J . Phys. Chem. 1987, 91, 1215-1222 (Table 111). In either mechanism, namely, nucleophilic attack
or charge transfer, one would expect a decrease in the reactivity as the ionization potential of polyenals decreases upon increasing the number of double bonds. This is in fact evident from the quenching rate constants given in Table 111. The fact that the absorption maxima (565-590 nm) of C,o aldehyde radical anion in CTAB and Triton X-100 micelles are comparable to those in alcohols (A, = 555 nm in methanol) suggests a polar, hydrogen-bonding nature of the environment for the long-chain polyenal. We note that, on going from nonprotic solvents (polar/nonpolar) to protic ones, the absorption maxima or radical anions of polyenals (e.g., retinal)7cundergo pronounced blue shifts. Thus, the maxima in both Triton X-100 and CTAB being located at longer wavelengths than in methanol indicate a less protic (hydrogen-bonding) nature of the environment in either of the micelles (relative to methanol). In agreement with the case of retinal,7cwe find the protonation of Cu,aldehyde radical anion to be slower in CTAB than in Triton X-100. This is interesting, because in the cationic micelle one would expect the anion to be located close to the positive interface, and hence water should be more easily accessible to the anion.
Mechanism and Kinetics of Br 4- HO, the Temperature Range 260-390 K
-
1215
Furthermore, the fact that the radical anion maximum is con= 565 nm) than in Triton spicuously blue-shifted in CTAB (A, = 590 nm) suggests a priori a higher protic character X-100 (A,of the environment (Le., greater availability of water) in the former. It appears that the ion pairing of the radical anion with the cationic head groups in CTAB results in both blue shift in ,A, and lower reactivity with water. Such effects of cation pairing have been revealed in our recent in~estigation'~ of the behavior of both retinal and C,o aldehyde radical anions in the presence of salts in weakly polar solvents.
Acknowledgment. We are grateful to Dr. E. J. Land and Hoffmann LaRoche for generous gifts of C3,, aldehyde and @cyclocitral, respectively. We thank Dr. P. Neta for many fruitful discussions. Registry No. 1, 432-25-7; 2, 79-77-6; 3, 6980-79-6; 4, 116-31-4; 5, 6985-27-9; 6, 106231-88-3; 7, 1107-26-2; HFIP, 920-66-1; TEA, 12144-8; THF, 109-99-9; CTAB,57-09-0; H20, 7732-18-5; Br-,24959-67-9; Triton X-100,9002-93-1; acetone, 67-64-1; methanol, 67-56-1. (19) Bobrowski, K.; Das, P. K., in preparation.
HBr 4- O2 and Br
+ H,02
-
Products over
Darin W. Toohey,* Wm. H.Brune, and J. G. Anderson Department of Chemistry and Center for Earth and Planetary Physics, Harvard University, Cambridge, Massachusetts 02138 (Received: August 18, 1986)
A discharge flow system employing simultaneous, direct detection of HOz and Br using laser magnetic resonance and resonance fluorescence, respectively, is used to study the kinetics of the title reactions. Over the temperature range 260 to 390 K, decays of HOZin excess Br yield the rate constant kl = (1.4 f 0.2) X 10-l' exp[(-590 f 140)/Tl cm3 molecule-' s-l for Br H 0 2 HBr 02.Experiments were carried out at 0.8 to 2 Torr total pressure in helium and in argon with multiple Br sources. The cited uncertainties include an estimate of the systematic errors at the 95% confidence level. Additional studies place cm3 rholecule-I s-l on the overall rate constant and product channels forming OH and H 0 2 for an upper limit of 5 X the reaction Br + HzOz products from 298 to 378 K. These results, in conjunction with data from other H 0 2 reactions, suggest that X + H 0 2 HX + Oz reactions (where X = F, C1, Br, and OH) proceed by direct attack on the H-0 u bond, rather than by radical addition to form a (HOOX)* intermediate followed by rearrangement and elimination of HX and O2from a modified four-center transition state. The implications of these studies for stratospheric chemistry are discussed.
