Chemical ionization mass spectrometric studies of the gas-phase

Mechanism of the Atmospheric Reactions between Methylperoxy Radicals and NO. A computational Study. Agnie M. Kosmas , Zoi Salta and Antonija Lesar...
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3750

J. Phys. Chem. 1993, 97, 3750-3757

Chemical Ionization Mass Spectrometric Studies of the Gas-Phase Reactions CF3O2 NO, CF30 NO, and CFJO RH

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Thomas J. Bevilacqua,+David R. Hanson,t and Carleton J. Howard' Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303 Received: September 30, 1992; In Final Form: December 22, I992

W e have used a flow tube reactor coupled to a chemical ionization mass spectrometric (CIMS) detector to study the reactions of trifluoromethylperoxy (CF3O2) and trifluoromethoxy ( C F 3 0 ) radicals with NO and the reaction of CF3O with isobutane. W e have determined the rate coefficient a t 297 K for the reaction CF3O2 NO to be (1.53 f 0.20) X 10-l'cm3 molecule-' s-l (all uncertainties are for 95% confidence limits), in excellent agreement with two previous measurements. The use of the C I M S detection technique has allowed us to observe both CF30 and N O 2 as the products of this reaction. Modeling of a secondary reaction between CF30 and NO observed in these studies has yielded a n estimate of k = (2 f 1) X IO-" cm3 molecule-' s-l for this reaction, in which FNO was observed as a product. A relatively rapid reaction was also observed between C F 3 0 a n d isobutane, for which a rate coefficient of ( 5 f 3) X 10-l2cm3molecule-' s-I is estimated. This reaction was seen to proceed by hydrogen abstraction, yielding trifluoromethanol, CF3OH. A much slower H-abstraction reaction was observed between CF30 and methane. The significance of these reactions for the atmospheric fate of the trifluoromethoxy radical is discussed.

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Introduction It is now well established that atomic chlorine, transported to the stratosphere by a variety of particularly stable chlorinecontaining compounds, is responsible for the catalytic destruction of ozone in that region of the atmosphere.' As substitutes for one class of these stable compounds-the chlorofluorocarbons (CFC's)-various hydrochlorofluorocarbons (HCFC's) and hydrofluorocarbons (HFC's) are presently being considered. The essential advantageshared by the proposed substituteswith respect to their ozone depletion potentials is their incomplete halogenation. The carbon-hydrogen bond(s) present in HCFC's and HFC's are prone to attack by tropospheric OH radicals, a reaction which produces a water molecule and a haloalkyl radical. The environmental acceptability of these proposed replacement compounds depends on the nature of the stable species into which such radicals are converted by their reactions with other atmospheric components. To address this question, efforts are underway in many research laboratories to determine the rates and products of the likely atmospheric reactions of these radicals. The ultimate goal of these studies is the development of complete oxidation mechanisms that provide the elementary steps in their conversion to the final stable species. The trifluoromethyl radical, CF,, is expected to be an intermediate in the atmospheric oxidation of several of the compounds proposed as substitutes for CFC's.2-3 Under atmosphericconditions, the immediate fate of this radical (as for other alkyl radicals) is the addition of an oxygen molecule to form the corresponding alkylperoxy r a d i ~ a l : ~ . ~

CF,

+ 0, + M

-

+

CF302 M

(1)

Recent review& have dealt with the likely subsequent atmospheric reactions of the trifluoromethylperoxy radical, CF302; some of these reactions have been the subject of experimental investigation^.^-^ Two kinetic studies of the reaction of CF302 with N O have been reported in the last 10 years. In 1982, Plumb and Ryan,s using a flow tube sampled by a mass ~~

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~

~~~

~

~~~~

NOAA/NRC Postdoctoral Research Associate. 1 Postdoctoral Research Associate, Cooperative Institute for Research in Environmental Sciences. University of Colorado, Boulder, CO To whom correspondence should be addressed at NOAA, R/E/ALZ. 325 Broadway, Boulder, C O 80303. +

spectrometer, measured the loss of CF302 and determined a roomtemperature rate coefficient for reaction with N O of (1.78 0.36) X 10-l'cm3 molecule-' s-l. Subsequently, Dognon et aL9 investigated the temperature and pressure dependence of this reaction, using flash photolysis and time-resolved mass spectrometry. They found no pressure dependence in the range of 2-10 Torr and only a weak, negative temperature dependence from 230 to 430 K. At 298 K, the rate coefficient was determined to be (1.45 f 0.2) X lo-" cm3 molecule-I s-I. In both of these studies the concentration of the peroxy radical was monitored mass spectrometrically. Electron impact ionization produced the CF202+ion, and the intensity at m / e 82 was used as a measure of the concentration of CF302. While Plumb and Ryan made no attempt to observe reaction products, Dognon et al. observed the production of NO2 in the photolysis of their CF3I - 02 - N O mixtures. These latter workers reported a yield of NO2 of 1.5 f 0.5, based on measurements of the production rate of NO2 and the loss rate of their CF31radical p r e c ~ r s o r . They ~ used this observation, the lack of pressure dependence for the reaction and the failure to detect any signal corresponding to CF302NO or CF30N02asstrong evidence that the reaction proceeds principally via the channel

