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Rate constants for the reaction of O(3P) atoms with benzene and toluene over the temperature range 299–440 K
RATE CONSTANTS FOR THE REACTION OF 0(3P) OVER THE TEMPERATURE RANGE 299-440 K R. ATKINSON and JN.
1 June 1979
CIiEhIICAL PHYSICS LEITERS
Volume 63. number 3
PUTS
ATOMS WITH BENZENE AND TOLUENE
Jr.
Deparmenr of Clretnisrry and Starewide Air Politttion Research Center, Uniz~ersir_tof Califorrrirr Rirede. Cidifornia 9252t. Um Received 1 September 1978; in final form 3 January 1979
Absolute rate constants for the reaction ofOC3P) atoms with ben&e and toluene hare been determmed oxer the temperature range 199-440 K usins a flash photoI)&-NO, chemiluminescence technique_The Arrhenius e\pressione obtained x\ere.X-2
I_ introduction The kinetics of the gas-phase reactions of O(3P) atoms with aromrttic hydrocarbons have been investigated using both relative [I-S] and absolute j6-121 rate techniques, most of these studies being for benzene [1,4,6-9,111 and toluene [24-6&9,11,12]. However, there are significant discrepancies between the rate-constant data obtained by the differing absolute techniques (pulsed radiolysis 161, discharge flow [7,121, and modulation-phase shift [$X,9,1 I 1) and also between the rate constants obtained by the two modulation-phase shift studies &9,11] for benzene and toluene. In this work, in an attempt to resolve these Iatter discrepancies. absolute rate constants have been determined for the reaction of O(3p) atoms with benzene and toluene over the temperature range 299-440 K using a flash photolysis-NO, chemiluminescence technique_
2. Experimentzxl The apparatus and techniques used have been described in detail [I 3,1-l], so only a brief summary is given here. O(3P) atoms were produced by the pulsed vacuum ultraviolet photodissociation of 0, (0.2-0.3
Torr) and NO (0_02-0.03Torr) at wavelengths longer than the CaF, cutoff (2 f 250 A). O(3P) atom concentrations were monitored as a function of time after the flash by NOz chemituminescence from the reaction [15,16]
0 + NO +
iv --fNO; I
-
f h¶
(1)
using a cooled EhlI 9659A photomultiplier fltred with an interference filter (center wakelength 5577 A_ halfbandwidth 96 A)_ The reaction cell was enclosed in a furnace which could be held constant to better than Fi K over the temperature range 295-450 K. The gas temperature was measured by a chromel/‘aIumel thermocouple mounted inside the reaction vessel- The flash lamp was operated at discharge energies of 12- 50 J per flash and a repetition rate of one flash e\ery three seconds_ Signals were obtained by photon counting in conjunction with multichannel scaling. Decays of the NO, chemiluminescence, and hence of 0{3P) atom concentrations, were accumulated from 65-I 601 flashes depending on the signal strengths. 0(3PJ atom half-lives ranged from I _72--25.3 ms and were folIowed over at least three half-Iives. All experiments were carried out under flow conditions such that the gas mixture in the reaction vessel was typically replenished between every 435
Volume 63. number 3
CHEYICAL
PHYSICS LfFI-TERS
of the NO, chemiiuminescent signal, and hence of the O(3Pj atom concentration, is given by the integgated rate expression
flash to avoid the accumubtion of photolysis or reaction products_ The gases used had the following purity teveis according to the manufacturer: Ar >99998%; O2 2 9999%; NO 299.0$& Ar and 02 were passed through traps containing Linde Molcular Sieve 4A, while NO was passed throeh Linde hiofecufar Sieve f 3X at 295 ECto remove any Hz0 and lW2 present _ A known fraction of the totaf argon flow was saturated with benzene or tohxene vapor (299% purity by gas chromatographic analysis] at 273 If and atmospheric pressure (or, for the use of toluene, also at =26Torr totaI pressure). The benzene and toluene partial pressures in this fkaction of the argon flow were determined by their ultraviotet absorption using a 9.0 cm pathIength cell and a Cary I5 spectrophotometer. The absorptions were calibrated using known pressures of
benzene or roluene as measured by an MKS Baratron capacitance manometer_ AI1 flows %irere~1oI~tored by calibrated f!owmeters and the gases were premised before entering the reaction vessei.