-
+
+
--
Introduction Reactions of HO, with atoms and radicals (here denoted by X) are of great interest to chemists because they represent a class of reactions that can proceed by two different mechanisms: direct or formation of a hydrogen attack by X to form H X and 0,; (HOOX)* intermediate by radical combination followed by either 0-0 bond cleavage to form XO and O H , or rearrangement and elimination of H X and 0,.Rate constants which display thirdbody effects and negative temperature dependences or reactions which proceed rapidly at room temperature have been cited as evidence of a mechanism dominated by radical addition.' Alternatively, a direct abstraction reaction is expected to be slower and exhibit a positive temperature dependence with no strong third-body effects. While the X O and OH products are almost certainly eliminated from the (HOOX)* intermediate, the mecan follow direct hydrogen attack tathesis products (HX and 0,) proceeding through the (X. .Ha SO,)*transition state or rearrangement of (HOOX). via a modified four-center transition state: (1) For a review of H 0 2 reactions see (a) Kaufman, M.; Sherwell, J. Prog. Reacr. K i m . 1983, Z2, 1. (b) Kaufman, F.J . Phys. Chem. 1984,88, 4909.
0022-3654/87/2091-1215$01.50/0
[q 020
Consequently, very little is known about the relative importance
of these two processes in the HO,-radical metathesis reactions. Very recently, the reactions of HOz with 0, OH, C1, C10, H , and HOz were investigated with an RRKM treatment developed for reactions proceeding over a potential ell.^.^ Because postulated (HOOX)* intermediates were unable to reasonably reproduce specific experimental temperature and pressure dependences and product distributions, it was concluded that bimolecular H abstraction occurs by direct attack at the hydrogen, even though the formation of (HOOX)* followed by 0-0 bond fission represented the major reactive channel.2 Unlike many other H02-radicalreactions which can proceed by several mechanisms simultaneously, forming products by either H-0 or 0-0bond fission, the reaction of Br with H 0 2 can only proceed by H - 0 bond fission ( la):495 (2) Mozurkewich, M. J . Phys. Chem. 1986, 90, 2216. (3) Mozurkewich, M.; Benson, S. W. J . Phys. Chem. 1984, 88, 6429.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
1216
Br
- +
+ HO,
HBr
BrO
+ 0,
AH = -39 kcal mol-' ( l a )
AH = +9.6 kcal mo1-l
OH
---
Experimental Section
The laser magnetic resonance (LMR)/resonance fluorescence (RF) discharge-flow system has been described e1~ewhere.l~The (4) ExceDt where noted. thermochemical auantities are from Stull. D. R.. Prophet, H.,'Eds. JANAF Thermochemical Tibles, 2nd ed.; U S . Department of Commerce: Washington DC, 1981; NSRDS-NBS 37. (5) To calculate AH for Br H02, a value of AHro300 = 3.5 kcal mol-' for H 0 2 was chosen from Shum, L. G. S.; Benson, S. W. J. Phys. Chem. 1983,
+
87 7479
(6) Day, M. J.; Stamp, D. V.; Thompson, K.; Dixon-Lewis, G. Symp. (Inr.) Combust., [Proc.],13rh 1971, 705. (7) Yung, Y. L.; Pinto, J. P.; Watson, R. T.; Sander, S . P. J . Armos. Sci. 1980, 37, 339. ( 8 ) Spencer, J . E.; Rowland, F. S. J. Phys. Chem. 1978, 82, 7. (9) Wofsy, S. C.; McElroy, M. B.; Yung, Y. L. Geophys. Res. Len. 1975,
2.215 (10) McElroy, M. B.; Salawitch, R. J.; Wofsy, S. C.; Logan, J. A. Nature (London) 1986. 321. 759. . (1 1) Poser, J.; Shkrwell, J.; Kaufman, M.Chem. Phys. Lett. 1981, 77,476. (12) Unless otherwise specified, rate constants data are from DeMore, W. B.; Margitan, J. J.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. "Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling: Evaluation Number 7", JPL Publication 85-37, Jet Propulsion Laboratory, Pasadena, CA, 1985. (13) Poulet, G.; Laverdet, G.; LeBras, G. J . Chem. Phys. 1984, 87, 3479.
f
LIGHT BAFFLES
LIGHT
F2
N,0
WINDOW
OUTLET
INLET
Figure 1. Cross-sectional view of the bromine atom RF detection system. Reactor flow is perpendicular into the page. Dotted lines indicate light path.
