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Kinetics of the R + NO2 Reactions (R ) i-C3H7, n-C3H7, s-C4H9, and t-C4H9) in the Temperature Range 201-489 K† Matti P. Rissanen, Suula L. Arppe, Arkke J. Eskola,‡ Matti M. Tammi, and Raimo S. Timonen* Laboratory of Physical Chemistry, UniVersity of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), Helsinki FIN-00014, Finland ReceiVed: September 30, 2009; ReVised Manuscript ReceiVed: December 22, 2009
The bimolecular rate coefficients of four alkyl radical reactions with NO2 have been measured in direct timeresolved experiments. Reactions were studied under pseudo-first-order conditions in a temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer (LP-PIMS). The measured reaction rate coefficients are independent of helium bath gas pressure within the experimental ranges covered and exhibit negative temperature dependence. For i-C3H7 + NO2 and t-C4H9 + NO2 reactions, the dependence of ordinate (logarithm of reaction rate coefficients) on abscissa (1/T or log(T)) was nonlinear. The obtained results (in cm3s-1) can be expressed by the following equations: k(n-C3H7 + NO2) ) ((4.34 ( 0.08) × 10-11) (T/300 K)-0.14(0.08 (203-473 K, 1-7 Torr), k(i-C3H7 + NO2) ) ((3.66 ( 2.54) × 10-12) exp(656 ( 201 K/T)(T/300 K)1.26(0.68 (220-489 K, 1-11 Torr), k(s-C4H9 + NO2) ) ((4.99 ( 0.16) × 10-11)(T/300 K)-1.74(0.12 (241-485 K, 2 - 12 Torr) and k(t-C4H9 + NO2) ) ((8.64 ( 4.61) × 10-12) exp(413 ( 154 K/T)(T/300 K)0.51(0.55 (201-480 K, 2-11 Torr), where the uncertainties shown refer only to the 1 standard deviations obtained from the fitting procedure. The estimated overall uncertainty in the determined bimolecular rate coefficients is about (20%. Introduction 1
Alkyl radicals are common intermediates in combustion and atmospheric chemistry,2 and they play a central role also in many other areas of chemistry.3,4 They are formed, for example, in bond-breaking reactions of large hydrocarbons under pyrolysis conditions5,6 or in hydrogen abstraction reactions by the reactive species like O(3P), O(1D), and Cl atoms and OH radicals.7 Photolysis of substituted and oxygenated hydrocarbon species (e.g., CH3I, C2H5Br, CH3C(O)CH3, etc.) is an important atmospheric source of alkyl radicals,2 for example, in the marine boundary layer of coastal regions.8,9 NO2 is a toxic substance that has an adverse impact on human health, mostly effecting through respiratory organs.10,11 It is an important constituent of the natural unpolluted atmosphere, but also one of the major pollutants from anthropogenic sources.12-15 Because oxides of nitrogen are ubiquitous to almost all combustion environments16 and alkyl radicals are found in the same environments as NO2,1,12,15 the rates of their mutual reactions may be valuable for combustion and atmospheric modelers as well as for other researchers. In the investigation of the reactivity of radicals in different environments, the alkyl radicals are good reference species, as they are free of interference from substituent effects and electronic resonances of heteroatoms and -groups and multiple bonds. Once the results exist for the corresponding alkyl radical (sharing the same basic structure), they can be used to compare, identify, and separate trends caused by the resonance, inductive, and steric effects of the heterosubstituents and multiple bonds. At the moment, the database of direct studies for R + NO2 †
Part of the special section “30th Free Radical Symposium”. * Corresponding author,
[email protected]. ‡ Current address: School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.
reactions (where R is an alkyl radical, containing only C and H atoms) covers the CH3 + NO217,18 and the C2H5 + NO219,20 reactions. We have now extended this database by performing direct experimental measurements for propyl (n-C3H7 and i-C3H7) and butyl (s-C4H9 and t-C4H9) radical reactions with NO2 as a function of temperature
n-C3H7 + NO2 f products
(1)
i-C3H7 + NO2 f products
(2)
s-C4H9 + NO2 f products
(3)
t-C4H9 + NO2 f products
(4)
To our knowledge, there have been two indirect experimental studies concerning the reactions shown above. Baulch et al.21 have studied the n-C3H7 + NO2, i-C3H7 + NO2, and s-C4H9 + NO2 reactions at 298 K and at about 300 Torr total pressure employing gas chromatography for the end product analysis of the C3H8 (or C4H10) + H2O2 + NO2 reaction mixtures. Alkyl radicals were produced from C3H8 (or C4H10) precursor by hydrogen abstraction reactions of hydroxyl radicals, which were formed in the H2O2 + NO2 reaction on the reactor wall. Jaffe et al.22 also used chromatographic end product analysis, with the help of the steady-state assumption, to deduce the branching fraction of n-C3H7 + NO2 reaction to nitro and nitrite products. This result was obtained from fitting experimental data to a complex set of reactions in their work on butyraldehyde + NO2 reaction at 298 K and at 2-20 Torr total pressure. There are no previous direct kinetic studies available for these reactions.