3, Rest&s The reaction 0f0(~P)
atoms with benzene and
tolnene \cere studied oter the temperature range 299443 K with =26Torr of argon as the diIuent gas_ Under the experimental
conditions employed,
the decay
1 June 1979
=
esp {(Xc1tlC_,[aromatic] i- ks PO] [M])(f-rO)),
where [O(3P)D] and [O(3P)r J are the concentrations of O(3P) atoms at times to and t, respectiveIy, So and S, are the corresponding NO, chemihnninescence intensities, ki is the first order rate for removal of 0(3P)
atoms in the absence of added reactant and NO (primarily attributed to diffusion out of the viewing zone anJ to reaction with O,),and ik? and k3 are the rate constants for the reactions O(3P) + aromatic-,
products,
(7-l
O(3P) +-NO f- hi + NO, + hl.
(3
In a11 experiments, exponential decays of the hf02 chemiluminescence signal were observed, and the measured de-y rates, defined asR = (t---~~)-~ In{S&), were found to depend IinearIy on the concentrations of added benzene or toiuene at constant total pressure and is0 concentration. Hence eq. (I) was obeyed and rate constants k, were accordingly derived from the slopes of plots of the decay rate R against the benzene or tofuene concentration_ Figs- I and 2 show typical plots of the 0(3P) atom decay rate against reactant
500
f
,-&?2
K
I
06
,@EINZENE]
molecule
t
08
t
IO
I
t2Ms6
cmm3
Fig. I. Ptvts 0f0(~P) atom decay raft a&as1 benzene concentration at 2989,337-0,385.4 and 440_2&L /Data points hs+e been displaced wrtieatl) by -36 s-t for T = 7989 I;: - 17 s-t for T = 337.0 K; +I 1 s-t for T = 385.4 K; and +20 s-l for T = 440.1 K to 1ield a cvmmon 0t3Pk svm decay sate of 50 s-t III - the abxnce of benzene-> Totat pressure==26Tvrr with argon diluent.
436
CHEhIICAL
63, number 3
1
PHYSICS LETTERS
I
05
I
t
IO [TOLUENE~
1 June 1979
20
15
mofecule
I
25x105
cmm3
Fig. 2- Plots of 0(3P) atom decay rate a&r& toluene concentmtion at 298-7, ** ~>6_9,385_4 and 410.4 K. (Data poinrs htiie been dispiaced vertically by -31 s+ for T= 298.7 EC;+2 s+ for T = 3369 K; +15 5-l for T = 385_4 K; and -23 s-t for T = 44OA Ii to yield a common Ot3P> atom decay rate of 50 s-r _m the absence of toiuene.) Total pressure z-26 Torr with argon drtutnt.
concentration for benzene and toluene, respectively. at the temperatures studied, whi!e table 1 gives the rate constants k2 obtained by Ieast-squares analysis from the slopes of such plots_ Duplicate determinations of the rate constants at 299 K for benzene, and at a!1 four temperatures for toluene (with the argon flow being saturated with toluene vapor at ~26 Torr total pressure, leading to potentially higher error limits for the toluene concentraiions in rhe reaction vessel. and involving the use of different flowmeters), yielded values of Ic, differing by 64% in all cases.
