2288
RICHARD B. TIMMONS AND B. DEB. DARWENT
with 20 mm of ONF and 1.78 for C02 and SFe, with 20 mm of ONF and 5 mm of C2H4. Assuming the light intensities were equal in those experiments, the fraction of collisions that lead to induced decomposition [k4/(ks kh)] is about 25% higher for C02 and SFe than for C2H4. This is to be expected in view of the much greater efficiency of C2H4 than of GOzor SF6 in removing the electronic energy of excited atoms and suggests that reaction 4 is actually a collision-induced predissoci-
+
ation, not involving any transfer of electronic energy to
M, and reaction 3 is strictly analogous to the quenching of the resonance radiation of metal atoms and involves the transfer of the electronic energy of ONF* into electronic or vibronic excitation of M. Quite strong support for the suggested mechanism is derived from the fact (Figure 2) that the addition of C02 increases R, from the value characteristic of C2H4to that of GOz.
The Decomposition and Deactivation of Chemically Activated ClNOl by Richard B. Timmons and B. deB. Darwent The Martin Maloney Chemistry Laboratory, The Catholic University of America, Washington, D . C . 90017 (Received September 17, 1968)
The photolysis of ClNO in the presence of added NO and inert gases (CR, N2, and He) has been studied at 25' From the observed quantum yield of Cl2 formation in the presence of these gases, it is possible to obtain a value for the rate constant ratio of the reactions
+ C1 ClNO* + M +-ClNO + M CINO* 4NO
where ClNO* represents the chemically activated species formed in the addition of a chlorine atom to NO. The rate constant ratios obtained were 20.1, 7.3, and 5.6 X 10'9 molecules/cc, where M is He, N2, and CF4, respectively. By assuming that collisional deactivation of the CINO* is possible on every collision, a minimum lifeseo. time for the ClNO is calculated to be of the order of 5 X
Introduction
+ +
+
Reactions of the type, A BC (M) --t ABC M, where A is an atom, BC is a diatomic molecule, and M is any third body, have been investigated with a variety of experimental techniques. I n general, the experimental results have most frequently been treated on the assumption that the above reaction is termolecular. Recently a compilation of termolecular rate constants for reactions of H, 0, and C1 atoms has been published together with new data on the reactions of C1 atoms with NO and The above type of reaction may also be treated as a combination of bi- and unimolecular reactions A+BCZABC* ABC*
+ M+ABC + M
where ABC* represents a chemically activated molecule (presumably in high vibrational excitation), The unimolecular decomposition of this excited molecule can then compete with the collisional deactivation process by some third body M. It is to be noted that relatively The Journal of Physical Chemiatry
little energy needs to be removed by the collisionaJ process to effectively stabilize the excited molecule with respect to its unimolecular decomposition. The reactions of H with 02*" and NOsb have been investigated and treated by the combined mechanism rather than as termolecular reactions. I n this paper we report the extension of this work to reactions of C1 atoms with NO in the presence of the inert gases, CF4, Nz, and He. The chlorine atoms, formed in the photolysis of CINO, are allowed to react competitively with ClNO to form C12or add to NO to form CINO*. From the pressure dependence of the C12quantum yield in the presence of added inert gases it is possible to obtain a value for the decomposition rate constant for the ClNO* molecule formed in the addition reaction and (1) This work was supported by the National Science Foundation, under research Grant GP-10353. (2) T. C. Clark, M. A. A. Clyne, and D. H. Stedman, Trans. Faraday
Soc., 62, 3354 (1966). (3) (a) R. L. Wadlinger and B. deB. Darwent, J . Phys. Chem., 71, 2057 (1967); (b) R. P. Borkowski, Ph.D. Dissertation, The Catholic University of America.
2209
DECOMPOSITION AND DEACTIVATION OF CHEMICALLY ACTIVATED ClNO information about the relative efficiencies of CF4, Nz, and He in the deactivation.
