J. Phys. Chem. 1986,90, 17-19
Temperature Dependence of the OH Reaction Channels
+ ClNO Reaction:
17
Evidence for Two Competing
B. J. Finlayson-Pith,* M. J. Ezell, and C. E. Grant Department of Chemistry, California State University, Fullerton, Fullerton, California 92634 (Received: October 2, 1985)
The temperature dependence of the reaction of OH with nitrosyl chloride, ClNO, has been studied from 263 to 373 K in a fast-flow discharge system. The decay of OH in the presence of excess ClNO was followed by resonance fluorescence at 309.5 nm. The total pressure was 1.05 f 0.05 torr in He as the carrier gas. The rate constant increases both above and below room temperature suggesting the existence of two competing reaction paths, a direct abstraction reaction, HO + ClNO HOCl + N O (la) and one involving formation of an intermediate complex, HO + ClNO s [HOClNO]* HQNO + C1 (lb). This supports the interpretation of the room temperature product data of Poulet and co-workers.
-
-
Introduction The kinetics and mechanism of the reaction of nitrosyl chloride (ClNO) with OH is of interest from the point of view of both atmospheric chemistry and chemical kinetics. Thus, nitrosyl chloride has been shown in laboratory studies' to be a product of the reaction of part-per-million (ppm) concentrations of NOz with NaCl and, hence, may be formed in polluted marine atmospheres. It is not clear whether reactions of ClNO with atmospheric species such as OH and NO3 can compete with its photolysis, which is rapid,' because there are few data available on the reactions of ClNO with such reactive species. Only one study2 of the OH-ClNO reaction has been reported in the literature; the room temperature rate constant was reported to be (4.3 f 0.4) X cm3 molecule-' s-', and two product channels, ( l a ) and (1 b), were identified through mass spectral analysis of the products:
HO
+ ClNO
-
+ NO H O N O + C1 HOCl
(la) (lb)
We report here the first determination of the temperature dependence of reaction 1. The results support the existence of two competing reaction paths, one proceeding by abstraction and one via complex formation. To the best of our knowledge, this is the first case of the reaction of O H with an inorganic which shows such behavior over a relatively narrow temperature range. Experimental Section The fast-flow discharge system used in these studies is similar to that described in detail e l ~ e w h e r eexcept ,~ that the flow tube was surrounded by a jacket through which heated or cooled ethylene glycol was circulated to provide temperature control. Hydrogen atoms were formed by a microwave discharge in a dilute (0.5%) hydrogen-argon mixture and further diluted by a factor of lo2with He ( U H P grade, 199.9999%). Hydroxyl radicals were formed by the reaction of hydrogen atoms with excess NOz from an NOZ-Hemixture (-20-25%) which was added through the upstream inlet of a double concentric movable inlet. The NO2 concentrations were kept sufficiently large ( 1 1 X lOI3 molecules that the hydrogen atom concentrations at the point of ClNO addition downstream were 51%of their initial value in order to minimize reaction4s5 of H atoms with ClNO. The decay of OH was followed by resonance fluorescence at 309.5 2.8 nm. Initial OH concentrations were in the range
-
*
(1) B. J. Finlayson-Pitts, Nature (London), 306, 676 (1983).
(2) G. Poulet, J. L. Jourdain, G. Laverdet, and G. Le Bras, Chem. Phys. Lett., 81, 573 (1981). (3) T. E. Kleindienst and B. J. Finlayson-Pitts, Chem. Phys. Lett., 61, 300 (1979). (4) M. A. A. Clyne and D. H. Stedman, Trans. Faraday. SOC.,62, 2164 (1966).
(5) M. R. Dunn, M. M. Sutton, C. G. Freeman, M. J. McEwan, and L. F. Phillips, J . Phys. Chem., 75, 722 (1971).
