J. Phys. Chem. 1986,90, 5937-5941 range 290-903 K. For reaction of O H a t the secondary site in propane, kHS/kDSvaries from 2.6 to 1.5 over a comparable temperature range. As discussed above, for reaction of OH at the tertiary site in isobutane, kHT/kDT varied from 1.9 to near (or below) the classical limit of 1.4 as the temperature increased from 300 to 750 K. Theoretical attempts to reproduce this trend in kH/kD magnitudes are in progress.
Summary We have accurately determined rate coefficients for H- and D-atom abstraction by OH from selectively deuterated isotopes of isobutane. Consistent with our results on the OH ethane and OH propane reactions, we find that the reactivity of an individual methyl or methylidyne group in isobutane is independent
+
+
5937
of the hydrogen isotopic content of the remainder of the molecule. From our measurements, we have determined temperature-dependent branching ratios for the formation of tert-butyl and isobutyl radicals in the reaction of OH with i-C4HI0and have characterized the kinetic isotope effect for tertiary H- and D-atom abstraction from isobutane by OH. Acknowledgment. This research was supported by the Division of Chemical Sciences, the Office of Basic Energy Sciences, the U. S. Department of Energy. We thank Dr. R. Atkinson for communicating his work prior to its publication. Registry No. (CH,),CH, 75-28-5; (CH3),CD, 13183-68-1; (CD,),CH, 13275-39-3; (CD,),CD, 19170-96-8; Dz, 7782-39-0; HO', 335257-6; N,O, 10024-97-2; 0, 17778-80-2; HzO, 7732-18-5; Hz, 1333-74-0.
Hydrogen-Atom Abstraction from Alkanes by OH. 5. n-Butane August T. Droeget and Frank P. Tully* Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550 (Received: March 14, 1986)
Absolute rate coefficients for the reactions of the hydroxyl radical with n-butane (k,) and n-butane-dlo (k,) were determined by using the laser photolysis/laser-induced fluorescence technique. Kinetic data were obtained at a pressure of 400 Torr of helium over the temperature ranges 294-509 K (k,)and 294-599 K (kz). Rate-coefficient results are fitted to the modified Arrhenius expressions kl(T) = (2.34 X 10-'7)T1.9s exp(+267 cal mol-'/RT) cm3molecule-' s-I and k,(T) = (2.92 X 10-L8)T2~20 exp(+65 cal mol-'/RT) cm3 molecule-' s-I. Utilizing the results of previous OH + alkane studies, we determine site-specific rate coefficients for H- and D-atom abstraction by OH from the methyl and methylene groups in n-butane. The kinetic isotope effect for abstraction of H- and D-atoms from the methylene sites in n-butane is characterized, and it is found to be very similar to that measured for H- and D-atom transfer to OH from the secondary site in propane.
Introduction Hydrogen-atom abstraction from hydrocarbons by hydroxyl radicals plays a fundamental role in the chemistry of atmospheric and combustion processes. Numerical modeling of these processes requires a large data base of accurate rate coefficients spanning a wide range of temperatures and pressures. Both the absolute rate coefficients and the branching ratios for formation of isomeric product radicals are required input to chemical models. In the previous four papers in this series,I4 we described measurements of the rate coefficients for H- and D-atom abstraction by OH from primary, secondary, and tertiary C-H and C-D sites in neopentane, 2,2,3,3-tetramethylbutane,ethane, propane, and isobutane. The reactivities of individual abstraction sites were found to be. independent of the hydrogen isotopic content of remote alkyl groups within the alkane molecule. Kinetic isotope effects were characterized, and for the reactions O H + C3Hs products and OH + i-C4HI0 products, branching ratios for the formation of isomeric product radicals were determined. In this paper we report kinetic measurements on the reactions of OH with n-butane and n-butane-dlo. We determined absolute rate coefficients, k l and k2, for the reactions
-
-
OH
+ D3C(CD2)2CD3
HDO +
HDO
+ D2C(CD,),CD, + D3CCDCD2CD3
(2)
respectively. Our data for reactions 1 and 2 are compared with measurements from prior studies, and our results for reaction 1 t Sandia
National Laboratories Postdoctoral Research Associate.
0022-3654/86/2090-5937$01.50/0
are compared with modified Arrhenius expressions derived by using either previous experimental data or transition-state theory. Using several assumptions based on the results of our previous OH alkane studies, these data permit us to determine sitespecific rate coefficients for H- and D-atom abstraction by OH from the methyl and methylene groups in n-butane. We characterize the kinetic isotope effect for abstraction of H- and Datoms from the methylene sites in n-butane and compare it to our previous measurement for H- and D-atom transfer to O H from the secondary site in propane.
