2262
J. Phys. Chem. 1981,85,2262-2269
NO3 due to the direct absorption of radiation energy by the NO, ion. As in methanol, dioxane, and ammonia NO3 was not observed we are of the opinion that production of NO3 may be better described by the reaction of the primary solvent cation with NO,, prior to the reaction with the solvent molecule, than by direct effect. Our results do not exclude the possibility that OH may play a role in
the formation of C1; in aqueous systems, but in those and other systems they strongly support the mechanism of oxidation of C1- by the primary solvent cation. Acknowledgment. We are indebted to Ms. B. Gawarska, Mr. L. Tarkowski, and the staff of the Linac group for their skillful assistance.
Kinetics of the Reactions of Hydroxyl Radical with Benzene and Toluene F. P. Tully,+ A. R. Ravishankara," R. L. Thompson, J. M. Nicovlch, R. C. Shah, N. M. Kreutter, and P. H. Wine Molecular Sciences Group, Engineering Experiment Statlon. Georgia Institute of Technology, Atlanta, Georgh 30332 (Received: October 31, 1980; In Flnal Form: February 13, 1981)
Absolute rate constants for the reactions of the hydroxyl radical with benzene and toluene were measured within the temperature and pressure ranges 213 IT I1150 K and 20 IP 5 200 torr by using He, Ar, and SF6 as diluent gases. To help elucidate the variations in reaction mechanism with temperature, we also studied OH reactions with deuterated benzene (C6D6)and with selectively deuterated toluenes (C6H5CD3,C&&D,, and C6D5CH3).Three major reaction channels were characterized kinetically. At T I298 K, electrophilicaddition of the OH radical to the aromatic ring is the dominant reactive pathway in all systems studied. At temperatures above 500 K, rapid decomposition of the thermalized adduct back to reactants diminishes the importance of the addition channel and leads to bimolecular reaction rate-constantvalues significantly lower than those measured near room temperature. At elevated temperatures, the ring hydrogen abstraction (for benzene) and side-chain hydrogen abstraction (for toluene) pathways are shown to be predominant. The measured bimolecular rate constants increase monotonically with increases in temperature above 500 K, and kinetic separation of the two hydrogen abstraction modes for toluene is achieved.
Introduction Detailed knowledge of the effects of temperature and pressure on the rate constants and reaction mechanisms of hydroxyl radical-aromatic hydrocarbon reactions is sparse. This sparsity exists because reactions of OH with aromatic hydrocarbons have previously been studied with only their tropospheric importance in mind. Combustion reactions of aromatic hydrocarbons take place over a wider range of system conditions than do their analogous atmospheric encounters. Present and future fuel concerns thus demand an improved characterization of the temperature and pressure dependences of aromatic hydrocarbon reactions, particularly a t elevated temperatures. We have undertaken a kinetic study in which radicalaromatic hydrocarbon reactions are investigated throughout a wide temperature interval, with acquisition of mechanistic information as a major goal; this paper is the first in a series which describes these measurements. Previous kinetic studies of OH-aromatic hydrocarbon reactions have demonstrated the existence of competing reaction channels. Davis et al.' found that rate constants for the reactions of OH with benzene and toluene at 298 K increase with pressure between 3 and 100 torr of He diluent gas. They interpreted this behavior as evidence that the reaction proceeds, a t least partially, via an addition channel in which an energy-rich adduct is produced; collisional stabilization of this adduct by the diluent gas would lead to the observed pressure dependence. Furthermore, on the basis of the differences in the pressure dependence of their measured bimolecular rate constants for the OH + benzene and OH + toluene reactions, Davis Applied Physics Division, Sandia National Laboratories, Livermore, CA 94550.
et al. suggested that OH reacts with toluene both through pressure-dependent OH addition to the aromatic ring and via pressure-independent hydrogen abstraction from the side-chain methyl group. Subsequent measurements by Lloyd et a1.,2 Hansen et al.,3 Perry et al.,435and Ravishankara et aL6of a variety of OH-aromatic hydrocarbon reaction rate constants indicate that the reactivity varies with both the nature and number of substituent groups on the aromatic ring. These studies, along with that of Hendry et al.,' strongly suggest that OH addition to the aromatic ring is the dominant reactive channel at 298 K. Prior to the present work, the only reported kinetic measurements of the temperature dependence of OHaromatic hydrocarbon reactions are those of Perry et aL4v5 Using the flash photolysis-resonance fluorescence technique over the temperature range 296-473 K, these authors found sharp structure in the In k vs. 1/T Arrhenius plots. Perry et al. interpreted their results in terms of variations with temperature of the branching ratios for addition and abstraction reaction channels. In the present investigation, the flash photolysis-resonance fluorescence technique was employed to measure (1) D. D. Davis, W. Bollinger, and S. Fischer, J. Phys. Chern., 79,293 (1975). (2) A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, Jr., J . Phys. Chern., 80, 789 (1976). (3) D. A. Hansen, R. Atkinson, and J. N. Pitts, Jr., J . Phys. Chern., 79, 1763 (1975). (4) R. A. Perry, R. Atkinson, and J. N. Pitts, Jr., J. Phys. Chern., 81, 296 (1977). (5) R. A. Perry, R. Atkinson, and J. N. Pitts, Jr., J. Phys. Chern., 81, 1607 (1977). (6) A. R. Ravishankara, S. Wagner, S. Fischer, G. Smith, R. Schiff, R. T. Watson, G. Tesi, and D. D. Davis, Int. J. Chern. Kinet., 10, 783 (1978). (7) D. G. Hendry, J. E. Davenport, and R. A. Kenley, Quarterly Progress Report (7/76-9/76), Environmental Protection Agency, Research Triangle Park, NC.
