1804
J. Phys. Chem. 1882, 86, 1684-1690
Rates of Reactions of O(3P) with Benzene and Toluene J. M. Nicovlch, C. A. Gump, and A. R. Ravlshankara" Molecular Sclences Branch, Englneering Experiment Station, Georgla Institute of Technokgy, Atlanta, Georgla 30332 (Received: October 1, 1981; I n Final Form: December 11, 1981)
Absolute rate coefficients for the reaction of w3P)with C6H6(kl), c& (/a2), C6H5CH3(k3),and C6H5CD3(k4) were measured, over the temperature range 298-950 K, by using the technique of flash photolysis-resonance fluorescence. The measured bimolecular rate coefficients increase monotonically with temperature and yield the following Arrhenius expressions (in units of cm3 molecule-' s-'): kl = (4.62 f 0.68) x lo-" exp[-(2.47 f 0.08) X 103/n, k 2 = (4.4 f 1.1)X lo-'' expH2.45 f 0.13) X 103/T], k3 = (4.26 f 0.58) X lo-" exp[-(l.gl f 0.07) X 103/T], and k4 = (3.71 f 0.77) X lo-" exp[-(1.88 f 0.10) X 103/T]. The isotopic substitution of ring hydrogen in the case of benzene and the side-chain hydrogen in the case of toluene did not affect the rate coefficient, i.e., kl = k2 and k3 = k4. Kinetic and mechanistic information derived from these measurements is discussed.
Introduction
The reactions of ground-state oxygen atoms, O(3P),with hydrocarbons are important in combustion chemistry (postflame chemistry), and to a lesser extent in atmospheric chemistry. Therefore, it is important not only to measure the rate coefficients for these reactions but also to understand the reaction mechanisms that are involved. Kinetic and mechanistic work has been carried out mostly for O(3P) reactions with aliphatic and olefinic hydrocarbons. Until recently, aromatic hydrocarbons have received very little attention, with all kinetic work that has been carried out restricted to low temperatures, i.e., T < 500 K. Since multiple reaction pathways are feasible at higher temperatures for these large molecules, it is important to study their reactions over a wide temperature range. We have recently initiated a series of studies using the flash photolysis-resonance fluorescence technique where the temperature range is extended to nearly 1000 K and where emphasis is placed also on understanding the mechanisms of the elementary reaction steps. This paper is the first in a series which deals with O(3P)reactions with aromatic hydrocarbons and describes the studies involving benzene and toluene. The kinetics of the reactions of O(3P)with benzene and toluene have been studied below 500 K by using a variety of techniques including pulse radiolysis,l discharge flowESR/mass spectroscopy,2modulation-phase shift,"5 and flash photolysis-NO, chemiluminescence.6 Furuyama and Ebara' did study reaction 2 at temperatures above 500 K, but they did not directly monitor any of the reacting species. Even though the kinetic data on O(3P) benzene/toluene reactions seem, upon a cursory examination, to be in reasonable agreement, there still remain certain discrepancies, especially regarding the exact value of the activation energies. Apart from the kinetic studies, two crossed molecular + benzene (toluene) reactions, beam s t u d i e ~ on * ~ O(3P) ~
+
(1) I. Mani and M. C. Sauer, Jr., Adu. Chem. Ser., No.82,142 (1968). (2)R.A. Bonanno, P. Kim, J. H. Lee, and R. B. Timmons, J. Chen. Phys., 57, 1377 (1972). (3)A. J. Colusai, D. L. Singleton, R. S.Irwin, and R. J. CvetanoviE, J. Phys. Chem., 79,1900 (1975). (4)R. Atkinson and J. N. Pitta, Jr., J. Phys. Chem., 78,1780 (1974). ( 5 ) R. Atkinson and J. N. Pitta, Jr., J. Phys. Chem., 79, 295 (1975). (6)R.Atkinson and J. N. Pitta, Jr., Chem.Phys. Lett., 63,485(1979). (7)S. Furuyama and N. Ebara, Znt. J. Chem. Kinet., 7,689 (1975). (8)T. M. Sloane, J . Chem. Phys., 67,2267 (1977). 0022-365418212086-1684$01.25/0
aimed at elucidating the reaction mechanisms, have been carried out. The conclusions of the two studies confirm some of the findings of previous end-point analysis inve~tigations'~'~ where O(3P)was shown to undergo electrophilic addition to the ring. In addition, many of the details of the 0(3P)-benzene encounter dynamics have been elucidated. The present work was undertaken to obtain rate-constant data as well as possible mechanistic information for the reactions
w3P)+ C6H6 -* products kl
q3P)
k2
products
-
+
q 3 P ) + C6H5CH3
k3
products
(1) (2) (3)
ki
w3P)+ C6H5CD3 products (4) using the technique of flash photolysis-resonance fluorescence over the temperature range 298-950 K. The deuterated species were included to help in the elucidation of the reaction mechanisms. Experimental Section
The utilization of the flash photolysis-resonance fluorescence technique in the study of O(3P)atom reaction kinetics is well established and is amply described in the literature.14 Recently, we have extended the temperature range of applicability of this method to permit rate-constant measurements up to -1000 K. A schematic diagram of the experimental apparatus is shown in Figure 1. The principal system components are (1)a thermostated all-quartz reaction cell equipped with long, jacketed arms connected to O-ring joints, (2) a spark discharge lamp perpendicular to one face of the cell, (3) a CW atomic oxygen resonance lamp perpendicular to the flash lamp, (4) a solar-blind photomultiplier tube per(9) S. T. Sibener, R. J. Buss, P. Caaavecchia, T. Hirooka, and Y. T. Lee, J. Chem. Phys., 72,4341 (1980). (IO) G. Boocock and R. J. CvetanoviE, Can.J. Chem., 39,2436(1961). (11) G. R. H. Jones and R. J. CvetanoviE, Can. J. Chem., 39,2444 (1961). (12)E.Grovenstein,Jr., and A. J. Mosher, J. Am. Chem. SOC.,92,3810 119701. ~. _,. .
