Gas-Phase Recombination of Chlorine Atoms
1325
On the Gas-Phase Recombination of Chlorine Atoms R. P. Widman and 6 . A. DeGraff*' Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 and Department of Chemistry, Madison College, Harrisonburg, Virginia 22801 (Received April 26, 1972) Publication costs assisted by Madison College
The recombination of chlorine atoms in the presence of various chaperons has been studied over the range 195-373°K using kinetic spectroscopy. The rate coefficients obtained, in M - 2 sec-l units and expressed as kM = A exp(E/RT) or log kM = B C log T, were kHe = 1.47 x lo9 exp(260 f 10IRT) or log kHe = 10.55 f 0.12 - (0.48 f 0.009) log T; k~~ = 1.26 X lo9 exp(660 f 20/RT) or log k~~ = 12.74 f 0.26 - (1.27 f 0.02) log T; kAr = 2.5 x los exp(1800 f 50/RT) or log kAr = 18.62 f 0.60 - (3.59 f 0.05) log T; k N z = 5.8 X lo8 exp(1600 f 140/RT) or log kNz = 15.61 f 0.65 - (2.32 f 0.02) log T; ksF6 = 2.4 x lo9 exp(970 f 100/RT) or log ksF6 = 13.72 f 0.76 - (1.47 f 0.02) log T; kCFi = l.62 x log exp(1200 f 200/RT) or log kCF4 = 14.43 f 1.36 - (1.77 f 0.04) log T; kcoz = 1.56 x 109 exp(1500 f 200/RT) or log k c O z = 15.75 f 0.60 - (2.22 f 0.018) log T. Where comparison values are available they are in good agreement with the results of this investigation. The values reported here are compared with the available data for iodine and bromine atom recombination.
+
Introduction The mechanism of atomic recombination in the presence of a third body is a fundamental process in chemical kinetics and continues to be the subject of numerous experimental and theoretical investigations. Experimentally, the recombination rates of bromine2 and iodine3 atoms have been extensively studied over a wide temperature range for many chaperons. However, there exist only limited data, especially at temperatures below 1000"K, on the gas-phase recombination of chlorine atoms. Because of the low extinction coefficients for chlorine, a formidable experimental problem is encountered in using the chlorine absorption to monitor the progress of the reaction. As a result, the presently available data from shock wave studies4-7 and flow system experiments8-10 were obtained using other detection means. Also, the high chemical reactivity of' chlorine atoms sharply limits the available chaperons. One of the most interesting results of the iodine and bromine studies is the wide range of efficiencies shown by the various third bodies in promoting recombination. Further, the efficiency of a chaperon can be correlated with the temperature dependence of kM, the recombination rate constant for the particular third body. Since chlorine atoms have the highest electron affinity of the halogen atoms, it is of interest to see if the recombination rate constants span a range similar to that observed for bromine and iodine and further to see if the same efficiencytemperature dependence relationship exists. To these ends we have determined the termolecular recombination rate constants and their temperature dependence for chlorine atoms in the presence of various chaperons of differing molecular complexity. This was done using the techniques of kinetic spectroscopy and utilizing the absorption of molecular chlorine to monitor the recombination. Experimental Section The reaction cell consisted of a length of Pyrex pipe (approximately 76 cm long with a 12.7-cm diameter) fitted with aluminum end plates. Multiple passes of the
monitoring light through the cell were effected by use of three mirrors in a modified White cell configuration.11J2 For this study, four traversals were used giving an effective light path of 2.8 m. Two flash lamps of 22-mm quart.z pass longitudinally through' the cell about 3 cm above and below the observed portion of the reaction zone. A piece of polished aluminum was curled to fit inside the reaction vessel and served as a reflector. Energy for the flash lamps was provided by two 14-pF capacitors charged to about 10 kV. With t h e flash tubes filled with a xenon-nitrogen mixture, the discharge time to 1% intensity was about 80 psec. A 100-W Osram mercury arc powered by two 12-V batteries served as the monitoring source. The wavelength used for this study, 365 nm, was isolated with a Spex 0.75-meter spectrometer fitted with a polychromator ai:tachment. Since considerable amplification of the photomultiplier output was necessary to obtain useful signals, baseline stability was a problem. The evaluation of th,e rate constants proved to be quite sensitive to baseline fluctuations and hence the baseline was monitored by a second photomultiplier set outside the chlorine absorption region. The relationship between the baseline as monitored by the two detectors was established by comparing scope trace photographs obtained on flashing the empty cell. The correlation between the two detectors was quite reproducible. Thus, the inverted output from the baseline monitor was added to the output from the photomultiplier (1) Present address: Madison College, Harrisonburg, Va. 22801. (2) (a) B. A. DeGraff and K. J. Lang, J. Phys. Chem., 74, 4181 (1970), and references therein: (b) J. K. K. Ip and George Burns, J. Chem. Phys., 51. 