0022-3654/81/2085-2262$01.25/00 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 15, 1981 2283
Reactions of Hydroxyl Radical with Benzene and Toluene
toring OH resonance fluorescence emitted perpendicular to both the photolysis and resonance radiation beams, and ( 5 ) a signal averager and fast photon counting electronics. During the course of this investigation two separate reactors were utilized. For low-temperature studies, Le., 213-350 K, a jacketed Pyrex reactor with an internal volume of 150 cm3 was used. The cell was maintained at a known constant temperature by circulating either methanol (213-298 K)or ethylene glycol (298-350 K) from a thermostated bath through the outer jacket. In hightemperature experiments, i.e., 298-1150 K, an all-quartz reactor with an internal volume of -300 cm3 was used. The reaction cell was resistively heated by using electrically insulated tantalum wire windings mounted to its graphite-coated outer surface. The temperature of the gaseous mixture inside the reactor was directly measured by using a retractable, quartz tube encapsulated, chromelalumel thermocouple introduced into the reactor through a Wilson seal. The temperature gradient across the reaction zone (-2 cm) was found to be very small (e.g., AT 5 K at T = 1000 K). A complete description of the high-temperature reactor is given elsewhere.12 In all of the experiments discussed below, OH radicals were produced by flash photolysis of H 2 0 at wavelengths between the Suprasil cutoff at 165 nm and the onset of continuum absorption at 185 nm (flash duration I 50 ps). Following the flash, weakly focused OH resonance radiation continuously excited a small fraction of the OH created in the reactor to the electronically excited A 2 P state; the resultant (0,O) band A X fluorescence emanating in the direction perpendicular to both the resonance excitation and photolysis beams was collected by a lens and focused onto a photomultiplier fitted with a band-pass filter (309.5-nm peak transmission, 10 nm fwhm). Signals were obtained by photon counting and then fed into a signal averager operated in the multichannel scaling mode. For each decqy rate measured, sufficient flashes were averaged to construct a well-defined temporal profile over at least a factor of 20 variation in [OH]. All experiments were carried out under pseudo-firstorder kinetic conditions with the aromatic hydrocarbon concentration [RH] in excess, [RH]/[OH] > 100. Initial OH concentrations were in the range 2 X lolo-1 X 10l1 molecules ~ m - With ~ , these experimental conditions it has previously been shown13that radical-radical recombinations such as those involving H + OH or OH + OH are not significant OH loss mechanisms. This conclusion has been verified experimentally, as has the lack of importance of processes such as OH + Re products,14via the photolysis flux and initial [OH] variations carried out in the present work. In the absence of secondary reactions which signjficantly deplete or reform the transient OH species, [OH] varies in an exponential manner with time: [OH], = [OH]oe-(k[RHl+kd)t I [OHIoe-kt (1) where k' is the measured pseudo-first-order rate constant, k is the bimolecular rate constant for the reaction
-
U
Flgure 1. Schematic diagram of a high-temperature flash photoiysis-resonance fluorescence apparatus: (AC) absorption cell: (AD) amplifier-discriminator; (D) diluent gas; (DVM) digital voltmeter: (EL) electrometer; (Fl)253.7-nm band-pass filter; (F2) 309.5-nm bandpass filter; (FL) flash lamp; (FT) flow transducer; (HC) high-voltage capacitor; (HV) high voltage; (L) lens; (LS) light source (Hg pen-ray lamp); (MC) mixing chamber: (MCA) multichannel analyzer; (MG) microwave generator; (NV) needle valve; (PD) photodiode; (PG) pressure gauge; (PM 1) photomultiplier RCA 8850; (PM2) photomultiplier 1P28; (PS) high-voltage power supply; (R/D) reactantldiluent gas: (RL) resonance lamp; (TC) thermocouple: (TTY) teletype; (VH) vacuum housing.
absolute rate constants for the reactions of OH with benzene and toluene over the temperature range 213-1150 K. To help elucidate the variations in reaction mechanism with temperature, we also studied OH reactions with deuterated benzene (C6D6)and with selectively deuterated toluenes (C6H5CD3,C6D5CD3,and C6D5CH3).The results clearly indicate that the addition channel is the dominant OH-aromatic hydrocarbon reaction pathway at T C 298 K and that the ring hydrogen abstraction (for benzene) and side-chain hydrogen abstraction (for toluene) channels are the dominant pathways at higher temperatures.
