J. Phys. Chem. 1989,93, 235-244 brational energy to a suitable electronic state is a question of enormous practical significance. Unfortunately, V-E transfer rates usually are slow. The rate constant for the nearly resonant energy transfer from HF(v) to NF(a), thus, assumes special significance because most other transfer systems probably are even less facile. Comparison can be made to another nearly resonant V-E energy transfer process, reaction 9. The rate constant for the reverse HF(u=2)
-
+ I(2P3/2)
HF(u=O)
+ I(2P1/2); AHo' = -154 cm-' (9)
process has been rather well studied and a value of 8.9 X cm3 molecule-' s-' seems established.22 The room-temperature equilibrium constant for (9) is 1.05, and from detailed balance cm3 molecule-' the V-E rate constant is approximately 1 X s-', which is surprisingly close to our result for reaction 1. This suggests that the critical factor for these two reaction rates is the coupling between HF(v=O) and HF(v=2), since the N F and I have very different interaction potentials with HF. In general, (22) Pritt, A. T.; Patel, D. J . Chem. Phys. 1984,81, 1337.
235
the rates for processes requiring large changes in the number of vibrational quanta will be slow, unless there are very specific types of intermolecular interactions. An enhancement in IF(B) formation was observed when HF(v) was added to a F2/12flow reactoraZ3 However, it was not possible to ascertain whether the IF(B) excitation was stepwise or from a single HF(v) collision. The latter requires, at least, HF(v=5). On the basis of our present knowledge, the most successful approach to utilization of HF(v) vibrational energy is probably via sequential mechanisms using a small number of quanta in each individual step. The E-V rate constants for I atoms and NF(a) molecules are about an order of magnitude smaller than the deactivation of HF(v=2) by HF, which is a rather undesirable situation for efficient utilization of HF(v) vibrational energy. Acknowledgment. This work was supported by the U.S. Air Force Weapons Laboratory. Registry No. NF, 13967-06-1; HF, 7664-39-3. (23) Tregay, G. W.; Raymonda, J. W.; Thompson, H.M.; Furner, T. E. Chem. Phys. Lett. 1986, 123,458.
NF(b) Quenching Rate Constants at 300 K: Electronic-to-Vibrational Energy Transfer Hyungki Chat and D. W. Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: March 4, 1988; In Final Form: June 20, 1988)
The total quenching rate constants for NF(b) have been measured with a variety of diatomic and small polyatomic molecules at 300 K using the Ar(3Po,2) NF2 flowing afterglow source of NF(b). The quenching rate constants are in the 10-'3-10-'5 cm3 molecule-' s-' range. The correlation of the rate constants with the highest vibrational frequency of the reagent suggests that the dominant quenching mechanism is electronic-to-vibrational(E-V) transfer with NF(a) being the product state. Large kinetic isotope effects were found for several deuteriated reagents. At 2.5 Torr Ar pressure and [NF,] = 1 X 1013molecules cm-3 and with no added reagent, the NF(b;ub-d3) vibrational distribution varied from 100:24:14:09 to 100:12:03:01 over the length of the reactor. However, vibrational excitation was found to have no significant effect on the quenching rate constants. The NF(b,u') vibrational relaxation rate constant with CF4 is approximately (3 2 ) X cm3 molecule-' s-'. Since CF4 has a small quenching rate constant (kQ = 0.6 X cm3 molecule-' s-I), the addition of CF, to the flow reactor can be used to prepare a Boltzmann distribution of NF(b) vibrational levels.