Table 1 Rate constants A-2 for the reaction of Of3P) atoms I\ith benzene and toluene. The indicated error limits include the leastsquares standard deviations (25-45&) as ixeI1 as the estimated accuracy limits of other parameters such as pressure and reactsnt eoncentmtions Aromatic
7-K)
1 Oz4k2
benzene
toluene
(cm3molecule-’ S-I )
&(benzene)
296.9 337-Q 385.4 440.2
2.00 4.32 9.15 17.3
t h i r
0.20 0.43 0.92 1.7
298.7
9-62 17.6 29.6 51-2
* t r t
0.96 1.8 3.0 5.1
3369 385.4 440.4
The feast-squrtres Intercepts of the 0(3P) atom decay rates obtained in the presence of benzene and ‘toluene were typically 3-20 s-l higher rhan the O@) atom decay rates obtained in their absence_ Such an effect has previousIy been noted for rz-butane [ 133 _ ethylene [14] _propylene (141 and the vinyl halides[17], presumably due to a small contribution from secondary reactions of O(3P) atoms wth reaction products at lo\\ aromatic concentrations. Rate constants k, were hence determined from Ieast-squares analyses of the 0(3P) atom decay rates obtained in the presence of the aromatics. In all cases, a variation of the flash energy by a factor of 3 or -+ had no effect on the rate constants within the experimental errors (?3%), indicating that secondary reactions of 0(3P) atoms with photofysis or reaction products had a negligibk effect on the measured rate constants_ Fig_ 3 shows the data plotted in Arrhenius form while least-squares analysis of the data yields the Arrhenius parameters: = I.68
cm3 molecuIe-l &(toluene)
X IO-”
= 1.64 X lo-11
cm3 molecule-l
exp [-(I3995
t 2OO)/RT]
exp [-(3050
rf:2OO)IRT]
s-1.
s-l,
where the activation energies are in cal moIe-1 and the indicated error limits in the activation energies are the estimated overaii error iimits. 457
Volume 63, number 3
CHEMICAL PHYSICS LETTERS
1
June 1979
4_ Discussion Secondary reactions of other atomic species formed in the flash can be shown to be of no consequence for the conditions used [13,14]_The reaction of 0(3P) atoms with benzene and toluene have been shown to proceed primarily via the addition of the O(3P) atom to the aromatic ring [I-4,6,7,18,19], followed by rearrangement of this initial biradical to form phenolic products, or ring cfeavage or contraction to form highly reactive conjugated oxygenates which wiIl react rapidIy with OCP) atoms [I ,181. Nowever, the observation that for both benzene and tohrene a variation of the fIash energy by a factor of 2 or 4 produced no variation in the rate coustants indicates that secondary reactions of OCP) atoms with reaction products had a negligible effect on the measured rate constants under the conditions used. Impurities in the benzene and toiuene (other aromatic hydrocarbons [Sl ) ause neghgible errors in the measured rate constants_ Table 2 compares the present room-temperature rate constams X-? and Arrhenius activation energies E with selected Iiterature values. It can be seen that the roomtemperature rate constants for both benzene and toluene obtained by Mani and Sauer f6], and that for benzene determined by Bonanno et aI_ 171, are sub-
Table 2 Comparison of the present room-temperature
2lromrttic ~-_
_---
--
____-__ 10’3kT (cm3 Gtotecu?e-’ 5-l) -
bcnztmc
.-
5.98 -t&5 2.39 t5-r t.eo roruror
--
_
304b)
ioa
= I.16 = I-16 5 033 2 0.075 z 0.20
hj
------.
9 8 d) 961 c 096 ----
_-__ - ------
E (cd mole-’ ) _ _ _ I. -
-
----Technique a)
__----
-_
---__---
4000 c)
r&tribe r3te
3400 3950 atoo 3995
PR DF WJS MPS i-P
= 500 s 400 5 330 * 300
3300 =)
23.2 f 5.0 1.4: z 0.75 5.68 J 0.15 -c--_ll--
_- _- .- ---
rate constmas kz and Arrhenius actRation
3100 -+ 300 3S60 t 40
2680 L 190 3050 If 200 -I- __-_ _ __ ._ _.^_ _
rehti\e rate b, PR MPS MPS DF F-P
energies E wth sekcted literature values Ref. --cI---~ 11231
I6J 171 IS,91 ItIf this \iork
rs.9; IIll 1121
this aork
1) PR: pulsed ndiolrsis: DF: discharge tIow; BIPS: modulation-phase shift; FP: fk~sb photot>sis--NO2 ciwmiIuminescence_ b, C\tnpohtcd front measurements at 393 and -I!?3 K; rrtiivr to a room temperature rate constant for the reaction of 0C3P) atoms xith eth, Iene of k = 7.6 1 x IOmu cm3 mokrufe-’ s-’ [ 14 j. c, Rehrke to 3n Arrhenius acrivation energy for the rextion ofOCP) atoms \&ithcyclopentene of zero ~11~4). d, CaIcukned from the quoted Arrhenius expression.
