ and laser-induced have all been Torr). Reactant gas is delivered to the flow tube through a 120 used to directly examine the kinetics of reaction 5. Several of cm long x 0.64 cm 0.d. Pyrex injector. The position of the these s t ~ d i e s ~ , ~ -reported ~ . ~ - ~room temperature rate constants injector in the flow tube can be varied along the 60 cm reaction zone which is the downstream half of the flow tube. The ' Cooperative Institute for Research in Environmental Science, University midpoint pressure of the reaction zone is measured by a of Colorado, Boulder, CO. Abstract published in Advance ACS Abstracts, August 1, 1995. differential capacitance manometer through a 0.64 cm 0.d. port. CH30,
(8)
@
0022-365419512099-12829$09.00/0 0 1995 American Chemical Society
Villalta et al.
12830 J. Phys. Chem., Vol. 99, No. 34, 1995
Neutral Flow Tube Mechanical Booster Pump
Fluid
I
T1Cs.WG
Gut
Mass Filter Ion Multiplier
Reactant Gari
I
I
Elccmn Impact Ion Source
Turbo Pumps
Ion Detection Chamber
Reagent Ion Recursor Gas
Ion Flow Tube
Figure 1. Experimental apparatus showing the separate neutral and ion flow tubes and the quadrupole mass spectrometer used for ion detection.
The reaction zone is jacketed to allow for cooling and heating by passing cooled methanol or heated ethylene glycol through the jacket. Chromel-alumel thermocouples monitor the temperature of the cooling and heating fluids at the inlet and outlet of the flow tube jacket. The fluid flow rate is sufficient to keep temperature differences between the inlet and outlet to 5 1 K. A 1.27 cm 0.d. port for the introduction of radicals is located 6 cm upstream of the variable temperature reaction zone. He carrier gas enters the flow tube through a 0.64 cm port 35 cm upstream of the reaction zone. The temperature of the carrier gas inside the flow tube is not measured; however, it has been in the past in this laboratory under similar conditions. It was found to differ by < l K from the temperature of the cooling/ heating fluid in the flow tube jacket within 5 cm of the beginning of the variable temperature zone. This is consistent with heat transfer calculations which show rapid heat transfer due to the high thermal conductivity of He. To minimize the effects of heterogeneous reactions, the inside of the Pyrex valve is coated with halocarbon wax, and the neutral flow tube is fitted with a 2.23 cm i.d. Teflon sleeve which runs from 4 cm upstream of the radical inlet to the downstream end of the flow tube. Holes are cut in the Teflon sleeve to accommodate the ports for the radical source and pressure measurement. The flowing afterglowI4 consists principally of a flow tube, an electron impact ion source, and a quadrupole mass filtedion multiplier. The 130 cm long x 7.30 cm i.d. stainless steel flow tube is coupled axially through a 0.5 mm diameter pinhole to the quadrupole mass filtedion multiplier detector via two differentially pumped stages. A large flow of He carrier gas (100 STP cm3 s-I) is pumped through the ion flow tube by a 500 L s-' mechanical booster pump. The electron impact ion source is at the upstream end of the flow tube and consists of a negatively biased thoriated-iridium filament surrounded by a piece of grounded copper mesh. The ions used for chemical ionization, the reagent ions, are produced by introducing the source gas into the ion flow tube either immediately upstream or downstream of the electron impact source. The ion source bombards the carrier gas with 5-75 pA of low-energy (20200 eV) electrons to produce ionization by e- impact or eattachment processes. The reagent ions travel down the flow tube to the point where the gas stream from the neutral flow tube enters. In the remaining 50 cm of the ion flow tube, a small fraction of the reagent ions react with the reactant andor product molecules from the neutral flow tube, generating new ions in proportion to the concentrations of the reacting neutral molecules and the ion-molecule reaction rate coefficients. At
the end of the ion flow tube, a portion of the ions pass through the pinhole which separates the ion flow tube from the detection region of the flowing afterglow, are mass-selected by the quadrupole mass filter, and are detected by the ion multiplier. Radical Generation. The CH3O2 radicals are generated by flowing a CH3CH20N0/02/He gas mixture through a heated quartz tube. The corresponding reaction sequence is as the following: CH,CH20N0 A CH, CH,
+ CH,O + NO
M + 0, CH,O,
(11)
(4)