(13)J. S. Gaffney, R. Atkinson, and J. N. Pitta, J. Am. Chem. SOC., 98,1828 (1976). (14)A. R.Ravishankara, G. Smith, R. T. Watson, and D. D. Davis, J. Phys. Chem., 81,2220 (1977).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982 1685
Rates of Reactions of O(3P) with Benzene and Toluene
continuum (175-130 nm), O2 is photolyzed to give O(lD) and O(3P)
O ~ D +) o ( 3 ~ )
o2
(5)
and O(lD) was quenched to O(3P)almost exclusively by Ar O(lD)
RESSURE GAUG XTURE OUT
Flgure 1. Schematic diagram of the flash photolysis-resonance fluorescence apparatus used in q 3 P ) atom reaction studies.
pendicular to both the photolysis and resonance radiation beams, and (5) a signal averager and fast photon counting electronics. The all-quartz reaction cell was resistively heated by using electrically insulated nichrome wire windings mounted on its outer surface. The heating element was covered with ceramic felt and layers of stainless-steel radiation shields. The air-cooled jackets on the arms of the reactor made it possible to keep the windows and the O-ring seals at a temperature close to 298 K. The cool joints allowed the use of a variety of window materials, many of which cannot be easily sealed to quartz and then subjected to temperature changes. The utilization of O-ring joints made frequent cleaning and replacement of windows convenient such that good signal levels could be maintained throughout the course of the work. The temperature of the gaseous mixture inside the reactor was directly measured by using a retractable chromel-alumel thermocouple introduced into the reactor through a “cajonnvacuum seal. The temperature gradient along the length of the reaction zone ( 1.5 cm) was found to be -20 K at lo00 K and negligible at 500 K. The radial temperature gradient across this zone (- 1.5 cm long) was essentially negligible at all temperatures. The quoted temperatures are those measured at the center of the reaction zone and thus represent average temperatures. We believe that the quoted temperatures are accurate to -f12 K at lo00 K, and f 2 K at 500 K. The resonance radiation was produced by subjecting a mixture of very small amounts of O2 in He to an electrodeless microwave discharge. The output of the lamp was checked by using a 0.5-m vacuum-UV monochromator. It was found that, in addition to oxygen atom lines at 130 nm, the lamp produced Lyman CY radiation and two very strong lines at 149 nm which are attributed to N atom transitions. Since these extraneous lines not only reduce the signal to noise (S/N) for O(T) detection but could also cause complications due to excitation of other transient species, the lamp output was filtered by 7.6 torr cm of O2 (to cutoff 149 nm) and a CaF2 window (to cutoff Lyman a). The photolysis radiation was produced by discharging a 0.3-pF capacitor through 100-500 torr of N2. The flash lamp light output was collimated and filtered by a sapphire window so that only radiation with X > 145 nm was transmitted into the reactor. In most of the experiments discussed in this paper, O W ) was produced by flash photolysis of O2 at wavelengths between the sapphire cutoff at 145 nm and the onset of absorption a t -195 nm (flash duration 550 ps). In the
-
-
-
+ Ar -,O(3P) + Ar
(6)
such that within -20 ps after the flash all O(lD) was converted to O(3P). Following the flash, the resonance lamp radiation continuously excited a small fraction of O(3P) to an electronically excited state; the resultant fluorescence emanating in a direction perpendicular to both the resonance excitation and photolysis beams was collected by a MgFz lens and focused onto the cathode of a solar-blind photomultiplier tube. Signals were obtained by photon counting and were then fed into a signal averager operated in the multichannel scaling mode. For each decay rate measured, sufficient flashes (10-1000) were averaged to construct a well-defined temporal profile over at least a factor of 20 variation in [O(3P)]. The sensitivity of our apparatus toward detection of O(3P)was determined by creating a known concentration of O(3P)and then measuring the resultant signal. O3 was photolyzed by using 266 nm15 (quadrupled Nd:YAG) in excess N2 hv
