207
Reaction of Oxygen Atoms with Carbon Disulfide
Temperature Dependence of Rate Constants and Branching Ratios for the Reaction of Oxygen Atoms with Carbon Disulfide Ronald E. Graham and Davld Gutman‘ Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 606 76 (Received August 4, 7976) Publication costs assisted by the National Science Foundation
-
The overall rate constant for the 0 + CS2 reaction ( k ) and the branching ratio ( k , / k ) for route 2 of this reaction, 0 + CS2 OCS + S, have been measured at seven temperatures between 249 and 500 K. The overall rate cms molecule-’ constant increases with temperature in a non-Arrhenius manner increasing from 2.9 X s-l at 249 K to 11.2 X lo-’, cm3 molecule-’ s-l at 500 K. The fraction of the reaction proceeding by route 2 decreases slightly with increasing temperature from 0.098 (f0.004)to 0.081 (*0.007) over the same temperature range.
Introduction The reaction between oxygen atoms and carbon disulfide has been shown to proceed by all three possible exothermic channel~l-~
+ cs,
+ SO + 31 kcal OCS t S + 5 5 kcal CO t S , + 83 kcal
[ T C S
(1) (2)
(3)
This reaction has received considerable attention in recent years because of its usefulness as a source of CS for producing chemical CO lasers through the important secondary reaction in these systems5 0t
cs+co* + s
(4)
The chemical kinetics of CS2+ 02(0) laser systems have been modeled to better understand the factors governing their p e r f ~ r m a n c e . ~It- ~has been found that the products of routes 2 and 3 are also involved in determining laser performance. The OCS produced by route 2 selectively relaxes the lower excited vibrational states of CO’,9 and can enhance laser output from higher levels. Nielsen has modeled an electrically discharged CS2-02 chemical laser, and has found that the OCS produced by route 2 is the dominant relaxer of C05 (CO in the fifth vibrational level) soon after the flash and continues to be one of the most important relaxers of CO near v = 5 at later timesS8The importance of OCS in determining laser performance has also been observed experimentally. Using a continuous wave chemical laser produced by reactions 1and 4, Suart, Arnold, and Kimbell found significant laser power enhancement (up to 500%) when small amounts of OCS were added to the system.1° Hudgens, Gleaves, and McDonald have found that the CO produced by route 3 is also vibrationally excited but has a markedly different vibrational population distribution than that of the CO produced in reaction 4, favoring the lower v levels instead of being peaked at v = 9.4 There is some evidence that this additional source of vibrationally excited CO effects laser performance. In Nielsen’s modeling study he did not include the production of excited CO from route 3, and he was unable to account for a “moderate rate of CO production in lower v levels”.8 For a better understanding of the chemical factors which govern the performance of CO lasers driven by the 0 CS2 reaction, more quantitative information is needed on the importance of its various routes. Although there have been several determinations of the overall rate constant
+
+
for the 0 CS2 reaction, very little is known about the rate constants into each reactive channel, kl-k3. Slagle, Gilbert, and Gutman have directly measured the branching ratio (R, = k 2 / k ) for route 2 at 302 K and find it is 0.093.3 Earlier estimates of R2,which vary from less than 0.015 to 0.22, were reviewed in their report of this s t ~ d y There .~ have been no direct determinations of R1 ( k , / k ) or R3 ( k , / k ) . Experimental evidence clearly suggests that route 1 is dominant (R, z O.8),l8s4 and that route 3 may account for about 10% of the overall reaction at room temperat~re.~,~ We have now measured R2 over an extended temperature range, 250-500 K, to learn whether the importance of route 2 changes with temperature. Estimates of kz over lo00 K suggest Rz might be increasing with temperature.!’ Such a suggested increase has already been used to model CS, O2 combustion behind shock waves.12 Calculation of “prior” branching ratios predict essentially no temperature dependence of R2.13 The temperature range covered overlaps that in which CS2 + O2 lasers operate, thus this study provides measured values of R2 for modeling these systems. Values of k, the overall rate constant for the 0 + CS2reaction, were also measured in the same temperature range and are also reported here.
+
-
Experimental Section Oxygen atoms (produced by the N + NO N2 + 0) reaction and CS2 were mixed in a 1.27 cm i.d. fast-flow reactor. The rate of CS, loss and OCS production were simultaneously monitored by mass spectrometric analyses of gas sampled through a 0.033-cm diameter hole in the end of the reactor. The temperature of the reactor was established by circulating a heating or cooling fluid (silicone oil or ethanol) from a thermostated bath through a concentric jacket surrounding the reactor. A chromelalumel thermocouple inside the movable central inlet tube was used to determine the temperature inside the flow reactor. Temperature uniformity along the reactor was within 1 K as was the temperature stability during an experiment. The experimental details and data analysis were the same as described before., The branching ratio for route 2 was obtained from the relation
Rz
=
k z / k = A [OCS],/A [CS,]t
(1)
where A[OCS], is the OCS concentration at time t , and A[CS2], = [CS2]0 - [CS,], is the decrease in [CS,] from its initial value.3 The Journal of Physical Chemistry, VoL 81, No. 3, 1977
208
R. E. Graham and D. Gutman
TABLE I: Results of Experiments to Measure 0 Rate Constants and Branching Ratiosa
~
0
6oti
7
401
0
20
~
1
376
138
1
285
249 27 3 295 335 376 431 500
sd-
c ./
OK
071
20
,6" 40
2.0094
60
80
t
I20
I40
Flgure 1. Plot of OCS production vs. CS2 loss from seven experiments at 335 K. Initial CS2 and 0 concentrations given in the figure. Slope of line through points yields R2 (see text). Error limits on R2 is the standard deviation of R2 values calculated for each point from the average slope.
At each of seven temperatures between 249 and 500 K,
Rz was measured in six-nine experiments and found to be independent of [O],, [CS2l0,[MI flow velocity, and extent of reaction. Graphical evidence for this independence of R2 on reaction variables can be seen in Figure 1, a plot of A [ 0 C S l t vs. A [ C S z ] measurements , obtained during the seven experiments at 355 K. Each point in Figure 1 yields a value of R2,and, according to eq I, the totality of points from all experiments should lie along a straight line passing through the origin. Within the experimental scatter this is indeed the case. The value of R 2 selected at each temperature was the slope of the straight line fitted through all data points plotted as in Figure 1. The line was not constrained to pass through the origin, but always passed extremely close to it. Implicit in the derivation of eq I is the assumption that the OCS produced by route 2 is not subsequently consumed by the 0 atoms in the system. Experiments were performed which verified the assumption. At each temperature OCS was mixed with the 0 atoms in the absence of CS2. Under the experimental conditions used to determine R z , there was no detectable OCS consumption (
