Reaction of TrifliJoromethyl Radicals with Nitric Oxide
1335
were the dominant mechanism.23 This prediction is related to the observation reported in the single-pulse shock tube study of the Hz Dz exchange that the rate depended more strongly on the concentration of Dz than on H2.21,24 In the temperature range 1050-1200”K, the rate of Dz with CzNz was found to be faster than that of HZ with CzN2 although the rate decreased with increasing temperat~re.~~ According to VEC, the important species is CzNz” and not H2U or D2U. Consequently, there should be a decrease in the rate constant as expected from the normal isotope effect when D:l rather than Hz is reacting with CzN2. The magnitude of the isotope effect, A D J A H ~calculated , from collision theory, is listed below for both atomic and VEC mechanisms. H,, D, CN HCN, D C N H, D 0.73 (15)
+
-
H,
+ + D + C2N2HCN, D C N + CN 0.715 H,, D z + C,N2” BHCN, DCN 0.72
-
(16)
(17) The experimental ratio (kD2/kHz) is about 0.6 which agrees with either the atomic or VEC mechanism within experimental error. The quadratic time dependence and the order with respect to reactants and total density are facts which can be rationalized in terms of an atomic or vibrational energy chain mechanism. The experimental activation energy of 62 kcal/mol is less than the 76 kcal/mol predicted by eq 10. This discrepancy plus the lack of any direct evidence for the presence of radicals argues somewhat against the atomic route. The observation of an appreciable vibra-
tional equilibration time for CzNz compared to the metathesis reaction time lends support to VEC. However, the activation energy predicted by eq 13 is not known and it is possible that it is high. A more definite statement may be made with regard to DZ exchange.15 The the role of cyanogen in the HCN proposal involved the following steps.
+
C2N2
+
D2
-
2DCN
( j) This work has demonstrated that the rate of reaction j is much slower than the exchange process and therefore cyanogen is not an important intermediate in the exchange sequence. The evidence is conclusive with respect to the complex nature of this metathetical reaction. The direct bimolecular combination of the reactants with the formation of a four-center transition state does not occur to any appreciable extent and provides another example of a “simple” chemical reaction that avoids the four-center pathway.
Acknowledgments. The authors would like to thank Mr. Tim Dupuy for his assistance with data reduction, MY. Darryl Olivier who helped maintain the equipment, and Professors Robert Flurry and Peter Politzer for fruitful discussions. We appreciate very much a critical review of the manuscript by Professor S . H. Bauer. (23) D. K. Lewis, Thesis, Corneli University, 1970; University Manu(24)
scripts, Ann Arbor, Mich. D.K. Lewis, private communication.
A Mass-Spectrometric Study of the Reaction of Trifluoromethyl Radicals with Nitric Oxide’ Hiok-Seng Tan and F. W. Lampe* Whifmore Laboratory, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received December 18, 1972) Publication costs assisted by The U.S. Atomic Energy Commission and The Petroleum Research Fund
Trifluoromethyl radicals, formed by the photolysis of hexafluoroazomethane, react with nitric oxide by successive addition to yield trifluoronitrosomethane and perfluorotrimethylhydroxylamine. Trifluoronitrosomethane is the sole product until the nitric oxide concentration has been reduced to very low levels, at which time the addition reaction to form perfluorotrimethylhydroxylamine can occur. After the trifluoronitrosomethane has been depleted the trifluoromethyl radicals can react with hexafluoroazomethane to yield perfluorotetramethylhydrazine and with each other to yield hexafluoroethane. Kinetic analysis o f the data subsequent to depletion of nitric oxide permits evaluation of a lower limit to the specific reaction rate for the addition of trifluoromethyl radicals to trifluoronitrosomethane. The value found is ks 2 9.7 i 0.7 X cm3 molecule-1 sec-I at 56”.