O3
Gi-GT O(lD) + O2
-+
+ o2 N2 -,O(3P) + N2
O(lD)
(7)
o(3~)
(8)
and the signal to noise per flash per 1 ms integration was measured. The photolysis laser energy was measured by using a calibrated radiometer, and the O3 concentration was obtained by UV absorption at 253.7 nm. It was found that -3 X 109 cmV3of O(3P)could be detected with a S/N of 1 for 1-ms integration. Based on this calibration, the concentrations of O(3P) used in the kinetic studies are calculated to range from -2 X 1O’O c m 3 (2.5 X 1015cm-3 O2and 2.5-5 flash energy) to -2 X 10l2 cm-3 (2.50 X 10l6 cm-3 O2 and 25-5 flash energy). In certain experiments above 700 K where an O(3P) photolytic precursor other than O2 seemed necessary, a mixture of N20 and N2 was photolyzed at 193 nm (ArF laser) to produce O(3P) N2O
+
O(lD)
193 nm
+ O(’D) N2 -,O(3P) + N2 N2
(10)
The concentration of N2 (-100 torr) was such that all O(lD) was quenched by N2. At high 193-nm laser powers benzene itself underwent some unknown photochemistry (most likely a multiphoton process) which resulted in vacuum-UV emissions. It was necessary to keep the laser energy below 25 mJ cm-2, in order to prevent these emissions from interfering with the resonance fluorescence signal. To increase the O(3P)concentration, we could not increase the N20 concentration indefinitely, since N 2 0 absorbs 130-nm resonance radiation. Therefore, a compromise in laser energy and [N20] was necessary which prohibited large changes (i.e., factors of 10) in either [N20] or laser energy. In order to avoid the accumulation of photolysis or reaction produds and to minimize any uncertainties in [RH] (15) J. C. Brock and R. T. Watson, Chem. Phys. Lett., 71,371 (1980), and references therein.
1686
The Journal of phvsical Chemistry, Vol. 86, No. 9, 1982
arising from aromatic-hydrocarbon adsorption on the reaction walls, we carried out all experiments under "slow flow" conditions. The flow rate through the cell (-3 cm s-l) was such that each photolysis flash encountered a fresh reaction mixture (photolysis repetition rate -0.5 Hz). The aromatic hydrocarbon was taken from a 12-L bulb containing an RH/diluent gas mixture. The RH/diluent gas mixture, 02,and additional diluent gas were mixed before entering the reaction cell. Concentrations of each component in the reaction mixture were determined from measurements of the appropriate mass flow rates (measured by using calibrated mass flowmeters) and the total pressure (measured by using either an MKS Baratron or a one-turn Bourdon gauge). The fraction of aromatic hydrocarbon in the RH/diluent gas mixture was checked frequently by simultaneous measurements of the aromatic-hydrocarbon absorption at 253.7 nm and the total pressure of the mixture. These determinations were carried out by using a Hg Pen-ray lamp as the light source, an 80-cm long absorption cell, and a photomultiplier tube fitted with a 253.7-nm isolation band-pass filter. The absorption cross sections at 253.7 nm used to calculate the RH concentrations in the source mixtures have been previously measured in this laboratory16to be 3.67 X lo-'' Cm2for C6H6, 2.39 x lo-'' Cm2for C6D6, 4.78 x lo-'' Cm2 for C6H5CH3,and 5.26 X cm2 for C6H6CD3. The absolute [RH] values calculated by using these cross sections are accurate to 8%. The stock gases used in this study were obtained from Matheson Gas Products and had the following stated purities: Ar, >99.9995%; He, >99.9999%; 02,>99.99%; N P , >99.9995%. These gases were used as supplied. Benzene and toluene were obtained from J. T. Baker Co. and had analyzed purities of >99.99%. The deuterated aromatics were purchased from Merck Sharpe and Dohme, Canada, Ltd. Their chemical purity was >99.99%, and their selectively labeled isotopic purities were as follows: D. AII aromatic C&, >99.99% D; and C6H&D3, hydrocarbons were degassed before use. N20 was obtained from Matheson Gas Products and had a stated purity of 99.99%; before use, N20 was degassed.
Nicovich et at.