Volume 63, number3
CHEMICALPHYSICS LE-PPERS
stantially higher than those determined in both the present work and the two modulation-phase shift studies [S,9,1 I ] _The relative rateconstants and Arrhenius activation energies of CvetanoviE and co-workers [l--3] are, considering that the values ofli2 at room temperature are extrapolated from data obtained at elevated temperatures (393-493 K), in good agreement with the present results. For benzene the present room-temperature rate constant X-2faUs in between the values obtained by the modulation-phase shift studies [S,9,1 I], being in agreement. within the cumulative error limits, with the rate constant k7 previously obtained in this laboratory [S, 91. In add&on, the present Arrhenius activation energy is in excellent agreement with those obtained by the modulation-phase shift studies [9,11] _ In the case of toiuene the present room-temperature rate constant is higher than either of the values obtained by the modulation-phase shift studies [&X,9, 111, but is in excellent agreement with the rate constant calculated from the Arrhenius expression of Furuyama and Ebara [I?] (although their _Arrhenius activation energy and preexponential factor are significantly tower than those obtained in this work). The Arrhenius activation energy determined in the present work is in escehent agreement with that obtained previously in this laboratory [9], but is significantly iower than that determined by Colussi et al [l 11. In conclusion, the present results (especially the Arrhenius activation energies) agree reasonably well, within 20-2570, with the modulation-phase shift data obtained in this laboratory [S,9], and it is thus likely that data for the sy!enes and trimethylbenzenes [S,9] are accurate to +20--25% [I 7]_
1 June 1979
References [If G_ Boocock and R_i_ Ctetanoti&
(1961) 2436_
Can. J. Chem_ 29
[2] G.R.H. Jones and R-J. Cvetano%irZ, Can. J_ Chem. 39 (1961) 2444. [3 j R.J- CvetsnovE, Adtxn- Photochcm. 1 (1963) I IS. 141 E. Gmvensteir; Jr_ snd A 3. Masher, J. Am_ Chem. Sot_ 92 (1970) 3810. [5) J.S. Caffney. R. Atkinson and J.N. Pittr Jr., J. Am. Chem. Sot. 97 (1975) 6481. [6] I. blani and WC. Sa~cr Jr., Advan. Chem. Ser. 82 (1966) 112. 171 R-A_ Bonanno. P_ Kun, J--H. Lee and R.B. Timmons. J. Chem. Phys. 57 (1972) 1377. [S] R. Atbinson and J.N. Pitts Jr., J. Phys. Chsm. 73 (1974) 1780_ [9] R_ Atkinson and J-N. Pnrs Jr., J. Ph>s. Chem. 79 i 1975) 295. [ 101 R_ Atkinson and J.N_ Pitts Jr., f. Phys. &hem. 79 (1975)
.541_ [ 11 I A-J_ Colussi. D-L_ Singieton. R.S. Irv,in and R.J. C%etanoviE_ J. Phq-s. CIzem. 79 (1975) 1900. [I21 S_ Furu_tAma and N. Ebara, Intern_ J. Chem_ Kinetics 7 (1975) 689. I131 Ii_ Atkinson, R.A. Perry and J.N. Pttts Jr., Chem PItqs. Letters 47 (I 977) 197. [ 141 R_ Atkinson and J-N_ Pit% Jr.. J. Chem. Ph_ts. 67 (1977) 38. [ 151 P. Kaufman,
Proc. Rej.