Experimental Technique and Data Reduction The utilization of the flash photolysis-resonance fluorescence technique in the study of OH(X2r) radical reaction kinetics is well established and is amply described in the literature.8-10 Recently, we have extended the temperature range of applicability of this method to permit rate-constant measurements to be made at 1000 K. A discussion of the apparatus modifications required to allow measurement at high temperature is given elsewhere.11J2 Hence, the following discussion will be limited to a brief review of the technique, with elaboration provided only for those features unique to this work. A schematic diagram of the experimental apparatus is shown in Figure 1. The principal system components are (1)a thermostated reaction cell, (2) a spark discharge flash lamp perpendicular to one face of the cell, (3) a CW OH resonance lamp perpendicular to the flash lamp, (4) a band-pass filter/photomultiplier combination for moni-
-
(8) D. D. Davis, S. Fischer, and R. Schiff, J . Chem. Phys., 59, 628 (1974). (9) A. R. Ravishankara, P. H. Wine, and A. 0. Langford, J. Chem. Phys., 70, 984 (1979). (10) A. R. Ravishankara, G. Smith, R. T. Watson, and D. D. Davis, J. Phys. Chem., 81, 2220 (1977). (11) F. P. Tully and A. R. Ravishankara, J. Phys. Chem., 84, 3126 (1980). (12) A. R. Ravishankara, J. M. Nicovich, R. L. Thompson, and F. P. Tully, J. Phys. Chem., in press.
-
-
-
OH + RH products [RH] is the (constant) aromatic hydrocarbon concentrak
(13) P. H. Wine, N. M. Kreutter, and A. R. Ravishankara, J. Phys. Chem., 83, 3191 (1979). (14) In addition t o the standard experimental tests used to demonstrate the independence of the [OH] decays from processes such as OH + R. products, Re, representing either a reaction product or an unwanted photolysis product, searches for aromatic hydrocarbon fluorescence were also made. Absorption of light in the wavelength region 230 < X < 280 nm induces fluorescence in benzene and toluene; any such flash-induced emission was not detected by our band-pass filter/photomultiplier tube combination.
-
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The Journal of Physical Chemistty, Vol. 85, No. 15, 1981
1013
0.93
Time, msec Figure 2. Typical [OH] temporal profile following flash photolysis of H,O/toluene/Ar mixture. Experimental conditions: T = 958 K, P = 100 torr (Ar), flash energy = 80 J, [H,O] = 180 mtorr. Concentrations of toluene in molecules cm3 are given next to the decay curves. 600 I
I
397 K
.Y
"
0
I
POLUENE]
2
3
, IOl4 molecule cm3
Figure 3. Typical plots of the pseudo-first-order rate constant vs. [toluene] at six temperatures. For linear plots, s o l i lines are obtained from linear least-squares analyses. The dashed line at 397 K represents the initial slope to the nonlinear k'vs. [RH] curve. For the sake of pictorial clarity, k'at [RH] = 0 is not included.
tion, and kd is the first-order rate constant for OH disappearance due to diffusion and reaction with background impurities in the absence of RH. Within the temperature intervals 213-320 and 400-1150 K, exponential [OH] decays such as those displayed in Figure 2 for reaction with toluene were observed. k'values were taken as the slopes of such decays. RH concentration variations over a factor of a t least 5 led to k'vs. [RH] plots of the type shown in Figure 3 for a series of temperatures. Bimolecular reaction rate constants k(T) were obtained as the slope of the least-squares straight line through the (k',[RH]) data points. In order to avoid the accumulation of photolysis or reaction products and to minimize any uncertainties in [RH] arising from aromatic hydrocarbon adsorption on the reactor walls, we carried out all experiments under "slowflow" conditions. The flow rate through the cell was such that each photolysis flash encountered a fresh reaction mixture (photolysis repetition rate i= 0.3 Hz). The aromatic hydrocarbon RH was taken from a 12-L bulb containing an RH/diluent gas mixture, and the water mixture was generated by bubbling diluent gas at 800 torr through distilled water at room temperature. The RH/diluent gas mixture, H20/diluent gas mixture, 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. 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, a 70-cm long absorption cell, and a photomultiplier tube fitted with a band-pass filter. The absorption cross sections at 253.7 nm used to calculate the RH concentrations in the source mixtures were measured during the course of the experiments; the obtained values are as follows: C6H6, 3.67 X cm2; C6H&H3,4.78 X lo-'' Cm2; C&, 2.39 X cm2;C6H5CD3,5.26 X cm2;C&CD3, 4.51 X cm2;C6D5CH3,4.99 X lO-l9 cm2. These cross sections and the resulting absolute [RH] values quoted for each experiment are thought to be accurate to better than determinations 10%. Any errors made in the aRH263*7 would result in systematic vertical shifts of an entire set of k(T) values in the In k ( T ) vs. 1/T Arrhenius graphs. The diluent gases used in this study had the following stated purities: Ar > 99.9995%, He > 99.9999%, and SF6 > 99.99%. 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 purities were >99.99%, and their selectively labeled isotopic purities were as fOllOWS: C6D6 > 99.96% D, C6H5CD3 > 99.0% D, C6D6CD3> 99.5% D, and C6D5CH3> 99.0% D. All aromatic hydrocarbons were degassed before use.
