J. Phys. Chem. 1982, 86, 2007-2011
Absolute Rate Constants of CFCI( ?A,)
2007
Reaction with Nitrogen Oxides
Stephen E. Blaikowsklt and William A. Guillory' Chemistry Department, University of Utah, Salt Lake Clty, Utah 84 1 12 (Received: September 29, 1980; I n Final Form: December 7, 198 1)
The chlorofluoromethylene (CFC1) molecule has been generated from the infrared multiple-photon dissociation (IRMPD) of 1,2-dichloro-l,2-difluoroethylene (CFCl=CFCl) with an apparent nascent vibrational temperature above 300 K. The overall vibrational rate constant of CFCl for relaxaJion into v" = 0 has been determined to be k , = (6.8 & 0.7) X cm3molecule-' s-'. Decay rates of CFC1(X'A1,OOO) with the stratospheric gases present in significant concentrations, N2, N20, NO, NOz, and 02,are also reported. Of these substrate gases, only NO and NO2 were observed to have significant rates of reaction. The others were found to be relatively unreactive. The decay rate constants for reaction with NO and NO2 were found to be k = (8.7 f 0.7) X and (1.6 f 0.2)X cm3molecule-' s-', respectively,obtained at room temperature and 100-torr total pressure.
Introduction A great deal of effort has recently been devoted to the modeling of upper atmospheric chemistry.' This effort has resulted from the fact that chemically inert halocarbons have been released into the atmosphere for years. As a result of their relatively low molecular weight, these substances diffuse into the stratosphere where they may be photolyzed by high-energy UV radiation. The resulting radical fragments, such as CF,, CFCl, CHF, etc., initiate reactions which can interfere with the natural ozone cycle and subsequently seriously disrupt the terrestrial ecosystem. Kinetic mechanisms and elementary rate data are of fundamental importance in atmospheric modeling. To date, kinetic data have not been obtained for the elementary gas-phase reactions of many of these transient halocarbon species. However, some halomethylene reactions (CF, and CHF) have been studied.24 In particular, the reaction rates for CFCl with several species have recently been r e p ~ r t e d . ~In, ~one s t ~ d yCFCl , ~ was produced in a discharge-flow system and CFCl concentration was detected by using the laser-excited fluorescence technique. The total pressure in these experiments was maintained below 5 torr in order for the discharge to occur. No reaction was observed between CFCl and either NO, NO2, or O2 a t 298 K, but rate constants for reactions with NO and NO2 were obtained a t 333 K. In this study we observe the formation of chlorofluoromethylene (CFCl) from the infrared (IR) multiple-photon dissociation (MPD) of 1,2-dichloro-1,2-difluoroethylene (CFCl=CFCl). Recent gas-phase spectral characterization of CFCl allows the use of the sensitive laser-excited fluorescence (LEF) technique to study the dynamics of this ~ y s t e m . The ~ overall vibrational relaxation rate in Ar and elementary gas-phase reaction rates of CFCl with the nitrogen-oxygen series N2/N20/N02/02 are examined. These substrates were chosen because of their presence in the stratosphere. The CFCl radical has also been produced from the IR photolysis of chlorotrifluoroethylene (CF2CFC1).6 In that study, CFCl was generated by C=C bond fission, with a vibrational temperature significantly above 300 K. It is likely that CFCl production from CFCl=CFCl also occurs by a carbon-bond scission. A high vibrational temperature of nascent CFCl could complicate analysis of the reaction data. At higher temperatures, the rate data would be a composite of vibrational relaxation and reaction. Therefore, IRMPD of CFCl=CFCl was performed Department of Chemistry and Chemical Engineering, Michigan Technological University, Houghton, M I 49931. 0022-3654f82f 2086-2007$0 1.25f 0
with a relatively high pressure of an inert rare gas buffer which served to reduce the vibrational temperature on a time scale short compared to chemical reaction and inhibited diffusion of the species out of the region probed by the monitoring laser.