+
*
Introduction The low-energy metastable a'A and b'Z+ electronic states of the group VIA diatomic molecules and molecules from intercombination of groups VA and VIIA elements are of current interest because of their possible use as chemical energy sources. The success of the singlet oxygen-iodine atom laser' has accelerated spectroscopic and kinetic studies of this class of molecules.2 The quenching reactions of 02(a1A) and 02(b1Z+) have been systematically studied, the rate constants are small, and electronic-to-vibrational (E-V) energy transfer is the accepted mechanism, except for special Davidson and Ogryz10,~ as well as Kear and Abrahamson: suggested that the magnitude of the E-V quenching rate constants depended upon the highest vibrational frequency of the reagent molecule, and such correlations exist for 02(b)5" and NH(b).9 The analogy between the quenching of SO(b) and 02(b) has been argued by Wildt et al." A few rate constants for NCl(b) have been reported by Pritt et al.1° This laboratory has developed a NF(b) source" based upon the metastable Ar flowing afterglow technique, and the quenching rate constants for halogens and interhalogens have been measured.'* The present goal was to acquire a comprehensive set 'Current address: Theoretical Physics and Chemistry Department, Korean Advanced Energy Research Institute, P.O.Box 7, Daeduk-Danji, ChoongNam, Republic of Korea.
0022-3654/89/2093-0235$01.50/0
of quenching rate constants for NF(b) with diatomic and small polyatomic molecules at 300 K. In contrast with the molecular (1) (a) McDermott, W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek, R. R. Appl. Phys. Lett. 1978, 32, 469. (b) Benard, D. J.; McDermott, W. E.; Pchelkin, N. R.; Bousek, R. R. Appl. Phys. Lett. 1979, 34,40. (c) Bachar, J.; Rosenwaks, S. Appl. Phys. Lett. 1982, 41, 16. (d) American Physics Society Study Group. Reu. Mod. Phys. 1987, 39, 541. (2) (a) Wildt, J.; Fink, E. H.; Winter, R.; Zabel, F. Chem. Phys. 1983, 80, 167. (b) Winter, R.; Fink, E. H.; Wildt, J.; Zabel, F. Chem. Phys. Lett. 1983,94, 335. (c) Winter, R.; Kruse, H.;Fink, E. H.; Wildt, J. Chem. Phys. Lett. 1983, 102, 404. (d) Bielefeld, M.; Elfers, G.; Fink, E. H.; Kurse, H.; Wildt, J.; Winter, R.; Zabel, F. J . Photochem. 1984, 25, 419. (e) Kruse, H.; Winter, R.; Fink, E. H.; Wildt, J.; Zabel, F. Chern. Phys. Lett. 1984,111, 100. (f) Winter, R.; Kruse, H.; Fink, E. H.; Wildt, J.; Zabel, F. Chem. Phys. Lett. 1984, 104, 383. (9) Bielefeld, M.; Wildt, J.; Fink, E. H. Chem. Phys. Lett. 1986, 126,421. (3) Wayne, R. P. Singlet 02;Frimer, A. A., Ed.;CRC Press: Boca Raton, FI, 1985; Chapter 4. (4) (a) Singh, J. P.; Bachar, J.; Setser, D. W.; Rosenwaks, S. J . Phys. Chem. 1985, 89, 5347. (b) Singh, J. P.; Setser, D. W. J. Phy. Chem. 1985, 89, 5353. (5) (a) Davidson, J. A,; Ogryzlo, E. A. Chemiluminescence and Bioluminescence;Plenum: New York, 1973; p 1 1 1 . (b) Davidson, J. A.; Ogryzlo, E. A. Can. J. Chem. 1974,52, 240. (6) Kear, K.; Abrahamson, E. W. J. Photochem. 1975, 3, 409. (7) Thomas, R. G. 0.;Thrush, B. A. Proc. R. SOC.London Ser. A 1977, A356, 287, 295, 307. (8) (a) Borrqll, P.; Borrell, P. M.; Richards, D. S.; Boodaghians, R. J. Photochem. 1984, 25, 399. (b) Boodaghians, R.; Borrell, P. M.; Borrell, P. J . Chem. Soc., Faraday Trans. 2 1984,80, 817.