SOS. A217
(1958)
123.
[ 161 11 A.X. CIyne and B.A. Thrush, Proc. Roy. Sot. A269
(1962)
404.
[ 17:
R. Xtkmson and J N. Pltts Jr, J. Chem. Phys. 67 (1977) 3488. [ 181 JS_ Gnffneq, R. Atkinson end J.N. Pirts Jr., J. AQL Client. Sot- 98 (1976) 1S33. [ 19 1 T.M_ Sloane, J_ Chem. Ph> s_ 67 (1977) 2267.
Acknowtedgement The authors grateful&p acknowiedge the support of NSF Grant No. CHE76 10447_
489
1 June 1979
CIiEhIICAL PHYSICS LEITERS
Volume 63. number 3
PUTS
ATOMS WITH BENZENE AND TOLUENE
Jr.
Deparmenr of Clretnisrry and Starewide Air Politttion Research Center, Uniz~ersir_tof Califorrrirr Rirede. Cidifornia 9252t. Um Received 1 September 1978; in final form 3 January 1979
Absolute rate constants for the reaction ofOC3P) atoms with ben&e and toluene hare been determmed oxer the temperature range 199-440 K usins a flash photoI)&-NO, chemiluminescence technique_The Arrhenius e\pressione obtained x\ere.X-2
I_ introduction The kinetics of the gas-phase reactions of O(3P) atoms with aromrttic hydrocarbons have been investigated using both relative [I-S] and absolute j6-121 rate techniques, most of these studies being for benzene [1,4,6-9,111 and toluene [24-6&9,11,12]. However, there are significant discrepancies between the rate-constant data obtained by the differing absolute techniques (pulsed radiolysis 161, discharge flow [7,121, and modulation-phase shift [$X,9,1 I 1) and also between the rate constants obtained by the two modulation-phase shift studies &9,11] for benzene and toluene. In this work, in an attempt to resolve these Iatter discrepancies. absolute rate constants have been determined for the reaction of O(3p) atoms with benzene and toluene over the temperature range 299-440 K using a flash photolysis-NO, chemiluminescence technique_
2. Experimentzxl The apparatus and techniques used have been described in detail [I 3,1-l], so only a brief summary is given here. O(3P) atoms were produced by the pulsed vacuum ultraviolet photodissociation of 0, (0.2-0.3
Torr) and NO (0_02-0.03Torr) at wavelengths longer than the CaF, cutoff (2 f 250 A). O(3P) atom concentrations were monitored as a function of time after the flash by NOz chemituminescence from the reaction [15,16]
0 + NO +
iv --fNO; I
-
f h¶
(1)
using a cooled EhlI 9659A photomultiplier fltred with an interference filter (center wakelength 5577 A_ halfbandwidth 96 A)_ The reaction cell was enclosed in a furnace which could be held constant to better than Fi K over the temperature range 295-450 K. The gas temperature was measured by a chromel/‘aIumel thermocouple mounted inside the reaction vessel- The flash lamp was operated at discharge energies of 12- 50 J per flash and a repetition rate of one flash e\ery three seconds_ Signals were obtained by photon counting in conjunction with multichannel scaling. Decays of the NO, chemiluminescence, and hence of 0{3P) atom concentrations, were accumulated from 65-I 601 flashes depending on the signal strengths. 0(3PJ atom half-lives ranged from I _72--25.3 ms and were folIowed over at least three half-Iives. All experiments were carried out under flow conditions such that the gas mixture in the reaction vessel was typically replenished between every 435
Volume 63. number 3
CHEYICAL
PHYSICS LfFI-TERS
of the NO, chemiiuminescent signal, and hence of the O(3Pj atom concentration, is given by the integgated rate expression
flash to avoid the accumubtion of photolysis or reaction products_ The gases used had the following purity teveis according to the manufacturer: Ar >99998%; O2 2 9999%; NO 299.0$& Ar and 02 were passed through traps containing Linde Molcular Sieve 4A, while NO was passed throeh Linde hiofecufar Sieve f 3X at 295 ECto remove any Hz0 and lW2 present _ A known fraction of the totaf argon flow was saturated with benzene or tohxene vapor (299% purity by gas chromatographic analysis] at 273 If and atmospheric pressure (or, for the use of toluene, also at =26Torr totaI pressure). The benzene and toluene partial pressures in this fkaction of the argon flow were determined by their ultraviotet absorption using a 9.0 cm pathIength cell and a Cary I5 spectrophotometer. The absorptions were calibrated using known pressures of
benzene or roluene as measured by an MKS Baratron capacitance manometer_ AI1 flows %irere~1oI~tored by calibrated f!owmeters and the gases were premised before entering the reaction vessei.