Flgure 2. Temporal profile of q3P) concentration in the presence of benzene: (a) T = 620 K, [CeHe]= 2.11 X 10" cm3, [O,] = 2.65 X 10i5~ m - flash ~ , energ! = 4 J, diluent gas = 100 torr of Ar; note the exponentiallty of [O( P)] decay over a factor of 50 change in [O(3P)]. (b) T = 895 K, [C,,H,] = 3.84 X 1013cm3, [O,] = 4.3 X 10" an3,flash energy = 6 J, diluent gas = 100 torr of Ar; note the ~ O W ~ ~ W H I"W r e of [ocp)]decay. (c) = 911 K, = 7-38 X 10' an3, [N,O] = 3.40 X lo", laser WIWS~= 15 ml an-', dikrent gas = 100 torr of N,; note the exponentiality of [O(3P)] decay.
r
[ce~]
500 I
400 -
300 -
-
'0
.0)
u)
Results All experiments were carried out under pseudo-firstorder kinetic conditions with the aromatic-hydrocarbon concentration in excess over O(3P);[RH]/[0(3P)] > 500. [RH] varied from 1 X 1013to 1 X 10l6cm-3 while [O(3P)] ranged from 2 X 1O'O to 2 X 10l2~ m - ~ In. the absence of secondary reactions which significantly deplete or reform the transient O(3P) species, [O(3P)]decreased exponentially with time: [0(3P)lt = [O(3p)~oe-(k[RHl+kd)t = [O(3P)]oe-k't (I) where k 'is the measured pseudo-first-order rate constant, k is the bimolecular rate constant for the reaction
k
O(3P) + RH products [RH] is the (constant) aromatic-hydrocarbon concentration, and kd is the first-order rate constant for O(3P)disappearance due to diffusion and reaction (with O2or any impurity) in the absence of RH. A plot of In [0(3P)]tvs. t yields a straight line whose slope is k'; one such plot is shown in Figure 2. k' values were, then, measured at various values of [RH]. Figure 3 shows the linear variation of k'with [RH]; the slope of such a plot yields k. The (16)F.P.Tully, A. R. Ravishanha, R.L.Thompson,J. M. Nicovich, R. C. Shah, N. M. Kreutter, and P. H.Wine, J.Phys. Chem., 85, 2262 (1981).
Y
200-
' . . ,O O ,'
0
01'
b
'
'
1
5
'
'
"
'
IO
[C6He] , I OI4 molec~llescm3
+
Flgwe 3. Plot of k'(=k [RH] kd) vs. [Cay]. T = 497 K. The sbpe of the plot yields the bimolecular rate coefficient, k , .
actual values of k were computed by subjecting the k'vs. [RH] data to a linear least-squares analysis. k,-k4 were measured at temperatures above 298 K. We could not measure rate coefficients at T < 298 K because of a limitation of our technique. The rate coefficients became so low that a very large concentration of RH was needed to obtain a measurable increase in k' above kd. When such a concentration of RH was used, however, the resonance lamp radiation was attenuated to an unworkable level due to absorption by RH. In the case of reaction 1 and to a lesser extent that of reaction 2, at temperatures above 700 K nonexponential [O(3P)]decays were obtained when O2photolysis was used as the O ( P ) source. One such nonexponential decay curve
Rates of Reactions of O('P) with Benzene and Toluene
The Journal of Physical Chemist?y, Vol. 86, No. 9, 1982 1687
TABLE I: Rate-Constant Data for the Reactions of O( 'P) with Benzenes and Toluenes lO1'kbimoledar, cm3 molecule-' s-' temp, K benzene benzene-d, toluene
toluene-di
298 0.158 f 0.039 0.714 f 0.089 0.860 f 0.093 326 1.15 f 0.13 364 0.529 * 0.070 376 0.597 0.068 2.36 t 0.26 2.85 * 0.27 392 0.979 i 0.108 3.43 * 0.30 402 1.28 * 0.13 41 7 1.35 t 0.10 440 1.55 t 0.14 4.40 9 0.42 6.12 f 0.51 452 2.39 5 0.21 7.42 f 0.63 484 2.61 +- 0.30 492 2.86 t 0.32 9.96 t 0.85 497 4.21 i 0.36 522 4.89 t 0.41 550 14.3 t 1.6 15.0 i 1.8 13.0 t 1.4 556 6.70 i 0.84 560 5.52 i 0.53 7.15 f 0.73 578 604 16.1 f 1.3 620 9.90 f 1.12 636 20.2 f 2.1 9.39 jl 0.76 640 24.1 t 2.5 660 13.6 f 1.2 718 16.3 t 1.6 30.0 t 3.1 724 17.8 f 1.9 7 29 15.8 i 2.3 74 2 20.2 r 1.7" 749 16.9 f 1.9" 42.3 5 6.4 807 18.3 f 2.3" 39.5 f 4.5 823 22.8 +- 2.2b 826 24.8 f 3.5b 39.3 t 4.2 833 25.5 t 2.ga 38.8 f 3.7 837 25.8 t 4.8" 867 30.1 i 3.6" 932 48.8 t 4.3 944 32.3 t 4.6" 56.0 t 7.6 Curved pseudo-first-order decays, k' estimated from the leading part of the decays. Laser photolysis (193 nm) of N,O/N,mixture as the source of O('P).