*

CF302+ NO

-

CF,O

+ NO,

(2) Unfortunately, their detection method did not permit the observation of the CFjO product or the simultaneous direct measurement of both a reactant and a product. The atmospheric fate of C F 3 0is considerably more uncertain than that of CF302.2.3Whileother haloalkoxy radicals are known to eliminate a chlorine atomlo or believed to have a hydrogen atom abstracted by 02?.3 available data on the heat of formation of C F 3 0 (AHf(CF30) = -157 kcal mol-')ll indicate that Fatom elimination is endothermic by -23 kcal mol-' and that formation of F 0 2(AHf = 6.2 kcal mol-1)12 via F abstraction by 0 2 is 10 kcal mol-' endothermic.'3 Ab initio theoretical calculations of the F elimination pathway by Li and Franciscoi4have indicated not only an endothermicity of 25.2 kcal mol-' but also an energy barrier of 29.1 kcal mol-', in good agreement with the activation energy of 3 1.O f 0.5 kcal mol-' estimated by Kennedy and Levyls from their experimental data. In 1989, the report of the Alternative Fluorocarbon Environmental Acceptability Study (AFEAS) noted that the detection and identification of the

0022-365419312097-3750%04.00/0 0 1993 American Chemical Society

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Reactions CF302

+ NO, CF3O + NO, and C F 3 0 + RH

reactions of C F 3 0 were some of the key remaining unknowns in the determination of the fates of the halocarbons under consideration as alternatives to CFC’s.I6 Since then, only one report of the detection of this species has appeared. In 1991, Li and Francisco,” following infrared multiphoton pumping of bis(trifluoromethyl) peroxide (CF300CF3, BTMP), detected a transient laser-induced fluorescence signal which they attributed to the CF30 radical. They also reported that the lifetime of the transient species grew shorter with increasing concentration of N O added to the gas mixture and cited evidence for the production of both NO2 and FNO. This report seems to represent the first experimental evidence relevant to the atmospheric fate of the C F 3 0 radical. The development of an alternative detection method and additional investigations of possible atmospheric reactions of this radical are clearly needed. To date, most investigations of alkylperoxy radicals have employed either ultraviolet absorption18or mass spectrometric8,9 detection methods. Absorption spectroscopy has the limitation of being relatively insensitive. Therefore, most applications of this technique have been limited to determination of peroxy radical absorption spectra and to studies of peroxy self-reaction kinetics. Mass spectrometry has been used in the studies noted above and in other investigations of h a l o a l k y l p e r ~ x yradical ~ ~ ~ ~ kinetics. The electron-impact ionization sources used in these experiments cause fragmentation; consequently, the parent ion is not seen in the mass spectrum. The development of an alternative ionization method, which provides both sensitive and selective detection of peroxy radicals, would aid further study of their kinetics. Chemical ionization mass spectrometry (CIMS) has been used in this laboratory for over a decade to detect a variety of radicals and stable compounds.20-25 In this detection method, the neutral compound of interest is ionized via an ion-molecule reaction with somesuitable reagent ion. Therelativegentlenessof this ionization method often allows for the detection of the neutral of interest as its parent ion, greatly facilitating assignment of the mass spectrum. Furthermore, this detection scheme has proven to be both sensitive and highly selective. For example, in one recent study,25both N ~ O and S ClONO2 were detected in the presence of H N 0 3 . This was possible because the two species of interest react readily with I- to give NO3-, while H N 0 3 undergoes no such reaction. The importance of haloalkylperoxy and haloalkoxy radicals in the atmospheric oxidation of halocarbons has led us to develop CIMS schemes for their detection. We have chosen the trifluoromethylperoxy and alkoxy radicals for this work because they are the simplest in the group, because they are degradation products of several of the proposed alternative compounds, and because the mechanism of C F 3 0 degradation is the least clear. Our goals were to remeasure the ratecoefficient k2 for the reaction of CF302 with N O and to definitively identify CF30as a product of that reaction. Furthermore, we wished to investigate some likely atmospheric reactions of the trifluoromethoxy radical.

Experimental Section Apparatus. The basic flow tube/CIMS apparatus used for these studies has been described in detail p r e v i o u ~ l y . ~For ~ -this ~~ work, the glass flow tube had an i.d. of 2.54 cm and a length of -2 m. To minimize heterogeneous effects, the tube was fitted with a Teflon sleeve of 2.2-cm i.d. which spanned the tube from a point 9 cm upstream of the radical inlet to - 8 cm from the downstream end of the flow tube. Holes in the sleeve afforded access to the radical inlet and pressure gauge ports. Helium carrier gas (3-15 STP cm3 s-I, STP being 273 K and 1 atm) entered the flow tube 30 cm upstream of the radical inlet port. A mixture of 2-3% N O in helium (0.005-0.05 STP cm3 s-I), further diluted in a carrier stream (0.2 to 0.5 STP cm3 s-I) of helium entered the flow tube through a movable inlet, whose position could be varied over a range of approximately 50 cm,