0022-3654/86/2090-0017$01.50/0
(5-10) X 10" radicals ~ m - as ~ ,measured by titration of the H atoms with NO2 and following the production of OH.3 ClNO concentrations were typically (0.8-20) X l O I 3 molecules cm-3 and were determined from the measured pressure drop in a calibrated volume, and the total flow rates and flow tube pressure. Total pressures were 1.05 f 0.05 torr, and the temperature was varied from -10 to 100 O C . The flow tube surface was coated with boric acid to minimize the surface recombination of OH. At each temperature at least three complete sets of runs, each set consisting of the addition of six different ClNO concentrations, were carried out. The initial NO2 and O H concentrations were varied from set to set to check for systematic trends in the rate constant; no such trends were observed, indicating that unrecognized systematic errors are unlikely. NO2 was prepared by the oxidation of NO (Linde, 198.5%) with excess Oz (Linde, UHP grade, 199.99%) followed by trapping the NOz at -77 O C and pumping off the 02.Mixtures in He were prepared by adding H e (199.9999%) to known pressures of NO2 in a 5-L bulb. Two sources of ClNO were used. The first was provided by Matheson Gas Co. and had a stated purity (liquid phase) of 97.0%. The second source was ClNO synthesized by repeatedly condensing mixtures of Clz and NO in a liquid-nitrogen-cooled cold finger. In both cases, the ClNO was purified by trapping it in a dry ice-acetone bath and repeatedly pumping off the vapor until a constant equilibrium vapor pressure (5 torr) characteristic of ClNO was obtained. The ClNO was then pumped over into a liquid-nitrogen-cooled trap. ClNO purified by this method was shown by FTIR and mass spectral analysis to contain c
,4
263 K
z
60-
9
40-
263'K
w 50W
8
0
30-
LT
0
3 20-
1.57
ii W
0
z
3.08 4.82
W
a
"E w 2 ~
G
5
'
_J
W
E
I
1
IO
20
\
7.93 I
30
c v)
REACTION TIME (ms)
Figure 1. Typical plot of OH resonance fluorescence intensity as a function of reaction time in the presence of increasing concentrations of ClNO at 263 K. Typical error bars shown represent two standard de-
viations. > 5c
Im
cr,
l
0
1
,
2
,
3
,
l
4
,
I
,
5
,
6
,
i
7
,
l
6
.
J
9
[CL NO] (molecules
Figure 3. Typical plot of observed first-order rate constants for the decay of OH as a function of the initial ClNO concentration at 263, 298, and 373 K. Errors shown are two standard deviations of the slopes of plots such as Figures 1 and 2.
z
T (OK)
W
417
t-
z
333
286
250
I
W
0
z W
0
m W
a 9 IC i LL
W
0
2
5 5 0
cn W
a
I 0
\
W
L
14.1
G-.
3.0
W
a
1 20
I IO
2.4
1
30
10311
REACTION TIME (ms) Figure 2. Typical plot of OH resonance fluorescence intensity as a function of reaction time in the presence of increasing concentrationsof ClNO at 373 K. Error bars are as in Figure 1.
TABLE I: Values of the Rate Constants Determined for the OH ClNO Reaction at Temperatures from 263 to 373 K 1013k(f24.0 no. of sets temp, K cm3 molecule-' s-I of runsb 263 7.8 f 0.9 5 276 6.7 f 0.9 3 298 3.9 f 0.2 10 329 4.5 0.2 3 358 4.7 f 0.4 4 373 5.6 f 0.1 5
4 .e
3.0
+
*
a Mean of sets of runs, each set consisting of addition of six different ClNO concentrations and determination of the slope of plots such as Figure 3 to generate k l for each set. Error is two standard deviations of the weighted mean, where the weighting was based on the standard deviation of the slope of plots such as Figure 3. bMeaning of a set of runs described in footnote a and in text.
0 in Figure 3 reflect the different rate constants for wall loss of OH in the absence of ClNO at the different temperatures. Table I summarizes the values determined for the rate constant k , at temperatures from 263 to 373 K. While the data have not been corrected for axial and radial diffusion, these corrections will
Figure 4. Arrhenius plot of rate constants ( k , ) from 263 to 373 K. Errors shown are two standard deviations of the mean of three or more runs carried out at each temperature.
be small, 6%; for example, when the data at 298 K are corrected for axial and radial diffusion by using the method of Brown,6 k , changes by only 2.6% to (4.0 f 0 . 2 ) X cm3 molecule-' s-I, An Arrhenius plot of the data in Table I is shown in Figure 4. The rate constant increases with temperatures both above and below room temperature. That this temperature dependence is not an experimental artifact associated with the system was established by determining the temperature dependence of the OH reaction with ethane from 283 to 377 K; the absolute values and the temperature dependence of the rate constant were in excellent agreement with those recommended by Atkinson,' ruling out artifacts associated with the system itself. The reactions of OH with various inorganics reported in the literature show either the usual positive or in some cases, a negative temperature dependence. A positive temperature dependence (Le., increasing rate constant with increasing temperature) is typically observed with simple abstraction reactions. A negative temperature dependence, where the rate constant decreases with increasing temperature, is found in such reactions as that of OH with HN03; this negative temperature dependence has been ( 6 ) R. L. Brown, J . Res. Natl. Bur. Stand., 83, 1 (1978). (7) R. Atkinson, Chem. Reu., in press, and references therein.