is shown in Figure 2. The leading part of these nonexponential decays was faster than the tail. The degree of curvature (i.e., deviation from exponential behavior) was reduced when the O2 (the photolyte) concentration was increased while [O(3P)]o was kept constant; also, the time constant for the decay was reduced. However, the curvature was not noticeably affected by changes in flash energy, total system pressure, linear flow rate of the mixture through the cell, or the benzene (C6D6) concentration. At these temperatures, to study reaction 1, we used 193-nm laser photolysis of N20/N2mixtures as the source of O(3P).This variation made the system wellbehaved, i.e., O(3P)decays were exponential, (see Figure 2) and were insensitive to [O(3P)]o or laser energy. The slopes of these [O(3P)] decays were identical with the fast initial slopes that were obtained when O2was used as the photolyte. Since, for reaction 2, the [O(3P)] decays were linear for at least 2 l / e times under all seta of conditions of pressure, [O,],and photolysis flux, and since the initial slope of O(3P)decay in the case of reaction 1was identical with that obtained by using the alternate O(3P)source, reaction 2 was not studied by using N20 as a photolytic source of O(3P). The rate constants kl-kqobtained by using 100 torr of diluent gas are listed in Table I. The same data are depicted as Arrhenius plots in Figures 4 and 5. The quoted errors are 2u and include not only the statistical errors obtained in the analysis of k'vs. [RH] data but also the estimated systematic errors. The main source of systematic errors in our data is the knowledge of absolute concentrations of the aromatic hydrocarbons. The con-
Flgure 4. A h e n i u s plots (In kvs. 1 / T ) for k , (U),k , (.), k , (O),and k , (0). The straight lines drawn for each case were obtained by subjecting the in k vs. 1 / T data to a linear least-squares analysis.
centrations were obtained by using measured UV absorption cross sections and the mass flow rate of gases measured by using calibrated mass flowmeters. The estimated overall error in concentration measurement is -8% and is based-on our estimates of errors in the above-mentioned quantities. Linear least-squares analyses of the In k vs. 1/T data
1688
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982
TABLE 11: Comparison of Present Results for O( 'P) Previous Measurements
Nicovich et ai.
+ Benzene and O( 'P)
t
Toluene Reactions with
1014k(298K),
10"A,
techniquea temp range, K cm3molecule-' s-l cm3molecule-' s-'
ref
Ea
9
cal mol-'
Benzene
1 2 4
PR DF/MS MPS MPS FP-NO, MPS FPRF
5
6 3
this work (C,H,)
5.98 * 4.51 2.39 t 2.39 f 2.00 t 1.54 t 1.58 t
255-305 299-392 299-440 298-462 298-867
1.16 0.33 0.33 0.20 0.075 0.39
6.31 t 2.5
4400 t 500
1.84 1.68 1.81 t 1.1 4.62
3980 t 3995 * 4200 t 4760 t
0.871 t 0.011
2680 t 190
1.36 1.64 3.82 t 0.18 3.8 t 1.1
3100 t 3050 t 3860 t 3722 t
400 200 430 150
Toluene
1 7
PR DF MP MPS FP-NO, MPS FPRF
4
5 6 3
373-648 299-392 299-440 298-462 298-930
23.2 t 5.0 9.43b 7.43 t 0.75 7.47 r 0.75 9.62 t 0.96 5.68 t 0.25 7.14 t 0.89
300 200 40 200
this work (C,H,CH,) PR = pulsed radiolysis; DF/MS = discharge flowlmass spectrometry; MPS = modulation-phaseshift; FP-NO, = flash photolvsis-NO, chemilumenescence:FPRF = flash photolysis-resonance fluorescence. Obtained by extrapolation of their Arrheniusexpression to 298 K.'
+
o ( ~ P ) BENZENE
10+1
;
'
'
I
'
2
a3P)+ TOLUENE
'
io00
3
T (K)
IO00
'T?iTT Flgure 5. Comparison of present results of k with those of previous measurements: (A)ref 1; (light dashed Hne) ref 2 (. .) ref 5; (heavy dashed line) ref 8; (heavy SOW line) ref 3; (light solid line) this work.