+ + C1 + O3 C10 + 0, C10 + BrO C1 + Br + O2 O3 + O3 30,
This mechanism has been implicated as a contributor to observed total ozone column depletions of 40% immediately following Antarctic winter within the southern polar vortex.I0 Because reaction l a followed by downward diffusion of HBr represents an important removal process for reactive bromine, accurate knowledge of k , and its temperature dependence is necessary in order to assess the impact of bromine compounds on the earth's ozone layer. The first measurement of k l by Posey et al." indicated that reaction 1 was slow. Using a mass spectrometer to monitor both Br concentrations and H 0 2 decays, they found k, = (2.2 f 1.1) X cm3 molecule-I s-', almost two hundred times slower than the C1+ HO, analogue.', More recently, Poulet et al.I3 examined reaction 1 simultaneously with Br + H 2 C 0 and reported a significantly larger value, k , = (7.6 f 0.9) X lo-', cm3 molecule-' s-I. In addition, the temperature dependence was not measured in either study. Therefore, our study, which employed multiple Br sources and simultaneous and direct detection of HO,and Br within the reaction zone, was carried out both to test the mechanism of H02-radical kinetics and to establish the quantitative importance of reaction 1 to the catalytic impact of bromine compounds in the stratosphere and troposphere.
Mg
PHOTOMULTIPLIER 8 ELECTRONICS
(Ib)
Bromine attack on the oxygen orbitals of H 0 2 therefore must be followed by migration and rearrangement to form the HBr and O2products. A study of reaction 1 and its temperature dependence represents an excellent opportunity to assess the mechanism of H0,-radical metathesis reactions. Reaction l a may have an important role as a chain termination step in combustion processes as bromine compounds have a pronounced inhibition effect on flames: and reaction l a represents the most important removal process for reactive bromine species in the earth's stratosphere.' Bromine atoms liberated by ultraviolet photolysis of bromine compounds used as agricultural fumigants, gasoline additives, and flame retardants engage in a series of homogeneous gas-phase catalytic reactions with odd oxygen to destroy 0zone.~3~Of particular species (odd oxygen = 0 + 0,) interest is the potential coupling of the chlorine and bromine cycles through the following reactions in the 20-km region of the atmosphere: Br O3 BrO 0,
net:
Toohey et al.
I
I
11
I
I
1
I
I
1
UNFILTERED SPECTRUM
1
1.2
145
150
155
160
Wavelength (nm) Figure 2. Spectrum of the lamp used for Br detection. The upper spectrum was obtained with the absorption cell purged with N2,the lower spectrum with the absorption cell filled with 1.7 atm of NzO. Arrows indicate emissions to the Br(2P3,2)ground state.
main reactor consists of 70 cm of 2.5-cm4.d. Pyrex immediately upstream from the LMR detection axis. This segment of the flow tube is surrounded by a jacket for circulation of heated or cooled fluids to control the temperature of the reactor. The downstream RF detection axes are connected to the LMR axis by a 30-cm length of 2.5-cm-i.d. Pyrex. When downstream detection axes are employed, the gaps of the LMR and R F axes are closed by insertion of retractable Pyrex tubing sections to minimize wall losses of reactive species. Except where noted, all surfaces are coated with teflon. H 0 2 and Br were produced in separate 6-mm-0.d. Pyrex movable injectors, the positions of which could be varied independently to monitor reaction, sources, and impurities anywhere within the flow tube. HOz was detected by LMR at a magnetic field of 2.3 kG by using the 163-pm CH30H laser emission (14) Brune, Wm. H.; Schwab, J. J.; Anderson, J. G. J. Phys. Chem. 1983, 87. 4503.
Br
+ HO, and Br + Hz02Reactions
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1217
-I
2.0
30t -1
1
[ ~ r ]in reactor -
1
I I I
7- 20
I
I
I
0
1 0
11
o First Calibration
Return Calibration ( 3 hours later)
tP
I I
(
I
8
o
=
;
=
-
W
1.0
[Br] in detector I
m
? E
-
-
7"
-
"
0 0
c
o
"
Br signal (SB')
4t 0
10
3.O
2.0
1.0
-
Figure 3. Examples of Br RF calibrations. Br is formed by the two-step reaction of Br2 in excess 0 (see text): Br2 + 2 0 2Br + 0,.[Br] is determined as twice the measured Br2 concentration.
