- Email: [email protected]
Kinetic studies of the reaction of atomic hydrogen with trifluoroiodomethane
4 December 1998
Chemical Physics Letters 297 Ž1998. 553–557
Kinetic studies of the reaction of atomic hydrogen with trifluoroiodomethane Jessie Yuan 1, Leah Wells, Paul Marshall
)
Department of Chemistry, UniÕersity of North Texas, PO Box 305070, Denton, TX 76203 USA Received 28 June 1998; in final form 25 August 1998
Abstract Rate constants for the reaction of H atoms with CF3 I have been measured using the flash-photolysisrresonance fluorescence technique over 295–730 K. The results are k s Ž6.7 " 0.3. = 10y11 expwŽy3.8 " 0.2. kJ moly1rRT x cm3 moleculey1 sy1, where the uncertainties represent "1 s in the Arrhenius parameters. Error limits for k are "10%. The reaction is shown to be the dominant pathway for consumption of CF3 I in a stoichiometric methanerair flame. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction
2. Experimental method
Production of halon fire extinguishing agents such as CF3 Br and CF2 ClBr is banned under the Montreal Protocols on Substances that Deplete the Ozone Layer, and CF3 I is a potential candidate for service in some applications as a new fire suppressant w1x, but there is a lack of kinetic information on its reactions at elevated temperatures. Here we present the first measurements of the rate constant k 1 for the reaction CF3 I q H ™ CF3 q HI
Ž 1.
above room temperature. The results are used to assess likely destruction pathways for CF3 I in a stoichiometric methanerair flame.
)
Corresponding author. E-mail: [email protected] Present address: Texas Instruments, 13553 Floyd Road, MS 374, Dallas, TX 75265, USA. 1
The experimental apparatus and modifications for H-atom kinetics have been described elsewhere w2–4x. In brief, atomic H was generated by pulsed flash lamp photolysis of NH 3 , through MgF2 optics, in the presence of a large excess of CF3 I. The nominal discharge time was 7 ms. All experiments were carried out in Ar bath gas at a total pressure P, and the reagent concentrations were derived from P and the mole fractions of reagents in mixtures made in darkened glass bulbs. The gas mixtures flowed slowly through the reactor, on a time scale long compared with that for loss of H atoms, so that the kinetic conditions were effectively static. The relative concentration of H was monitored using time-resolved resonance fluorescence at a wavelength of 121.6 nm. Fluorescence was detected with a solar-blind photomultiplier tube ŽPMT. employed with pulse counting and signal averaging. A filter of dry air was employed in front of the PMT to help separate the
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 1 7 0 - 1
554
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
Lyman-a radiation from other scattered vacuum UV radiation from the microwave-powered resonance lamp. Under pseudo-first-order conditions and constant wNH 3 x, d w H x rdt s y Ž k 1 w CF3 I x q k diff . w H x s yk ps1 w H x ,
Ž 2. where k diff accounts for loss of H atoms out of the reaction zone other than by reaction with CF3 I, primarily via diffusion to walls of the reaction cell. Depending on the conditions, k diff varied between 50 and 300 sy1 . k ps1 was obtained by fitting the observed fluorescence intensity If vs. time profiles to an exponential decay Žan example is shown as the inset on Fig. 1. over typically ; 4–8 lifetimes: If s A exp Ž yk ps1 t . q B .
Ž 3.
The second-order rate constant k 1 was obtained as the slope of a linear plot of k ps1 vs. typically five values of wCF3 Ix, from 0 to wCF3 Ix max Žsee Fig. 1 for an example.. An important verification that pseudofirst-order conditions were achieved is that the measured k 1 should be independent of the initial concentration of H atoms. wHx 0 was altered by changing the concentration of NH 3 Žby up to a factor of ; 3. andror the energy discharged through the flash lamp, F, which was varied by a factor of 2. The temperature T in the reaction zone was monitored with a thermocouple, corrected for errors which arise from radiative heat transfer between the thermocouple and
the heated reactor walls of up to 10 K, before and after each set of k ps1 measurements, and is expected to be accurate to within "2% w5x. The average residence time of gas mixtures in the heated reactor before photolysis, tres , was varied by up to a factor of 4 to check for possible pyrolysis of CF3 I. The Ar ŽAir Products, 99.997%. was used directly from the cylinder, and NH 3 ŽMG Industries, 99.99%. and CF3 I ŽAlfa Æsar, certified analysis 99.95%, air 0.05%, HI not detectable. were purified by freeze– pump–thaw cycles at 77 K.