The Journal of Physical Chemistry, Vol. 97, No. IS, 1993 3751 beginning from a point 2 cm downstream of the radical inlet. Kinetic studies were carried out under a variety of carrier flow velocities (800-2400 cm/s) and flow tube pressures (0.8-2.0Torr). Reagent flow rates were measured with calibrated mass flow meters. All data were taken at 297 f 2 K. The effluent of this glass flow tube passed through a glass/ Kel-F valve into a stainless steel flowing afterglow reactor. In this ion flow tube, small flows of reagent ion precursors (either SF6 or 02 in this work) were diluted in a large excess of helium (typically 100 STP cm3 SKI). The reagent ions were generated by flowing this He/SF6 or He/02 mixture past an electron impact ionizer operated at 45 V and 10 PA. The plasma was carried downstream some 60 cm to the point at which the gas from the neutral flow tube enters. A small fraction of the reagent ions react with the incoming neutrals, creating new ions in proportion to the concentrations of the reacting neutral species and to the rate coefficients of the corresponding ion-neutral reactions. A portion of the ionized gas is sampled by an on-axis pinhole, after which it enters a quadrupole mass filter and an ion multiplier. Radical Generation. The CF3 radicals were generated by pyrolysis of one of three precursor compounds in a quartz sidearm connected to the main flow tube. The precursor was diluted in a stream of helium (- 1-2 STP cm3 SKI), and the pyrolysis cell was held at -730 OC. In most of the studies described here the precursor was CF31(iodotrifluoromethane), which has beenshown to yield CF3 upon C02 laser irradiation2, or shock tube heati11g.2~ Someof the kineticdata wereobtained using CF3CHtONO (2,2,2trifluoroethyl nitrite) or (CF,C0)20 (trifluoroacetic anhydride) as the precursor. Trifluorcethyl nitrite thermolysis has been shown previously to yield CF3 radicals.28 While UV photolysis of trifluoroacetic anhydride has been used to generate trifluoromethyl radi~als,2~+30 an investigation of its pyrolysis between 216 and Our studies of 320 O C found no evidence of CF3 productiot~.~~ the pyrolysis of dilute, low-pressure flowing mixtures of (CF3C 0 ) 2 0in helium indicate that these conditions lead to the ample production of CF3 from this molecule. Oxygen (-0.3 STP cm3s-I) was added to the furnace effluent 11 cm upstream of the main flow tube, providing sufficient interaction time (-20 ms) to ensure complete conversion of CF3 to CF3O2 before the stream entered the main flow tube. The concentration of trifluoromethylperoxy was estimated by assuming that at high [NO] and long reaction times CF3O2 was completely converted to NO2. The concentration of NO2produced under these conditions was estimated by using 0 2 + to ionize both N O and NO2: by knowing [NO] and the approximate relative rate coefficients of the reactions of 02+with the two nitrogen oxides,3* [NO21 was estimated from the relative ion signals at m / e 46 and 30 amu. The typical concentration of trifluoromethyl peroxy radicals in the flow tube was thus estimated to be < l o ” molecule cm-). To ensure pseudo-first-order decay of this radical, [NO] in the flow tube was at least 7.3 X loll molecule ~ m -and ~, in some experiments it was as high as 5.8 X l o f 2molecule ~ m - ~ . CIMS Detection Reactions. For most experiments described below, SF6-was used as the ionizing reagent. This ion is known to react with NO2 by charge transfer33and with CF2O via fluoride ion transfer:25

SF;

+ NO,

SF,-

+ CF,O

-

-

NO;

+ SF,

CF30-+ SF,

(3) (4)

In the course of this work, we have found that SF6-reacts with CF3O2 and C F 3 0 by charge transfer:

SF,- + CF30

-

CF30-+ SF,

3752 The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 The signal at m / e 101 is assigned to CF302- based on the dependence of its appearance upon the presence of 0 2 and CF3 precursor and the temperature of the pyrolysis furnace and its removal upon addition of N O to the neutral flow tube. The production of this ion by reaction with SFb- provides a lower limit on the electron affinity of CF302of EA(CF302) 1 EA(SF6) = 1.05 eV. This value is comparable to that of H02, the only other proxy radical for which electron affinity data are available: EA(H02) = 1.08 eV.I3 The CF3O- signal at m/e 85 depends on the presence of CF302 and increases as CF302 is lost upon the addition of NO. The formation of thisionvia either reaction 4 or reaction 6 is facilitated by its remarkable stability: the electron affinity of CF30 has been found to be 4.35 f 0.48 eV.I3 The two routes to CF30could be distinguished by exploiting reaction 10, discussed below, between CF30 and isobutane. Nearly all the m/e 85 signal was removed upon addition of isobutane to the flow tube. The existence of reaction 10 and the lack of an analogous reaction between CF2Oand isobutanedemonstrates that most of the mass 85 signal is generated from CF3O via reaction 6. The small residual signal at m/e 85 was attributed to CF2O via reaction 4; C F 2 0 could be generated by the decomposition of hot C F 3 0 or CF30H.j4 A small amount of m / e 85 signal was present whenever CF302 was made. This background was attributed to CF30 production from the self-reaction of CF302 in the radical source, reaction 7. This reaction has recently been studied by Nielsen et al.,35