3414 (1969), and references therein. (3) J. A. Blake and %eorge Burns, J. Chem. Phys., 56, 3155 (1972:1, and references therein. (4) R. W. Diesenand J. Felmlee,J. Chem. Phys., 39, 2115 (1963). (5) T. A. Jacobs and R. R. Giedt, J. Chem. Phys., 39, 749 (1963). (6) R. A. Carbettaand H. B. Palmer, J. Chem. Phys., 46, 1333 (1967). (7) M. van Thiel, D. J. Seery, and D. Britton, J. Phys. Chem., 69, 834 (1965). (8) E. Hutton and M. Wright, Trans. FaradaySoc., 61,78 (1965). (9) L. W. Baderand E. A. Ogryslo, Nature (London), 201,491 (1964). (10) M. A. A. Clyne and D. H. Stedman, Trans. Faraday SOC., 64, 2698 (1968). (11) J. U. White, J. Opt. SOC.Amer., 32,285 (1942). (12) R. V. Fitzsimmons and E.J. Bair, J. Chem. Phys., 40,451 (1964). The Journal of Physical Chemistry, Vol. 77, No. 11, 197'3
R. P. Widman and
1326
B. A. DeGraff
m
5
0 >
TIME (msec) Figure 1. Oscilloscope trace for the recombination of CI atoms in Ne: Pci, = 0.91 Torr; = 290.0 Torr; T = 327.2'; scale
factors: 0.01 V/division; 5 m sec/division.
monitoring the Cl2 absorption. This technique gave a reproducible empty and filled cell baseline and the data scatter was reduced to ca. *5% random about the leastsquares line. The rate of appearance of Cl2 after the flash was followed by displaying the photomultiplier output for X 365 nm on a Tektronix 547 oscilloscope. Photographs of the oscilloscope traces were measured with precision calipers and the deflection converted to Cl concentration. A typical oscilloscope trace is shown in Figure 1. A conventional glass vacuum line was used to fill the reaction cell. To load the cell, chlorine was expanded from a storage bulb into a 5-1. mixing bulb and the pressure measured with a sulfuric acid manometer. The mixing bulb was then filled with chaperon gas to a pressure of about 1 atm. The gases were mixed by mechanical stirring for -30 min, and then expanded into the reaction cell. The total pressure in the reaction cell was measured with a Wallace-Tiernan absolute pressure gauge. As all the volumes involved were calibrated, the partial pressure of chlorine in the reaction cell could be readily calculated. The mixtures obeyed Beer's Law and for 23", e 24 M-1 cm-1 a t 365 nm, in good agreement with the data of Gibson and Bayliss.l3 After the correlation between the manometric and photometric values for the chlorine pressure was established, the photometric method was used to determine the Cl2 pressure in the cell. The cell was surmounted in a Styrofoam box and the cell itself was wrapped with asbestos tape over which was wound nichrome heating wire. Temperatures above room temperature were obtained by adjusting the current through the coils until the desired temperature was reached and. remained constant over a 2-hr period. The mixture was flashed several times during the 2-hr equilibration period to assure thermal equilibrium between the cell walls and the gas mixture. A calibrated iron-constantan thermocouple was used to monitor the cell wall temperature. Measurements at -78" were performed by packing the Styrofoam box with Dry Ice and allowing at least 4-hr for equilibration. The chaperon gases used in this study did not react chemically with either Cl2 or C1 and did not undergo direct photolysis. The concentration of chlorine in the cell was checked both before and after a set of experiments to determine if any permanent chemical change had occurred. For the data reported here this change was never greater than the photometric error in measuring the 612 concentration (Le., -1%). All gases used were of reagent grade and were used without further purification. Stopcocks which came in contact with chlorine or its mixtures were greased with either Dow Corning silicone grease or 3-M Kel-F stopcock grease. The Journal of Physical Chemistry, Vol. 77, No. 7 1, 7973
Figure 2. Plot of k&d vs. (C12/M) for M = He, H; M = Ar, 0 . The dashed lines define our estimated error limits for the average value of kobsd.