The CH3CH20NO is synthesized from ethanol and sodium nitrite following a literature procedure'* and purified by trapto-trap distillation. It is stored in the dark and used direc!ly from a trap at 196 K by eluting with UHP He. The radical source consists of a 20 cm long x 1.27 cm 0.d. quartz tube with 0.025 cm diameter platinum wire wound -30 times around a 5 cm long section of the tube. The high temperature of the radical source is generated by electrically heating the platinum wire. A chromel-alumel thermocouple is placed under the heater and in contact with the quartz tube at the midpoint of the heated region. It is insulated from the platinum wire by a thin piece of ceramic fiber insulation. A 0.64 cm thick sheath of ceramic fiber insulates the heated section. A temperature controller along with a variable transformer and the thermocouple are used to regulate the temperature at 973 K. To minimize loss of radicals on the glass surface of the radical source, a Teflon sleeve is used to cover the wall of the radical source inlet into the flow tube, and halocarbon wax was applied from the beginning of the sleeve to within 3 cm of the heated region. The heated region itself was not covered or coated. Experimental Conditions. He (99.9999%) flows of 1822 STP cm3 s-' and flow tube pressures of 2.0-5.5 Ton are used in the neutral flow tube. These conditions result in flow velocities of 600-2600 cm s-l. NO flows ranging from 4.2 x to 3.3 x lo-, STP cm3 s-I enter the flow tube through the movable injector and result in NO concentrations of 2.3 x 10"-8.6 x lo1*molecules cm-3 in the neutral flow tube. The NO is delivered to the flow tube in two ways, either as pure NO or out of Pyrex bulbs containing NO/He mixtures of -0.1 0.2% NO, The pure NO flows are determined by flowing the NO into a calibrated volume and measuring the pressure change
Kinetics Study of the CH3O2
+ NO Reaction
J. Phys. Chem., Vol. 99, No. 34, 1995 12831
for a given time interval. The NO/He mixture flow rates are measured with calibrated mass flowmeters. The concentrations of the NO bulbs are determined by comparing the NO+ signals (from the NO 0 2 + charge transfer reaction) when using the NO/He mixtures and known flows of pure NO. In both cases, the NO (299.0%) used is passed under high pressure through a dry ice cooled silica gel trap to reduce the amount of nitrogen oxide impurities present. The radical source temperature is maintained at 973 K. The CH3CH20NO reservoir is kept either at 196 K, in which case a needle valve is placed downstream to reduce and regulate the flow of the CH3CH20NO/He mixture into the radical source or at 186 K, in which case no needle valve is used and the mixture flowed directly into the radical source. The flow of He through the reservoir is typically 0.2-0.5 STP cm3 s-l, and at times an additional 0.5-1.0 STP cm3 s-l flow of He is added after the reservoir and before the radical source to optimize the production of CH302. A 1.0 STP cm3 s-l flow of 0 2 (299.99%) is added to the source reactor and results in 0 2 concentrations in the radical source of 1 x 10l6-1 x 10'' molecules ~ m - ~The . rate constant for the CH3 0 2 reaction at 300 K and pressures of 2.0-5.5 Torr is -(2-4) x cm3 molecule-' s-I. Under these conditions, '95% of the CH3 is converted to CH302 before it enters the reaction zone of the neutral flow tube. The CH3O2 concentrations are 51.2 x 10" molecules cm-3 with a typical value of 5 x 1o'O molecules ~ m - ~The . procedure for estimating these values is described in the next section. The 0 2 and total He flows are measured with calibrated mass flowmeters. The flowing afterglow operates with a He flow of 100 STP cm3 s-' and at a pressure of 0.5 Torr. The He (99.5%) is passed through a liquid nitrogen cooled zeolite filled trap to reduce impurities. The filament of the electron impact ion source is typically biased at -150 eV with a 50 p A emission current. Several STP cm3 s-I of 0 2 (199.99%) is added just downstream of the filament to produce the 0 2 + reagent ions. Ion Detection Scheme. The reagent ion used in the experiments described here is 0 2 + . Signals at mle 47 and mle 15 are observed in the mass spectrum upon generation of CH3O2 in the neutral flow tube and 0 2 + in the ion flow tube. The signal at mle 47 is assigned as CH3O2+ on the basis of its dependence on heating the radical source and on the addition of 0 2 and CH3 precursor (CH3CH20NO) to the radical source as well as its depletion upon addition of NO to the neutral flow tube. The signal at mle 15 is likewise assigned to CH3+. A large mle 15 signal remains upon elimination of the 0 2 flow. This is attributed to the charge transfer reaction of 0 2 ' with the CH3 present in the neutral flow tube in the absence of 0 2 . The ion-molecule reactions which explain these assignments are as follows:
reaction for reaction 12 cannot be directly determined because the heat of formation of CH302+ is not known. Although the ionization potential of CH3O2 is not known, it was bracketed by examining the charge transfer reaction of CH302 with N02+ and Br2+. The charge transfer reaction is observed to occur with Br2+ ( I P B = ~ ~10.51 eVlZ0but not with N02+ (IPNo~= 9.75 eV)?O From this observation, the IP of CH302 is most likely 510.51 and 29.75 eV. This bracketed IP along with the heat of formationI2of CH302 determines a heat of formation for CH3O2+ of AHf = 238 f 10 kcal mol-' and heat of reaction for reaction 12 of AH: = -44 f 10 kcal mol-'. The mle 15 signal generated by reaction 13 is used as the signature of CH3O2 in the experiments described here. The signal at mle 47 generated by reaction 12 is not used because of significant background at this mass when trying to follow the decay of CH3O2 while adding NO. This background is attributed to the production of the isotopomers of N02+ at mle 47 (15N02+and NOI7O+)by the charge transfer reaction of 0 2 + NO2 where the NO2 is a product of reaction 5 and also an impurity in the NO. This interference can be corrected for by monitoring N02+ (mle 46), calculating the contribution of the NO2 isotopomers at mle 47 by using the isotope abundances of I5N (0.37%) and I7O (OM%), and subtracting this quantity from the mle 47 signal for each data point. While this procedure will allow for the use of the signal at mle 47 to follow the decay of CH3O2, the monitoring of mle 15 is more straightforward, and there are no interferences at mle 15 due to other ions while possible interferences from other ions exist for mle 47. The approximate CH3O2 concentration in the neutral flow tube is determined by estimating the CH3O2 flow from the radical source. The CH3O2 flow is proportional to the CH3+1 0 2 + signal ratio, and this proportionality is determined by depleting the CH3+ signal by adding a known quantity of NO and measuring the resulting NO+ and N02+ signals. From the quantity of NO added and the rate constantsIg for the charge transfer reactions of 0 2 + with NO and NO2, the quantity of NO2 produced can be estimated and used as a measure of the CH302 present initially.
+ -.CH302+-4- 0, -25% CH302+ 02+-CH3+ + 20, -75% CH, + 0; - CH,' + 0,
d[CH302]ldt= -k:[CH302]
(1)
ln[CH302]= -kit -I- c
(11)
+
+
CH302 0,'
+
(12)
(13)
(14)
The CH302 0 2 + branching ratio estimated above is simply a measure of the relative heights of the mle 15 and 47 peaks and does not take into consideration the effects of mass discrimination. Allowing for typical mass discrimination for the quadrupole mass filter, ranges of 25-50% and 50-75% can be estimated for reactions 12 and 13, respectively. The standard heats of reaction for reactions 13 and 14 can be calculated to be -21 and -52 kcal mol-', respectively, from the heats of formation'2s20of the molecules involved. The standard heat of
+
Results The reaction of CH3O2 and NO was studied at temperatures ranging from 199 to 429 K. The methylperoxy radicals were generated in the radical source at a fixed position on the flow tube. The NO was added through the movable injector in large excess so that the reaction kinetics were pseudo-first-order in CH3O2. The NO concentrations were 2 8 times the CH302 concentrations. The variation of CH3O2 concentration with reaction time is descriljed by eqs I and 11.
where k: = ks[NO] - k,, c is a constant, ks is the bimolecular rate coefficient for reaction 5, and k, is the rate coefficient for loss of CH302 on the wall of the movable injector. The wall loss on the injector enters because the amount of injector surface exposed to the radical stream changes as the injector is moved. The first-order rate coefficient k, is determined by measuring the CH3O2 decay when [NO] = 0 and is found to be small with values of 5 2 s-l. The reaction time is varied by changing the NO injector position in the reaction zone of the flow tube. The reaction time is related to the injector position by the simple equation t = dv where t is time (s), z is distance from the tip of the injector
Villalta et al.
12832 J. Phys. Chem., Vol. 99, No. 34, 1995
loooo
1.8e-11
1
1.6e-11
[NO]=O
i
m
3
/
0.