3. Results The experimental conditions and results for k 1 are summarized in Table 1. The independence of k 1 from F and wNH 3 x shows that secondary chemistry involving photolysis or reaction products was negligible and that pseudo-first-order conditions were attained, and the lack of dependence of k 1 on tres shows that pyrolysis of CF3 I was unimportant. Table 1 also lists the weighted mean values w6x of k 1 at each temperature. The Arrhenius plot for reaction Ž1. is shown in Fig. 2, and a weighted linear fit w8x to the set of 24 measurements yields k 1 s Ž 6.7 " 0.3 . = 10y1 1 exp Ž y3.8 "0.2 kJ moly1 rRT . cm3 moleculey1 sy1 .
Ž 4. The quoted errors in the Arrhenius parameters are 1 s and are statistical only. Consideration of the covariance leads to a 2 s precision for the fitted k 1 of 3–5%, and allowance for possible systematic errors Žup to 2% each in the flow rates, T and P . leads to 95% confidence intervals of about "10%.
4. Discussion Fig. 1. Plot of pseudo-first-order rate constant k ps1 vs. wCF3 Ix at P s69 mbar and T s 415 K. The inset shows the decay of time-resolved fluorescence intensity If for the filled point. The error bars represent "2 s statistical precision in k ps1 .
Our k 1 values are compared with previous measurements by Morris et al. w7x over 196–293 K in Fig. 2. It may be seen that there is excellent accord
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
555
Table 1 Rate constant measurements of the reaction H q CF3 I T ŽK.
P Žmbar.
tres Žs.
F ŽJ.
wNH 3 x Ž10 15 molecule cmy3 .
wCF3 Ix max a Ž10 12 molecule cmy3 .
k 1 " sk 1 Ž10y11 cm3 moleculey1 sy1 .
295 295 295 295
64.5 66.0 66.0 73.7
1.3 1.4 1.4 3.1
4.05 5.00 2.50 4.05
3.32 2.63 2.63 4.24
3.79 3.16 3.16 5.25
1.39 " 0.04 1.35 " 0.01 1.39 " 0.04 1.37 " 0.06 1.35 " 0.02 b
295 354 354 354 354
92.6 92.6 64.7 74.1
2.4 2.4 0.8 1.3
6.05 1.80 4.05 4.05
2.12 2.12 0.75 1.47
2.20 2.20 1.15 1.47
1.74 " 0.06 b
354 415 415 415 415
102.6 69.4 105.0 59.7
2.2 1.0 1.2 0.7
4.05 4.05 4.05 4.05
2.00 1.35 1.38 0.96
1.56 0.96 1.33 0.93
59.3 58.5 58.5 76.5
0.7 0.7 0.7 1.6
4.05 5.00 2.50 4.05
1.98 1.96 1.96 2.24
0.83 0.83 0.83 1.39
87.8 87.4 51.1 122.8
0.9 1.7 1.0 1.2
4.05 4.05 4.05 4.05
1.51 1.53 1.72 1.37
0.98 1.08 1.07 1.02
111.7 66.9 113.8 151.0
1.8 0.5 1.1 1.9
4.05 4.05 4.05 4.05
1.64 0.74 1.08 2.71
731 a b
3.36 " 0.11 2.77 " 0.12 3.07 " 0.25 3.19 " 0.08 3.14 " 0.12 b
622 731 731 731 731
2.73 " 0.17 2.78 " 0.09 2.40 " 0.15 2.76 " 0.14 2.70 " 0.12 b
502 622 622 622 622
2.44 " 0.10 2.50 " 0.09 2.30 " 0.05 2.15 " 0.04 2.26 " 0.06 b
415 502 502 502 502
1.69 " 0.06 1.66 " 0.05 1.74 " 0.07 1.91 " 0.06
2.07 0.71 1.20 2.13
3.22 " 0.15 3.85 " 0.20 3.41 " 0.22 3.46 " 0.07 3.45 " 0.12 b
Smallest non-zero wCF3 Ix employed was typically about 0.25wCF3 Ixmax . Weighted mean value "2s statistical precision.