Experimental Section Reagents. Nitric oxide (CP grade, Matheson Co.) was further purified by distillation, under vacuum, from - 185 to - 196". The last traces of NO2 were removed by condensing the gas on silica gel a t - 196", allowing the Si02 to warm slowly and collecting only the first fraction from the column in a 5-1. storage bulb. The chlorine (research grade, Matheson Co.) was treated in the usual manner of trap distillations with only the middle fraction retained for reaction. The ClNO was prepared by the direct reaction of NO and Clz. A large excess of NO was condensed on solid chlorine at -196". The liquid nitrogen was then removed and the reaction between NO and Clz was allowed to proceed. The yellow liquid Clz was rapidly converted to the reddish brown color of liquid ClNO. The process of freezing and warming the gases was repeated several times after which the excess NO was removed by distillation at - 185". The inert gases, helium and nitrogen (research grade, Matheson Co.), and the carbon tetrafluoride (Columbia Organic Chemical Co.), were used without further purification with the exception that the Nz and CF4 were passed through a furnace at 450" which was packed with pure copper gauze to remove any oxygen which might have been present. Apparatus. Reactions were carried out in a conventional high-vacuum all-glass apparatus. tince ClNO is decomposed up to wavelengths of 6400 A, all glassware with which this gas came in contact was painted black to avoid photolysis by the lights of the laboratory. The Pyrex reaction vessel was spherical in shape and had a total volume of 2195 cm3. The reactant gases were mixed by a magnetically driven glass rod equipped with several vanes mounted in the center of the cell. The reaction cell also contained a small cold finger (volume of 15 cm3) which was used to condense the ClNO and NO. The initial pressures of the reactants were measured either with a silicone oil manometer (ClNO and NO), which could be used as a null instrument, or with a mercury manometer for the inert gases. A cold trap between the mercury manometer and the reaction vessel effectively prevented mercury from diffusing into the reaction system. The requisite amounts of ClNO were measured in a small. (22.15 cm3) volume and frozen into the cold finger attached directly to the reaction vessel. For higher pressures of ClNO the process was repeated as often as necessary; although those pressures were known accurately, it was difficult to always measure out exactly the same quantity from one run to another. ClNO is slightly soluble in the silicone oil; however, the dissolution is slow enough to allow quite accurate measurements to be made. I n preparing the
reaction mixtures, the ClNO was measured and frozen in the small cold finger, and the NO was treated similarly. Then the inert gas was added and its pressure was measured. The reaction vessel was then isolated, the cold finger was warmed, and the gas was mixed for 20-30 min before the photolysis was started. After the photolysis, the Cla and ClNO were slowly condensed in loop traps a t - 185" and the NO and inert gas were pumped off. The condensed residues were then analyzed for Clz as described below. The analysis of small amounts of Clz in the presence of large amounts of ClNO was the most difficult analytical problem. The analysis was accomplished satisfactorily by removing the ClNO, by allowing the mixture to react with a large excess of concentrated H2S04, thereby establishing the equilibrium NOCl
+ HzS04 J_ "OS04
+ HC1
then treating the residual Cl2 with K I and determining the 1 3 - complex spectrophotometrically. The experimental details of the analysis are as follows. Approximately 10 cm3 of concentrated K2S04 was placed in a 50-cm3bulb, attached to the apparatus, and evacuated. The was cooled to -195" and the mixture of ClNO and Clz was distilled onto it from the loop traps. The mixture was warmed to room temperature, after which it was cooled to 0" and the unreacted Clz was distilled into 5 ml of a 2 M solution of K I at - 195". The bulb containing the K I and Clz was then removed and allowed to warm up to room temperature and the concentration of 1 3 - was determined (Beckman DU spectrophotometer) from the measured optical density at 295 mp. Since the extinction coefficient of 13-depends4 on the concentration of I-, the I3--Isolutions were diluted to maintain the concentration of I- constant. The validity and accuracy of the analysis were demonstrated by measuring the NO and Clz produced in the photolysis of ClNO. From a calibration curve obtained by the reaction of known amounts of Clz with the K I solution, it was found that the Clz produced was equal to one-half the number of moles of NO formed in the photolysis, within an experimental error of i s % . It was also determined that for a concentration range of Clz from 0.96 X to 2.91 X mol, the optical density of the resulting K I solution obeyed Beer's Law.
Results and Discussion Although we are confident of the analysis for C12, that concentration may not represent the rate of formation of C12 if the back-reaction 2 N 0 Clz + 2C1NO were significant. Experiments showed that the rates of formation of NO and Cl2 from a fixed pressure (1.0 Torr) of ClNO were independent of time, up to 2% conversion which was the maximum attained in these
+
(4) J. J. Luster and S. Natelson, Anal. Chem., 21, 1005 (1949). Volume 73, Number 7 July 1960
2210
RICHARD B. TIMMONS AND B. DEB. DARWENT
experiments, and that the amount of Clzwas not reduced by the addition of 16 Torr of NO after the photolysis and allowing the gases to stand for 30 min before analysis. This is consistent with the value5 of 1.6 X lo7 om6 sec-’ at 22’ for the termolecular rate constant. In addition, the rate of formation of NO was directly proportional to the pressures of ClNO up to 2.5 Torr. Other experiments showed that the addition of He, up to a pressure of 700 Torr, did not affect the rate of photolysis of ClNO. Thus the kinetics are not complicated by the formation of long-lived electronically excited molecules as was found for ONFn6 The variation of the Clz yield with added NO and helium as the inert gas is shown in Table I over a range of reactant pressures. For a fixed ratio of ClNO to NO the effect of an increase in helium pressure is to reduce the yield of Clz. I n addition, the yield of Clz is observed to be strongly dependent on the ratio of ClNO to NO and independent of the individual pressures of ClNO to KO when the ratio of those pressures is constant.
