T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in U. S. Patent Office 0 Copyright, 1975, by the American Chemical Society
VOLUME 79, NUMBER 4
FEBRUARY 13, 1975
Temperature Dependence of the Absolute Rate Constants for the Reaction of O(3P) Atoms with a Series of Aromatic Hydrocarbons over the Range 299-392'K R. Atkinson and J. N. Pitts, Jr." Department of Chemistry and Statewide Air Pollution Research Center, University of California, Riverside, California 92502 (Received August 21, 1974) Publication costs assisted by the University of California
Rate constants, h 2 , for the reaction of O(3P) atoms with a series of aromatic hydrocarbons have been determined over the temperature range 299-392°K using a modulation technique. The Arrhenius expressions obtained are as follows (1. mol-l sec-l): hz(benzene) = 11.1 X 109e-(39so*400)/RT, hz(to1uene) = 8.2 X loge -(3100f300)/RT, h 2 (0-xylene) = 6-25 X loge -(2430*300)lRT, k (m- xylene) = 7.7 X loge -(2150*300/RT, h 2 ( p - xylene) = 7.9 X loge-(2540*300)/RT, h 2 (1,2,3-trimethylbenzene)= 10.3 X 10% -(1600*300)'RT, 122 (i,2,4-trimethylbenzene) = 9.35 X loge -(1650*300)/RT, h 2 (1,3,5-trimethylbenzene) = 6.05 X l o g e -(770&300)/RT.
Introduction Although there is a considerable body of reliable rate constant data available in the literature for the reactions of O(3P) atoms with alkenes and alkanes, there have been few rate constant determinations for the reaction of O(3P) atoms with aromatic hydr0carbons.l Thus benzene is the only aromatic compound for which absolute rate constants have been obtained over a range of temperature.2 Recently we determined the absolute room-temperature rate constants for the reaction of O(3P) atoms with a series of aromatic hydrocarbons using a modulation phase-shift techniques3In this work we have extended these measurements to elevated temperatures in order to obtain the Arrhenius parameters.
Experimental Section The apparatus and techniques used have been described p r e v i o u ~ l y , and ~ - ~ hence only a brief summary will be given here. O(3P) atoms were generated by the sinusoidally modulated mercury photosensitization of NzO and their concentrations monitored by the NO2 emission using a cooled EM1 9659A photomultiplier fitted with a cutoff filter transmitting X 14700 A. Phase shifts 4 between the incident 2537-A radiation and the NO2 emission were measured using a PAR HR-8 lock-in amplifier fitted with a Type C preamplifier.4 The reaction cell was enclosed by a furnace whose tem-
perature, monitored by a thermocouple probe, could be held constant to f0.2"K. The reactant gas stream was determined to be within f0.5OK of the furnace temperature under all conditions used. The gases used, their purities, and their purifications were as described p r e v i ~ u s l y . ~
Results Under the experimental conditions used, the phase shift,
4,is given by3>4 tan 4 = 2 ~ v / ( k ~ [ N 0 ] [ M ]+ k,[aromatic])
(I)
where h 1 and h 2 are the rate constants for the reactions 0 + N O +M--NO, 0
+
aromatic
+ M
(1)
--+ products
(2)
and u is the modulation frequency (1200 f 1 Hz). Total pressures were in the range 51-75 Torr. Rate constants k 2 were determined over the temperature range 299-392°K as described p r e v i ~ u s l y .Thus, ~ for example, Figure 1 shows plots of 2rrultan 4 against [aromatic] for m-xylene at the three temperatures studied. The rate constants ilz2 obtained from such plots are given in Table I and are plotted in Arrhenius form ( h 2 = A 2e-E2/RT) in Figure 2. The Arrhenius parameters A and E2 determined by least-squares analysis of the data are given in Table 11. 295