Experimental Section The experimental setup and apparatus used in this study have been previously described,' and only those aspects unique to this study will be discussed. The 2kand 2; (_367and 378 nm) vibronic transitions of the CFCl(A'A" X'A') transition were used to monitor the ground vibronic state population. The probe dye laser (PDB) delivered 200 pJ in 5 ns. The beam diameter of the dye laser at the reaction cell was 3 mm. A perpendicular photolysis beam-probe beam arrangement was used. The TEA-C02 photolysis laser was operated on the R branch of the 10.6-pm laser transition. A 15-cm focal length lens was used to focus the IR radiation into a stainless-steel photolysis cell. The laser delivered 150 mJ at 20 Hz with a pulse in which greater than 70% of the energy was contained in a 150-ns fwhm spike and the remaining energy in a tail of approximately l-ps duration. Pulse-to-pulse energy instability of this laser was the main source of noise in these experiments and was about 5% of the total energy. The LEF signal was collected through a lens system and a monochromator. The entrance slit of the monochromator was aligned parallel to the photolysis beam, and the lens system served to image the photolysis zone onto the entranse slit of the monochromator. Fluorescence from CFCl(A X) was observed at 450 nm in order to minimize the scattered dye laser light. To verify that only CFCl was being monitored a t this wavelength, the LEF excitation spectra were taken at several delay times and corresponded only to CFC1. The time-resolved LEF signals were obtained by scanning the delay between the firing of the
-
-
(1) H. S. Johnston, Annu. Reu. Phys. Chem., 26, 315 (1975);J. S. Chang and W. H. Duewer, ibid., 30,443 (1979). (2)J. Heicklen, Adu. Photochem., 7,57 (1969);D. S. Hsu, M. E. Umstead, and M.C. Lin, ACS Symp. Ser., No.66,128 (1978). (3)J. J. Tiee, F. B. Wampler, and W. W. Rice, Jr., Chem. Phys. Lett., 73,519 (1980). (4)H. Heunier, J. R. Purdy, and B. A. Thrush, J.Chem. Soc., Faraday Trans. 2,76,1304 (1980). (5)S. E. Bialkowski, D. S. King, and J. C. Stephenson, J. Chem. Phys., 71,4010 (1979). (6)J. C. Stephenson, S. E. Bialkowski, and D. S. King, J. Chem. Phys., 72,1161(1980). (7) M. L. Lesiecki, K. W. Hicks, A. Orenstein, and W. A. Guillory, Chem. Phys. Lett., 71,72 (1980).
0 1982 American Chemical Society
2000
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
photolysis and probe lasers. A transient digitizer-MCA was used to record the time vs. LEF intensity data. These data were finally transferred to a microprocessor for storage and analysis. It was experimentally determined that a flowing gas photolysis system was not required for these measurements. This fact was confirmed by two methods. Both time- and excitation-wavelength-resolved spectra of CFCl were taken at the beginning of photolysis and after 1h of 20-Hz photolysis. No apparent change was observed in these spectral determinations as a function of time, even in the presence of a reactive substrate. This is probably due to the fact that the ratio of the photolysis volume to the cell volume was -lo? The time required to obtain a time-resolved spectrum was typically 7 min, and thus a static gas system was used. The excitationwavelength-resolved spectra further served as a means to verify that only CFCl was giving rise to the LEF signal. That is, no product species were being formed that could be excited within the scanned wavelength to give rise to fluorescence at 450 nm with a LEF intensity comparable to that of CFC1. By comparison of these excitation spectra to those taken a t lower temperature^,^ it was estimated that the LEF time-resolved signals were due to mostly CFC1(XIA1,OOO),and hot-band contribution was under 10%. LEF spectra of CFCl from CF,=CFCl IRMP photolysis were obtained and compared to those from CFCl=CFCl. The vibronic structure was the same in both cases, and thus the photolysis mixtures were both 300 K by comparison.6 Pressures of CFCl=CFCl were always below 20 mtorr, and added argon (Ar) buffer gas pressure was always below 150 torr. If either,of these two pressure limits were exceeded, optical breakdown would result.8 No optical breakdown thresholds were observed for the added reactive substrate gases as long as the total pressure was below 150 torr. A capacitance manometer was used to record pressures between 0 and 10 torr, and a calibrated pneumatic pressure gauge for pressures above 10 torr. The gases used were a 50% mixture of cis- and trans-CFCl=CFCl (PCR, >95% purity), and Nz, CzF3C1,N20, NO, NO2,02,and Ar (Matheson, all >99% purity). The purity of the CFCl= CFCl mixture was further confirmed from a high-resolution spectrum with the use of a Nicolet FT IR spectrometer. These gases were used without further purification.
-
Bialkowski and Guillory
-
[L W
0 250;
B 8.30
11 ,
0 0
I
I
50 0
100
TIME
I
a
150 0
Cpnacl
-
Figure 1. Time-resolved- scan of CFCl in the ground vibronic state (noisy trace). The CFCI(A'Arr R'A'), 2: transition was pumped at 377.9 nm and fluorescence was monitored at 450 nm. CFCl was produced from 983-cm-' COPlaser multiphoton photolysis of C F C F CFCl gas at 5 mtorr with a total pressure of 20.5 torr in Ar. The fast component of the production is due to those CFCl formed initially in the ground vibronic state while the slower-rise component is due to vibrational relaxation into the ground vibronic state of those CFCl formed vibratlonally hot. The relaxation rate of CFCl in Ar was determined to be k , = (6.8 f 0.7) X lo-'' cm3 molecule-' s-' (solid trace). The decay of the signal is due to diffusion.