-

CF302+ CF302 2 CF30 + 0,

(7)

who reported a rate coefficient of k7 = (1.8 f 0.5) X 1Ci2cm3 molecule-’ s-1. The background CF30-signal was readily reduced by increasing the helium carrier flow through the furnace. This decreased both the CF302 concentration and the reaction time in the source reactor and hence the extent of self-reaction. Any CF30- observation using SF6-was invariably accompanied by a stronger ion signal at m / e 105, whose kinetics closely paralleled those of the CF30- signal. As the mass 105 peak had an isotope pattern consistent with a ion containing one carbon and one oxygen atom, and the mass number 105 is consistent with an empirical formula of CFdOH, and since no other explanation fit these facts, a working hypothesis was developed which assigned the signal at m / e 105 to the ion CFjOHF-. This ion could be formed from CF30 in a two-step process. First, CF30 may abstract hydrogen from background organic material (e.g., back-streamed pump oil vapors) in the neutral flow tube:

CF,O + RH

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CF30H + R

(8) The 0-H bond energy in C S O H is to be approximately 105 f 4 kcal mol-’; thus, reaction 8 is at least slightly exothermic for all alkanes. Second, trifluoromethanol formed in the neutral flow tube could then reaction with SF6- in the flowing afterglow via fluoride ion transfer to give the proposed CF30HF-:

SF;

+ CF,OH

-

CF,OHF

+ SF,

(9) The fluoride ion affinity of CF30H has not been measured, but may be estimated from the results of Larson and McMahon.3’ These workers found that for a wide range of alcohols fluoride ion affinity was linearly related to gas-phase acidity. While the acidity of CF3OH is not known, an estimate of -323 kcal mol-’ may be readily derived from the heats of formation of CF30H (--211 f 2 kcal mol-i),11.’3 CF30-, and H+. Assuming Larson and McMahon’s linear relationship may be extrapolated beyond the most acidic alcohol they studied, we predict a fluoride affinity for C S O H of approximately 63 kcal mol-’. This is larger than for any of the compounds studied by Larson and McMahon. Using this estimate of the fluorideion affinity of trifluoromethanol, the enthalpy of reaction 9 is estimated to be -28 kcal mol-!.

Bevilaqua et al.

10

30

70

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Reaction Time (10.’ s) Figure 1. Variation of CF3O2- signal with reaction time. Flow tube conditions: (A) 0, [NO] = 0,P = 1.33 Torr, D 5 1000 cm s-I; (B) B, [NO]= 2.2 X 10’2 molecule cm-3, P = 0.73 Torr, 0 = 1570 cm SKI; (C) e, [NO] = 3.5 X 10l2 molecule ~ m - P~ = , 1.60 Torr, B = 1730 cm SKI; (D)A, [NO]= 5.8 X 10l2molecule cm-), P = 1.60 Torr, B = 1720 cm S-1.

Two experiments served to confirm this assignment of the m / e 105 signal. First, heating the neutral flow tube to -150 OC while passing 0 atoms (generated by a -30-W microwave discharge in 0 2 ) through it significantly reduced the ratio of mass 105 to mass 85 signals. We attribute this to removal of the background organic material by the oxygen atoms. Second, upon addition of isobutane to the neutral flow tube, the CF3O signal declined and the m/e 105 increased. This was atrributed to the reaction

+

-

+

CF,O i-C,H,o CF30H t-C,H, (10) In studies of the yield of reaction 2,02+ was used to ionize both NO and NO2 by charge transfer.32 02+is also known to charge transfer efficiently with CF3 radicals;32 unfortunately, it reacts with both CF3P2and (CF3C0)20viadissociativecharge transfer to form CF3+. Therefore, 02+ was not suitable as a reagent ion for the detection of CF3 in the presence of residual amounts of these precursors. 02-undergoes charge transfer with CF3 but does not react with CFJ; therefore, it was used to monitor CF3 production. A minor complication with this reagent ion is that the ion source also produces 0-, which can produce CF3- by reaction with CF31.32However, by adding a small flow of N 2 0 to the ion flow tube, the 0- concentration could be reduced to a noninterfering level.32 Reagents. CFJ (197%) and (CF3C0)20 (199%) were obtained commercially; CF3I was used without further purification, while (CF3CO)20was freeze-pumpthaw degassed before use. CF3CH20N0 was synthesized from 2,2,2-trifluorathanol and sodium nitrite using the scheme outlined by Haszeldine and M a t t i n ~ o n .NO ~ ~ was purified by passing it through a silica gel filled trap held at -78 OC; the concentration of NO in the dilute helium mixture was determined manometrically, using capacitance manometers. Helium (>99.999%) and oxygen (>99.97%) were used without further purification. Results CF302+ NO. Kinetic Studies. In these studies, trifluoromethylperoxy radicals were generated in a flow tube sidearm, and an excess of NO, dilute in helium, was introduced through the movable injector. SF6-was used as the CIMS ionization reagent todetect CF302,CF30,CF30H,andN02in the flowtubeeffluent. The dependence of CF302 radical concentration (detected as CF302-,m/e 10l)in theneutral flow tube,asafunctionofreaction time, is shown in Figure 1 for several NO concentrations and flow tube conditions. It can been seen in the figure that in the absence of N O there is no measurable change in [CF302], indicating a negligible loss of CF302 on the surface of the movable inlet. The figure demonstrates our observation, upon addition of NO through the injector, of pseudo-first-order loss of CF302, with a loss rate proportional to the N O concentration.