R. Atkinson and J. N. Pitts, Jr.
296
TABLE I: Rate Constants kz for the Reaction of O(3P) Atoms with Aromaticsa Aromatic Benzene Toluene o-Xylene m-Xylene #-Xylene l72,3-Trirnethylbenzene 1,2,4-Trimethylbenzene 1,3,5-Trimethylbenzene
Temp,"K
10-8k,, 1. mol" sec-*
300.3 f 0.3 341.2 f 0.5 392.2 f 0.5 300.4 i 0.2 341.2 i 0.5 392.2 * 0.5 299.1 f 0.3 341.2 f 0.5 392.2 f 0.5 299.5 f 0.5 341.2 f 0.5 392.2 f 0.5 300.4 f 0.5 341.2 i 0.5 392.2 rt 0.5 299.7 f 0.4 341.2 f 0.5 392.2 f 0.5 300.2 i 0.3 341.2 f 0.5 392.2 f 0.5 299.9 i: 0.5 341.2 f 0.5 392.2 f 0.5
0.144 f 0.02 0.303 f 0.035 0.69 f 0.08 0.450 f 0.045 0.85 f 0.09 1.52 fO.15 1.05 fO.11 1.72 f 0.18 2.77 i 0.3 2.12 f 0.21 3.09 f 0.3 5.0 f 0.5 1.09 f O . 1 1 2.01 f 0.2 2.94 f 0.3 6.9 f 0.7 9.9 f 1.0 13.0 f 1.5 6.0 f 0.q 8.1 f 0.4 11.5 f 1.2 16.8 f 2.0 18.9 f 2.0 22.8 f 3.0
I
6 [m-XYLENE]
i
1oL10-5
mole lite;'
Flgure 1. Plots of 2nvltan r#~ against [m-xylene]. (Total pressures and NO pressures are as follows: 299.5'K, 53 and 0.66 Torr; 341.2'K, 65 and 1.08 Torr; 392.2'K, 68 and 1.45 Torr. The data points at 392.2'K have been displaced vertically 4-0.2 X lo4 sec-' for clarity.)
5*
a The errors include precision limits derived from least-squares standard deviations and estimated accuracy of other parameters such as pressure and composition of mixtures.
TABLE 11: Arrhenius Parameters for t h e Reaction of O(3P) Atoms with Aromaticsa Aromatic Benzene Toluene o-Xylene m-Xylene p-Xylene 1 2 ,3 -Tr imet hylbenzene 192,4-Trimethylbenzene 1,3,5 -Trimethylbenzene
10-9~~~ 1. mol'' sec-'
11.1 8.2 6.25 7.7 7.9 10.3 9.35 6.05
1.2.4.19lMETHYLBENZENE
Ez,kcal mol-' 3.98 f 0.4 3.10 f 0.3 2.43 f 0.3 2.15 f 0.3 2.54 f 0.3 1.60 f 0.3 1.65 f 0.3 0.77 f 0.3
aThe indicated errors in the activation energies E2 include the least-squares standard deviations and the estimated overall error limits. Discussion
As discussed previously3 the reactions of O(3P) atoms with reactant impurities and secondary products are estimated to cause errors in the measured rate constants of less than 5% a t room temperature. At elevated temperatures, such errors should be lower because the O(3P) atom reaction rate constants for the reactant are expected to approach those for the reaction products. It can be seen from Table I1 that, within the likely experimental errors, the preexponential factor A is reasonably constant for all the aromatics studied, while the activation energy E2 decreases with the degree of methylation. Hence, in a similar manner to the reaction of O(3P) atoms The Journal of Physical Chemistry, Vo/, 79,No. 4, 7975
IOOO/T
(OK)
Flgure 2. Arrhenius plots of log k2 against 1/ T for the aromatic hydrocarbons. (The error bars on the data points for o-xylene and 1,2,3-trimethylbenzenehave been omitted for clarity.) with olefins,6-8 the different reactivities shown by the aromatics are primarily due to differences in the activation energies. The preexponential factors for the reactions of O(3P)atoms with the aromatic hydrocarbons are somewhat higher (by a factor of -2) than those recently determined for the simple o l e f i n ~ . ~ ~ ~ - ~ ~
Reaction of O p P ) Atoms with Aromatic Hydrocarbons
297
TABLE 111: Comparison of the Arrhenius Activation Energies ( E 2 ) a n d the Room-Temperature Rate Constants ( K 2 ) from the Present Work with Selected Literature Values
IO%,, 1. mol-i sec-'
E,, kcal mo1-l
Aromatic
Present work
Benzene
3.98
Toluene o-Xylene m-Xylene $-Xylene
3.10 f 0.3 2.43 f 0.3 2.15 P 0.3 2.54 f 0.3
f 0.4
Lit. 4.4
f
0.5b
Derived from re1 rates"
Present work
Lit.
f 0.7
0.144 f 0.02
2.47 f 0.4
0.45 i: 0.045 1.05 & 0.11 2.12 f 0.21 1.09 * 0.11
0.28 * 0.OTh 0.36 i 0.07" 1.4 * 0.3c 6.7 f 1.6c 7.7 * 2.OC 4.5 1.4'
3.27
Derived from re1 ratesa 0.16 0.57
a References 6, 7, 12, and 13. Placed on an absolute basis using k z = 3.37 X 109e-1270/RT for ethylene.5 Error limits on E2 are those associated with [&(aromatic) - Ez(cyclopentene)];12.13the overall errors may thus be larger. Reference 2. c Reference 11.