10.1021/jp909396v 2010 American Chemical Society Published on Web 02/04/2010
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Rissanen et al.
Experimental Section Details of the experimental apparatus and the procedures employed to determine the bimolecular reaction rate coefficients of the studied reactions have been presented previously,23,24 and only an overview is given here. Reactor tubes used in the measurements were made of seamless stainless steel and had an inner diameter of 6, 8, or 17 mm. They were typically coated with halocarbon wax (HW, Supelco) and cooled or heated with a circulating liquid in a temperature controlling mantle (T ) 201-363 K).23 For a few measurements at higher temperatures (T ) 387-489 K) poly(dimethylsiloxane) (PDMS) coating and a resistive electrical heating mantle24 were employed. In the experiments, a gas mixture flowing through the temperature-controlled tubular reactor contained the radical precursor ( k300K(nC3H7 + NO2) > k300K(i-C3H7 + NO2), is in agreement with the work of Baulch et al.21 Jaffe et al.22 studied the n-C3H7 + NO2 reaction using chromatographic end product analysis in their work on the butyraldehyde + NO2 reaction. They measured the relative importance of nitrite (eqs 11 and 12) and nitro (eq 10) channels (k(R-ONO)/k(R-NO2)) and arrived at a value of 2.2 in favor of the nitrite route but they did not determine the bimolecular reaction rate coefficient k(n-C3H7 + NO2) that was one of the main subjects of this work. According to the current and previous21,22 results, the mechanisms of the studied R + NO2 reactions can be qualitatively described with eqs 10-12. However, not much can be said about the relative importance of the channels (10) and (11), the formation of nitro and nitrite adducts, respectively. On the basis of the study by Baulch et al.21 and on the present study, one can conclude that under the conditions of the present experiments the step (11) is almost exclusively followed by the step (12); in low pressures the nitrite product cannot be stabilized efficiently, resulting in the formation of alkoxy radicals and nitric
Rissanen et al. oxide. Baulch et al.21 also observed that even though the yield of nitrite products was increasing with increasing pressure from 100 to 700 Torr, the yield of nitro products was constant within the experimental uncertainty. As has been discussed above, the nitro products were sought in the studied reactions but not found, probably because (i) the bath gas pressures used were not high enough to stabilize them, (ii) they were photodissociated to other products in the photoionization, or simply because (iii) the detection sensitivity of the apparatus is too low for these species, mostly due to the instability of their cations, under these experimental conditions. It is also possible that the studied reactions proceed straight to bimolecular products in concerted steps, without going through steps (10) and (11). This is supported by the rate coefficients’ lack of pressure dependence at low pressure. More experiments with methods suitable for observing alkoxy and nitro species are needed to validate the mechanism of R + NO2 reactions. The bimolecular rate coefficients of all four reactions measured in this study exhibit negative temperature dependence, which is typical for barrierless radical-radical reaction. One can clearly observe differences in the reactivities of n-C3H7, i-C3H7, s-C4H9, and t-C4H9 radicals toward NO2 at different temperatures (Figure 2a,b). The value of the temperature coefficient n in expression (A), k ) k300K (T/300 K)n, changes from -0.14 for n-C3H7 + NO2 reaction to -1.74 for the s-C4H9 + NO2 reaction while the observed curvature in the temperature dependences of the i-C3H7 + NO2 and t-C4H9 + NO2 reaction rate coefficients makes the comparison to these reactions less meaningful. If only the two parameter expressions are considered (Table 1), then the order of influence of temperature on the reactivity is: n-C3H7 < i-C3H7 < t-C4H9 < s-C4H9. All R + NO2 reactions measured in this work are faster than the corresponding R + O2 reactions40,41 and can present a noteworthy loss process for these radicals in oxygen-deficient polluted environments. It is interesting to observe that the isomeric species have notably different behavior in analogous reactions in the same temperature interval. For most of the temperatures covered in this work, the reactivity toward NO2 is lower for the radicals that are sterically more hindered. This can be seen by comparing the rate coefficients of the i-C3H7 + NO2 with n-C3H7 + NO2 reactions and those of t-C4H9 + NO2 with s-C4H9 + NO2 reactions, respectively. This is true in all except for the extremes of the temperature range; the kinetics of the studied reactions cross when the temperature is increased, at about T ) 232 K for propyl radical reactions and at about T ) 425 K for butyl