Reactions CF3O2

+ NO, C F 3 0 + NO, and C F 3 0 + RH

The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3753

I . 0

2

[NO]( 10l2 molecule cm") Figure 2. CF302 pseudo-first-order decay rate constants vs [NO] for the three trifluoromethyl radical precursors used: 0,CFd; m, (CF3C0)20; 0 , CF3CHlONO. The line represents the weighted fit to the data, with intercept=(0.7* l.O)s-~andslope=(1.53fO.O4)X10-11cm3molecule-1 s-l, where the uncertainties given are two standard deviations.

The plots in Figure 1 and others like them were used to determine pseudefirst-order decay rate coefficients k'for a variety of N O concentrations

or ln[CF,O,] = -k't

+c

where k' = k2[NO] - k,, c is a constant, k2 is the bimolecular rate coefficient for reaction 2 between CF302 and NO, and k, is the rate coefficient for loss of CF302 on the wall of the movable inlet. In Figure 2 these pseudo-first-order CF302 loss rate coefficients, k', are plotted as a function of [NO] in the flow tube. Data are included for all three radical precursors used. It can be seen that the loss rate is linear over a large range of NO concentrations and independent of the CF3 source used. The solid line in the figure is a linear fit to the data, weighted by the uncertainties in the pseudo-first-order decay rate coefficients. The y-intercept of this line, k, = 0.7 f 1.O s-l (where the uncertainty is 2u), is not significantly different from zero, corroborating the evidence of Figure 1 that there is no change in [CF302] as the injector is moved without NO. The slope of the line is (1.53 f 0.04 (2a)) X lo-" cm3 molecule-' s-I. Allowance for possible systematic errors of lo%, predominantly in [NO], flow tube pressure and mass flow rates, yields a value for k2 = (1 -53f 0.20) X lo-" cm3 molecule-' s-1,where the uncertainty represents a 95% confidence limit. Products. When N O is added through the movable injector, the CF302-,CF30-, CF30HF-, and NO2- ion signals are all seen to vary with reaction time. An example of the observed variation is shown in Figure 3 for an experiment in which CF3CH20NO was used as the trifluoromethyl radical source and for which the N O concentration was 9.0 X 10') molecule ~ m - ~The . figure demonstrates clearly that loss of CF3O2 is accompanied by production of both C F 3 0 and NO2. The figure also shows that the production of CF3OH closely parallels that of CF30,but that the response of our SF6- detection scheme to CF3OH is much greater than to CF30: while the CF30- signal increases by approximately 50 counts s-I, the CFjOHF- signal increases by -300 counts s-I. The relative response of the SF6- detection scheme to C F 3 0 H and C F 3 0 was confirmed by adding excess isobutane to the flow tube toconvert C F 3 0to CF3OH via reaction 10. When this was done, the mass 105 signal increased by approximately six times as much as the mass 85 signal decreased. Applying this relative sensitivity factor to the data of Figure 3, we find that approximately one-quarter of the C F 3 0is converted toCF30H by the background organic material in the neutral and ion flow tubes. The similarity in the temporal behavior of the

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..

I . . . I

IO

6

4

...

I

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.

30

.

I

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.

.

l

50

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I

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.

.

l

70

Reaction Time (10' s) Figure 3. Variation of reactants and products of reaction 2 with reaction time. (A) A,CF302; (B) 0, NO&(C) +,CFjO; (D) m, CF3OH (product of secondary reaction 8). Flow tube conditions: P = 0.83 Torr, 0 = 1360 cm s-l; [NO] = 9.0 X loll molecule ~ m - CF3 ~ ; precursor = CF3CH2ONO.

85 and 105 amu signals in Figure 3 would seem to indicate that the hydrogen abstraction reaction 8 is rapid in comparison with the production of C F 3 0 via reaction 2. The greater response of our SF6- CIMS detector to CF3OH relative to C F 3 0implies that the rate coefficient for the fluoride transfer reaction 9 is significantly larger than that for the charge transfer reaction 6. Note also that the CF30- signal initially rises with reaction time and then gradually falls at the longest reaction times. This observation can be explained by consideration of a secondary reaction between CF3O and NO. This subsequent reaction will be discussed below. Yield Measurements. 0 2 + is known to undergo charge-transfer reactions with both N O and so it was used as a CIMS reagent ion in an attempt to determine the quantitative yield of NO2 in reaction 2. In these studies, the neutral flow tube conditions were similar to those used for the determination of the rate coefficient. An essential difference was that for the yield measurement the concentration of N O used was similar to that of CF302. The yield was measured as