The activation energies E 2 and the room-temperature rate constants k 2 determined in this work are compared to selected literature values in Table 111. The room-temperature rate constants obtained by Mani and Sauerll using a pulsed radiolysis technique are a factor of 3-6 higher than the present values, as discussed previously.3 Similarly, the room-temperature rate constant k 2 for benzene of Bonanno, et a L 2 determined by discharge flow mass spectrometry is a factor of 2 higher than the present value, although their activation energy of 4.4 f 0.5 kcal mol-], obtained over the range 255-305OK, is in good agreement with the activation energy E 2 = 3.98 f 0.4 kcal mol-l determined here. The relative rate constant data of Cvetanovic and co~ ~ r k e r ~obtained ~ t ~ at , ~393-493OK ~ J ~ have been placed on an absolute basis using h2 = 3.37 X 109e-1270'RT1. mol-l sec-' for e t h ~ l e n e .They ~ are seen to be in reasonable agreement with the present work for both benzene and toluene within the overall experimental errors. Recently the room-temperature relative rate constants for the reaction of OH radicals with n- butane and a series of aromatic hydrocarbons have been determined.14 The aromatic hydrocarbons studied show a similar order of reactivity toward OH radicals as with O(3P) atoms, although with a smaller range of rate constants. These rate constants, placed on an absolute basis14 using the rate constant for OH n- butane of 1.8 X lo9 1. molm1sec-1,15-18 range from (2.5 f 0.9) X lo9 1. mol-' sec-l for toluene to (3.1 f 0.4) X 1O1O 1. mol-] sec-l for 1,3,5-trimethylbenzene, demonstrating the higher reactivity of the OH radical
+
toward the aromatic hydrocarbons compared to O(3P) atoms.
Acknowledgments. The authors are grateful to Drs. A. C. Lloyd and J. L. Sprung for helpful discussions. The financial support of NSF Grant GP-38053X and Environmental Protection Agency Grant 800649 is gratefully acknowledged. The contents do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. References a n d Notes (1) J. T. Herron and R. E. Huie, J. Phys. Chem. Ref. Data, 2, 467 (1973). (2) R. A. Bonanno, P. Kim, J. H. Lee, and R. B. Timmons, J. Chem. Phys., 57, 1377 (1972). (3) R. Atkinson and J. N. Pitts, Jr., J. Phys. Chem., 78, 1780 (1974). (4) R. Atkinson and R. J. Cvetanovic, J. Chem. Phys., 55,659 (1971). (5) R . Atkinson and J. N. Pitts, Jr., Chem. Phys. Left., 27, 467 (1974). (6) R. J. Cvetanovic, J. Chem. Phys., 33, 1063 (1960). (7) R. J. Cvetanovlc, Advan. Photochem., 1, 115 (1963). (8) D. D. Davls, R. E. Huie, and J. T. Herron, J. Chem. Phys., 59, 628 (1973). (9) D. D. Davls, R. E. Huie, J. T. Herron, M. J. Kurylo, and W. Braun. J. Chem. Phys., 56, 4888 (1972). (IO) M. J. Kurylo, Chem. Phys. Lett., 14, 117 (1972). (1 1) I. Mani and M. C. Sauer, Jr.. Advan. Chem. Ser., No. 82, 142 (1968). (12) G. Boocock and R. J. Cvetanovic, Can. J. Chem.. 39,2436 (1961). (13) G. R. H. Jones and R. J. Cvetanovic, Can. J. Chem., 39,2444 (1961). (14) G.J. Doyle, A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, Jr., Environ. Sci. Techno/.,in press. (15) N. R. Greiner, J. Chem. Phys., 53, 1070 (1970). (16) E. D. Morris, Jr. and H. Niki, J. Phys. Chem., 75, 3640 (1971). (17) F. Stuhi, 2.Natufforsch. A, 28, 1383 (1973). (18) R. A. Gorse and D. H. Volman, J. Photochem., 3, 115 (1974).
The Journal of Physical Chemistry, Vol. 79, No. 4, 7975