3
2
4
5
l / T x IO'
Figure 3. Arrhenius plots of kobsd Ne, E ; and M = Ar, 0 .
k~ for M = He, A ; M =
Results For this study flash energies centered around 1500 J which resulted in ca. 10% initial dissociation of the chlorine. Further, PclzN 1.0 Torr and 2 x 5 Xclz 5 1 C x with most experiments done at Xclz = 4 x 10-3.14 If the average wavelength absorbed by the chlorine is taken as h(max), we calculate that under the worst conditions the maximum temperature rise expected a t the completion of the recombination was 10" and that the usual rise lay in the range of 3-5" for most of our experiments. To minimize any possible error due to thermal effects, the data points were taken only over the first two half-lives of the reaction. The data obtained in this study were treated according to the following rate expression
+
where kobsd = k M [C12]/[M]kcl,. The contribution from atomic chlorine was neglected. The observed rate constant, kobsd, was obtained through a least-squares fit of the data to the integrated form of eq 1. To assess the contribution of the second term to k&sd under our conditions, a series of experiments was performed in which the C12-M ratio was varied over a five- to tenfold range. The chaperons chosen for these experiments were helium and argon since the knf for these gases was expected to be among the smallest observed and hence any contribution from the (13) G. E. Gibson and N. S. Bayliss, Phys. Rev., 44, 188 (1933). The temperature dependence of e at this wavelength is within experimental error for the temperature range used. (14) Xciz = P c i 2 / ( P c i Z P M ) N P C I ~ / P M .
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Gas-Phase Recombination of Chlorine Atoms
1327
TABLE I: Rate Constants for Chlorine Atom Recombination Chaperon
No. of expt
T, "C
He
10 20 8 9 8 11 11 9 40 7
- 68
Ne
Ar
6 Nz
8 9 8
sF6
10 9 7 8 8 9 9 9 4 9
CF4
CQz
20
100
- 78 20 54 100 - 78 20
50 100 20 50 100 20 52 100 20
50 100 20
50 70 100
kM X
-
IO-$, M - 2 sec-'
2.77 f 0.26 2.28 f 0 . 1 3 2.09 f 0 . 1 5 6.85 & 0.45 3.78 f 0.37 3.45 & 0.15 3.06 f 0.30 25.3 f 2.5 5.32 f 0.33 4.02 f 0.20 2.79 f 0.18 8.08 f 0.81 6.09 f 0.29 4.60 & 0.46 12.0 f 1.2 11.0 f 0.4 8.48 f 0.80 12.3 .f1.2 9.19 f 0.49 7.93 f 0.69 18.8 f 1.6 15.7 f 0.5 12.7 f 0.9 11.2 f 0.9
k,,, term would be more readily detected. The plots of kobsd us. [C12]/[M] for M = He and Ar, Figure 2, indicate that for all our experiments the contribution of Clz to the recombination was within experimental error. Thus, the approximation that kobsd = k M is valid for our experiments. Table I presents a summary of our experimental results. The rate constants are cast in both an Arrhenius form and as log kM = El C log T. The error limits shown in Table I represent the standard deviation of the appropriate least-squares line and should be taken as a measure of the precision of the data. However, due to the difficult nature of the experiments (eg., such problems as thermostating a 14-1. cell at -78") the accuracy is likely less by about a factor of 2. Arrhenius plots of log k M us. 1/T for M = He, Ne, and Ar are shown in Figure 3. Similar plots were obtained for all chaperons studied.