pumped by the 9(R)38 laser line from a 20-W C W C 0 2 laser and calibrated as before.I4 Under favorable operating conditions 2 X lo8 cm-3 could be detected at S / N = 1. Hydroxyl was monitored with the same laser line at 3.8 kG as described previouslyI4 with a typical sensitivity of 2 X lo7 cm-3 at S / N = 1. A diagram of the bromine atom detection system appears in Figure 1. R F detection of Br employed the ( ~ S ) ~-P(4p5)2P312 ~/~ transition at 158 nm excited by a 30-W, 2.45-GHz discharge of trace Br, in He in a flowing lamp at 2 Torr. The sapphire window of this lamp removed all emissions at wavelengths shorter than 145 nm. A 0.5-cm cell situated between the lamp and flow tube through which N 2 0 flowed at 1.5 atm served as a filter to pass 158-nm emission. The Br resonance fluorescence (RF) signal was monitored with a pulse-counting photomultiplier (EMR 541 J) mounted perpendicular to the lamp axis. The vacuum-UV spectrum of this lamp system appears in Figure 2. For calibration of the Br axis, measured flows ( 5 X lOI3 ~ m - ~ ) produced by discharge of 0, in He. The rapid reactions 0 Br, BrO Br and 0 BrO Br 0, produce two bromine atoms for each bromine molecule consumed, with negligible wall loss of Br under conditions of excess O.I5 The 0 atom concentration and addition point of Br, were carefully varied to maximize production of Br. An example of the Br R F calibration plots appears in Figure 3. The calibrations were linear up to 1.4 X 1013cm-3 with negligible intercepts. Typical R F sensitivity was 2X cm3 counts s-l with a background signal of 20 counts s-I from Br2 photolysis within the detection region. For 10-s integration periods, the useful detection limit was 0.5[0] to eliminate 0 atoms before injection of Br into the main reactor. As a precaution, the absence of 0 atoms was confirmed 0 triplet for each experiment by R F detection of the (3S1-3P2,1,0) emission at 130 nm as described p r e v i o ~ s l y with , ~ ~ an 0 atom detection limit of 2 X lo8 ~ m - ~Investigations . described in the next section were carried out to ensure that impurities did not interfere with the kinetics of reaction 1. Laboratory gases were supplied by Matheson with the following stated purities: H e (HP, 99.99%) for bulk flow, H e (UHP, 99.999%) for all discharges, H e (99.9999%) for all mixtures, CF4 (99.7%), NO2 (99.5%), N O (CP, 99.0%), N2 (UHP, 99.999%), 0, (UHF, 99.99%), N 2 0 (UHP, 99.99%), Ar (prep, 99.998%), Ar (UHP, 99.999%), and ClNO (97.0%). N O was passed through an Ascarite trap at atmospheric pressure before use. ClNO was further purified before use by vacuum distillation. H 2 0 2(FMC Corp., >83% mole fraction) was further purified to >99% by vacuum distillation (reducing the volume over 75%) to reduce the partial pressure of H20to below 10% of the H202partial pressure. Br2 (J.T. Baker Chemical Co., >99.8%) was passed over P2OS (J.T. Baker Chemical Co.) to remove H 2 0 and trapped and pumped on at 234 K. CF4, NO,, Br,, and ClNO concentrations were all determined by measured rate of pressure drop in a calibrated volume. All experiments were carried out under pseudo-first-order conditions with Br atoms in excess. For most experiments, the reaction time was varied by moving the Br injector relative to a fixed HOz injector to minimize wall loss corrections; H 0 2 decays were monitored over a factor of 1.2 to 10. Forty-seven experiments were carried out at a temperature of 298 f 2 K, ten at 344 f 2 K, five at 350 f 2 K, and five at 390 f 5 K. Several measurements were made at 260 f 4 K; however, increased H0, and Br wall losses eliminated the possibility of conducting more thorough studies at low temperatures.
Results The rate constant for the Br + HOz reaction was measured by following decays of H 0 2 in the presence of excess Br, a series of which appear in Figure 5 . The observed first-order decays of H02were linear down to the detection limit. Linear least-squares fits (weighted and unweighted) to the observed decays provided the first-order rate constant (kIobd) which was further corrected for loss of decaying reactant on the movable injector (probe loss)
[ ~ r (~10-l~) ] atoms
Figure 6. Plot of k', vs. Br concentration for Br + HO,
at room temperature.