.
for reactions 1-4, shown in Table I, yield the following Arrhenius expressions: kl = (4.62 f 0.68) X lo-'' exp[-(2.47 f 0.08) X 103/r] cm3 molecule-'s-' 298 C T k2
= (4.4 f 1.1) x lo-'' exp[-(2.45 f 0.13)
< 867 K X
103/T] cm3 molecule-l s-'
376 < T < 944 K k3
= (4.26 f 0.58) X lo-'' exp[-(1.91 f 0.07) X 103/T] cm3 molecule-' s-'
298 < T C 932 K
k4 = (3.71 f 0.77) X exp[-(1.88 f 0.10)
X
103/T] cm3 molecule-' s-'
298 < T C 944 K All quoted errors are 2u and uA = AuM. As mentioned earlier, all experiments were carried out under pseudo-first-order conditions. To ensure that sec-
Flgwe 6. Comparison of present results of k, with those of previous measurements: (A)ref 1; (heavy SOHd line) ref 3 (.-.) ref 5; (heavy dashed line) ref 6; (light dashed line) ref 7; (light s d i line) this work.
ondary reactions did not affect our measured rate constants, we varied [O(3P)]o,O2 concentration, photolysis flux, system pressure, and photolysis wavelengths (Le., X > 145 nm using sapphire window or X > 165 nm using quartz window). (In experiments where N20 was subjected to 193-nm laser photolysis, both [N20]and laser flux were varied.) The measured values of kl-k4 were insensitive to these variations. This invariance indicates that O(3P) reactions with photolysis products and reaction products do not contribute to the measured rate coefficients. In addition, it also indicates that O(3P) is not formed by secondary reactions; the exception, that of curved [O(3P)] decays at T > 700 K for reaction 1,will be discussed later.
Discussion The rate coefficients for reactions 1 and 3 have been measured directly by various investigators using many different techniques. Table I1 lists the results of all previous direct measurements that are reported in the literature; the Arrhenius plots are shown in Figures 5 and 6 for benzene and toluene, respectively. It is very clear that the pulse radiolysis study of Mani and Sauer' yielded results which are much higher than those of all subsequent studies. These authors calculated k , and k , based on the
Rates of Reactions of q 3 P ) with Benzene and Toluene
The Journal of Physlcal Chemistry, Vol. 86, No. 9, 1982 1889
growth rate of an uncharacterized UV absorption feature following O(3P) production. Bonanno et al.,2on the other hand, monitored both O(3P) (using ESR)and c6& (using mass spectroscopy) to follow the course of reaction 1 in a discharge flow tube. However, they obtained inconsistent results, Le., kl measured in excess C6Hs did not agree with that measured in excess O(3P). I t seems that in both the pulse radiolysis and the flow tube that the reaction studies were not truly isolated from other secondary reactions. All other direct measurements of kl and k3 involved monitoring O(3P)temporal profiles in the presence of excess C6H6and C7H8. In three of the studies:+ q 3 P )was created via Hg-sensitized decomposition of N20 and then monitored by using its chemiluminescent reaction with NO. In the fourth investigation, that of Atkinson and Pitts? O(3P) was created via vacuum-UV photolysis of O2 and, as before, monitored by using the 0 + NO chemiluminescent reaction. As seen from Table 11, our 298 K values of k1 and k3 are in good agreement with all four studies, the best agreement being with the results of Colussi et al.3 A comparison of the Arrhenius parameters for reactions 1 and 3 obtained in the present study with those from previous investigations reveals a satisfactory agreement. In all previous studies, the temperature range covered was small and hence a large error in the A factor is possible. Inspection of Figures 5 and 6 shows that, over the entire range of temperatures covered by previous studies, all results overlap within the combined error bars. However, the calculated A parameters vary significantly. This is not surprising since small systematic errors (such as concentration error) can lead to large errors when a large extrapolation is needed to obtain the A factor. Furuyama and Ebara' measured k3 in the temperature range of 373-648 K. Even though an extrapolation of their results to 298 K yields a value of k3 which is in good agreement with other studies, their measured temperature dependence is very weak. Their experiments were hampered by surface effects and inexact knowledge of [O(3P)]. Also their experiments yielded relative values of k3. Since the apparatus used in this study was newly assembled and in certain aspects much different from our other flash photolysis-resonance fluorescence apparatus, we measured the rate coefficient for the reaction
-
O(3P) + C2H4
products
(11)
over a temperature range of 298-945 K to check the apparatus. The measured value of kll at 298 K was (6.94 f 0.58) X cm3molecule-' 5-l in excellent agreement with previous measurements."-20 In addition, the fit of lowtemperature (i.e., T < 500 K) data yields an Arrhenius expression k l l = (1.36 f 0.69) X lo-'' exp[-(903 f 182)/T] cm3 molecule-' s-l where errors are 2u and uA = A u v . This expression is in good agreement with other investigations and in excellent agreement with that of Singleton and Cvetanovi~.'~(At higher temperatures we observed a definite non-Arrhenius behavior.) On the basis of these results, we are confident that our measurements are devoid of any major systematic errors. (17)D. L.Singleton and R. J. CvetanoviE, J.Am. Chem. Soc., 98,6812 (1976). (18)D. D. Davis, R. E. Huie, J. T. Herron, M. J. Kurylo, and W. Braun, J. Chem. Phys., 56,4868 (1972). (19)J. T.Herron and R. E. Huie, J. Phys. Chem. Ref. Data, 2, 467 (1973). (20)R. Atkinson and J. N. Pith, Jr., J. Chem. Phys., 67,38 (1977).