0 1989 American Chemical Society
236 The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 halogens, these NF(b) quenching rate constants are small and in the 10-13-10-15cm3 molecule-' s-I range. The magnitude of the rate constants correlates with the highest vibrational frequency of the reagent molecule. There is no evidence for quenching by chemical reaction, and the quenching mechanism is assigned as E-V transfer with formtion of NF(a). The kinetic isotope effect also can be used as a diagnostic test for E-V transfer and a few examples are presented to verify the E-V aspect of NF(b) quenching. A more detailed discussion of the kinetic isotope effects will be given in a later publication. The quenching rate constants for NF@) can be compared with those for 02(b), SO(b), NH(b), and NCl(b); the (b-a) energy separations for SO, 02,NCl, NF, and N H are 4360, 5251,5720,7470, and 8649 cm-', respectively. The NF(b,v'p;) vibrational distribution at the entrance to the flow reactor was 100:24:14:09 for normal operating conditions. At the end of the flow reactor, which corresponded to -25 ms of reaction time, the distribution was 100:12:03:01. Vibrational relaxation occurs via collisions with N F 2 and with the walls of the reactor, and the NF(b) vibrational distribution depended upon the flow time, the [NF,], and the Ar pressure. The vibrational relaxation rate of NF(b,u') by CF4 is faster than the electronic quenching rate and CF4 can be added to obtain only NF(b,u'=O) in the reactor. Most quenching measurements were done using an interference filter, to monitor all u' levels from the NF(b-X) transition. However, the possible coupling of the effects of vibrational relaxation and the quenching rate was examined for a few reagents by using a monochromator to monitor only NF(b,u'=O) and comparing this rate constant with results from monitoring NF(b,u'l2) with the interference filter. There was no difference in the quenching rate constants for several molecules that were studied by the two methods. The Ar(3Poz) NF, source has the disadvantage that F atoms are generated, as well as NF(b). As far as could be ascertained, no N atoms are produced. The products from F atom reactions with some reagents introduce complications to the NF(b) quenching kinetics. This problem was circumvented by adding a small amount of C2H6, which removed F atoms, but did not cause appreciable quenching of NF(b).
+
Experimental Techniques A . Flow Reactor and Sample Preparation. The NF(b) radicals were prepared by the dissociative excitation-transfer reaction between Ar(3Po,z)atoms and NF2 in a prereactor.11-'2 The Ar ~ ) generated by passing metastable atoms (- 1Oloatoms ~ m - were Ar through a weak discharge maintained between two hollow tantalum electrodes (separated by -3 cm).I3 The NF, radicals were obtained from thermal (-200 "C) dissociation of a flow of N2F4. The quenching of NF(b) was studied in a 41-mm-diameter tubular reactor, the typical [NF,] and Ar pressure were 1 X 1013molecules cm-3 and 2.0 Torr at a flow speed of 16 m s-l. The length of the reactor was -38 cm, which gave a residence time of approximately one NF(b) radiative lifetime. The reaction time, At, was calculated by the plug flow approximation; At = A x / ( u ) . The reactor was coated with halocarbon wax (HALOCARBON Products Corp.) to minimize wall related quenching problems. However, successful experiments also were done in an uncoated Pyrex reactor. The reagent molecules were introduced into the reactor through a ring-shaped Pyrex glass inlet, which was placed 2-3 cm downstream from the entrance of the NF(b) flow to the reactor. The flow rate of the reagents were measured by monitoring the pressure rise in a calibrated volume with a 10 Torr MKS Baratron transducer. The Ar flowmeter (FischerPorter) was calibrated by observing the pressure rise in a 12-L standard flask. The reliability of the flow calibrations was es-
-
~
~~~
(9) (a) Zetzsch, C.; Stuhl, F. Ber. Bunsen-Ges. Phys. Chem. 1976, 80, 1362. (b) Zetzsch, C.; Stuhl, F. J . Chem. Phys. 1977, 66, 3107. (10) Pritt, A. T.; Patel, D.; Coombe, R. D. J . Chem. Phys. 1981,75,5720. (11) (a) Lin, D.; Setser, D. W. J . Phys. Chem. 1985, 89, 1561. (b) Lin, D. M. S. Thesis, Kansas State University, 1984. (12) (a) Cha, H.; Setser, D. W. J . Phys. Chem. 1987,91,3758. (b) Cha, H.Ph.D. Dissertation, Kansas State University, 1987. (13) Kolts, J. H.; Setser, D. W. Reactive Intermediates in the Gas Phase; Setser, D. W., Ed.; Academic; New York, 1979; Chapter 3.