+
Experimental Technique and Data Reduction We performed all experiments using the laser photolysis/laser-induced fluorescence technique. This technique and its application to OH-alkane kinetic studies have been described in detail previously.2 Hence, only a brief summary of the experiment is given here. The basic components of the apparatus are (1) a gas handling and pressure/flow control system that maintains known concentrations of gases in a reactor; ( 2 ) a heated Pyrex reactor mounted inside a vacuum housing; (3) an excimer-laser photolysis source that irradiates the reactor perpendicularly to one of its faces; (4)an intracavity-doubled, single-mode, C W ring dye laser of exciting OH fluorescence in the region of the reactor defined by the perpendicular intersection of the dye-laser and photolysis-laser beams; ( 5 ) a band-pas filter and a photomultiplier for detecting fluorescence emitted at right angles to both laser (1) Tully, F.P.;Koszykowski, M.L.;Binkley, J. S. S y " . (Int.) Combust., [Proc.],20th 1984, 715. (2) Tully, F. P.; Droege, A. T.; Koszykowski, M. L.; Melius, C. F. J. Phys. Chem. 1986, 90, 691. (3) Droege, A. T.; Tully, F. P. J . Phys. Chem. 1986, 90, 1949. (4) Tully, F. P.; Goldsmith, J. E. M.; Droege, A. T. J . Phys. Chem., preceding paper in this issue.
0 1986 American Chemical Society
5938 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Droege and Tully
beams; and (6) fast photon-counting electrgnics, a multichannel TABLE I: Rate Coefficients for the Reactions of OH with n-C4H,, analyzer and a minicomputer for recording and analyzing laand n-C,Dln ser-induced fluorescence signals. We carried out all experiments under slow-flow conditions, so that each photolysis pulse initiated reaction within a locally fresh 294 2.42 f 0.10' 0.893 f 0.037 gas mixture. This procedure eliminated kinetic complications 332 2.95 f 0.12 1.13 f 0.05 caused by unwanted accumulation of photolysis or reaction 377 3.53 f 0.15 1.49 f 0.06 products. Individually controlled gas flows of n-butane/He, 439 4.56 f 0.19 2.07 f 0.09 N20/He, H,O/He, and He dihent were turbulently mixed before 509 5.84 f 0.25 2.87 f 0.12 entering the reactor. The composite flow conditioned the reactor 599 3.98 f 0.17 for several minutes prior to the onset of data collection, thereby Uncertainties represent f 2 u estimates of the total experimental minimizing any effects due to reactant adsorption on the cell walls. error. Total pressure in the reactor was measured at the gas outlet side. The gas temperature in the reaction zone was measured with a 1 1 retractable chromel/alumel thermocouple and was constant to within f 2 O C over both the dimensions of the probed volume and the duration of the experiment. Hydroxyl radicals were formed by 193-nm photolysis of N 2 0 to O(lD) and N2 and the subsequent reaction of O(lD) with H20. For our experimental conditions, this process was 95% complete in less than 20 ps, and niore than 95% of the O(lD) formed in the primary photodissociation process was converted to OH.5 Typical reaction mixtures used in these experiments consisted of the following concentrations in molecules ~ m - ~N,O, : 1.3 X H 2 0 , 3.3 X 10l5;n-butanes, (0-3.8) X He, (6.4-13.2) X loi8. We varied the photolysis flux density over the range 1-4 mJ cm-2 and observed no significant changes in the kinetics. However, I * 4 secondary-reaction kinetic interference was observed for reaction * 1 at T = 599 K and for both reactions 1 and 2 at temperatures above 600 K, data from these experiments are not reported in this 1 0 " 1 paper. Typical values fbr the total flow rate and the initial O H 1 2 3 4 concentration were 380 sccm and 7.5 X lolo molecules ~ m - ~ , 10001T(K) respectively. Figure 1. Arrhenius plot of the kinetic data for reaction 1: 0,-, this Following reaction initiation, we measured time-resolved [OH] work; +,Anderson and Stephens;' 0,Greiner:! V,Gorse and V~lman;~ profiles as functions of the n-butane number density using laV, Campbell et a1.;I0A, Perry et al.;" U,Paraskevopoulos and Nip;I2 X, ser-induced fluorescence. The Rl(3) line in the (0,O)band of the Atkinson et aI.;l3 8 , Stuhl;I4 +, Behnke et a1.;I5 0 , Atkinson and AsA2Z+ X211 OH transition was excited by ultraviolet radiation chmann;I6X,Schmidt et al.;" +, Morris and Niki;'* m, Gordon and near 307 nm produced by the dye laser. OH concentration decays Mulac;I90 , Hucknall et a1.;20A, Baldwin and Walker.21The inset is an enlargement of the near-room-temperature data. were obtained by signal averaging 50-400 excimer-laser shots. In all of these experiments, [OH]