Table I: Effect of Added KO and Helium on Clz Production in Photolysis of CINO at 25” PClZQ!
PC123
PNOn
mm X
mm X
mm
mm
102 a
102
1.03 1.03 1.03 1.05 1.05 1.04 1.07 1.02 1.03 0.29 0.29 0.29 0.29 0.29 0.29 0.29 0.29 0.29
8.24 8.24 8.24 8.40 8.40 8.32 8.56 8.16 8.24 2.32 2.32 2.32 4.64 4.64 4.64 4.64 4.64 4.64
PNOCli
P H ~mm ,
100 100 150 200 200 400 600 600 755 100 175 500 80 100 150 400 650 700
1.55 1.55 1.55 1.58 1.58 1.57 1.62 1.53 1.55 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37
0.94 0.93 0.79 0.71 0.74 1.51 0.31 0.34 0.30 0.23 0.19 0.10 0.19 0.16 0.12 0.06 0.05 0.04
Y
1.54 1.50 1.03 0.82 0.88 0.48 0.24 0.29 0.24 1.65 0.95 0.37 0.95
0.77 0.48 0.19 0.16 0.15
aP C I represents ~ the pressure of Clz which would have been formed from the photolysis of ClNO in the absence of added gases.
The quantum yield of NO formation in pure ClNO Qhotolysis is 2 over the wavelength range of 3000-6800 A.7 More recently, the photolysis has been extended to 2537 a t which wavelength the ATO quantum yield is also equal to 2.8 From the work in ref 7 and 8, it is established that the quantum yield of NO in this photolysis is 2 over a ClNO pressure range of 6-680 mm pressure. We have assumed that the quantum yield is also The Journal of Physical Chemistry
2 at the lower ClNO pressures used in this work. The extinction coefficients of ClNO and Clz are almost equal in the wavelength region of 3100-3800 A, whereas at longer wavelengths the ClNO extinction coefficient is much greater than that of C1,. This fact plus the low per cent conversion of the ClNO allows us to avoid any difficulties which might be introduced via the subsequent photolysis of Clz. The following reaction mechanism is proposed.
+ hv +NO + C1 C1 + ClNO +NO + C1z C1 + NO ClNO* ClNO* +NO + C1 ClNO* + Ril +ClNO + M ClNO
.--)
(0) (1)
(2)
(3)
(4)
Application of the usual steady-state treatment to the mechanism yields y = RC12’/(RC12
- Rcl,’)
= (kl/k&(l
+ kdkm)
(I)
where Rc12is the rate of production of Clzin the photolysis of pure ClNO and Rc12’ is the rate of Clz production in the presence of added NO; r is the ratio of ClNO to NO and m is the concentration of all substances capable of deactivating the excited ClNO. It is assumed that the excited ClNO species refers to a molecule in high vibrational excitation. No visible fluorescence was observed. Equation I predicts a linear relationship between y and the reciprocal total pressure m for a constant value of r. That this corresponds to the experimental facts is shown in Figure 1. The dependence of the ratio of ClNO to NO is also shown in this figure. Several runs were carried out with a reduced ClNO pressure but a constant ratio of ClNO to NO. These runs, indicated by triangular points, give essentially the same y value as those runs in which the ClNO pressure was greater by a factor of 3. Thus no marked effect of hot C1 atoms is observed in these experiments. I n addition, the incident intensity was reduced by a factor of 6.3 in runs at a ClNO pressure of 1.03 mm. These experiments gave the same y values as those at full intensity with the same pressure. A number of experiments were carried out with inert gases CF, and Nz in place of helium. The results of these experiments are shown in Table I1 and in Figure 2. From these experiments it is concluded that there is a dependence of the observed kinetics on the nature of the inert gas. For example, the slope of the y vs. m-l plot is different with NZ than with He for a fixed ratio of ( 5 ) G. Waddington and R. C. Tolman, J . Amer. Chem. SOC.,57, 689 (1935). (6) A. Flores, Ph.D. Thesis, Catholic University of America. (7) G. B. Kistiakowsky, J . Amer. Chem. SOC.,52, 102 (1930). (8) R.P.Wayne, Nature, 203,516 (1964).