Cha and Setser timated as &lo%. The NF(b-X) emission was detected with either a 0.3-m monochromator (McPherson Model 218 with resolution of 12 A/mm) and a RCA C31034 PM tube or with a moveable PM tube (Hamamatsu R212) and a 530-nm interference filter with IO-nm band-pass. The signals from the PM tubes were measured with an electrometer and displayed on a strip chart recorder. The interference filter viewed the 0-0 (528.9), 1-1 (527.3), and 2-2 (525.8 nm) bands. The reagents were degassed, distilled, and stored in reservoirs as pure gases. The source of the reagents with the quoted purity is listed, because some rate constants are small and the impurity level could prove to be important. The suppliers and specified minimum purities for the chemicals are as follows: CH4 (Matheson; 99.95%), CH3Cl (Matheson; 99.5%), CH3Br (Matheson; 99.5%), CD3Br (MSD; 99.5%), CH3CN (Aldrich; 99.9%), CD3CN (MSD; 99.7%), C H 3 0 H (Fisher; 99.9%), CHzCIF (Dupont; 99.5%), CHzClz (Aldrich; 99.9%), CHC13 (Aldrich; 99.9%), CDC13 (MSD; 99.8%), CHF3 (Matheson; 98%), CHFCl, (K&K; 99%), CC14 (Fisher; 99.9%), CF31 (PCR; 99%), cyclopropane (Matheson; 99.9%)) C2H6 (Matheson; 99.9%), C2H4 (Matheson; 99.98%), C2H2 (Matheson; 99.6%), CH3COCH3 (Fisher; 99.5%), CD3COCD3(MSD; 99.9%), 0, (Kansas Oxygen; 99.9%), N O (Matheson; 98.5%), CO (Matheson; 99.99%) Hz (Matheson; 99%), D2 (Matheson; 99.5%), C 0 2 (Midwest; 99.5%), COS (Matheson; 96%), N,O (Matheson; 98%), SO, (Matheson; 99.98%), NH3 (Matheson; 99.95%), NF3 (Ozark Mahoning; 98%), SF6 (Matheson; 98%), H F (Matheson; 98%), HBr (Matheson; 99.8%), H2S (Matheson; 99.5%), HCN (Matheson; 99%), BrCN (Fisher; 97%), ICN (Eastman; 97%), C2N2(Matheson; 98.5%), Ar (Knoll Welding Supply; 99.995%). Anhydrous HI was synthesized in our laboratory by the reaction of solid iodine with boiling tetrahydr~naphthalene.'~The N2F4 was provided by the U S . Air Force Weapons Laboratory. B. Characterization of the NF(b) Source and Kinetic Treatments of Data. The NF(b) relative concentration was monitored by the NF(b-X) emission intensity. The NF(b-X) intensity from NF, was about 4 times stronger than that from the same flow of NZF4. Since [N,(A)] could be generated in the same reactor by adding Nz to the flow of Ar(3P0,2),13 the [NF(b)] was estimated from the ratio of Z N F ( ~ ~ ) / Z N which ~ ( ~ - ~was ) ; -60 f 30 at the entrance to the flow reactor. Since both N,(A) and NF(b) are long-lived, the intensity ratio is given by