Results and Discussion Itl this work bimolecular reaction rate-constant measurements were made within the pressure and temperature intervals 20 I P 5 200 torr and 213 I T I 1150 K for the following processes: OH OH
+ C6H6 + C6D6
kl
ka
-+
+ C6H5CH3 OH + C6H5CD3 OH + C6D5CD3
OH
OH
+ C6D5CH3
products
(1)
products
(2)
k8
k4
k6
k8
products
(3)
products
(4)
products
(5)
products
(6) Over 900 pseudo-first-order [OH] decay rates, k', were measured. The resultant values of k'are listed along with all pertinent experimental conditions in Tables IV-IX, which are available as supplementary material to this manuscript. (See paragraph a t end of text regarding supplementary material.) Table I lists, as a function of temperature, the highpressure limit bimolecular rate constants, k1-k6, measured in this work; f 2 a precision limits, derived from the deviations of the (k',[RH]) points from best-fit straight lines, are included. A t certain temperatures, as illustrated in Figures 3 and 4, nonlinear k'vs. [RH] plots and/or nonexponential [OH] decay curves were measured. For the former case, k values were estimated from the steep initial tangent to the k'vs. [RH] curves; for the latter case, k' values were approximated by the slope of the leading portion of the [OH] decay curve, and k values were derived as above from these (k',[RH]) points. These estimated k values are listed without precision limits and are identified with superscripts in Table I. Their inclusion is intended
The Journal of Physical Chemistry, Vol. 85, No. 15, 198 1 2265
Reactions of Hydroxyl Radical with Benzene and Toluene
T (K)
I
0
1
I
I
1
1
I
1
1
2
3
4
5
6
7
Time, msec Flgure 4. Typical nonexponentiai [OH] temporal profile obtained between 325 and 380 K for reaction 3. For comparison an exponential [OH] decay obtained at 504 K is also shown. T (K) 301
I
Y
yG
I
500 400
1000
1
I
I
300
1
250
I
1
e o
, I/
2
I
c&;Ol
3
4
j
1000 T (K)
Figure 5. Arrhenius plots of In k vs. 1000/ T(K)for reactions of OH with CEHE and CEDe Points denoted by X were obtained under conditions where nonexponential [OH] decays and/or nonlinear k'vs. [RH] curves were observed.
merely to reflect the decreasing trend in reactivity throughout this temperature region. The rate-constant data compiled in Table I are also plotted in Arrhenius form in Figure 5 for the benzenes and in Figure 6 for the toluenes. The OH-aromatic hydrocarbon reactive systems are discussed individually below. OH C6r,(c&) Products. The k1(298 K) values obtained in this and in previous investigations are listed in Table 11. In contrast to the findings of Davis et al.,I no significant k1(298 K) pressure dependence was observed in this work over the range 25-200 torr with He, Ar, and SF6 as diluent gases. Furthermore, while our 100-torr results for k1(298 K) = (1.21 f 0.09) X and (1.25 f 0.06) X cm3 molecule-' 5-l with Ar and He diluent gases, respectively, are in remarkably good agreement with the measurements of Hansen et aL3and Perry et al.,4 they are significantly lower than the high-pressure limit value of k1(298 K) = (1.59 f 0.12) X cm3 molecule-l s-' reported by Davis et al. No obvious reason for this discrepancy is apparent, particularly in light of the good agreement between the present measurements and those of Davis et al. on k3(298 K). Figure 5 displays our results for k,(7') and k2(7') in Arrhenius form. For T I298 K, the rate constants for
+
-
3
2
4
5
Figure 6. Arrhenius plot of in kvs. lOOO/T(K)for reactions of OH with four isotopically labeled toluenes. Points denoted by X were obtained under conditions where nonexponential [OH] decays and/or nonlinear k'vs. [RH] curves were observed.
reactions 1and 2 are, within experimental error, identical, and both increase slightly as the temperature is raised. Between 320 and 400 K, nonexponential [OH] decays were observed, and the approximated bimolecular reaction rate constants drop sharply with increasing temperature. In the interval 400 < T < 500 K, nonlinear k'vs. [RH] plots were obtained, and estimated k values continue to decrease and show a significant kinetic isotope effect. Above -500 K, kl(T) and k 2 ( T )display a strong positive temperature dependence, and a marked kinetic isotope effect is maintained up to the limiting measurement temperature of 1150 K. These results are in qualitative agreement with those of Perry et al.4 These authors measured kl(T) between 296 and 425 K, finding slowly increasing values from 296 to 325 K, nonexponential [OH] decays and rapid falloff in reactivity between 325 and 380 K, and, again, increasing values from 380 to 425 K. In explaining these observations, Perry et al. proposed the following mechanism: OH 4C6H5' + H2O (a) [C6&OH]* (b) [C,&OH]* -.+ C&& 4- OH (4
-
+
+M
(d) [C6H6OHl (e) [C&jOH] -.+ C & 3+ OH Davis et al. interpreted their observed pressure dependence of k1(298 K) in terms of competition between collisional stabilization (eq d) and decomposition back to reactants (eq c) of the energized addition complex formed in the primary reaction pathway (eq b). Our measurements, however, indicate that channel d is the dominant pathway above 25-torr total pressure. This reduces the mechanism to one characterized by competition among the forward hydrogen abstraction (eq a) and OH addition (eq d through eq b) channels and the reverse, thermalized adduct decomposition channel (eq e). At low temperatures (250-298 K) our measurements yield an expression for k,(!I') of ki(7') = (3.1 f 2.6) X exp[-(2.7 f 2.2) x 102/T] cm3 molecule-' 5-l where the listed errors represent f 2 a values and UA E Aa+. This expression is in good agreement with that derived by Perry et al.: k1(n= (5.0 f 2.0) X exp[-(4.5 f 5.0) X lO2/U cm3 molecule-l 8-l I_,
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Tully et al.