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Discussion
The rate constant for C1(2P3/z) recombination with argon as the chaperon has recently been obtained using a low-pressure flow system.1° The value of kAr = 2.0 x lo8 exp(1800 f 700/RT) M - 2 sec-l reported in that study is in good agreement with the present results as is the value for kAr reported in ref 9 and the reinterpretationlo of the data from ref 8. The only other chaperon for which a rate constant has been reported and with which we can compare our results is that for helium, kHe N 3 x 109 M-2 sec-l at 25°,9 which is in satisfactory agreement with our value. That the contribution to kobsd from the kclz term in (1) is negligible for the [Clz]/[M] ratios used in this study is consistent with the reported k c l z = 20 f 3 x 109 M-2 sec-I at 25".lo Referring to Table 11, it appears that Clz is the least efficient of the halogens as a chaperon. Porter,15a in his classic studies on iodine atom recombination, observed a distinct correlation between the efficiency of a chaperon and the temperature dependence of
k~
= A exp(E/RT); l o g k = ~ B f c i o g r, M-2sec-'
k ~ =r 2.50 X
l o 8 exp(1800 f 5 0 / R T ) ;
= 18.62 f 0.60 -
log k~~
(3.59 f 0.05)log 7
k N z = 5.80 X 1O8exp(1600 f 140/RT); log k N z = 15.61 f 0.65 - (2.32 f 0.02) log
r
kM as expressed in the form k. = A exp(E/RT). A similar relationship is found for bromine atomsg2aTo rationalize and correlate these and other experimentally observed trends in atom recombination data, two recombination mechanisms have been used as the basis for several formal approaches to the calculation of kM. The "bound cornplex" (BC) mechanism15Je can be represented as C1 MC1*
+
+
M -=c= MC1*
M e MCl
h,, h - l
+
M h,, h-2
+
MC1 -k C1 -C1, M h, where E(MC1") 5 0 and E(MC1) < 0 khile the energy transfer (ET) appr0achl7-~9can be represented as
+ C1 z== C1,* + M C1
-
Clz*
kq,
h-4
C1, + M h , Also, Keck and coworkers have developed a classical phase-space theory which includes contributions from both mechanisms.20 There has been considerable discussion as to the relative contribution of the BC and ET mechanisms. Simple calculations by Johnstonz1 and more detailed work by Clarke and B ~ r n s suggest ~ ~ , ~that ~ for I atom and Elr atom recombination in the room temperature region, the BC mechanism is dominant for all but the lightest chapDiscuss. Faraday SOC., 33, 198 (1962);(b) D. I.. Bunker and N. Davidson, J. Amer. Chem. SOC., 80,5085 (1958). (16) S.K. Kim, J. Chem. Phys., 46,123 (1967). (17) E. Rabinowitch, Trans. Faraday Soc., 33,283 (1937). (18) D. L. Bunker, J. Chem. Phys., 32,1001 (1960). (19) S.W. Benson and T. Fueno, J. Chem. Phys., 36,1597(1962). (20) V. H. Shui. J. P. Appleton, and J. C. Keck, J. Chem. Phys., 53, 2547 (1970),and references therein. (21) H. S.Johnston, "Gas-Phase Reaction Rate Theory," Ronald Pres!;, New York, N. Y., 1966. (22) A. G.Clarkeand G. Burns, J. Chem. Phys., 55,4717(1971). (23) A. G.Clarkeand G. Burns,J. Chem. Phys., 56,4636(1972). (15) (a) G. Porter,
The Journal of Physical Chemistry, Vol. 77, No. 7 7, 7973
R. P. Widman and B. A. DeGraff .E 11: Room Temperature (20’) Rate Constants and E / R Values for Halogen Atom Recombination
He Ne Ar N2
SFs
2.3 3.8 5.3 8.1 12.0
131 331 904 803 487
0.44 0.72
12.3 18.8
602 753 803
2.3
1.26 1.56
1.o
2.26
1.5 2.3
3.36 8.1
10.ld
CF4
coz X2
(halogen)
a Reference 10.