a = -R-S

KD

where Rs is the ratio of the increase in the NO2+ signal to the decrease in the NO+ signal with reaction length and RD is the ratio of detection responses of our CIMS apparatus to NO2 and NO. Thedetection sensitivities were calibrated immediately after the yield measurements were made, under the same flow tube conditions, except that the pyrolysis furnace temperature was well below that needed togenerate CF3 radicals. Thecalibrations were performed by flowing manometrically prepared mixtures of N O or NO2 in helium into the flow tube and determining the appropriate ion signal as a function of the concentration of the nitrogen oxide in the flow tube. In five separate measurements of the sensitivity ratio we obtained an average value of RD = SNOJSNO = 1.1 f 0.3. This is in reasonable agreement with the ratio, 1.3 f 0.4, of the averages of previously reported rate coefficients for the reactions of 02+ with NO2 and NO, respecti~ely.329~~~ Twenty-eightmeasurementsof Rs were made, each accompanied by one of the five measurements of RD,to yield 28 measurements of @. The average of these measurements was 1.6 f 0.7 NO2 produced for every N O consumed, where the stated uncertainty is twice the standard deviation of the set of measurements and includes no estimate of systematic errors. Reactionsof CF30. In our studies of reaction 2 between CF302 and NO, we found evidence suggesting that the product CF3O radical was undergoing secondary reactions with at least two species present in the flow tube. We have already discussed the formation of CF3OHF- from CF3OH, generated by hydrogen abstraction by C F 3 0 from either background or added hydrocarbons (reaction 8). We also observed a reaction between CF3O and NO.

Bevilacqua et al.

3154 The Journal of Physical Chemistry, Vol. 97. No. 15, 1993 h

-i 0

u,

The reaction of CFjO with NO was confirmed by using 02as the ionization reagent. When CF302 was reacted with NO under conditions similar to those used to study the kinetics of that reaction ([NO] = 3.3 X 10l2molecule cm-3, V = 1670 cm s-l), we observed a signal at m / e 49 which increased with reaction length. We assign this signal to FNO-, formed via fluorine abstraction from CF30 by NO, reaction 12, within the radical

I_j L

CF,O 10

20

30

40

Reaction Time ( s) Figure 4. Dependence of kinetics of C F 3 0 product from reaction 2 on nitric oxide concentration. Flow tube conditions were: (A) 0, [NO] = 1.3 X 10l2molecule ~ m - P~ =, 1.10 Torr, 0 = 2340 cm SKI;(B) m, [NO] = 2.2X 10~2moleculecm-3,P=0.73Torr,0= 157Ocm~-~;(C)A [NO] , = 5.8 X 10l2 molecule ~ m - P~ =, 1.60 Torr, 0 = 1720 cm s-l.

TABLE I: Parameter Ranges Used in ACUCHEM Modeling of Reaction 11 ~

~

~~

parameter kz kw(CF302)" kw(CF3O)" kn ki I [CF302lo [RHIO [NO10

value range 1.53 X lo-" cm3 m ~ l e c u l e -s-I ~

-0.7 s-I 5 s-I (0.4-1.6) X cm3 molecule-I s-I (0.1-3.0) X IO-II cm3 molecule-Is-I (0.5-1.0) x 1011 molecule cm-3 (1.0-10) X 10'2 molecule cm-3 (1.3, 2.6, 4.5) X 10l2molecule ~ m - ~

a Note that loss of either of these radicals on the walls of the movable inlet is manifested as an apparent increase in their concentration with reaction length (or time). Such an increase is modeled with k, > 0, as k, is defined in the text. The fit to the data of Figure 2 indicated that kw(CF302), though within its uncertainty of zero, was slightly negative; that fit served as the basis for the choice of kz(CF302) values used in these modeling studies.

TABLE II: Commrison of k2Values methoda

k2/(10-I1 cm3 molecule-1 s-1)

ref

FT'/EIMS LP/TRMS FT/CIMS

1.78 f 0.36 1.45 f 0.2 1.53 f 0.20

8 9 this work

FT = flow tube; EIMS = electron impact mass spectrometry; LP = laser photolysis; T R M S = time-resolved mass spectrometry; C I M S = chemical ionization mass spectrometry.

Our studies of reaction 2 were made over a range of NO concentrations which varied by nearly a factor of 8. Figure 4 depicts the time dependence of the CF30- ion signal observed in our measurements of the kinetics of reaction 2 a t several different concentrations of NO. The figure shows that a t low [NO] the CF3O signal rises as a function of reaction time, a t higher NO concentrations it increases and then decreases with time, and a t the highest [NO] values the signal monotonically decreases (the peakoccurring prior toour observation Ywindows). This behavior of the CF30 radical signal can be explained by a secondary reaction of CF3O with the excess NO present in the flow tube:

CF30+ NO

-

products

(11) The ACUCHEM kinetic modeling program45was used to analyze plots such as those of Figure 4. The kinetic scheme employed for these calculations included reactions 2,8, and 11, as well as slow wall loss processes for CF302 and CF30. The ranges of initial reactant concentrations and rate coefficients used in these modeling studies are listed in Table I. Modeling theCF3Okinetics for three different concentrations of NO yielded an estimate of the rate coefficient for reaction 1 1 of k lI = (2 f 1) X 10-I I cm3 molecule-' s-I.