--
INF(b)
INz(A)
- [NF(b)17N2(A) [N2(A)17NF(b)
(1)
Since 7N2(A) is -2 and TNF(b) is -20 mS,1'1'6[NF(b)] Z 0.6 [N,(A)]. The [NF(b)] was also estimated by comparing the IF(B-X) emission intensity from the NF(b) and N,(A) excitation-transfer reactions to IF for the same [IF(X)] and [Ar(3Po,z)]. The IF(B-X) emission intensity follows first-order kinetics for low [IF(X)] and ZIF(B) = k*[WX)I[NF(b), or Nz(A)I
(2)
The excitation rate constant, k*, from N2(A)15is 8.0 X lo-" and that from NF(b)12 is 1.4 X lo-', cm3 molecule-' s-'. Since the measured IF(B-X) intensity ratio was -80 f 30, the [NF(b)]/[N2(A)] is -0.7. In this flow r e a ~ t o r ' ~ [Ar(3Po,2)] *~~~'~ = [N,(A)] = 1 X 1Olo molecules ~ m - and ~ , the [NF(b)] must be (7 f 3) x io9 molecules ~ m - ~ . The NF(b-X) bands were the only emission detected in the 200-700-nm range from Ar(3Po,2)+ NF,. Very weak NF(a-X) emission at 874 nm could be observed at the entrance to the reactor, but the [NF(a)] was too low for useful kinetic studies. Considering the Einstein coefficient for the NF(a-X) transition (14) Hoffmann, C. J. Inorg. Synth. 1963, 7, 180. (15) Tennyson, P. H.; Fontijn, A,; Clyne, M. A. A. Chem. Phys. 1981, 62, 171. (16) Piper, L. G.; Marinelli, W. J.; Rawlins, W. T.; Green,B. V. J. Chem. Phys. 1985, 83, 5602. (17) Sadeghi, N.; Setser, D. W. Chem. Phys. Lett. 1981, 82, 44.
NF(b) Quenching Rate Constants at 300 K
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 237
(about 1/300 of that of the NF(b-X) transition) and the relative response at 874 nm, the estimated [NF(a)]/[NF(b)] is 10.l.lZb Thus, the Ar(3Po,2) N F 2 source gives NF(b) together with F, plus a small amount of NF(a) in the presence of excess NF,. As will be discussed in the next section, consideration should be given to the vibrational distribution of NF(b) for some types of experiments. The decay of NF(b) occurs by radiative decay, wall quenching, and quenching by Ar, NF2, and added reagent, Q. The interaction of NF(b) with itself (or with NF(a)) and with F atoms can be ignored at the low concentration ( - 7 X lo9 molecules cm-7 in our reactor. The NF(b) decay follows a pseudo-first-order differential rate law. -d[NF(b)]/dt = (7NF(b)-I + kw + kk[Ar1 + ~ N F z [ ~ + ~ Z~ Q I [ Q I ) [ N F ( ~ )(3) I
10 9
+
8 7 6
5 m
- 4
t 3.