-
HBr + 0,
-
TABLE I: Summary of Data for Br + H 0 2 HBr + O2 temp, no. of 10I2k,,cm3 K conditions' experiments molecule-' s-I 298 0 + Br, source 13 2.06 f 0.04b 0 + Br,, Ar carrier 4 1.95 f 0.10 0 + Br2, 3.0-cm reactor 11 1.98 f 0.10 Br2 discharge, ClNO titration 10 1.85 f 0.06 Br2 discharge, RF detection 9 1.96 f 0.20
334 351 390 262
all results
41
0 + Br2 0 + Br,, 3.0-cm reactor 0 + Br,
10
2.43 f 0.10
5
2.82 f 0.10 3.15 f 0.30 1.6 f 0.4'
5
4
1.98 f 0.05
"Except where indicated, He was the carrier gas, and the reactor tube diameter was 2.5 cm. bError is la. CRougherror based on scatter of results. and axial diffusion of H 0 2 (98%) were achieved.22 In our studies, Br2 discharge efficiencies of less than 15% were typically observed, mainly due to large wall recombination losses of Br. Because recombinative losses were small in the VLPR experiments, Br atoms were likely injected into the reactor shortly after discharge of Br2. The quenching of Br(2Pl/2)by helium is very thus, with the lack of significant wall deactivation, some Br(2P,/2)from the discharge is likely to enter the VLP reactor. As Br(2Pl/2)is 0.48 eV above the ground state, its reaction with H 2 0 2to form HBr and H 0 2 is 10.5 kcal mol-' exothermic and should be faster than the analogous reaction of ground-state Br atoms. Thus, mass spectrometric observations of Br atom consumption would yield the sum of the rates of reaction of ground- and excited-state bromine atoms with H202. Reactions of H 0 2 with Radicals: Reactivity Trends. The large difference between the.activation barriers for reactions 1 and 2a suggests a correlation of the rate constants with the exothermicities of the reactions, in particular, a correlation of the experimental activation energies with the heats of reaction. A continuation of the figure for the X + H 2 0 2series presented by Friedl et al.24 to include the X + H 0 2 HX + O2 series appears in Figure 10. A lower limit for the activation energy of the Br + H 2 0 2 cm3 reaction was obtained by assuming an A factor of 4 X molecule-' s-' (estimated for Br + H 2 0 2 previously22)in conjunction with the experimental upper limit on the high temperature rate constant of Br + H202. The activation energies for the F + H 0 2 and F + H 2 0 2reactions were estimated by assuming A factors of 1 X lo-'' cm3 molecule-' s-' (similar to Br + H 0 2 and C1 + H 0 2 ) and 4 X lo-'' cm3 molecule-' s-' (analogous to F + H 2 0 ) respectively in conjunction with the room temperature rate constant measurement^?^*^^ and these seem reasonable since both reactions proceed rapidly at room temperature. The X H 2 0 2 series was described well by the bond energy-bond order (BEBO) and, indeed, these reactions fall well within the range of a variety of hydrogen abstraction reactions by OH, C1, and F.
4 3
f -
X+H,O,dHX+HO, X +H02-
HX+O,
X+HO,=
(HOOX?4HX+O2
-
-
+
(22) Heneghan, S. P.; Benson, S. W. Znt. J. Chem. Kinet. 1983,15, 131 1. (23) Donovan, R. J.; Husain, D. Chem. Rev. 1970, 70, 489. (24) Friedl, R. F.; Brune, Wm. H.; Anderson, J. G. J. Phys. Chem. 1985, 89, 5505. (25) Zahniser, M. S.; Stanton, A. C. J . Chem. Phys. 1984, 80, 4951. (26) Hampson, R. F. "Chemical Kinetic and Photochemical Data Sheets for Atmospheric Reactions", U.S. Department of Transportation, Report No. FAA-EE-80- 17, Washington, DC, 1980. (27) Johnston, H. S. Gas Phase Reaction Rate Theory; Ronald Press: New York, 1966.
-a L 7
.L
I
-80
I
I
I
1
I
-60 -40 -20 AHreaction (kcal mol-')
0
Figure 10. Diagram of experimental activation energy vs. heat of reaction for radical/H02 and radical/H202 metathesis reactions. The shaded region and dashed line indicate the range of the BEBO model as discussed by Friedl et al.24 All data is from reference 12 except for S H + H202?4Br H 0 2 (this work), F H202,20F + H02?5 and H + H202.26 See text for estimation of the activation energies for F HOz, F + H202, and Br + H202. For the OH H 0 2 reaction, only the pressure-independent results are plotted.