221 1
DECOMPOSITION AND DEACTIVATION OF CHEMICALLY ACTIVATED ClNO ~
~~
Table I1 : Effect of Added NZand CFa on the Productioh of Clz in the Photolysis of ClNO NO at 25"
+
PNOClo
PNO.
Pinert gsn,
PClr'o
m 2 ,
mm
mm
mm
mma
mm X 102
y
0.29 0.29 0.29 0.29
2.32 2.32 2.32 2.32
100 150 300 300
0.37 0.37 0.37 0.37
0.14 0.12 0.06 0.07
0.61 0.47 0.20 0.23
0.29 0.29 0.29 0.29 0.29 0.29 0.29
0.58 0.58 0.58 0.58 0.58 0.58 0.58
120 120 120 200 600 600 600
CF4 0.37 0.37 0.37 0.37 0.37 0.37 0.37
0.24 0.23 0.24 0.20 0.11 0.11 0.12
1.84 1.65 1.84 1.20 0.42 0.42 0.48
Nz
PcQ represents the pressure of Clz which would have been formed in the photolysis of ClNO in absence of added gases. 1031
rn(mm')
Figure 1. The effect of increase in helium gas pressure on the Clz quantum yield in the photolysis of ClNO a t fixed ratios of ClNO to NO: 0 , r = l/s; H, r = 1/18. (Note: A represent r = but a t pressures of ClNO and NO reduced by a factor of 3.)
h / k z obtained from a least-squares treatment of all the data for various values of r and different inert gases was 0.10. If equal preexponential factors are 0.26 assumed for reactions 1 and 2 and zero activation energy for reaction 2, one calculates an activation energy for reaction 1 of 0.8 kcal/mol. The value agrees with the published value of 1.06 kcal/mol for this r e a ~ t i o n . ~ Using the value of 0.26 0.10 for the kl/k2 ratio, the ratio of k3/h4 was calculated for each of the three inert gases. The ratios of ka/k4 are tabulated (Table 111). Of the three inert gases employed in this work CF4 was the most effective deactivator of ClNO*. If we assume that every collision of CF4 with ClNO* is effective in deactivating the excited molecule, we can calculate a maximum rate for k k by assuming that it equals 2 4 , the collision frequency. I n calculating Z4 we have used collision diameters of 4.70 8 for CF4 and 4.00 for ClNO*. From the ratio of k3/kr determined experimentally and using the calculated value for 24, we obtain a maximum value for k3 of 1.8 X 1O1O sec-l.
*
*
Table I11 : Comparison of the Relative Efficiencies of He, Nz, and CFI in Deactivating ClNO* a
103/m ( m m 3
Figure 2. The effect of added CF4 or NZon the quantum yield of Clz in the photolysis of ClNO: 0, using CF4 and r = m, using NZand r =
ClNO to NO. A considerably smaller difference is observed in comparing Nz and CF4. From eq I it is apparent the intercept of the plot of y ws. wz-l is equal to (k1/k2)r and the ratio of slope to intercept yields the ratio of ks/k4. The average value of
Added inert gas
ka/kr, molecules/cms
CF4 N2 He
5 . 6 X 10'9 7 . 3 x 10'9 20.1 x 10'9
24
(calcd), cma molecule-1 sec-1
3.14 X 3.40 x 10-lo 5 . 7 x 10-10
Relative efficiency in deactivating CINO*
1.00 0.72 0.16
a Calculations based on ks equal to 1 . 8 X 10'0 sec-' obtained in the experiments with CF4.
(9) W. G. Burns and F. S. Dainton, Trans. Faraday SOC.,48, 39 (1952).
Volume 73, Number 7 July 1969
2212
The reciprocal of k3 represents the minimum lifetime of ClNO* which in this case is 5.5 X lo-" sec. From the observed ratios of k3/k4 for each of the inert gases it is possible to obtain the relative efficiency of these inert gases in deactivating ClNO". We base the calculation relative to the value of unity for C R . The assumption of unity efficiency for CF4may be incorrect; however, the calculated relative efficiencies would still be correct. These values are tabulated in Table 111. Quite clearly the deactivating efficiencies of the inert gases used in this work are in the order He