Previous indirect studies3JJ0have shown that the O(3P) reaction with benzene proceeds mainly through electrophilic addition to the aromatic ring. This hypothesis has been further substantiated by crossed molecular beam 1 and 2. I t is believed that O(3P) s t u d i e of ~ ~reactions ~~ adds to benzene to form a triplet biradical?P This biradical can subsequently undergo rearrangement to form phenol, H atom elimination, or CO elimination
q 3 P ) + C&
-
-
C&0:
(12)
C6H@: --+ C6H60H CeH.60: C6H60' + H
(13)
+ co
(15)
C6H60:
+
C6&
(14)
Sibener et al.9 have shown that branching ratios for the rearrangement and hydrogen atom elimination are both collision-energy and isotope-identity (i.e., H vs. D) dependent. Our results show that kl and k2 are identical (within experimental error) and that the variations of In kl and In k2 with 1 / T are linear. These observations suggest that the reaction of O(3P)with benzene proceeds via a single elementary pathway over the temperature range of our study and, furthermore, that this channel is addition of O(3P) to benzene. The observed activation energy must then be for the process that leads to the formation of the triplet biradical. On the basis of our kinetic results, it is not possible to obtain any information regarding reactions 13-15, i.e., the fate of the triplet biradical. Sloane observed CO formation in his crossed molecular beam studies of O(3P) plus benzene reaction. This seemed reasonable in light of previous CO observations in high-pressure studies of Boocock and Cvetanovicloand of Gaffney et al.13 However, Sibener et did not see any evidence for CO production. In fact, they showed that CO seen by Sloane8 was not a primary product of O(3P) f C6H6reaction but rather a fragment formed in his mass analyzer. We checked for CO production in this system, albeit under multicollision conditions, by using resonance fluorescence detection of CO. The O(3P) lamp was replaced by a CO 1amp2l(emitting the fourth positive band) and the experiment repeated. We did not detect any CO. On the basis of calibration experiments which determined the sensitivity of our CO detection system, it is estimated that the amount of CO produced is less than 5% of O(3P) that was generated. The pressure in the system was varied not only to check for any pressure dependence but also to quench any CO that could have been formed vibrationally hot. It should be noted, however, that since CO is a stable species, if it had been formed, we would have detected it at least at longer times. This observation tends to support that of Sibener et al. and indicates that CO seen in the previous end-product analysis experiments was probably due to secondary reactions. In the case of toluene, a reaction mechanism for the initial step similar to that for reaction 1 would be operative. In addition, the side-chain hydrogen abstraction is energetically feasible, being approximately 17 kcal mol-' exothermic. However, we see that k3 is essentially identical with k4 over the entire range of temperature and that In k3 and In k4 are linear functions of 1/T. This indicates the absence of a second reaction, i.e., side-chain hydrogen abstraction, in addition to the primary ring-addition pathway. If the side-chain hydrogen abstraction had been (21)T.G.Slanger and G. Black, J.Chem. Phys., 51,4534(1969),and references therein.
1690
J. Phys. Chem. 1982, 86, 1690-1694
as much as 30% of the net even at 900 K, we would have been able to detect a difference in k3 and k4 due to the primary kinetic isotope effect. The exclusion of hydrogen abstraction channels then leads to the conclusion that the increased reactivity of toluene with respect to benzene, i.e., k3(298K) = 5k1(298K), is due to the electrophilic nature of the reaction aided by the methyl group attached to the ring increasing the electron density on the ring. Such a correlation has been seen for the electrophilic addition of OH to a series of aromatic hydrocarbons.22 Finally, a few comments regarding the nonexponential decay of the [O(3P)]in the case of reactions 1 and 2 at temperatures above 700 K are necessary. The nonexponential decays could be caused by either regeneration of O(3P)due to secondary reactions or unwanted detection of some other species (e.g., a product). Since it has been speculated that H/D atoms and CO could be formed in reactions 1 and 2, it was essential to find out whether our system was unintentionally detecting these species. Melton and ~ o - w o r k e r have s ~ ~ shown that there is an accidential coincidence between the oxygen lamp emission and a CO transition. However, with our oxygen lamp configuration (0, filter and CaF, window) it was demonstrated that neither H atoms nor CO could be detected. Therefore, we are led to the conclusion that O(3P)was being regenerated in the system. Such a regeneration could be due either to a secondary reaction or to the thermal decomposition of (22)A. R.Ravishankara, S. Wagner, S. Fischer, G. Smith, R. Schiff, R. T. Wataon, G. Tesi, and D. D. Davis, Znt. J. Chem. Kinet., 10,783 (1978). (23)L.A. Melton and K. C. Yiin, J. Chem. Phys., 62,2860(1975),and references therein.