\
In (3) we have assumed that there are no complications from the interaction of the added reagent with N F 2 or F atoms; 7NF(b)-l is the radiative decay constant, k, is the wall quenching rate constant, and kk, kNF2,and k, are the quenching rate constants for Ar, NF2, and reagent, Q. Since the kA,[Ar] and kNF2[NF2] terms are very ~ r n a l l , ’ Ithe * ~ ~integrated rate law can be simplified. In ([NF(b)l,/[NF(b)lo) = -ktotalAt ktotal =
7NF(b)-1
+ ~Q[Q+ I kw
\
\
C H30 H
(4b)
+
+
Experimental Results A . Vibrational Relaxation of NF(b,v’). The nascent NF(b,u’=0-6) vibrational distribution, which can be observed in the Ar(3Po,2) N F 2 mixing zone, is 100:47:35:27:20:15:12. The distribution entering the quenching reactor, a delay of 8 ms, depended upon [NF,] and Ar pressure. A surprising observation was the increase in vibrational excitation of NF(b) as the Ar pressure was rasised from 2 to 5 Torr; this increase existed throughout the quenching reactor. Collisions of NF(b,u’) with the walls must be partly responsible for vibrational deactivation and higher pressure reduces the rate of NF(b,u’) diffusion to the walls. The data in Figure 3 were taken to study the vibrational relaxation along the reactor for various [NF,] and fixed Ar pressure. For [NF,] 1 X 1013molecules ~ m - which ~ , was the concentration for most of the quenching experiments, -90% of the NF(b) was in the u ’ = 0 level after -25 ms, i.e., at the end
-
\ 2
(4a)
The relative NF(b-X) emission intensity can be substituted for the concentration with proper attention given to the vibrational levels being monitored. The quenching rate constants can be measured by either the moving detector or the fixed point methods. Nearly all quenching rate constants in this work were measured by the second method, and INF(b) was observed 38 cm downstream from reagent inlet, At = 25 ms, while the reagent concentration was varied. The slope of these plots gives the product of k&. Figure 1 shows some typical results obtained by this method. The reproducibility of such experiments with no special complications was typically f 15%. An advantage of the moving detector technique is the ability to measure 7NF(b)-l and k,, as well as k,. Pseudo-first-order decay constants, k’ = (sNF(b)-l kQ[Q] + k,), are measured from the slope of the In (INF(b)) vs At plots for several reagent concentrations. The slope and the intercept from the plot of k’vs [Q] give k, and the decay constant in the absence of reagent, respectively. A sample plot for which N F ( b p ’ l 2 ) was monitored with the interference filter with CH3C1 as the reagent is shown in Figure k, values from several independent 2. The average 7NF(b)-I experiments with various reagents in a halocarbon wax coated reactor’, was 51.5 f 3.0 s-l, which is consistent with the value obtained previously from the uncoated reactor” (and with the result in Figure 2). The upper limit for 7NF(b) obtained from all of these measurements is 19.4 f 2.5 ms. We found no evidence for a significant contribution of k, to k’for halocarbon wax coated reactors or clean Pyrex reactors. However, after extended use with corrosive or acidic gases, quenching by the activated Pyrex glass walls was found. In such instances the reactor was discarded.
+
\
1 ,
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
C h a and Setser 1. o j
1088-
I
765-
0’
*
4-
>.
t
’
c
3-
2
P 5
2-
P Y
Y
2 19-
87-
6-
54-
0.0
10.0
30.0
20.0
TIME
3-
msec
Figure 3. Vibrational distribution of NF(b,u? vs time for various [NF,] -
1
18
12
6
0
At,
28
24
msec
for an Ar pressure of 2.3 Torr. Zero time correspondsto the position of the reagent addition inlet, which is 8 ms downstream of the Ar* + NF2 mixing zone. The nascent distribution measured in the Ar(3Po,2)+ NF2 mixing zoneis u b - u k = 10047:35:27:2015:12. Theub-ukdistribution at the entrance to the flow reactor for [NF,] = 7.5 X lo’, molecules ~ r n - ~ was 100:26:19:15:9:5:3. The curves are the results from the model calculation for [NF2] = 1.6 X l O I 3 molecules cm3.
.-I 0
1.o
2.0
[CH~CI] ;
3.0
\
d4molecule cm-3
1
Figure 2. (A, top) Plots of log [NF(b)] vs A f for several CH3CI concentrations. The [NF(b,u’lZ)] was monitored with the 530-nm interference filter at various positions along the reactor. (B, bottom) The slopes from each line in Figure 2A plotted vs [CH3Cl]. The slope gives kQ = 22.0 2.0 x cm3 molecule-I s-]. individual vibrational bands with the monochromator. For [CH,Cl] = 4.5 X lOI4 molecules cm-3 and At = 16 ms, the u ’ = 0-2 distribution was 100:5:1 with negligible emission from the higher u’levels; the electronic quenching was a factor of 4. T h e estimated vibrational relaxation rate constant was k( 1 4 ) , 1 X cm3 molecule-’ S-I with a factor of 2 uncertainty. T h e quenching rate constant for CH3CI was measured with the fixed
-
1
\
molecule-' s-l, relaxation can be neglected, which is the case for the halogenated methanes. Except for the quenching rate constants mentioned above, no explicit tests were made for possible vibrational effects on the quenching rates. Since the v' 1 1 population is only -20% at the entrance to the reactor and 10% at the end of reactor, any effect from vibrational relaxation on kQ will be less than the absolute uncertainty of k,. If higher Ar pressure, lower [NF,], or shorter observation times had been used, consideration of the NF(b,v'L 1) vibrational relaxation might have been necessary.
