The Journal of Physical Chemistty, Vol. 83, No. 12, 1979
Rate Constants for HO, Reactions
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(34) S. W. Benson and J. H. Buss, d . fhys. Chem., 61, 104 (1957). (35) M. Szwarc, Chem. Rev., 47, 75 (1950). (36) "Partlcubte Polycyclic Organic Matter",National Academy of Sciences,
(31) R. D. Smith, manuscript in preparation. (32) (a) D. M. Golden, G. N. Spokes, and S.W . Benson, Angew. Chem., Int. Ed. Engl., 12, 534 (1973);(b) S.W. Benson, "Thermochemical Kinetics", 2nd ed., Wiley, New York, 1976. (33) R. D. Smith, unpublished work.
Committee on Biologic Effects of Atmospheric Pollutants, Washington, D.C., 1972.
Rate Constants for the Reaction of HO, with HOP, SO2, CO, N20, trans-2-Butene, and 2,3-Dimethyl-2-butene at 300 K Richard A. Graham," Arthur
M. Winer, Roger Atkinson, and James N. Pltts, Jr.
Statewide Air Pollution Research Center and Department of Chemistry, University of California, Riverside, California 9252 1 (Received December 27, 1978) Publication costs assisted by the National Science Foundafion
Relative rate constants have been measured at 300 K and 1atm total pressure for reactions of HO2 with HO2, SOz, CO, N20,truns-2-butene, and 2,3-dimethyl-2-butene. The thermal decomposition of HOZNO2was used as a source of H02 radicals, and rate constants were measured relative to that of the H02 + NO2 reaction. A rate constant of 3.8 X 10-l2 cm3 molecule-' s-l (total uncertainty is a factor of 2) was measured for the H 0 2 disproportionation reaction; the good agreement of this value with the average literature value of -3 x cm3 molecule-l s-l supports the validity of the present experimental techniques. Rate constant upper limits 2X and 4 X 10-ls cm3molecule-' s-l were obtained for the reactions of H02with S02, NzO, of 1 X and trans-2-butene, respectively; these values indicate that the reactions will be of negligible atmospheric importance. Significant reaction rates were observed with CO and 2,3-dimethyl-2-buteneand may correspond cm3 molecule-l s-l, respectively; these values must and 4 X to HO, rate constants as large as 2 x be regarded as only upper limits because of the possible effects of large amounts of the reactants on H02N02 in the gas phase or on the cell surfaces.
Introduction Hydroperoxyl radicals (HO,) have long been recognized to play an important role in hydrocarbon oxidation processes a t combustion temperatures.lI2 Furthermore, the important role of H 0 2 in the chemistry of the tropo~phere~ and - ~ the s t r a t ~ s p h e r e ~has - ~ become ~ recognized in recent years. However, it is only in the past 2 years that directly measured rate constants for the reactions of HOz radicals with 0,13OH,13N0,14J5and have become available. The reaction of the H 0 2 radical with SOz has been considered to be a significant process for SO2oxidation in both clean and polluted air in the tropo~phere.'~The only measurement of the rate constant for this reaction to date used a photochemical competitive isotope labeling technique and obtained a value of (8.7 f 1.8) X cm3 molecule-* s-l a t 300 K.18 However, this rate constant clearly needs to be redetermined since the rate constant for the reaction of H 0 2 with NO determined in the same study18 was a factor of -25 lower than the recently measured absolute value.14J5 The thermal decomposition of H 0 2 N 0 2has been well c h a r a c t e r i ~ e dand l ~ ~can ~ ~ serve as a kinetic source of H 0 2 radicals. Competitive rate constants for H 0 2 reactions can then be determined by comparison to directly measured values of the H 0 2 + NO2 rate constant:16 HOz + NO2 H02+ R
M
-
HOzN02 products
(12)
(3) This method has been used in the present work to measure relative rate constants for the reactions of H 0 2 with HO,, SOa, CO, N20, trum-2-butene, and 2,3-dimethyl-2-butene 0022-3654/79/2083-1563$0 1.OO/O
a t 300 K and 1 atm total pressure. The two alkenes were studied since no reliable data presently exists21for the rate constants for alkene reactions with the H 0 2 radical at ambient conditions, and until recently these reactions were considered to be significant in photochemically polluted N20 was also included since no data are atmo~pheres.~ available for its reaction with H02,21and this reaction might thus be an unrecognized tropospheric sink for N20.22 Measurement of the rate constant for the H02 disproportionation reaction can be compared against results in the literature to confirm the validity of the experimental system: HO2 + HO2 H202 + 02 (4) +
The technique is further supported by a preliminary rate cm3 molecule-l s-l constant m e a s ~ r e m e n of t ~ 7.5 ~ x (total uncertainty is a factor of 2) for the HOz + NO reaction at 269 K and 1atm total pressure which compares favorably with absolute room temperature measurements of this rate constant.14J5 Experimental Section The apparatus and experimental techniques used in this study have been described p r e v i o u ~ l yand , ~ ~only ~ ~ ~a brief summary will be given here. The temperature inside a cylindrical, Teflon-lined 5800-L evacuable chamber was controlled to 300.0 f 0.2 K by ethylene glycol circulated through channels on the chamber's outer surface. A Teflon disperser tube for reactant injections and two 250-L s-l stirring fans inside the chamber ensured rapid mixing. The chamber pressure was measured by an MKS Baratron capacitance manometer. The chamber was evacuated to 10.02 torr between experiments by a liquid-ring pump and 0 1979 American
Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 12, 1979
Graham et al.
TABLE I: HO, Reaction Rate Data Summary IRIS [NOzlav? compd
torr
ppm
1o5kwa1i,s - ' (tot press., torr)
kexpt 5-l)
k, (cm3molecule-' s-l)
recommended rate const (cm3 molecule-' s-' )
1
6.5 7.6 i 0.9 (760) (3.9'::; ) x 10-lZC 3.8 x 6.5 4.5 i 2.6 (760) (3.81;:; ) x trans-2-butene 5.0 39 3.3 i 0.4 (755) 3.6 t 0 . 3 (2.9 i 1.2) x