The Journal of Physical Chemistry, Vol. 85, No. 15, 198 1
TABLE I: Rate Constants for the Reaction of OH with Aromatic Hydrocarbons as a Function of Temperature
1012k,dcm3 molecule"s-' temp, K 213 231 250 260 270 298 3 20 332 352 3 58 3 83 390 397 412 442 470 498 504 518 54 2 56 8
621 630 6 53 666 67 5 694 700 7 15 734 74 2 793 817 830 842 86 8 895 917 958 966 981 1002 1017 1046 1150
'6%
6 '
D6
1.04 f 0.08
1 . 0 8 5 0.05
1.20 f 0.09 1.24 f 0.09
1.19 f 0.05
0.7a,b
' 6 H5CD3
'SHSCH3
8.20 i 0.54 8.73 f 0.39 7.97 i 0.56 8.53 f 0.37 7.44 f 0.55 6.36 f 0.69 6.3 f 0.6a 5.4 f 1 . 1 a 3.6a1b
C6D5CH3
' 6 D5CD3
5.62
f
0.52
6.04
0.48
6.11 f 0.40
5.97 5.63
f f
0.17 0.30
6.36 f 0.52 6.40 i 0.20
6 . 0 2 i 1.68 6.47 f 0.65
j:
3.Oalb
O.Bb 0.3a
0.f~~
1.4b l.lb
0.4b
1.7b 0.73 f 0.07 1.17 f 0.09
0.4b 2.16
f
0.08 1.23 f 0.09
2.04
0.543 f 0.023 0.227
0.639 i 0.029 0.68, f 0.07, 0.60, + 0.03,
f
0.03
2.45 2.49
f i
1.66 f 0.10
0.05c 0.12
f
0.14
1.27 f 0.03 1.32 f 0 . 0 V 1.41 f 0.06 1.46 f 0.09 1.40 f 0.08
1.15 i 0.05
2.52 c 0.14
1.97 i 0.12 0.42, t 0.04, 0.30, i 0.03,
2.10 3.26
0.43,
f
f
i
0.07
2.69 3.29
f f
0.28 0.25
4.53 5.08
f
f
0.52 0.32
0.29
0.02, 3.58f 0.16 2.35 f 0.16 2.18+ 0.10
1.02 f 0.04 0.48,
f
0.01,
1.20 i 0.16
3.01 4.67
f
1.59 c 0.09
3.59 0.72,
f
i
0.10
0.19 f
f
0.23
2.20
i
0.34
0.28
2.76 3.18 4.05
i t i
0.14c 0.30 0.30
0.13
4.55
i
0.34
5.92
f
0.42
6.48 f 0.41
1.04 f 0.03 6.87
2.35
i
0.04, 5.54 f 0.27
1.90 f 0.20 2.26 f 0.13
3.53
f
0.23
1 . 0 8 f 0.10 1.47 f 0.07
4.52
f
6.9,
f
0.25
8.5,+ 1.2,
6.52
i
0.91
7.97
f
0.73
1.3,
9.5 f 1.0 1.91 I 0.28
6.5, f 1.5, First-order [OH] decay plots are nonexponential; k' 2 initial slope. k' vs. [RH] plots are nonlinear; k f tangential slope of k ' vs. [ RH] plot. At these temperatures multiple experiments were carried out at different times to cross-check results. At T < 298 K, k values corresponding to the high-pressure bimolecular limit are listed. a
TABLE 11: Comparison of Present Results for OH t Benzene Reaction with Previous Measurements at 298 K
10IZk,cm3 molecule-' diluent
press., torr
He He He He He Ar Ar Ar Ar
3 20 25 50 100 50 100 200 600 100
s F6
this worka
ref
I b
5-l
ref 4c
ref 3b
0 . 8 5 f 0.08 1.36 f 0.09 1.10 i 0.05 1.11f 0.24 1.25 + 0.06 1.20 f 0.06 1.21 f 0.09 1.19 f 0.12
1.59 f 0.12 1.20
f
0.15
1.28 f 1.22 rt 1.20 1.24 ?:
0.04 0.06
0.06 0.11
1.11c 0.05
a Quotedwror is 20. and refers t o the precision of the measurement. the measurement. Quoted error is the estimated overall accuracy.