+
+ +
+
The metathesis reactions of OH, Br, C1, and F with H 0 2 , which are considerably more exothermic than the reactions with H202, are also described well by the BEBO model, which predicts negligible activation energies for reactions with greater than 40 kcal mol-' exothermicity. This trend is also reminiscent of that recently discussed by Dunning et a1.28for H + H X abstraction reactions. Only the reactions of H 0 2 with the larger radicals (C10, H02, and CH3O2) deviate significantly from the BEBO curves. In addition, these three reactions have A factors of (0.8-5) X cm3 molecule-' s-I, much smaller than the (1-2) X lo-'' cm3 molecule-' s-' range displayed by the other X + H 0 2 metathesis cm3 reactions, and not unlike the estimate of (0.2-4.2) X molecule-' s-' for the Br H 0 2 four-cefiter channel. At room temperature, these reactions very likely involve fast radical combination, followed by rearrangement of a long-lived intermediate complex into a tighter cyclic intermediate before elimination. It is interesting to note that the rate constant for the C10 + H 0 2 cm3 molecule-' s-' at reaction reaches a minimum of 5.5 X high temperatures, displaying only a slight negative temperature dependence (-0.2 kcal mol-') between 340 and 393 K.29 Above room temperature, where the direct hydrogen attack mechanism should dominate, this reaction also fits the curve in Figure 10. The BEBO model for hydrogen atom transfer predicts an asymptotic value of 0 kcal mol-' for the activation energy at large reaction e ~ o t h e r m i c i t y ;however, ~~ the reactions of C1 and O H with H 0 2 have negative experimental activation energies. This might suggest the existence of a long-range interaction to form a X-*H02complex early along the reaction coordinate (see Figure 9) as proposed recently for C1+ H 0 2 HCl + O2 to account for somewhat large A factors for H atom abstraction.2 A recent ab initio investigation of the hydrogen-bonded OH-. H 0 2 intermediate30 found a 4.7 kcal mol-' stabilization at an equilibrium distance of 2 A. This represents a significant interaction which can likely account for the negative temperature dependence observed for the pressure-independent channel of the O H H 0 2
+
-
+
(28) Dunning, T. H., Jr.; Harding, L. B.; Bair, R. A.; Eade, R. A.; Shepard, R. L. J. Phys. Chem. l986,90, 344. (29) Stimpfle, R. M.; Perry, R. A.; Howard, C. J. J . Chem. Phyi. 1979, 71, 5183. (30) Jackels, C. F.; Phillips, D. H. J. Chem. Phys. 1986, 84, 5013.
J . Phys. Chem. 1987, 91, 1222-1225
1222
-
+
H 2 0 O2 reaction. However, it is unlikely that a hydrogen-bonded intermediate can produce a negative temperature dependence for the atom + H 0 2 reactions in Figure 10 as atoms possess no permanent dipole moment. Alternatively, it does not appear necessary to invoke a long-range attraction for these atom + HO, reactions since only the C1 HO, HCl + O2reaction exhibits a negative temperature dependence, and indeed only a negligible one within experimental error.31 More likely, the explanation for the somewhat large A factors for the X HOz HX 0, series lies in the Hammond postulate, which in essence states that the transition state of a direct reaction increasingly resembles reactants as the exothermicity of the reaction increases.32 The X H02 HX + O2reactions, with unusually large exothermicities (40 kcal mol-' greater than the X + H202 counterparts), would thus be expected to have somewhat large A factors as the transition states would occur early along the reaction coordinate. The following picture then emerges for the reactions of HOz with radicals. Formation of the (XOOH)* complex is a rapid process, occurring at or near the gas kinetic rate. At high pressure, this intermediate can undergo collisional stabilization, but at low pressure the lifetime of the excited complex is long enough for the excess energy to induce bond breakage of either the H-O bond (reformation of reactants) or the 0-0 bond (formation of X O and OH products). Much slower is the direct attack on the hydrogen atom to form the H X and O2products; however, this
+
-
-
+
+
+
-
(31) Lee, Y.-P.; Howard, C. J. J . Chem. Phys. 1982, 77. 756. (32) Hammond, G. S. J . Am. Chem. SOC.1955, 77, 334.