3, Rest&s The reaction 0f0(~P)
atoms with benzene and
tolnene \cere studied oter the temperature range 299443 K with =26Torr of argon as the diIuent gas_ Under the experimental
conditions employed,
the decay
1 June 1979
=
esp {(Xc1tlC_,[aromatic] i- ks PO] [M])(f-rO)),
where [O(3P)D] and [O(3P)r J are the concentrations of O(3P) atoms at times to and t, respectiveIy, So and S, are the corresponding NO, chemihnninescence intensities, ki is the first order rate for removal of 0(3P)
atoms in the absence of added reactant and NO (primarily attributed to diffusion out of the viewing zone anJ to reaction with O,),and ik? and k3 are the rate constants for the reactions O(3P) + aromatic-,
products,
(7-l
O(3P) +-NO f- hi + NO, + hl.
(3
In a11 experiments, exponential decays of the hf02 chemiluminescence signal were observed, and the measured de-y rates, defined asR = (t---~~)-~ In{S&), were found to depend IinearIy on the concentrations of added benzene or toiuene at constant total pressure and is0 concentration. Hence eq. (I) was obeyed and rate constants k, were accordingly derived from the slopes of plots of the decay rate R against the benzene or tofuene concentration_ Figs- I and 2 show typical plots of the 0(3P) atom decay rate against reactant
500
f
,-&?2
K
I
06
,@EINZENE]
molecule
t
08
t
IO
I
t2Ms6
cmm3
Fig. I. Ptvts 0f0(~P) atom decay raft a&as1 benzene concentration at 2989,337-0,385.4 and 440_2&L /Data points hs+e been displaced wrtieatl) by -36 s-t for T = 7989 I;: - 17 s-t for T = 337.0 K; +I 1 s-t for T = 385.4 K; and +20 s-l for T = 440.1 K to 1ield a cvmmon 0t3Pk svm decay sate of 50 s-t III - the abxnce of benzene-> Totat pressure==26Tvrr with argon diluent.
436
CHEhIICAL
63, number 3
1
PHYSICS LETTERS
I
05
I
t
IO [TOLUENE~
1 June 1979
20
15
mofecule
I
25x105
cmm3
Fig. 2- Plots of 0(3P) atom decay rate a&r& toluene concentmtion at 298-7, ** ~>6_9,385_4 and 410.4 K. (Data poinrs htiie been dispiaced vertically by -31 s+ for T= 298.7 EC;+2 s+ for T = 3369 K; +15 5-l for T = 385_4 K; and -23 s-t for T = 44OA Ii to yield a common Ot3P> atom decay rate of 50 s-r _m the absence of toiuene.) Total pressure z-26 Torr with argon drtutnt.
concentration for benzene and toluene, respectively. at the temperatures studied, whi!e table 1 gives the rate constants k2 obtained by Ieast-squares analysis from the slopes of such plots_ Duplicate determinations of the rate constants at 299 K for benzene, and at a!1 four temperatures for toluene (with the argon flow being saturated with toluene vapor at ~26 Torr total pressure, leading to potentially higher error limits for the toluene concentraiions in rhe reaction vessel. and involving the use of different flowmeters), yielded values of Ic, differing by 64% in all cases.