The assigned errors in both of these expressions are large because only the rate constants obtained over a very limited temperature range are used and the rate constants do
Quoted error is 10 and refers to the precision of
not change rapidly with temperature. The lack of a significant kinetic isotope effect in our low-temperature measurements of kl(T) and k 2 ( T )demonstrates the unim-
Reactions of Hydroxyl Radical with Benzene and Toluene
portance of the hydrogen abstraction reaction, channel a, a t these temperatures. Also, the exponentiality of the [OH] decays and the linearity of the k'vs. [RH] plots in this interval confirm that decomposition of the thermalized adduct via channel e is unimportant at these temperatures over the experimental measurement times utilized. Thus our calculated low-temperature expression for kl(T ) may be associated entirely with the process
and k l ( T ) = kladd(T) for T I298 K. This equality is strictly true if and only if OH addition to C6H6is not followed by elimination of a fragment other than OH. Extrapolation of the resultant kladd(T)expression to higher temperatures, at which the thermalized adduct lifetime is shortened, should provide fair estimates of the rate constant for the forward-step formation of the thermalized adduct. At temperatures in the interval 320 < T < 400 K, we observed nonexponential [OH] decays in our -20-ms duration studies of reactions 1 and 2. Our results are consistent with the suggestion of Perry et al. that at these temperatures and over these time scales OH is regenerated via the decomposition back to reactants of the thermalized addition complex. At very long times we observed that the [OH] decays became exponential with slope = -kd;this situation could only be reached if the OH + RH + M + RHOH M reaction had established dynamic equilibrium, with the thermalized adduct serving, in effect, as a temporary sink for OH. Additional evidence for the validity of the above mechanism is provided by our results for reaction 1 at 352 K. k'values as high as 700 s-l were measured (approximated for nonexponential [OH] decays) at this temperature. The plot of k'vs. [RH] at 352 K is concave upward for low values of k', and linear, with slope yielding kl = 1.1X cm3 molecule-' s-', at high values of k'. Similarly, for low k' (small [RH]), the [OH] decays were markedly nonexponential, while for high k 'they remained exponential over two l / e decay periods. Since the use of increasing [RH] concentrations required progressively shorter experimental measurement times, it may be argued, by analogy with Perry et al.'s discussion concerning nonexponentiality and experimental time scales, that the adduct decomposition channel e had not yet become important during the -2.85-ms (-2tlie for k' = 700 s-l) period of exponential [OH] decay. If such were the case, one would then expect, as was observed, k1(352 K) to approach its calculated addition channel value of -1.4 X cm3 molecule-' s-' at high k'. Our observations of exponential [OH] decays and concave downward k' vs. [RH] plots for reactions 1 and 2 between 400 and 500 K cannot satisfactorily be explained at this time. While at first glance these observations may appear to be in conflict with those of Perry et al., this is not necessarily the case. In our experiments, the observed curvature in the k'vs. [RH] plots became marked only for k ' 2 200 s-'; this k'value was the typical upper limit in Perry et al.'s measurements at T > 400 K. Because the overall reaction rate constants approach their minima in this temperature regime, large [RH] concentrations were required to obtain even moderate k'values. One may only speculate that such large [RH] concentrations could have effected a mechanism modification and/or enhanced the probability of kinetic interference due to secondary reaction processes. Further studies of this complex 300-500 K temperature region are in progress. Reactions 1and 2 were kinetically well-behaved at T 1
+
The Journal of Physical Chemistry, Vol. 85, No. 15, 198 1 2267
40 30-
T(K) 750
1000
500
I
I
I
(2A*0.9)x 16" exp~(2.56*O.301xIO3/T]
-
xx -
(1.3C0.6) xld"
exp [-(2.30? 0.34)x103/ T]
1
cm3 mol uledsec"
1.0
0.8
1.2
i.4
1.6
1.8
2.0
1000 T(K) Figure 7. Arrhenius plot of In k vs. 1000/ Tfor reactions of OH with OH f CBHB C6H6 and C&e at temperatures greater than 550 K: (0) reaction; (0) OH C B Dreaction. ~ Solid lines were obtained by linear least-squares analyses on these data; the obtained Arrhenius expressions are shown next to the lines.
+
500 K; measured In [OH] vs. time and k'vs. [RH] plots were both linear. Our high-temperature measurements of k l ( T ) and k 2 ( T )are plotted in Arrhenius graph form in Figure 7. Linear least-squares fits to the included points generate the expressions k l ( T ) = (2.4 f 0.9) X exp[-(2.26 f 0.30) X 1 0 3 / q cm3 molecule-l s-l k * ( T ) = (1.3 f 0.6) X lo-'' exp[-(2.30 f 0.34) X 1 0 3 / q cm3 molecule-' s-l quoted errors representing f 2 g values and uA AuM. The temperature dependences observed for k l ( T ) and k 2 ( T )at T > 500 K are much stronger than those found at T I298 K. Also, the magnitude of the kinetic isotope effect for T I 500 K, kl = 2k2,may be contrasted with that observed for T I298 K, kl = k2. Both of these observations strongly suggest that the predominant reaction taking place at T I 500 K is ring hydrogen abstraction, channel a. Extrapolation of the high-temperature kl(T) expression to 298 K yields klabst(298K) = 1.2 X cm3molecule-' s-l. Channel a thus contributes only 1% of the total reaction at room temperature, and the measured absence of a kinetic isotope effect below 298 K is easily understood. Summarizing the OH + benzene products reaction system, then, we find clear evidence for addition channel domination at low temperature and hydrogen abstraction channel domination at high temperature. While a thorough characterization of the 300-500 K reaction mechanism transition region awaits further study, the qualitative interpretation involving strongly temperature-dependent rates of decomposition of the thermalized adduct back to reactants (and perhaps to other species) appears verified. OH + C6H&H3 (C6HsCD3, C6DBCD3, C6D5CH3) Products. The pressure dependence of k3(298K) from 20 to 100 torr was investigated in this work using He, Ar, and SF, as diluent gases. Our room-temperature results are presented along with those of previous investigations in Table 111. A significant pressure dependence was observed for k3(298K), in agreement with the results of Davis et al.' k3(298 K) appears to reach its high-pressure limit value at -100-torr total pressure, and our 100-torr values are in excellent agreement with those of all prior s t u d i e ~ . ' l ~ , ~
-
4
2268
The Journal of Physical Chemistry, Vol. 85,No. 15, 7981
Tully et ai.