process is still significantly faster than the rearrangement of the (HOOX)* intermediate into a modified four-center transition state, unless the intermediate is sufficiently large (Le., X = C10, H 0 2 , etc.) to allow for a favorable geometry for elimination of H X and 02. The implications of our results for stratospheric chemistry are significant. Above 40 km, the Br HO, reaction represents the major pathway for conversion of odd-oxygen-destroying bromine radicals into the temporary HBr reservoir. Current evaluations'2 cm3 molecule-' s-I for use in suggest a value of kl = 8 X computer modeling schemes of the stratosphere. At stratospheric temperatures our value is 25100% larger than the recommended value, and calculated profiles for bromine species should be very sensitive to this change. In addition, reaction 1 followed by downward diffusion of HBr represents the major removal process for stratospheric bromine. Since most of the bromine in the stratosphere is in the reactive BrO form,'v9 a larger rate constant for reaction 1 would imply an increase in the ratio of [HBr] to [Br] + [BrO] and thus a decrease in the catalytic destruction rate of ozone for a given total amount of bromine. The fractional contribution of bromine to the total global budget of stratospheric ozone awaits measurement of the concentration of BrO throughout the middle atmosphere.
+
Acknowledgment. We acknowledge valuable discussions with Charles Jackels and, in addition, thank Lee Loewenstein and Jonathan Abbatt for experimental assistance. This work is supported by the National Science Foundation, Grant ATM-8 1151 12. Registry No. H 0 2 , 3170-83-0; H202, 7722-84-1; Br, 10097-32-2.
Hydrogen-Atom Abstraction from Alkanes by OH. 6. Cyclopentane and Cyclohexane August T. Droege? and Frank P. Tully* Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550 (Received: September 11, 1986)
Absolute rate coefficients for the reactions of the hydroxyl radical with cyclopentane-hlo ( k J , cyclopentane-dlo (k,), cy~lohexane-h,~ ( k 3 ) ,and cyclohexane-d12(k4)were determined by the laser photolysis/laser-inducedfluorescence technique. Kinetic data were obtained at a pressure of 400 torr of helium over the temperature ranges 295-491 (kl), 295-602 ( k , ) , 292-491 ( k 3 ) ,and 292-603 K (k4). Rate-coefficient results are fitted to the modified-Arrhenius expressions k , ( T ) = 6.04 X 10-'6T1.52 exp(+220 cal mol-l/RT) cm3molecule-' s-l, kt(7') = 4.50 X 10-15T'.21exp(-511 cal mol-'/RT) cm3molecule-' SKI, k3(T) = 1.09 X 10-15T1.47 exp(+249 cal mol-'/RT) cm3molecule-' s-I, and k,(T) = 3.48 X 10-16T'.62 exp(-112 cal mol-'/RT) cm3 molecule-' s-l. Rate coefficients for abstraction of hydrogen atoms from methylene sites are determined and compared with our previous measurements on other alkanes. The kinetic isotope effect for abstraction of H and D atoms from cyclopentane and cyclohexane is characterized, and it is found to be very similar to that measured for H- and D-atom transfer to OH from the secondary sites in propane and n-butane.
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 detailed chemical models. In the previous five papers in this series,'-5 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, isoSandia National Laboratories Postdoctoral Research Associate. Present address: Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA 94550. *Author to whom correspondence should be addressed.
0022-3654/87/2091-1222$01.50/0
butane, and n-butane. The reactivities of individual abstraction sites were found to independent of the hydrogen isotopic content of neighboring alkyl groups within the alkane molecule. Kinetic isotope effects were characterized, and, for the reactions of O H with propane, isobutane, and n-butane, branching ratios for the formation of isomeric product radicals were determined. In this paper we report kinetic measurements on the reactions of O H with cyclopentane, cyclopentane-dlo, cyclohexane, and (1) Tully, F. P.; Koszykowski, M. L.; Binkley, J. S. Symp. ( h t . ) Combusr. [Proc.],20th 1984, 715-721. ( 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. 1986,
90, 5932. (5) Droege, A. T.; Tully, F. P. J . Phys. Chem. 1986, 90,5937.
1987 American Chemical Society