-
B. Quenching Rate Constantsfor Polyatomic Molecules. Lin and Setserlla made preliminary measurements for quenching of NF(b) by several reagents. Some of those rate constants were remeasured in the present system with better reliability. Good agreement was generally found between the two studies, but the new measurements are preferred because greater attention was given to flow measurements and to reagent purity. Table I summarizes the NF(b) quenching rate constants at 300 K for the polyatomic molecules; the listed uncertainty is the standard deviation from multiple experiments. If a prior rate constant measurementlla was found to be suspect, it was not included in the summary. The absolute uncertainty in the rate constants is estimated to be f20% for well-behaved reagents and f40% for reagents that required C2H6as a scavenger for F atoms. The methyl halides, including CH3CN, have about the same quenching rate constants, - ( 2 0 f 2) X cm3 molecule-' s-l; however, CH31may have a somewhat larger rate constant. The rate constants for methylene halides, kcH2CIF and kCH2Cl are (1 7 f 2) X cm3 molecule-' s-', but k~~~~~~ is slightly larger, (21 f 2) X cm3 molecule-' s-l. The CHC13, CHF3, and CHFC12 0 2.5) X cm3 molecules have rate constants of ~ ( 8 . f molecule-' s-l. The deuteriated methyl compounds, CD3Br and CD3CN, have about 60 times smaller rate constants than those
-
240 The Journal of Physical Chemistry, Vol. 93, No. I, 1989
0.0
2.0
4.0
6.0
Cha and Setser
8.0 0.0
-
for the undeuteriated molecules; CDC13also showed a large kinetic isotope effect with kCHCI,/kCDCI, 21. The rate constant for C H 3 0 H was about 4 times larger than that for the methyl halides, which suggest a role of the OH bond in quenching. The fully halogenated methanes (CF4, CCl,, CF3Br, and CF31) have small rate constants -( 1.0 f 0.5) X cm3 molecule-' s-'. The rate constant for CF,NO is larger, (-55 f 5) X cm3 molecule-' s-', than those for other halogenated methanes or for NO, which suggests a different kind of quenching mechanism. Experiments were done to search for the C F 3 N 0 singlet emission bands at 750-800 nm,18 but no emission was observed. However, excitation transfer to the C F 3 N 0 triplet state is possible. The C2 and C3 hydrocarbons have rate constants of (1 1-26) X cm3 molecule-' s-I. Acetylene has a smaller value, (4.2 f 0.2) X cm3 molecule-' s-', because there are only two C-H bonds. From the magnitudes of the rate constants, there is no evidence that the unsaturated hydrocarbons or acetone quench by chemical reaction; i.e., the rate constants are comparable to those for methyl halides after adjustment for the number of C-H bonds. The kNH,value reported earlier" was confirmed and we believe the indirectly measured value given by Zhuang and co-workers to be too large.19 The quenching rate constant for NF3 was remeasured and a small value was confirmed." The rate constants for NF3 and NzF4 (and NF2) are comparable, as would be expected for quenching by E-V transfer. Given the expense of NF3 and NzF4 and the difficulty of measuring small rate constants, no attempt was made to obtain especially reliable values for these rate constants. There is a large discrepancy in kNF, between this cm3 molecule-' s-l) and the report (180 X work (-0.5 X cm3 molecule-' s-I) by Clyne and ~o-workers.~~ In their work, the NF(b) radicals were generated by a pulsed Tesla discharge in a NF3/Ar mixture and the kNFI was inferred by monitoring the NF(b) for variable [NF,]. Their kNF, value probably is an effective rate constant for NF, plus other species (possibly F2) generated by the discharge. Quenching by a NF,/Ar flow that has been passed through a microwave discharge has a large effective rate constant.lh The kNFlvalue is comparable with the rate constants of other perfluorinated molecules, such as SF6 and CF4, and we believe our directly measured small kNF, value to be reliable. The quenching rates of PCl,, BC13, and C2N, were too slow to measure for concentrations that could be conveniently added to the flow reactor. There is no suggestion for enhanced quenching via adduct formation from Lewis basic or acidic reagents. N
(18) Dyet, J . A.; McCoustra, M. R.S.; Pfab, J. Chem. phys. Lett. 1987, 135, 534.