TABLE 111: Comparison of Present Results for OH t Toluene Reaction with Previous Measurements at 298 K 10IZh,cm3 molecule-'s-' diluent press., torr this worka ref I b ref 4c ref 3 b He He He He Ar
Ar Ar Ar
SF,
3 20 40 100 25 100 250 619 100
4.72 2 5.44 i 6.00 2 5.78 i 6.36 i
3.60 f 0.26 5.00* 0.18
0.46 0.07 0.43 0.62 0.69
6.11 f 0.40 6.402 0.54
6.55 2 0.49
a Quoted error is 20 and refers to the precision of the measurement. Quoted error is the estimated overall accuracy. the measurement.
OH reacts with toluene in a manner analogous to its reaction with benzene. The major active channels include the previously documented OH addition to the aromatic ring and the inverse decomposition of the thermalized adduct, hydrogen abstraction from the aromatic ring, and a new pathway, hydrogen abstraction from the side-chain methyl group. These processes are as follows: OH
5.90 f 0.16 5.69 ? 0.27 5.75 j. 0.16
ke
+ C6H5CH3 + M
[CGHsCH30H] + M
F=
k-r
kr"
OH
+ CsH5CH3
OH
+ C6H5CH3 2C&$H2
kH
CcH4CH3 + HzO
+ H2O
(f) (8)
(h)
As seen from Table I and Figure 6, for T I 298 K, k3(T)-k6(T) are, within 25%, equal. In this region the bimolecular reaction rate constants are only weakly dependent on temperature, and only in the case of toluene were enough points taken to justify generation of a rateconstant expression: k3(T) = (3.8 f 2.5) X exp[+(l.8 f 1.7) X 1 0 2 / q cm3 molecule-l s-l Assigned errors represent f 2 u values, g A AgM, and the expression is valid only for 213 IT I298 K and P I100 torr, Best-fit activation energies (kcal mol-') for reactions 3-6 were also shown to be small: E,(3) = -0.36, EL4)ir. +0.18, EL5)2: +0.17, and E,(,) 2: 0 when 250 IT I298 K. The above expression for k3(T) is in qualitative but not good quantitative agreement with that recommended by Perry et al.: k3(T) = (3.2 f 1.1) X exp[+(0.8 f 0.5)
X
103/TJ cm3 molecule-' s-l
While both formulas produce decreasing k3 values with increasing temperature, the expression based on the present work is probably more reliable because of the greater number of points measured and the wider temperature range covered. The reaction rate constants k3-k6 were found to be only weakly dependent on the isotopic hydrogen content of toluene for T I 298 K. In agreement with Perry et al., we interpret this observation as evidence that the addition channel f is the dominant reaction pathway at low temperatures. Hydrogen abstraction channels g and h are minor routes for T 5 298 K. Our measured ratio, k3(298 K)/k1(298 K) 2: 5, is then interpreted as arising from ring activation resulting from the electron-donating character of the side-chain methyl group. Such a correlation between (addition channel) reaction rate constants and the Hammett functions of the side-chain substituent groups has been previously discussed.6
Quoted error is l o and refers to the precision of
Between 320 and 380 K, nonexponential [OH] decays were observed in all studies of reactions 3 4 , As is the case for the benzene reactions, the approximated overall rate constants drop sharply throughout this temperature interval. We again attribute this pronounced decrease in reactivity to the increasingly rapid thermal decomposition of the collisionally stabilized (and, of course, activated) addition complex back to reactants. In the temperature range 380-450 K, exponential [OH] decays and concavedownward k'vs. [RH] plots were obtained in all experiments. The measured k'values at a given [RH] concentration within this temperature interval were found to be moderately strong functions of [OH],,. Above 450 K, the addition channel f becomes ineffective, and k3(T)-k,( T )were observed to increase monotonically from indefinitely located minima as the temperature was raised. These increasing rate-constant values reflect the sum of the temperature-dependent contributions to the ring H (D) and side-chain H (D) abstraction channels g and h. In order to separate and assign rate-constant expressions for each abstraction channel, we investigated the reactions (3-6) of OH with the four isotopically substituted toluenes. As shown in Figure 6, our results may be summarized as follows: k3(T) i= k,(T), k 4 ( T )x k5(T),and k 3 ( T ) / k 4 ( T )x k6(!i?/k5(T)2: 2. These relationships strongly suggest that side-chain hydrogen abstraction is the dominant pathway for the OH reaction with toluene at T > 450 K. The single measurement of k3(424.4 K)/k5(432.2 K) = 2.5 previously made by Perry et aL4 is in good agreement with the present results. Furthermore, since the ratio k3(500 K)/k,(500 K) x 5 may be estimated from the present measurements, the contribution of the ring hydrogen abstraction channel g to the overall rate constant at T = 500 K may be roughly approximated at 20%. A more quantitative estimate of the relative importance of the ring and side-chain hydrogen abstraction processes g and h may be made by using the following two assumptions: (1)the rate constant for H (D) abstraction from a ring position by OH is