-

+ NO

CF20+ FNO

flow tube followed by charge transfer from 0 2 - in the flowing afterglow. The reaction of 02-with FNO has been observed previously; the rate coefficient was reported to be (6 f 3) X 10-Io cm3 molecule-' s-I, but no product information was reported.46 The observation of FNO-as a product of this reaction implies EA(FN0) 1 EA(02) = 0.44 eV.I3 The observation of the reaction of CF30 with some background hydrogen-containing species led us to test this hypothesis by studying the reaction of C F 3 0with isobutane. The evidence for reaction with (CH3)jCH is a reduction of the CFjO- and a simultaneous increase of the CF3OHF- signals upon addition of isobutane to the flow tube while reacting CF302 with NO. We estimated the rate coefficient for the reaction of isobutane with CF30 by noting the conditions under which the CF30- signal was reduced to the background level. First, the source conditions are held constant: the CF302and NO concentrations, the injector position, and the carrier gas velocity are fixed. Then isobutane is added through a port near the downstream end of the flow tube until the CF30- and CF30HF- signals reach levels that do not change with increasing amounts of isobutane. The reaction is assumed to take place solely within the -20-cm length of the radical flow tube which is downstream of the port used to add (CH&CH because the contents undergo a dilution of a factor of 10-20 upon entering the ion flow tube, essentially stopping neutral bimolecular reactions. As the region between the isobutane addition port and the entrance to the ion tube is of irregular cross section, the residence time in this area must be estimated. With the average radius between the port and the ion tube estimated as 0.5 cm, for a total mass flow rate of 8 STP cm3 s-l, and a pressure of 1 Torr, the approximate reaction time is 5 ms. We find that the addition of -3 mTorr of (CH3)$2H to the flow tube under these conditions is just sufficient to reduce the CF30 signal to within the noise of the background. Equating this level with -5-10% of the signal in the absence of isobutane, we infer a CF30 loss rate of 500 s-l and crudely estimate a rate coefficient for the reaction of CF30 with isobutane of klo ( 5 f 3) X 10-12 cm3 molecule-' SKI. We also observed a slow reaction to occur between trifluoromethoxy radical and methane. However, this reaction is so slow that a reliable estimate for its rate coefficient could not be established using the method described above for isobutane. No reaction was observed between CF3O and D2. We estimate an upper limit on the rate coefficient for this reaction of k ~ I , 10-15 cm3 molecule-' s-1. Attempts to generate trifluoromethoxy radicals in quantities sufficient to make more accurate measurements of the rate coefficients of these reactions were unsuccessful. Reaction 2 fails because the amount of NO required to drive the reaction to completion removes nearly all the CF3O via reaction 11. We briefly investigated reaction 13 employing 0 2 - as the reagent ion, and determined a value of k 1 3 (1 f 0.7) X 10-It cm3molecule-'

-

-

-

=

CF,

-

+ NO2

CF30 + NO

(13a)

CF20+ FNO

(13b)

CF,ONO

Reactions CF302 + NO, C F 3 0

+ NO, and C F 3 0 + RH

s-1 for the rate coefficient by following the loss of CF3. We

obtained evidence for all three reaction channels listed, observing production of CF3O (confirmed by addition of isobutane to the downstream flow tube port), CF2O (signal at m / e 85 which persisted upon addition of isobutane), FNO ( m / e 49), and CF3N02or C F 3 0 N 0 (at m / e 115). Unfortunately, our data indicate that channel 13b, leading to the formation of CF20, is predominant, precluding the utility of channel 13a as a route to CF30. No reaction was observed between CF3 and N20, and we estimate an upper limit of k I1 X lO-I4 cm3 molecule-' s-I for this reaction.

Discussion

+

CF102 NO. Two previous measurements of the rate coefficient of reaction 2 have been r e p o ~ t e d . ~Thevalues .~ obtained in those studies, along with the result of our measurement, are compiled in Table I. Note that previously we reported4' a value 10% lower than the value reported here. The correction is due to an error in the calculation of N O concentrations used. Our corrected value lies between the previous two and within their experimental uncertainties. We found k2 to be independent of pressure in the range 0.8-2 Torr and of the CF3 radical precursor. The other essential findings of our investigation of reaction 2 are the direct mass spectrometric detection of the CF302 reactant as the parent ion, and the simultaneous observation of the C F 3 0 product, which has previously only been assumed to be the reaction product. In a preliminary study the pentafluoroethyl peroxy radical, C2F502,has also been observed by using charge transfer from SF6- ions. Thus, the CIMS detection method appears to be a general means of studying the kinetics of peroxy radicals. In light of the uncertainty of the mechanism of reaction 2 and the rate of the conversion of the C F 3 0 product to CF20, the detection of CF30 is a more significant finding. Having only very recently been detected in the gas phase," little experimental data are available on the fate of C S O . The development of this method for its detection will facilitate the study of its likely reactions, at least two of which we have observed in this work. The distribution of our measurements of the yield of NO2 in reaction 2, CP = 1.6 f 0.7, does include the likely value of 1. However, the average value of 1.6 is puzzling, as a yield greater than 1 is not physically possible. Several steps were taken to rule out possible systematic errors: different mixtures of N O and of NO2 in helium were used, and these mixtures were introduced to the flow tube through various pathways. There are several other possible systematic errors. We know that there is a secondary reaction of CF30 with NO which yields FNO. The production of NO+ via a reaction of FNO with 0 2 + is exothermic by at least 8 kcal mol-'. If such a reaction should occur with a rate coefficient significantly larger than that for the reaction of NO with 0 2 + ,the NO+ signal from FNO would be larger than from an equivalent amount of NO. A simple calculation shows that for conditionstypical of those used in our yield measurements ([CF302]o= 1 X loll moleculecm-3, [Nolo= 2.9 X 101'molecule cm-9, if the rate coefficient for FNO 02+ were -1.2 X cm3 molecule-I SKI(Le., twice that for NO + 02+):w4 we would observe a yield 20% larger than the actual yield. While this cannot account for the entire deviation of our result from the assumedvalueof 1, thisdoes demonstrate how a secondary reaction can affect our measured yield. Another possible explanation of our anomalous yield is that the NO2 product of reaction 2 may be formed in a vibrationally excited state, which may react with 0 2 + more rapidly than ground-state NO2 does. If this were the case, the relative detection efficiencies for NO2 and NO which we measure with our samples of these stable species would not be the relevant calibration factors. Other possibilities are the presence of some unknown ion chemistry or other systematic errors. Reactions of CFJO. There have recently been two reports of the reaction of CF3O with NO. Li and FranciscoI7 detected