at room temperature where the two temperature ranges meet. Given that the earlier work used a different technique, ESR detection of H atoms in a dischargerfast-flow apparatus, there seems to be little evidence of systematic error in either data set. Apparently k 1 exhibits a linear Arrhenius dependence over a wide range of temperature, so that Eq. Ž4. is also a reasonable representation of the com-
bined data set, over 200–730 K. For comparison, an unweighted fit to the points on Fig. 2 yields k 1 s Ž 6.4 " 0.3 . = 10y1 1 exp Ž y3.7 "0.1 kJ moly1 rRT . cm3 moleculey1 sy1
Ž 5. and the parameters are not significantly different from those in Eq. Ž4..
556
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
Reaction Ž1. has similar rate constants to its methyl analog CH 3 I q H ™ CH 3 q HI .
Ž 6.
The Arrhenius parameters for k 6 are A s Ž6.3 " 0.6. = 10y1 1 cm3 moleculey1 sy1 and Ea s y5.0 " 0.3 kJ moly1 w9x, which are very close to those for k 1 summarized in Eq. Ž4.. We have shown that reaction Ž6. proceeds via I-atom abstraction rather than substitution of I by H or H-atom abstraction w9x, and ab initio calculations have indicated very large barriers to F-atom abstraction in reactions of H with hydrofluoromethanes w10x, so the product assignment in reaction Ž1. seems reasonable. The C–I bond dissociation enthalpy in CH 3 I at 298 K is 237.2 " 1.3 kJ moly1 w11x, somewhat greater than 227.2 " 1.3 kJ moly1 in CF3 I w12x, so that reaction Ž1. is more exothermic than reaction Ž6.. Possibly this contributes to the marginally smaller Ea for reaction Ž1., although the two reactions have very similar kinetics. Reaction Ž1. is fairly fast even at room temperature. We now consider the likely fate of CF3 I in combustion systems. We have calculated effective loss rates for CF3 I via attack by the flame radicals H, OH w13x, O w14x and CH 3 w15x, and by unimolecular decomposition w16x, using concentration and temperature profiles calculated using the Chemkin package w17x and the GRI mechanism w18x for an atmospheric pressure, stoichiometric methanerair flame. The results apply to traces of added CF3 I, where the CrHrO chemistry is unperturbed. As may be seen
Fig. 3. Pseudo-first-order consumption rates of CF3 I by various pathways in a stoichiometric atmospheric pressure methanerair flame Žsolid lines, left-hand axis., and temperature profile Ždashed line, right-hand axis..
from Fig. 3, at all points in the flame, reaction Ž1. is the most important pathway for destruction of CF3 I. This reflects both the large value of k 1 and the high relative abundance of H atoms in flames, where they are important chain carriers. The reactivity of CF3 I makes it an efficient scavenger of H atoms at low and moderate temperatures, so this agent might be a useful inhibitor of ignition and other lower-temperature combustion processes.
5. Conclusions The rate constant for reactions of H atoms with CF3 I has been measured at temperatures up to ; 730 K. The results agree well with earlier work at room temperature and below, and are similar to those for the analogous reaction H q CH 3 I. Modeling of a stoichiometric methanerair flame indicates that the H q CF3 I reaction is the fastest destruction pathway for CF3 I in all parts of the flame.
Acknowledgements
Fig. 2. Arrhenius plot of rate constants for HqCF3 I Žv, this work; I, Ref. w7x.. Error bars represent "2 s statistical uncertainty at each temperature.
We thank the Robert A. Welch Foundation ŽGrant B-1174., the Air Force Office of Scientific Research, and the UNT Faculty Research Fund for financial support.