7.0e-12
A
I
i
0
0
5.0e-12
0
5
10
15
20
30
25
Relative Reaction Time
~
2.0
s)
,
,
2.5
3.0
, 3.5
4.0
4.5
5.0
5.5
100OR (K-l)
Figure 2. Typical semilog plot of CH3+ signal vs. relative reaction time. Conditions were T = 298 K, v = 1630 cm s-', P = 2.70 Torr, and [NO] = 0-6.67 x 10l2 molecules ~ m - NO ~ . concentrations are listed to the right of each decay with units of molecules ~ m - ~ . 70
Figure 4. Arrhenius plot of data from present study (O), along with data from Ravishankara et al. (ref 10, A) and from Simonaitis and Heicklen (ref 5, m). Data points from the present study are listed in Table 1, and those for the other two studies are listed in Table 2. The line is a nonlinear least-squares fit of the data from the present study giving k ( T ) = (2.8 f 0.5) x lo-'' exp((285 f 60)IT) cm3 molecule-' S-1.
60 50
20 10 0
0
1
2
3
[NO] (1 0
4
5
6
7
8
9
molecule c d 3 )
Figure 3. Plot of k: vs NO concentration for temperatures of 200 (B), 298 (O), and 429 K (A).The 298 K values are from nonlinear leastsquares fits of the decays shown in Figure 2. Linear least-squares fits of the data shown give rate constant values of (12.40 f 0.49) x lo-'', (7.96 rt 0.23) x lo-]', and (5.84 f 0.30) x cm3 molecule-' s-', respectively, where the error bars are the 95% confidence level of the fits.
to the end of the reaction zone (cm), and v is average gas flow velocity (cm s-l), which is determined by the total gas flow rate and flow tube temperature, pressure, and cross-sectional area. Thus, the reaction time is determined by the injector position and the flow velocity. The concentration of CH302 is monitored via the m/e 15 signal resulting from reaction 13. For a given temperature, a series of these decay plots are taken at different NO concentrations. A series of typical experiments are shown in Figure 2 , where the log(m/e 15 signal) is plotted versus the relative reaction time on a linear scale. A nonlinear least-squares fit of a particular decay determines k: for the values corresponding NO concentration. In Figure 3, the from Figure 2 are plotted against their NO concentrations along with data from measurements made at 200 and 429 K. The slopes of the linear least-squares fits to these plots are the bimolecular rate constants (k5(T)) for the reaction of CH302 and NO at temperature T. The k: axis intercepts of these fits determine k,. The values of k, in this study did not deviate significantly from zero. The bimolecular rate constants for a series of different temperatures measured in the present study are listed in Table 1 and shown in an Arrhenius plot in Figure
4. A nonlinear least-squares fit to our data gives k ( 0 = (2.8 f 0.5) x 10-l2 exp((285 f 60)/ncm3 molecule-' s-l, where the error bars represent the 95% confidence level of the fit plus a factor of f 1 0 % for possible systematic error. Fitting the Arrhenius equation with and without weighting for precision of individual rate constants makes little difference because the rate constants have similar uncertainties. The quoted parameters are obtained with a fit which uses no weighting. The f10% factor for systematic error is incorporated into the A factor. The uncertainty on the Arrhenius A coefficient represents our estimated accuracy of the calculated rate constants for the temperature range of the study. An average of our measurements of k5 at 298 K gives k298K = (7.5 f 1.3) x cm3 molecule-' s-'. The error bar is the sum of -7% random error and 10% possible systematic error. The random error is determined by the propagation of the uncertainties of the total flow (2%),temperature (l%), reactant flow (4%),total pressure (l%), flow tube radius (l%), and slope of the decay plots (5%). The uncertainty in the slopes from the linear least-squares fits are typically about f3%.