Letters
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 19
showns-I0 to be consistent with the formation of an intermediate complex between OH and the reactant, which can decompose back to reactants, as well as go on to form new products:
limited data, we have found that the OH ClNO reaction is very sensitive to even small amounts of wall contamination, more so than the OH + C& reaction for example. Hence, much more data are needed to better define the Arrhenius parameters in this temperature region. Our kinetic data are consistent with the two reaction channels ( l a ) and ( l b ) proposed by Poulet and co-workers;2 thus, ( l a ) HO ClNO HOCl N O (la)
OH
+ HN03
$
(OH.HN03)*
-
products
At temperatures 2298 K, a least-squares straight line analysis of the data in Figure 4 gives
k = (2
X
10-'2)e-(466*272)/T
(11)
(This is not to imply that a straight line is necessarily the best fit; however, until more data are obtained at intermediate temperatures, a nonlinear analysis is not justified over such a narrow temperature range.) The preexponential factor and activation of O H with energy in (11) are similar to those for the C10N02 where the recommended valued3 of A and E , / R are 1.2 X cm3 molecule-' s-' and 333 f 200, respectively. While the products of the OH C 1 0 N 0 2 reaction have not been determined, a chlorine atom abstraction has been suggested" as the most likely mechanism. We thus suggest that, at temperatures above 298 K, the OH + ClNO reaction occurs primarily by chlorine atom abstraction. At temperatures below 298 K, the kinetic data indicate that an intermediate complex which can decompose back to reactants is formed, resulting in a negative temperature dependence. Benson and co-workerss-10have shown that activation energies as negative as -2 kcal mol-I are expected from such intermediate complex formation under certain conditions. Consistent with this, data below room temperature in Figure 4 and Table I suggest a negative activation energy in the range of -2 to -3 kcal mol-'. While the rate constant clearly rises again at temperatures below 298 K, derivation of Arrhenius parameters from these two data points is not justified; in addition to the dangers of over-interpreting such
+
(8) M. Mozurkewich and S. W. Benson, J. Phys. Chem., 88,6429 (1984). (9) M. Mozurkewich, J. J. Lamb, and S. W. Benson, J. Phys. Chem., 88,
6435 (1984), and references therein. (10) J. J. Lamb, M. Mozurkewich, and S. W. Benson, J. Phys. Chem., 88, 6441 (1984), and references therein. (1 1) M. S. Zahniser, J. S. Chang, and F. Kaufman, J . Chem. Phys., 67, 997 11977). (i2)A.'R. Ravishankara, D. D. Davis, G. Smith, G. Tesi, and J. Spencer, Geophys. Res. Lett., 4, 7 (1977). (131 W. P. DeMore, J. J. Marnitan. M. J. Molina, R. T. Watson. D. M. Golden, R. F. Hampson, M. J. Kirylo, C. J. Howard, and A. R. Ravishankara, JPL Publ. No. 85-37, July 1 , 1985.
+
+
-
+
appears to be a simple chlorine atom abstraction, whereas (1b)
is clearly more complex and may occur by the formation of an intermediate complex analogous to the OH-HN03 reaction.8-10 We therefore tentatively identify the predominant reaction path in the OH-C1NO reaction at T > 298 K as the chlorine atom abstraction (la), and that at T < 298 K as intermediate complex formation leading to products in (1b). At 298 K, the kinetic data (Figure 4) suggest these two paths are of equal importance. This is also consistent with the work of Poulet et ale2who showed, based on the product yields, that kl, E k l bat room temperature. To the best of our knowledge, this is the first case of a reaction of OH with an inorganic which shows two different temperature dependencies over a relatively narrow temperature range due to competing reaction pathways with different mechanisms. However, an analogous situation occurs in the O H reactions with aromatics' where addition occurs at lower temperatures ( T < 300 K), and abstraction at higher temperatures ( T > 380 K). More detailed kinetic studies as well as product analysis using a modulated beam mass spectrometer interfaced to a fast flow discharge system are currently underway. The products HOC1, HONO, and C12 (from the reaction of C1 with the excess ClNO) have been identified and their yields are being determined as a function of temperature. If our interpretation of the kinetic data is correct, the yield of HOCl should increase with increasing temperature above 298 K, whereas the yield of C12 should increase below 298 K.
Acknowledgment. The authors are grateful to the National Science Foundation for support of this work through Grant No. ATM-8315323 and to J. N. Pitts, Jr., for his generous assistance with the preparation of this manuscript.