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
References w1x R.E. Tapscott, S.R. Skaggs, D. Dierdorf, in: A.W. Miziolek, W. Tsang ŽEds.., Halon Replacements, ch. 14, ACS Symp. Ser. 611, Am. Chem. Soc., Washington, DC, 1995. w2x Y. Shi, P. Marshall, J. Phys. Chem. 95 Ž1991. 1654. w3x L. Ding, P. Marshall, J. Phys. Chem. 96 Ž1992. 2197. w4x A. Goumri, W.-J. Yuan, L. Ding, Y. Shi, P. Marshall, Chem. Phys. 177 Ž1993. 233. w5x L. Ding, P. Marshall, J. Chem. Soc., Faraday Trans. 89 Ž1993. 419. w6x P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969, p.88. w7x R.A. Morris, K. Donohue, D.L. McFadden, J. Phys. Chem. 93 Ž1989. 1358. w8x J.A. Irvin, T.I. Quickenden, J. Chem. Educ. 60 Ž1983. 711. w9x J. Yuan, L. Wells, P. Marshall, J. Phys. Chem. A 101 Ž1997. 3542. w10x R.J. Berry, C.J. Ehlers, D.R. Burgess Jr., M.R. Zachariah, P. Marshall, Chem. Phys. Lett. 269 Ž1997. 107.
557
w11x S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin, W.G. Mallard, Gas-phase ion and neutral thermochemistry, J. Phys. Chem. Ref. Data 17 Ž1988. Suppl. 1. w12x R.L. Asher, B. Ruscic, J. Chem. Phys. 106 Ž1997. 210. w13x R.J. Berry, J. Yuan, A. Misra, P. Marshall, J. Phys. Chem. A 102 Ž1998. 5182. w14x J. Yuan, A. Misra, L. Wells, S. Hawkins, A. Krishnan, R.B. Nathuji, P. Marshall, R.J. Berry, 4th Int. Conf. on Chemical Kinetics, Gaithersburg, MD, 1997, Pap. J5. w15x R.J. Berry, P. Marshall, Int. J. Chem. Kinet. 30 Ž1998. 179. w16x S.S. Kumaran, M.-C. Su, K.P. Lim, J.V. Michael, Chem. Phys. Lett. 243 Ž1995. 59. w17x R.J. Kee, F.M. Rupley, J.A. Miller, Chemkin-II: A Fortran chemical kinetics package for the analysis of gas-phase chemical kinetics, Sandia Natl. Lab., Albuquerque, NM, 1991, Rep. No. SAND89-8009B. w18x M. Frenklach, H. Wang, C.-L. Yu, M. Goldenberg, C. T. Bowman, R.K. Hanson, D.F. Davidson, E.J. Chang, G.P. Smith, D.M. Golden, W.C. Gardiner, V. Lissianski, GRIMech 1.2, http:rrwww.gri.org.
Chemical Physics Letters 297 Ž1998. 553–557
Kinetic studies of the reaction of atomic hydrogen with trifluoroiodomethane Jessie Yuan 1, Leah Wells, Paul Marshall
)
Department of Chemistry, UniÕersity of North Texas, PO Box 305070, Denton, TX 76203 USA Received 28 June 1998; in final form 25 August 1998
Abstract Rate constants for the reaction of H atoms with CF3 I have been measured using the flash-photolysisrresonance fluorescence technique over 295–730 K. The results are k s Ž6.7 " 0.3. = 10y11 expwŽy3.8 " 0.2. kJ moly1rRT x cm3 moleculey1 sy1, where the uncertainties represent "1 s in the Arrhenius parameters. Error limits for k are "10%. The reaction is shown to be the dominant pathway for consumption of CF3 I in a stoichiometric methanerair flame. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction
2. Experimental method
Production of halon fire extinguishing agents such as CF3 Br and CF2 ClBr is banned under the Montreal Protocols on Substances that Deplete the Ozone Layer, and CF3 I is a potential candidate for service in some applications as a new fire suppressant w1x, but there is a lack of kinetic information on its reactions at elevated temperatures. Here we present the first measurements of the rate constant k 1 for the reaction CF3 I q H ™ CF3 q HI
Ž 1.
above room temperature. The results are used to assess likely destruction pathways for CF3 I in a stoichiometric methanerair flame.