photolyzing pulse is terminated. Under comparable conditions, rotational relaxation of CF2is complete in about 20 ns (k, = 2.1 X 10-locm3molecule-' s-l)l0and rotational relaxation of CFCl probably occurs on a similar time scale. Thus, the slow-rise component is probably not due to rotational relaxation. The eventual decay of the signal is due to diffusion of CFCl out of the region probed by the dye laser. This suggestion was confirmed by obtaining the decay of CFCl from the IRMPD of 10 mtorr of CFCl=CFCl at various pressures of Ar from 20 to 120 torr. Two general trends were observed with increasing Ar pressure. First, the decay component decreased with increasing pressure. A decrease in decay rate with increasing pressure is indicative of a diffusion process since the diffusion coefficient scales linearly with pressure and the decay rate due to diffusion is inversely proportional to the diffusion coefficient. Second, the slow-rise component increased in rate with increasing pressure. This increase in production rate suggests that a collisional process is probably responsible for this production component. It is likely that this slower production component is due to vibrational relaxation. Since the u = 0 level of CFCl was monitored in these experiments, the fast-rise portion of the LEF signal is probably due to those CFCl molecules initially formed in the ground vibronic state. Thus, it appears that the slower-rise component is due to population of the ground vibronic level by overall relaxation of excited vibrational states. T o determine the initial population of CFCl formed in the ground vibrational state and the overall rate of relaxation, k,, we fitted the data to the diffusion equation"
Results and Discussion Production and Vibrational Relaxation of CFC1. The CFCl radical was produced in the ground electronic state solely from the IRMPD of cis-CFCl=CFCl (the trans isomer does not have a COz pumpable IR absorption). The C02 photolysis laser was tuned to overlap the v4, asymmetric stretch of the cis isomer: having a band center a t 972 cm-l. There was adequate dissociation to study the dynamics of CFCl when this laser was tuned to the R(34) through R(26) lines of the 10.6-pm transition (from 985 to 981 cm-'). It should be pointed out that the choice of these lines was based on the infrared absorption character of the parent, and no attempt was made to determine whether other laser wavelengths could cause dissociation. Figure 1illustrates a typical time-resolved trace of CFC1. In this experiment, 5 mtorr of CFCl=CFCl was photolyzed with 20.5 torr of added Ar. The fast rise of the CFCl signal followed by a slower rise and decay a t longer times is indicative of vibrational relaxation of CFCl long after the
where S(t) is the LEF signal at time t, S ( 0 ) is the total population at t = 0, P is a parameter which depends on the apparent radii of the laser beams, D is the diffusion coefficient, and g is a geometrical factor. Rate processes
(8) Optical breakdown was assumed to be occurring when a bright flash was associated with irradiation. (9)D. E. Mann, L. Fano, J. M. Meal, and T. Shimanouchi, J. Chem. Phys., 27, 51 (1957).
(10)J. C.Stephenson, D. S. King, M. F. Goodman, and J. Stone, J . Chem. Phys., 70, 4496 (1979). (11)S. E. Bialkowski, D. S. King, and J. C. Stephenson, J . Chem. Phys., 72, 1156 (1980).
s ( t ) = S(O)lP/(P + 4Dt)lgJ'(t)
(1)
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982 2008
CFCI(R'A,) Reaction with Nitrogen Oxides
ratio CFCl(v = O)i/CFCl(v = 0)f was 0.37 f 0.07. Interpretation of the calculated overall vibrational relaxation rate constant is not as detailed as desired since the relaxation process is not state-to-state specific but represents the rate at which the ground-state population increases because of contributions from all higher-lying levels. Previous measurement of the overall relaxation rate constant for CF2 in Ar has been compared to the rate constant for vibrational relaxation of the w2 bending motion of COz. In this case, both have similar w2 vibrational frequencies CF2(010)+ Ar CF2(000) + Ar + 666 cm-' (3a)
+
CO2(O10) Ar 0 0a0
0 500
i 000
i
500
2 000
2 sa0
3 000
3 500
TIME Cmrecl
Flgure 2. Time-resolved scans of CFCi in the ground vibronic state. Trace a obtained from 10 mtw of CFCI=CFCI photolyzed in 100 torr of Ar. The decay is due to diffusion since vibrational relaxation occurs on a very fast time scale compared to the experimental time. Trace b is obtained from 10 mtorr of C F C M F C i photolyzed in 5 torr of NO and 95 torr of Ar. Accounting for diffusion, a rate constant of k = (1.6 f 0.2) X lo-'' cm3 molecule-' s-' was calculated for CFCl NO products.