radical reactions. Above these limiting temperatures, the reactivities toward NO2 are reversed, i.e., k240K(n-C3H7 + NO2) > k240K(i-C3H7 + NO2) and k430K(t-C4H9 + NO2) > k430K(s-C4H9 + NO2). This is an interesting observation and clearly shows that in certain cases unpredictable temperature dependencies can be observed. Substituting one R-hydrogen atom in n-C3H7 with a methyl group to form s-C4H9 radical increases electron density in the radical center. Upon this substitution, the reactivity increases at low temperatures but decreases at temperatures higher than about 330 K. The temperature has a large influence on the reactivity of s-C4H9 toward NO2 but in contrast almost no effect on the reactivity of n-C3H7. On inspection of the results, it seems that the methyl group brings more sterical hindrance (than H atom) to the radical center making it less reactive at temperatures above 330 K. However, with the ability of a methyl group to serve as an electron donor, it increases the electron density in the radical center and has been observed to increase the R + NO2 reaction rate coefficients.20,32
Kinetics of the R + NO2 Reactions In comparison with the previously measured CH3 + NO217,18 and C2H5 + NO219,20 reaction rate coefficients, the obtained results appear reasonable. The CH3 + NO2 reaction has the lowest rate coefficient of the alkyl radical reactions with NO2 measured so far k295K(CH3 + NO2) ) (2.5 ( 0.5) × 10-11 cm3 s-1.17 In the ethyl radical, an R-hydrogen atom is replaced with a methyl group in the radical center and consequently it has a higher room temperature rate coefficient with NO2 k300K(C2H5 + NO2) ) (4.33 ( 0.87) × 10-11 cm3 s-1 than CH3.20 The n-C3H7 + NO2 reaction measured in this study has the same rate coefficient at room temperature k300K(n-C3H7 + NO2) ) (4.34 ( 0.87) × 10-11 cm3 s-1 as with the ethyl reaction20 and only a small dependence on temperature. As was noted above, the reactivity toward NO2 is somewhat lower for the radicals that are sterically more hindered although this does not apply in the whole experimental temperature range. Also the dependence on temperature seems to become stronger as the branching of the radical increases. However, s-C4H9 is not as much branched as t-C4H9 but still its reaction rate with NO2 shows a stronger dependence on temperature. It is reasonable to suppose that the inductive effect of the substituents decreases as the distance from the reaction center is increased.42 On comparison of the results obtained for the i-C3H7 + NO2 and s-C4H9 + NO2 reactions, it seems that the ethyl group acts as a stronger electron donor for the radical center than the methyl group does; the rate enhancing effect is not yet the strongest for a methyl substituent but increases still at least from methyl to ethyl as a substituent. On comparison of the rate coefficients of the C2H5 and n-C3H7 reactions with NO2 the same conclusion cannot be drawn because the room temperature rate coefficients are almost the same and the temperature dependences are similar. The straight chain radicals show a much weaker dependence on temperature than the branched ones do and the R,R-alkyl substitution in the radical center changes the observed dependences considerably, at least in the propyl radical reactions measured in this study. No conclusive reasons for the distinct k(T) dependences of the reactions studied can be given, but the probable explanation can be connected to the effects discussed above: the steric differences in the radical sites as well as the electron-donating inductive effects of the alkyl substituents in the radical center. Conclusions Four alkyl radical reactions with NO2 have been studied in direct time-resolved measurements. All the obtained rate coefficients are independent of bath gas pressure and exhibit negative temperature dependence under the experimental conditions covered. The observed k(T) dependences are not easily anticipated between the studied reactions. The lack of pressure dependence and the high reaction rates which increase with decreasing temperature suggest that the potential energy surfaces for these reactions are either barrierless or the barrier is submerged below the energy of the separated reactants. To uncover the details of the reaction mechanism at low pressures, a method suitable for observing nitro and alkoxy species would be helpful. Acknowledgment. R.S.T. and M.P.R. appreciate the support from the CoE of the Academy of Finland. References and Notes (1) Warnatz, J.; Maas, U.; Dibble, R. W. Combustion, 4th ed.; SpringerVerlag: Berlin, 2006. (2) Sander, S. P.; Friedl, R. R.; Ravishankara, A. R.; Golden, D. M.; Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Keller-Rudek, H.; Finlayson-Pitts, B. J.; Wine, P. H.; Huie, R. E.; Orkin, V. L. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modelling:
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