Discussion The room temperature rate constant (ks) found in this study is listed in Table 4 together with previously reported values. Our value agrees well with the recommended value of k298 K in the JPL NASA Evaluation.I2 The recommendation is the average of the values from refs 2,4,5, and 9- 11, each of which agrees with the present study within the error bars listed as do the values reported in refs 6 and 8. This is not the case for refs 3 and 7 . It has been suggested4 that ref 3 was probably in error because of interference from CH3ONO formation. The bimolecular rate constants measured in the previous temperature dependence studies5.I0are shown in Table 2 and are plotted with our measurements in Figure 4. Our Arrhenius parameters are shown in Table 3 together with the currently recommended valuesI2 and the previously reported values of Simonaitis and Heicklen5 and Ravishankara et a1.I' The parameters found in the study by Ravishankara et a1.I0 show a very small temperature dependence, and the authors reported that k5 was independent of temperature due to the uncertainties in the individual k(T) values. The deviation of their parameters
Kinetics Study of the CH302
+ NO Reaction
J. Phys. Chem., Vol. 99, No. 34, 1995 12833
+
TABLE 1: CHJOZ NO Temperature Dependence Data temp (K)
no. of exps
NO source"
199 200 202 223 223 248 248 273 298 298 298 298 298 298 298 329 372 394 410 429
6 7 6 6 8 7 7 6 6 7 4 6 7 10 6 6 7 5 7 7
bulb 1 PRC bulb 1 bulb 1 PRC PRC bulb 1 bulb 1 PRC PRC PRC bulb 1 bulb 1 bulb 2 bulb 2 bulb 1 PRC bulb I PRC PRC
press. (Torr)
flow velocity (cm s-l)
5.30 1.95 4.80 4.41 2.5 1 2.70 4.55 5.40 2.64 2.70 5.81 5.35 4.45 2.55 2.84 4.97 2.90 4.90 2.24-5.33 2.60
620 1760 715 725 1410 1410 800 830 1840 1630 780 895 1040 1830 1670 1050 2020 1180 1100-2630 2620
[NO] range (1012 molecules ~ m - ~ ) 0.5-2.6 1.3-5.3 0.5-2.3 0.6-3.0 2.4-8.3 1.3-5.7 0.6-3.6 0.8-2.4 1.5-8.4 1.2-6.7 1.4-3.6 0.8-4.2 1.4-4.1 0.5-6.8 1.1-7.8 0.3-3.4 1.6-6.2 1.1-4.3 2.8-6.6 1.2-8.6
ksb
cm3molecule-' s-I) 11.00f0.21 12.40 f 0.49 12.30 f 0.38 9.76 f 0.28 10.11 f 0.27 9.09 f 0.30 8.79 f 0.14 7.40 f 0.13 7.23 f 0.24 7.96 f 0.23 7.94 f 0.48 7.04 f 0.18 7.59 f 0.20 7.06 f 0.27 6.88 f 0.10 6.33 & 0.35 5.93 f 0.19 5.49 f 0.30 5.92 f 0.23 5.84 f 0.30
PRC indicates that the NO flow rate is determined by pressure rate of change fiom a pure NO sample. Bulb 1 and bulb 2 indicate that the NO flow came from mixtures of 0.0944% and 0.242% NO in He, respectively, where the mixture flows were measured with a calibrated mass flowmeter. See experimental conditions section for a discussion of the mixtures concentration determination. Errors are 2a from linear least-squares fits of k: vs [NO] plots.
TABLE 2: Results from Previous Temperature-Dependent Studies of the CH302 NO Reaction
+
TABLE 4: Comparison of Room Temperature Rate Constant Values for CH302 NO Reaction
+
k5 (10-l2 cm3 molecule-' s-I) 7.5 f 1.3
k",b
study Simonaitis and Heicklen (ref 5)
Ravishankara et al. (ref 10)
temp (K)
press. (Torr)
(10-12 cm3 molecule-' s-I)
218 218 296 296 296 365 240 250 270 298 298 298 339
-200 -600 s 100 -300 -600 -200 40 40 40 40 100 50 40
13.5 f 1.4 17.0 f 2.2 7.7 f 0.9 7.1 f 0.3 8.2 f 0.9 6.3 f 0.5 8.4 f 1.1 8.60 f 0.27 9.03 f 0.35 8.01 f 0.21 7.5 f 0.55 7.80 =k 0.27 7.80 f 0.92
Error bars for the Simonaitis and Heicklen study are one standard deviation. Error bars for the Ravishankara et al. study are two standard deviations and are measures of the precision of their data.