+
-
which may either add to the signal by relaxation processes or subtract from the signal by reaction are accounted for in F(t). These processes are restricted to those which start at t = 0 and which do not lead to significant pressure changes. For vibrational relaxation and diffusion, eq 1 becomes S ( t ) = A{P/(P + 4Dt)P{CFCl(v= 0)i + (CFCl(v = O), - CFCl(v = O)illl - exp(-kJ't)l) (2) In eq 2, A is a scaling factor, the i and f subscripts refer to initial and final populations of the v = 0 level, k, is the overall vibrational relaxation rate constant, and P is the total pressure. The relaxation data were fitted to eq 2 by using the standard x 2 minimization routines. For perpendicular pump-probe beam geometries, the exponential factor, g, in eq 1and 2 should equal l/pll However, when the data were fitted to eq 2, a minimum in x2 was found for g = 1. This was further supported by plotting the inverse of the data taken over longer experimental scan times. It was found that the inverse of the data was linear in time according to eq 1. An exponent of 1 corresponds to a collinear pump-probe geometry, and it is probable that the light collection apparatus emulated a collinear geometry by imaging the photolysis laser axis onto the entrance slits of the monochromator. A calculated fit to the data is illustrated in Figue 1(solid trace). For fits of this nature, the diffusion parameters, D, P, and g, were first obtained from data taken on millisecond time scales, but under the same experimental conditions as those used for obtaining parameters in microsecond time frames. These parameters were obtained by expanding the time scale to 3.5 ms, and a typical plot of this nature is shown in Figure 2. These data were more sensitive to the diffusion parameters than those at microsecond times. On the other hand, k, and the initial and final populations of the CFCl(v = 0) are less sensitive to the data obtained over millisecond time frames. Thus, the millisecond and microsecond data are complementary and both are required for accurate determination of the dynamic behavior of CFC1. From this information, it was found that the overall vibrational relaxation rate constant was k, = (6.8 f 0.7) X cm3molecule-' s-l and that the
--
+
CO2(0O0) Ar
+ 667 cm-'
(3b)
The overall relaxation rate constant for CF2 is k, = (2.0 f 0.3) X cm3molecule-' s-l (ref ll),and for the bend em3 molecule-' s-',12 only a in C02 it is k, = 1.0 X factor of 2 smaller. Thus, the CF2 rate constant appears to characterize eq 3a because of its similarity to eq 3b and the bending vibration may in fact be the rate-limiting step for V R/T. The CFCl relaxation CFCl(010) Ar CFCl(000) + Ar 448 cm-' (4) involves a bending vibration which is lower in frequency than those of CF2 and C02. The difference in relaxation rates between these two species and CFCl is probably due in part to this frequency difference and in part to an increase in the collision cross section. Bondybey has observed multiquantum vibrational relaxation of CFCl in an Ar matrix,13 and Akins et al. have observed similar behavior for CF2 in the gas ~ h a s e . ' ~ JThis ~ multiquantum relaxation may be responsible for the rapid decrease of the population in the stretching vibrations and thus lead to an overall vibrational relaxation rate which is relatively independent of the bending vibrational relaxation in eq
-
+
-
+
4.
Reactions of CFCI(g"Al).The decay rates of CFCl from the ground vibronic state with a series of nitrogen-oxygen substrates were determined. In these experiments, the total gas pressure was considerably higher than that for the vibrational relaxation experiments. This higher pressure (-100 torr) was utilized to cause complete vibrational relaxation on a time scale much shorter than chemical reaction and to minimize loss of fluorescence signal due to diffusion. Of the substrate gases used, N2, N20,NO, NO2,and 02,decay rates that were competitive with diffusion were observed only for NO and NO2. No reaction was observed for CFCl with N2,N20,or 02,even at substrate gas pressures exceeding 60 torr. Reaction of CFCl with CFC14FCl was also not observed; that is, the decay rate of the time-resolved spectra did not change significantly with initial pressures of CFCl=CFCl varied between 2 and 20 mtorr. Figure 2 shows the results of the relative rates of relaxation and NO reaction with CFCl. In the upper trace (a), which has the slower decay, 10 mtorr of CFCl=CFCl was photolyzed in 100 torr of Ar. The lower trace (b) with the fast decay is that of 10 mtorr of CFCl=CFCl and 5 torr of NO and a total pressure of 100 torr with added Ar. It is apparent that CFCl reacts with NO since all other parameters are held constant in both cases. Data of this type were taken at several pressures of reactive substrate, (12)H.0.Kneser in 'Physical Acoustics", W. P. Mason, Ed., New York, 1965. (13)V. E. Bondybey, J. Chem. Phys., 66,4237 (1977). (14) D.L.Akins, D. S. King, and J. C. Stephenson, Chem. Phys. Lett., 65,257 (1979). (15)J. Stone, E.Thiele, M. F. Goodman, J. C. Stephenson, and D. S. King, J. Chem. Phys., 73,2259 (1980).
2010
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
TABLE I : Rate Constants for Bimolecular Reactions of CFCl' CFCI(%IA,) ref 4 b
0, NO2