)
Corresponding author. E-mail: [email protected] Present address: Texas Instruments, 13553 Floyd Road, MS 374, Dallas, TX 75265, USA. 1
The experimental apparatus and modifications for H-atom kinetics have been described elsewhere w2–4x. In brief, atomic H was generated by pulsed flash lamp photolysis of NH 3 , through MgF2 optics, in the presence of a large excess of CF3 I. The nominal discharge time was 7 ms. All experiments were carried out in Ar bath gas at a total pressure P, and the reagent concentrations were derived from P and the mole fractions of reagents in mixtures made in darkened glass bulbs. The gas mixtures flowed slowly through the reactor, on a time scale long compared with that for loss of H atoms, so that the kinetic conditions were effectively static. The relative concentration of H was monitored using time-resolved resonance fluorescence at a wavelength of 121.6 nm. Fluorescence was detected with a solar-blind photomultiplier tube ŽPMT. employed with pulse counting and signal averaging. A filter of dry air was employed in front of the PMT to help separate the
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 1 7 0 - 1
554
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
Lyman-a radiation from other scattered vacuum UV radiation from the microwave-powered resonance lamp. Under pseudo-first-order conditions and constant wNH 3 x, d w H x rdt s y Ž k 1 w CF3 I x q k diff . w H x s yk ps1 w H x ,
Ž 2. where k diff accounts for loss of H atoms out of the reaction zone other than by reaction with CF3 I, primarily via diffusion to walls of the reaction cell. Depending on the conditions, k diff varied between 50 and 300 sy1 . k ps1 was obtained by fitting the observed fluorescence intensity If vs. time profiles to an exponential decay Žan example is shown as the inset on Fig. 1. over typically ; 4–8 lifetimes: If s A exp Ž yk ps1 t . q B .
Ž 3.
The second-order rate constant k 1 was obtained as the slope of a linear plot of k ps1 vs. typically five values of wCF3 Ix, from 0 to wCF3 Ix max Žsee Fig. 1 for an example.. An important verification that pseudofirst-order conditions were achieved is that the measured k 1 should be independent of the initial concentration of H atoms. wHx 0 was altered by changing the concentration of NH 3 Žby up to a factor of ; 3. andror the energy discharged through the flash lamp, F, which was varied by a factor of 2. The temperature T in the reaction zone was monitored with a thermocouple, corrected for errors which arise from radiative heat transfer between the thermocouple and
the heated reactor walls of up to 10 K, before and after each set of k ps1 measurements, and is expected to be accurate to within "2% w5x. The average residence time of gas mixtures in the heated reactor before photolysis, tres , was varied by up to a factor of 4 to check for possible pyrolysis of CF3 I. The Ar ŽAir Products, 99.997%. was used directly from the cylinder, and NH 3 ŽMG Industries, 99.99%. and CF3 I ŽAlfa Æsar, certified analysis 99.95%, air 0.05%, HI not detectable. were purified by freeze– pump–thaw cycles at 77 K.
3. Results The experimental conditions and results for k 1 are summarized in Table 1. The independence of k 1 from F and wNH 3 x shows that secondary chemistry involving photolysis or reaction products was negligible and that pseudo-first-order conditions were attained, and the lack of dependence of k 1 on tres shows that pyrolysis of CF3 I was unimportant. Table 1 also lists the weighted mean values w6x of k 1 at each temperature. The Arrhenius plot for reaction Ž1. is shown in Fig. 2, and a weighted linear fit w8x to the set of 24 measurements yields k 1 s Ž 6.7 " 0.3 . = 10y1 1 exp Ž y3.8 "0.2 kJ moly1 rRT . cm3 moleculey1 sy1 .
Ž 4. The quoted errors in the Arrhenius parameters are 1 s and are statistical only. Consideration of the covariance leads to a 2 s precision for the fitted k 1 of 3–5%, and allowance for possible systematic errors Žup to 2% each in the flow rates, T and P . leads to 95% confidence intervals of about "10%.