TABLE 3: Comparison of Arrhenius Parameters for the CH302 NO Reaction temp A range (lo-'* cm3 reference method" (K) molecule-' s-]) EIR ( K )
+
this work FT-CIMS 199-429 Simonaitis and FP-UVA 218-365 Heicklen (ref 5) Ravishankara et al. LP-LIF 240-339 (ref 10) recom valueb (ref 12)
2.1 f 1
-(285 f 60) -(380 f 250)
6.3 h 2.5
-(86 f 112)
2.8 f 0.5
4.2
-(180 f 180)
a FT-CIMS = flow tube-chemical ionization mass spectrometry, FP-UVA = flash photolysis-ultraviolet absorption, LP-LIF = laser photolysis-laser-induced fluorescence. The recommended values were derived from a least squares fit to the data from the studies of Simonaitis and Heicklen5 and Ravishankara et a1.I0
from those presented here is most likely due to the small temperature range (240-339 K) of their study and the uncertainty (-20%) associated with their individual k(T) values. Figure 4 shows that their individual points fall close to those of the present study. The Arrhenius parameters reported by
reference this work recom value (ref 12)b
method" FT-CIMS
7 6 11 10 9 5 2 4 8 3
LP-PIMS PR-UVA LP-UVLA LP-LIF DF-EIMS FP-UVA MMS FP-UVA DF-EIMS FP-KS
7.7
11.2 f 1.4 8.8 f 1.4 7 f 2 8.1 f 1.6 8.6 f 2.0 7.7 f 0.9 6.5 f 2.0 7.1 f 1.4 8.0 f 2.0 3.0 f 0.2
a FT = flow tube, PR = pulse radiolysis, LP = laser photolysis, DF = discharge flow, FP = flash photolysis, CIMS = chemical ionization mass spectrometry,PIMS = photoionization mass spectrometry,W L A = ultraviolet laser absorption, EIMS = electron impact mass spectrometry, UVA = ultraviolet absorption,MMS = molecular modulation spectrometry, KS = kinetic spectroscopy. Value is the average of the values found in refs 2, 4, 5 , and 9- 11.
Simonaitis and Heicklen5 agree with those reported here within their relatively large error bars. Their EIR value is slightly more negative than the value found in the present study. An examination of Figure 4 indicates that this is primarily due to their two points taken at 218 K which are significantly higher then the data of the present study. Greater CH3ONO formation might be suspected to interfere at lower temperatures, especially with the measurement made at 2 18 K and -600 Torr; however, CH30NO formation results in low measured rate constants as proposed4 in the case of ref 3. The recommended values'* listed in Table 3 were derived from a least-squares analysis of the data from the studies of Simonaitis and Heicklen5 and Ravishankara et a1.I0 Due to the relatively large temperature range, large number of data points, and improved precision of the data, the Arrhenius parameters of the present study are in our opinion more reliable than the previously recommended va1ue.I2 It should also be pointed out that temperature-dependent studies of the C2H5O2 NO and CH$(0)02 NO reactions have not been carried out, and their EIR values have been recom-
+
+
12834 J. Phys. Chem., Vol. 99,No. 34, 1995
+
Villalta et al.
+
mended'* by analogy with the CH302 NO and RO2 NO reactions to be 0 f 300 and 0 f 200 K, respectively. In view of the Arrhenius parameters found in the present study, the EIR values for the C2H5O2 NO and CH3C(0)02 NO reactions should be examined. We intend to study these reactions in the future. The slight negative temperature dependence of the CH3O2 NO reaction observed here is expected, since this type of temperature dependence is common for fast radical-radical reactions. For example, the following reactions
+
+
+
+ NO -CF30 + NO, CF,C10, + NO - CF,CIO + NO, CFCl,O, + NO - CFC1,O + NO2 CCl,O, + NO - CC130 + NO, CF,O,
(15)
(16) (17) (18)
have measured temperature coefficients of approximately -375, -470, -405, and -310 K, respectively.21 These values are similar to the value of -(285 f 60) K reported here for the CH3O2 NO reaction. This type of temperature dependence is qualitatively explained by the formation of a reaction intermediate, as shown below for the present case.
+
CH,O,
+ NO zz [CH300NO]* - C H 3 0 + NO,
(19)
This type of mechanism has been examined by Golden2, in terms of transition state theory. The analogous reaction intermediateformation has been postulated23for the case of H02 NO to explain the observed negative temperature dependence. Alkyl nitrate formation via ROZ NO reactions has been ~ b s e r v e dfor ~ ~R. ~groups ~ larger than methyl. The branching ratio for nitrate formation goes up with increased pressure, reduced temperature, and increased size and branching of the R group. It has been suggested26that methyl nitrate may be formed in the CH3O2 NO reaction by isomerization of the CH3OONO reaction intermediate. While this product channel is expected to be negligibly small at the low pressures used in the present study, the fraction converted to nitrate should increase with pressure. Although the fraction would be expected to be very small (