2.0
4.0
6.0
8.0
10.0
The quenching plots by CF31 were abnormal, giving two first-order decay regimes as shown in Figure 5. The faster decaying component for low [CF31] had a rate constant of 2 1 X lo-" cm3molecule-1 s-I; the rate constant of the slower decaying component associated with high [CF31] was 55 X cm3 molecule-' s-'. Experiments with higher flows of NF2 seemed to have a larger first component. Exchanging NF, by N2F4 as the NF(b) source gave the same pattern, but with a smaller first component. Quenching studies with C2FSIand C6FI3Igave similar plots.Izb This suggests that the species responsible for the fast quenching component was I F formed from the F RI reaction. The somewhat larger first component for NF,, relative to N2F4 as the NF(a) source, suggests that [F] is larger for the former system, which is consistent with the fact that [NF(b)] is -4 times larger with NF2.11a The plots could be converted to single-component first-order decay plots by adding a F atom scavenger, either ethane or cyclopropane, to the reactor. Some typical results are shown in Figure 5. The scavanger were added through a bulb inlet, which was placed 3-4 cm upstream from the reagent inlet. Enough (-3 X lo', molecules cm-,) ethane or cyclopropane was added to quench about 30% of the NF(b). The rate constant obtained in the presence of the scavanger matched the rate constant of the slow component from the two-component exponential decay plot. Ethane seemed somewhat the better F atom scavenger giving more linear log [NF(b)] vs [Q] plots. The rate constants for the hydrogen abstraction reaction of atomic fluorine with and 0.56 X cm3 ethane and cyclopropane are 1.0 X molecule-I s-l, respectively.z6 Ethylene and acetylene also gave two-component quenching plots; however, the first components for CzH4 and CzH2were much smaller than for CF31. The rate constants reported in Table I for C2H4and C2H2were obtained in the presence of added ethane. The explanation for the kinetic complication arising in the CzH4 and CzH2systems is not obvious, but the presence of H atoms from F atom displacement probably is responsible for the difficulty. The HF formed by H abstraction also can cause problems for reagents with sufficiently small quenching rate constants. C. Quenching Rate Constants for Diatomic and Triatomic Molecules. The rate constants for diatomic and triatomic molecules, are summarized in Table 11. Quenching plots for H2 and D2 exhibited two first-order regimes in the absence of F atom scavenger; see Figure 6. These molecules can react with F atom and the product molecule, HF, is a more effective quencher for NF(b) than H2 or D2; furthermore, H and D atoms can interact with NF or NF,. With added ethane, the quenching plots for H2 and D, were linear and the rate constants were the same as that deduced from the slow component in the absence of C&; see Figure 6. With the exception of the hydrogen halides, the rate constants for diatomic and triatomic molecules generally are
+
NF(b) Quenching Rate Constants at 300 K
The Journal of Physical Chemistry, Vol. 93, No. I, 1989 241
TABLE II: Quenching Rate Constants for Diatomic and Triatomic Molecules at 300 K (1O-l' cm3 molecule-' s-l)
H2 D2
3.8 f 0.6d