Table 1 Rate constants A-2 for the reaction of Of3P) atoms I\ith benzene and toluene. The indicated error limits include the leastsquares standard deviations (25-45&) as ixeI1 as the estimated accuracy limits of other parameters such as pressure and reactsnt eoncentmtions Aromatic
7-K)
1 Oz4k2
benzene
toluene
(cm3molecule-’ S-I )
&(benzene)
296.9 337-Q 385.4 440.2
2.00 4.32 9.15 17.3
t h i r
0.20 0.43 0.92 1.7
298.7
9-62 17.6 29.6 51-2
* t r t
0.96 1.8 3.0 5.1
3369 385.4 440.4
The feast-squrtres Intercepts of the 0(3P) atom decay rates obtained in the presence of benzene and ‘toluene were typically 3-20 s-l higher rhan the O@) atom decay rates obtained in their absence_ Such an effect has previousIy been noted for rz-butane [ 133 _ ethylene [14] _propylene (141 and the vinyl halides[17], presumably due to a small contribution from secondary reactions of O(3P) atoms wth reaction products at lo\\ aromatic concentrations. Rate constants k, were hence determined from Ieast-squares analyses of the 0(3P) atom decay rates obtained in the presence of the aromatics. In all cases, a variation of the flash energy by a factor of 3 or -+ had no effect on the rate constants within the experimental errors (?3%), indicating that secondary reactions of 0(3P) atoms with photofysis or reaction products had a negligibk effect on the measured rate constants_ Fig_ 3 shows the data plotted in Arrhenius form while least-squares analysis of the data yields the Arrhenius parameters: = I.68
cm3 molecuIe-l &(toluene)
X IO-”
= 1.64 X lo-11
cm3 molecule-l
exp [-(I3995
t 2OO)/RT]
exp [-(3050
rf:2OO)IRT]
s-1.
s-l,
where the activation energies are in cal moIe-1 and the indicated error limits in the activation energies are the estimated overaii error iimits. 457
Volume 63, number 3
CHEMICAL PHYSICS LETTERS
1
June 1979
4_ Discussion Secondary reactions of other atomic species formed in the flash can be shown to be of no consequence for the conditions used [13,14]_The reaction of 0(3P) atoms with benzene and toluene have been shown to proceed primarily via the addition of the O(3P) atom to the aromatic ring [I-4,6,7,18,19], followed by rearrangement of this initial biradical to form phenolic products, or ring cfeavage or contraction to form highly reactive conjugated oxygenates which wiIl react rapidIy with OCP) atoms [I ,181. Nowever, the observation that for both benzene and tohrene a variation of the fIash energy by a factor of 2 or 4 produced no variation in the rate coustants indicates that secondary reactions of OCP) atoms with reaction products had a negligible effect on the measured rate constants under the conditions used. Impurities in the benzene and toiuene (other aromatic hydrocarbons [Sl ) ause neghgible errors in the measured rate constants_ Table 2 compares the present room-temperature rate constams X-? and Arrhenius activation energies E with selected Iiterature values. It can be seen that the roomtemperature rate constants for both benzene and toluene obtained by Mani and Sauer f6], and that for benzene determined by Bonanno et aI_ 171, are sub-
Table 2 Comparison of the present room-temperature
2lromrttic ~-_
_---
--
____-__ 10’3kT (cm3 Gtotecu?e-’ 5-l) -
bcnztmc
.-
5.98 -t&5 2.39 t5-r t.eo roruror
--
_
304b)
ioa
= I.16 = I-16 5 033 2 0.075 z 0.20
hj
------.