20a
3.5 3.8
10.2d 7.8c 446
438= 4946 703c 755c 906b 1045d 116gd 1O6Oc
0.54
1.5e
201“
0.68
1.9f 2.ge 4.5f
15.9 554e
13.4e
a8oe 22151~
1.o 1.5 3.7
3.5 .20
1600h
0.52 0.65
1.o 1.6
4.6 550
*
Reference 1. Reference 2. Reference 25. e Reference 26. f Reference 27. g Reference 28. Reference 15. This work
erons. However, recent work by Pack, Snow, and Smith24 suggests that the E T mechanism dominates for H atom recombination in He, Ar, and H2 except at very low temperatures. A comparison of the room temperature kM values shown in Table 1125-28 indicates that kM for C1 atom recombination is somewhat greater than for I atoms and about twice the corresponding value for Br atoms. The singular exception is the case of the halogen itself acting as a chaperon. The recombination in this case would, according to the BC model, take place uia an X3 intermediate. While C13 has been observed in low-temperature infrared experim e n t ~ direct , ~ ~ measurements of the bond energies for Is, Bra, and cl3 are not available. However, the kinetic data would indicate that I3 > Br3 N C13. The formulations of k~ which are based on the BC mechanism and also the phase-space approach of Keck, which includes both mechanisms, suggest that when the atom-chaperon interaction is large with respect to RT, the temperature dependence of K M is approximately exponential. -Thus, it is instructive to compare the observed , in exponential temperature dependence of k ~ expressgd form, for the three halogen atoms for which data is available (see Table 11). Interestingly, the correspondence between the efficiency of a chaperon and the E I R value which is observed for I atom (see extensive data of ref 15a) and Br atom recombination is not well defined for the C1 atom case with the chaperons used here. In the simple+ formulations based on the BC mechanism E / R is interpreted as a measure of the atom-chaperon potential well. In recent work, Burns30 using a modified BC approach and Keck31 using the phase-space approach have used the atom-chaperon potential well as the adjustable parameter in order to fit the available C1 atom experimental data. Interestingly Keck
The Journalof Physical Chemistry, Vol. 77,No. 11, 1973
found that the best fit to the available chlorine atom data in excess argon was obtained with a C1-A well depth of 900”. This may be compared with the value of 904°K from this work using the very simple Arrhenius formulation. However, this is likely just fortuitious. Finally, the lack of a simple correlation between the efficiency of the chaperon and the temperature dependence of kM as expressed in exponential form may be due to the fact that the simplifying assumptions required to obtain an Arrhenius expression are not valid under our conditions. Further, while an Arrhenius formulation of kM may be used to fit the data, a simple correlation between the argument of the exponent and the depth of the Cl-chaperon potential well as suggested for I and Br atom recombination is not valid. Indeed, recent scattering experimentsaz suggest that even when the BC mechanism represents the major pathway for recombination, the relationship between the experimental temperature dependence of kM and the atom-chaperon potential well may not be as straightforward as the simpler formulations based on the BC mechanism suggest. R. T. Pack, R. L. Snow, and W. D. Smith, J. Chem. Phys., 56, 826
(1972).
S. K. Chang, A. G. Clarke, and G. Burns, J. Chem. Phys., 54, 1835 (1971). G. Porter and J. A. Smith, Proc. Roy. SOC.,Ser. A, 261,28 (1961). K. E. Russell and J. Simons. Proc. Roy. SOC., Ser. A, 217, 271 (1853).
G. Porter, 2. G. Szabo, and M. G. Townsend, Proc. Royal SOC., Sef. A, 270,493 (1962). L. Y. Nelson and G. C. Pimentei, lnorg. Chem., 7, 1695 (1968). G.Burns and R. J. Browne, J. Chem. Phys., 53,3318 (1970). V. H. Shui, J. P. Appleton, and J. C. Keck, Symp. (Int.) Combust., [Proc.], 72th, 7958, 21 (1969). M. J. Cardlilo. M. S. Chou, and E. F. Greene, Abstracts of the 163rd National Meeting of the American Chemical Society, Boston, Mass., paper 77,Physical Division, Apr 1972.