independent of the isotopic identity of the side-chain hydrogen species, and conversely the rate constant for side-chain H (D) abstraction by OH does not depend on the isotopic identity of the ring hydrogen species; and (2) for T > 500 K, the ring hydrogen and deuterium abstraction rate constants for the toluenes, krH(7') and k,D(T), may be approximated as 5 / 6 of the corresponding best-fit ring hydrogen and deuterium abstraction rate constants, k1(Uand k2(T),for the benzenes. We have carried out point-by-point subtractions of kF(!i") = 5/skl(T) from k 3 ( T ) and k 4 ( T ) and of krD(T) 5 / 6 k 2 ( T ) from k 5 ( q and k,(T). These subtractions yield data on kaH(T) (from k , and k,) and kaD(T) (from k4 and k5),the rate constants for side-chain hydrogen and deuterium abstraction, respectively. These channel-specific reaction
Reactions of Hydroxyl Radical with Benzene and Toluene (l.4t0.5)x
lo" exp [-(l.5*0.2)x103/T)
5
A
1.0
2.0
1000 T(K) Figure 8. Arrhenius plot of In k vs. 1000/ T ( K ) for the side-chain H (squares) and D (circles) abstraction from toluenes by OH. The arrows refer to the ordinate of reference. Open points were obtained by subtracting k r ( T ) values and closed points by subtracting kF( T ) values from the k,, k4, k g , or k e data points.
rate constants are plotted in Arrhenius graph form for T > 500 K in Figure 8. Open points were obtained via krH(T ) subtraction and closed points via kF(T) subtraction; it may be seen that the ksH(T)and ksD(T)data derived from the two sources are in good agreement. Linear least-squares analyses give the following expressions for the side-chain hydrogen and deuterium abstraction channels in the toluenes: k s H ( T ) = (2.1 f 0.6) X exp[-(1.3 f 0.2) x 1o3/q cm3 molecule-l s-l
k s D ( T ) = (1.4 f 0.5) X exp[-(1.5 f 0.2) x i 0 3 / q cm3 molecule-'^-^ where the included errors represent f2a values, UA 5 ABM, and the expressions are considered most reliable in the interval 500 I T I 1000 K. Extrapolation to low temperatures of the expressions derived above for krH(T)and k s H ( T )confirms the dominance of the addition reaction channel f a t T 5 298 K. A quantitative comparison of our approximated results for ksH(7')and ksH(298K)/k3(298 K) with corresponding estimates from previous studies yields only fair agreement. k s H ( T ) values calculated from the above expression fall
The Journal of Physical Chemistry, Vol. 85, No. 15, 198 1 2269
30-50% below the k3(T) values measured from 380 to 473
K by Perry et al.4 The branching ratio at T = 298 K for side-chain hydrogen abstraction calculated from the present results, ksH(298 K)/k3(298 K) = 0.042:!, is significantly lower than the 0.16 and 0.15 values reported by Three factors probably Perry et aL4and by Kenley et account for these discrepancies. First, in the present work we have found indirect evidence that the addition channel contributes to the reaction on a millisecond time scale up to temperatures of -450 K; the 380-473 K results of Perry et al. are thus likely to contain a decreasingly important (with increasing temperature) contribution from the addition reaction channel, and the separated, channel-specific, side-chain hydrogen abstraction rate constant may be expected to possess a steeper temperature dependence than that characteristic of Perry et al.'s observed data. Second, it must be remembered that the expression generated in this work for k s H ( T ) was derived, following inexact assumptions, from differences between measured quantities. Third, the probable contributions of tunneling at lower temperatures, which would result in increased estimated values of ksH(T) at 298 K, have been ignored. Substantial scatter exists in the data plotted in Figure 8, and likely kinetic subtleties, such as concave-upward Arrhenius graph c u r ~ a t u r e , ~could ~ J ~ ,not ~ ~ be considered resolvable. Finally, it is instructive to use our approximate channel-specific, rate-constant expressions for the OH-toluene system to calculate the relative importance of the ring hydrogen and side-chain hydrogen abstraction reaction channels at high temperatures. It is easily shown that k r H ( T ) / k s H ( T5) 0.5 for all T < 1500 K. Thus side-chain hydrogen abstraction remains favored relative to ring hydrogen abstraction as the temperature is raised to the aromatic hydrocarbon pyrolysis regime; subsequent reactions involving benzyl-like radicals are then more likely than those involving methylphenyl-like radicals. Acknowledgment. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. ER-78-S-05-6030. Supplementary Material Available: Tables IV-IX, listing rate-constant data for the reactions of OH with C&6, C6D6, C&&H3, C&&,CD,, C&&D3, and C&CHs (16 pages). Ordering information is available on any current masthead page. (15) R. A. Kenley, J. E. Davenport and D. G. Hendry, J. Phys. Chen., 82, 1095 (1978). (16) W . C. Gardiner, Jr., Acc. Chen. Res., 10, 326 (1977). (17) R. Zellner, J. Phys. Chem., 83, 18 (1979).