4. Discussion Fig. 1. Plot of pseudo-first-order rate constant k ps1 vs. wCF3 Ix at P s69 mbar and T s 415 K. The inset shows the decay of time-resolved fluorescence intensity If for the filled point. The error bars represent "2 s statistical precision in k ps1 .
Our k 1 values are compared with previous measurements by Morris et al. w7x over 196–293 K in Fig. 2. It may be seen that there is excellent accord
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
555
Table 1 Rate constant measurements of the reaction H q CF3 I T ŽK.
P Žmbar.
tres Žs.
F ŽJ.
wNH 3 x Ž10 15 molecule cmy3 .
wCF3 Ix max a Ž10 12 molecule cmy3 .
k 1 " sk 1 Ž10y11 cm3 moleculey1 sy1 .
295 295 295 295
64.5 66.0 66.0 73.7
1.3 1.4 1.4 3.1
4.05 5.00 2.50 4.05
3.32 2.63 2.63 4.24
3.79 3.16 3.16 5.25
1.39 " 0.04 1.35 " 0.01 1.39 " 0.04 1.37 " 0.06 1.35 " 0.02 b
295 354 354 354 354
92.6 92.6 64.7 74.1
2.4 2.4 0.8 1.3
6.05 1.80 4.05 4.05
2.12 2.12 0.75 1.47
2.20 2.20 1.15 1.47
1.74 " 0.06 b
354 415 415 415 415
102.6 69.4 105.0 59.7
2.2 1.0 1.2 0.7
4.05 4.05 4.05 4.05
2.00 1.35 1.38 0.96
1.56 0.96 1.33 0.93
59.3 58.5 58.5 76.5
0.7 0.7 0.7 1.6
4.05 5.00 2.50 4.05
1.98 1.96 1.96 2.24
0.83 0.83 0.83 1.39
87.8 87.4 51.1 122.8
0.9 1.7 1.0 1.2
4.05 4.05 4.05 4.05
1.51 1.53 1.72 1.37
0.98 1.08 1.07 1.02
111.7 66.9 113.8 151.0
1.8 0.5 1.1 1.9
4.05 4.05 4.05 4.05
1.64 0.74 1.08 2.71
731 a b
3.36 " 0.11 2.77 " 0.12 3.07 " 0.25 3.19 " 0.08 3.14 " 0.12 b
622 731 731 731 731
2.73 " 0.17 2.78 " 0.09 2.40 " 0.15 2.76 " 0.14 2.70 " 0.12 b
502 622 622 622 622
2.44 " 0.10 2.50 " 0.09 2.30 " 0.05 2.15 " 0.04 2.26 " 0.06 b
415 502 502 502 502
1.69 " 0.06 1.66 " 0.05 1.74 " 0.07 1.91 " 0.06
2.07 0.71 1.20 2.13
3.22 " 0.15 3.85 " 0.20 3.41 " 0.22 3.46 " 0.07 3.45 " 0.12 b
Smallest non-zero wCF3 Ix employed was typically about 0.25wCF3 Ixmax . Weighted mean value "2s statistical precision.
at room temperature where the two temperature ranges meet. Given that the earlier work used a different technique, ESR detection of H atoms in a dischargerfast-flow apparatus, there seems to be little evidence of systematic error in either data set. Apparently k 1 exhibits a linear Arrhenius dependence over a wide range of temperature, so that Eq. Ž4. is also a reasonable representation of the com-
bined data set, over 200–730 K. For comparison, an unweighted fit to the points on Fig. 2 yields k 1 s Ž 6.4 " 0.3 . = 10y1 1 exp Ž y3.7 "0.1 kJ moly1 rRT . cm3 moleculey1 sy1
Ž 5. and the parameters are not significantly different from those in Eq. Ž4..
556
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
Reaction Ž1. has similar rate constants to its methyl analog CH 3 I q H ™ CH 3 q HI .
Ž 6.