the 0(3P)-benzene adduct on the same time scale as the forward reaction, as has been observed in the OH-benzene system.16* Our experiments using N20 photolysis as the O(3P) source where exponential O(3P) decays were observed demonstrated that O2was in some way involved in the secondary O(3P) production. One possible source of this O(3P) production was H attoms reacting with O2 to produce O(3P)
H + 0, A, o ( 3 ~+) OH
(16)
This hypothesis would explain why the secondary production was seen only at high temperatures. (At low temperatures, H atoms add to 0, to form HOD) It is worth noting that such an O(3P)production was not seen in the case of reactions 3 and 4, where it has been postulated that H2 from the methyl group, rather than H atoms from the ring, is eliminated.8 However, reaction 16 seems unlikely since the rate coefficient k16 is not fast enough25to reproduce the observed curvature. It is possible that some other reaction product was responsible for O(3P)production, but we cannot at this point identify the source. Experiments designed to measure H atom yields in these systems are currently underway. Acknowledgment. We thank Dr. Paul Wine for all of his help during the course of this work. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy, under Contract No. ER-78-S-05-6030. (24)R.A. Perry, R. Atkinson, and J. N. Pitta, Jr., J. Phys. Chem., 81, 296 (1977). (25)N.Cohen and K. Westberg, J. Phys. Chem. Ref. Data, in press.
Rates of Reactlons of O(3P) with Xylenes J. M. Nlcovlch, C. A. Gump, and A. R. Ravlshankara" Molecular Sclences Branch, Englneering Experiment Station, Gmrgla Instltute of Techno-, (Received: October 22, 198 1; In Flnal Form: December 11, 198 1)
Atlanta, Gmrgla 30332
Absolute rate coefficients for the reactions of O(3P)with o-xylene (kl), m-xylene ( k J , p-xylene (k3),o-xylene-dlo (k4), and m-xylene-dlo( k b ) were measured over the temperature range 298-970 K by using the technique of flash photolysis-resonance fluoresence. Between 298 and 600 K, the rate coefficients obeyed the following Arrhenius relationships (in cm3 molecule-' s-'): k l = (3.90 f 0.82) X lo-'' exp[-(1.54 f 0.08) X 103/Z'l,k2 = (3.78 f 0.84) X lo-" exp[-(1.35 f 0.09) X 103/T],k3 = (3.91 f 0.77) X lo-" exp[-(1.54 f 0.09) x 103/T),k4 = (3.42 f 0.61) X exp[-(1.55 f 0.07) X 103/fl, and k5 = (2.78 f 0.59) X lo-" exp[-(1.29 f 0.08) x 103/T'l. Above 600 K, the Arrhenius plots were found to curve upward. The isotopic substitution of deuterium for hydrogen decreased the rate coefficient at T > 600 K, while not affecting the values at T < 600 K. Kinetic and mechanistic information derived from these measurements is discussed. Introduction Ground-state oxygen atoms, O(3P),play an important role in hydrocarbon combustion processes. Therefore, a great deal of kinetic and mechanistic work on O(3P) reactions with aliphatic and olefinic hydrocarbons has been carried out. However, aromatic-hydrocarbon reactions have not received much attention. Even the kinetic studies that have been carried out on O(3P)-aromatic hydrocarbon reactions have been restricted to temperatures less than 500 K. Since multiple reaction pathways are feasible at higher temperatures for the aromatic hydrocarbons, it is important to study their reactions over a wide temperature 0022-3654l82l2086-1690$01.2510
range. We have recently initiated a series of kinetic studies using the flash photolysis-resonance fluorescence technique with the temperature range extended to nearly lo00 K, and with emphasis placed on understanding the mechanisms of the elementary reaction steps. This paper, the second in a series which deals with O(P) reactions with aromatic hydrocarbons, describes the studies involving xylenes. The kinetics of the reaction of O(3P)with xylenes have been studied at 298 K by Mani and Sauer' and between (1) I. Mani and M. C. Sauer, Jr., Adu. Chem. Ser., No.82,142 (1968).
0 1982 American Chemical Society