9 8 d) 961 c 096 ----
_-__ - ------
E (cd mole-’ ) _ _ _ I. -
-
----Technique a)
__----
-_
---__---
4000 c)
r&tribe r3te
3400 3950 atoo 3995
PR DF WJS MPS i-P
= 500 s 400 5 330 * 300
3300 =)
23.2 f 5.0 1.4: z 0.75 5.68 J 0.15 -c--_ll--
_- _- .- ---
rate constmas kz and Arrhenius actRation
3100 -+ 300 3S60 t 40
2680 L 190 3050 If 200 -I- __-_ _ __ ._ _.^_ _
rehti\e rate b, PR MPS MPS DF F-P
energies E wth sekcted literature values Ref. --cI---~ 11231
I6J 171 IS,91 ItIf this \iork
rs.9; IIll 1121
this aork
1) PR: pulsed ndiolrsis: DF: discharge tIow; BIPS: modulation-phase shift; FP: fk~sb photot>sis--NO2 ciwmiIuminescence_ b, C\tnpohtcd front measurements at 393 and -I!?3 K; rrtiivr to a room temperature rate constant for the reaction of 0C3P) atoms xith eth, Iene of k = 7.6 1 x IOmu cm3 mokrufe-’ s-’ [ 14 j. c, Rehrke to 3n Arrhenius acrivation energy for the rextion ofOCP) atoms \&ithcyclopentene of zero ~11~4). d, CaIcukned from the quoted Arrhenius expression.
Volume 63, number3
CHEMICALPHYSICS LE-PPERS
stantially higher than those determined in both the present work and the two modulation-phase shift studies [S,9,1 I ] _The relative rateconstants and Arrhenius activation energies of CvetanoviE and co-workers [l--3] are, considering that the values ofli2 at room temperature are extrapolated from data obtained at elevated temperatures (393-493 K), in good agreement with the present results. For benzene the present room-temperature rate constant X-2faUs in between the values obtained by the modulation-phase shift studies [S,9,1 I], being in agreement. within the cumulative error limits, with the rate constant k7 previously obtained in this laboratory [S, 91. In add&on, the present Arrhenius activation energy is in excellent agreement with those obtained by the modulation-phase shift studies [9,11] _ In the case of toiuene the present room-temperature rate constant is higher than either of the values obtained by the modulation-phase shift studies [&X,9, 111, but is in excellent agreement with the rate constant calculated from the Arrhenius expression of Furuyama and Ebara [I?] (although their _Arrhenius activation energy and preexponential factor are significantly tower than those obtained in this work). The Arrhenius activation energy determined in the present work is in escehent agreement with that obtained previously in this laboratory [9], but is significantly iower than that determined by Colussi et al [l 11. In conclusion, the present results (especially the Arrhenius activation energies) agree reasonably well, within 20-2570, with the modulation-phase shift data obtained in this laboratory [S,9], and it is thus likely that data for the sy!enes and trimethylbenzenes [S,9] are accurate to +20--25% [I 7]_
1 June 1979
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[2] G.R.H. Jones and R-J. Cvetano%irZ, Can. J_ Chem. 39 (1961) 2444. [3 j R.J- CvetsnovE, Adtxn- Photochcm. 1 (1963) I IS. 141 E. Gmvensteir; Jr_ snd A 3. Masher, J. Am_ Chem. Sot_ 92 (1970) 3810. [5) J.S. Caffney. R. Atkinson and J.N. Pittr Jr., J. Am. Chem. Sot. 97 (1975) 6481. [6] I. blani and WC. Sa~cr Jr., Advan. Chem. Ser. 82 (1966) 112. 171 R-A_ Bonanno. P_ Kun, J--H. Lee and R.B. Timmons. J. Chem. Phys. 57 (1972) 1377. [S] R. Atbinson and J.N. Pitts Jr., J. Phys. Chsm. 73 (1974) 1780_ [9] R_ Atkinson and J-N. Pnrs Jr., J. Ph>s. Chem. 79 i 1975) 295. [ 101 R_ Atkinson and J.N_ Pitts Jr., f. Phys. &hem. 79 (1975)
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Acknowtedgement The authors grateful&p acknowiedge the support of NSF Grant No. CHE76 10447_
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