-

+

The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3755 a laser-induced fluorescence signal following IR multiphoton dissociation of CF300CF3 (bis( trifluoromethy1)peroxide) and attributed it to CFjO. They found that addition of N O to their mixture of CF3OOCF3 and argon quenched the fluorescence signal. FT-IR product analysis of a photolyzed mixture of the peroxide and N O demonstrated the appearance of NO2 and FNO in the mixture. These products were attributed to abstraction of 0 and F atoms, respectively, from CF30 by NO. We have recently learned that Chen et a1.,48,49 in studies performed simultaneously to our own, photolyzed mixtures of CF3NO and N O in O2/N2 diluent and used FT-IR spectroscopy to analyze the stable products. They, too, found FNO and NO2 among the products and attributed FNO generation to reaction 12. In their experiments, however, NO2 production could be attributed to reaction 2. Indeed, it is difficult to understand the observation of NO2 in the experiments of Li and Francisco. The oxygen atom abstraction they propose, reaction 14, is approximately 32 kcal

CF,O + NO

-

CF,

+ NO,

(14)

mol-' endothermic." This barrier seems very unlikely to be surmounted, even if the dissociation of CF3OOCF3 yields highly excited CF30. The unimolecular elimination of a fluorine atom from such an energetic trifluoromethoxy radical should be fast," and greatly favored over elimination of an oxygen atom, because fluorine loss leads to production of a very stable carbonyl compound. In our experiments NO2 generated by reaction 2 would tend to mask any produced by the occurrence of reaction 14. However, our observation of the variation of C F 3 0 with N O concentration shown in Figure 4, as well as our observation of FNO production, agrees with the observations of both Li and Francisco and Chen et al. suggesting the occurrence of reaction 12. Neither of these groups reported an estimate of the rate coefficient kI2,so no value exists to which we may compare our estimate of k12 = (2 f 1) X cm3 molecule-' s-I. It is likely that reaction 12 between C F 3 0 and N O forms a [CF,ONO]* intermediate, which then decomposes to CF20 and FNO via a four-centered mechanism similar to that proposed by Sugawara et al. for reaction 13b between CF3 and NO2. Very recently, there have been two reports of the reaction of C F 3 0with small alkanes.so.5' When Chen et alas0added ethane or propane to their photolysis mixtures of CFJNO, NO, and artificial air, they saw reduced yields of CF2O and FNO and the appearance of alkane oxidation products in their product mixtures: ethane and propane yielded acetaldehyde and acetone, respectively. They explained these observations by proposing hydrogen abstraction from the alkanes by CFsO, reaction 15,

CF30+ CH3CH,R

-

CF30H

+ CH3CHR

(1 5 )

where R = H for ethane and R = CH3 for propane as the added gas. The alkyl radicals resulting from the H abstraction are then oxidized by 0 2 and NO to the carbonyl compounds.s2ss3 These workers observed the trifluoromethanol product of reaction 15 in their product spectra, noting that it decomposed rapidly following its production,s0although somewhat more slowly than previously reported by Kl6terand S e ~ p e l t .Chen ~ ~ et al. observed no reaction of C F 3 0 with CH,. Sehested and Wallingtons' observed the formation of CF3OH in their studies of the CIinitiated oxidation of CF3CFH2 in air and attributed it to the reaction of CF,O, formed in the degradation of C F ~ C F H Zwith , the parent hydrofluorocarbon. As a check of their assignment of the CFjOH spectral features, they looked for, and observed, the same features in spectra of photolyzed mixtures of CFlNO and CH4 in air. Again, they attributed the CFjOH to hydrogen abstraction by CF30. These workers reported even longer lifetimes for the CF3OH product and noted that the half-life increased from 1 to 5 h during the course of their experiments; they attributed this increase to the gradual conditioning of their reactor walls.

3756 The Journal of Physical Chemistry, Vol. 97, No. 15, I993

TABLE III: R-H Bond Strengths and Relative Rate CoeMcients for Hvdrotzen Abstraction bv CFIO ~

~~~

comvd

BDE(R-H) (kcal mol-')

ref

CFaOH HjC-H H5C2-H (CH3)lHC-H (CH3)jC-H

105 f 4 105.1 0.2 100.6 i 0.5 97.1 f 1 93.6 h 0.5

11,36 57 57 57 57

estimated k,"