The Arrhenius parameters for k 6 are A s Ž6.3 " 0.6. = 10y1 1 cm3 moleculey1 sy1 and Ea s y5.0 " 0.3 kJ moly1 w9x, which are very close to those for k 1 summarized in Eq. Ž4.. We have shown that reaction Ž6. proceeds via I-atom abstraction rather than substitution of I by H or H-atom abstraction w9x, and ab initio calculations have indicated very large barriers to F-atom abstraction in reactions of H with hydrofluoromethanes w10x, so the product assignment in reaction Ž1. seems reasonable. The C–I bond dissociation enthalpy in CH 3 I at 298 K is 237.2 " 1.3 kJ moly1 w11x, somewhat greater than 227.2 " 1.3 kJ moly1 in CF3 I w12x, so that reaction Ž1. is more exothermic than reaction Ž6.. Possibly this contributes to the marginally smaller Ea for reaction Ž1., although the two reactions have very similar kinetics. Reaction Ž1. is fairly fast even at room temperature. We now consider the likely fate of CF3 I in combustion systems. We have calculated effective loss rates for CF3 I via attack by the flame radicals H, OH w13x, O w14x and CH 3 w15x, and by unimolecular decomposition w16x, using concentration and temperature profiles calculated using the Chemkin package w17x and the GRI mechanism w18x for an atmospheric pressure, stoichiometric methanerair flame. The results apply to traces of added CF3 I, where the CrHrO chemistry is unperturbed. As may be seen
Fig. 3. Pseudo-first-order consumption rates of CF3 I by various pathways in a stoichiometric atmospheric pressure methanerair flame Žsolid lines, left-hand axis., and temperature profile Ždashed line, right-hand axis..
from Fig. 3, at all points in the flame, reaction Ž1. is the most important pathway for destruction of CF3 I. This reflects both the large value of k 1 and the high relative abundance of H atoms in flames, where they are important chain carriers. The reactivity of CF3 I makes it an efficient scavenger of H atoms at low and moderate temperatures, so this agent might be a useful inhibitor of ignition and other lower-temperature combustion processes.
5. Conclusions The rate constant for reactions of H atoms with CF3 I has been measured at temperatures up to ; 730 K. The results agree well with earlier work at room temperature and below, and are similar to those for the analogous reaction H q CH 3 I. Modeling of a stoichiometric methanerair flame indicates that the H q CF3 I reaction is the fastest destruction pathway for CF3 I in all parts of the flame.
Acknowledgements
Fig. 2. Arrhenius plot of rate constants for HqCF3 I Žv, this work; I, Ref. w7x.. Error bars represent "2 s statistical uncertainty at each temperature.
We thank the Robert A. Welch Foundation ŽGrant B-1174., the Air Force Office of Scientific Research, and the UNT Faculty Research Fund for financial support.
J. Yuan et al.r Chemical Physics Letters 297 (1998) 553–557
References w1x R.E. Tapscott, S.R. Skaggs, D. Dierdorf, in: A.W. Miziolek, W. Tsang ŽEds.., Halon Replacements, ch. 14, ACS Symp. Ser. 611, Am. Chem. Soc., Washington, DC, 1995. w2x Y. Shi, P. Marshall, J. Phys. Chem. 95 Ž1991. 1654. w3x L. Ding, P. Marshall, J. Phys. Chem. 96 Ž1992. 2197. w4x A. Goumri, W.-J. Yuan, L. Ding, Y. Shi, P. Marshall, Chem. Phys. 177 Ž1993. 233. w5x L. Ding, P. Marshall, J. Chem. Soc., Faraday Trans. 89 Ž1993. 419. w6x P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969, p.88. w7x R.A. Morris, K. Donohue, D.L. McFadden, J. Phys. Chem. 93 Ž1989. 1358. w8x J.A. Irvin, T.I. Quickenden, J. Chem. Educ. 60 Ž1983. 711. w9x J. Yuan, L. Wells, P. Marshall, J. Phys. Chem. A 101 Ž1997. 3542. w10x R.J. Berry, C.J. Ehlers, D.R. Burgess Jr., M.R. Zachariah, P. Marshall, Chem. Phys. Lett. 269 Ž1997. 107.
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