The photolysis of hexafluoroazomethane in the visible and near-ultraviolet regions of the spectrum has been well studiedz-? and often used8-12 as a convenient source of tri-
fluoromethyl radicals at near room temperature. The products of the photolysis are nitrogen, hexafluoroethane, perfluorotetramethylhydrazine, and perfluorohexamethylThe Journal of Physical Chemistry, Voi. 77, No. 11, 19:73
Hiok-Seng Tan and F. W. Lampe
1336
tetrazine and it is generally acceptedz-7 that these arise from the mechanism
+
---
CF3N2CF3 hv 2CF3* + N, CF,. CF3N2CF3 --+ (CF,),NNCF.,
+
CF,
+
+
CF3. CF3. (CF,),NNCF3
+
(CF,),NNCF,
(1)
(2) C2Fe (3) (CF3)2NN(CF3)2 (4)
(CF,),NNCF,
(CF,)zNN(CF,>N(CF,)N(CF,),
(5)
If nitric oxide is added to a system in which trifluoromethyl radicals are present the trifluoromethyl radicals will be scavenged by nitric oxide, via (6),12-18 to form triCF,
+
NO
----t
CF,NO
(6)
fluoronitrosomethane, a stable blue gas that can be isolated.19-20 The scavenging reaction, (6), is very efficient, the rate constant having been shown21 to be at least cm3 molecule-1 sec-1, and it will, therefore, be the essentially exclusive fate of trifluoromethyl radicals until the nitric oxide concentration has been reduced to very low levels. Trifluoronitrosomethane, the product of (6), will also scavenge trifluoromethyl radicals by successive addition of the radicals to form perfluorotrimethylhydroxylamine,22-26as shown by (7) and (8). Indeed, the photolysis of CF,. CF3
+
+
CF3N0
(CF,),NO.
--
(CF,),NO.
(7)
(CF3),NOCF3
(8)
2: 1 mixtures of trifluoroiodomethane and trifluoronitrosomethane has been used to prepare perfluorotrimethylhy5 7 and 8 are thus droxylamine in 90% ~ i e l d . ~Reactions similar to reactions occurring when small amounts of nitric oxide are added to systems in which hydrogen atoms,27 methyl radicals,28 and ethyl radicals29 are present. Since its discovery and isolation by Blackley and Reinhard,30 the nitroxide radical produced in (7) has been well studied.25331-35 It abstracts hydrogen atoms from alkane@ and adds to 0lefins3~~33 in a reaction predominating over hydrogen atom abstraction33 from olefins. It reacts very rapidly with free radicals, as would be expected, since it is itself a free radical. Therefore, since activation energies for addition to olefins are 7-10 k c a l / m 0 1 ~ ~ and addition appears to predominate over hydrogen abstraction, one may conclude that in the presence of trifluoromethyl radicals (7) will be followed rapidly by (8) to yield the perfluorotrimethylhydroxylamine. This paper describes a mass-spectrometric study of the photolysis of hexafluoroazomethane in the presence of up to 10% of nitric oxide. Experimental Section Trifluoromethyl radicals were generated by the photolysis of hexafluoroazomethane in the presence of up to 10% of nitric oxide. The reactions were carried out in a photolysis cell containing a pinhole leak leading directly into the ionization chamber of a somewhat-modified Bendix Model 14-101 time-of-flight mass spectrometer. The photolysis cell was 8.35 cm in length with a diameter of 2.10 cm and was connected uia 3-mm stainless steel and 6-mm Pyrex tubing to a large reservoir (5-12 1.) containing the reactants, hexafluoroazomethane and nitric oxide. The The Journal of Physical Chemistry, VoI. 77, No. 1 1 , 1973
apparatus and general techniques have been fully described in previous publications from this laboratory.2'-30ys6 An Osram HBO-100 high-pressure mercury arc was used as a light source. The spectral emission of this lamp and the wavelength dependence of the hexafluoroazomethane absorption coefficient6 are such that the average effective photolytic wavelength was about 3660 A. The lamp was operated a t 5.2 A using a power supply obtained from George W. Gates Co. (Model P109); with this power supply the lamp current could be reproduced to within k2%. Hexafluoroazomethane was purchased from Merck Sharpe and Dohme, Ltd. It contained 1-2% of trifluoroiodomethane which was not removed by repeated freezepump-thaw cycles in liquid nitrogen. However, the long wavelength limit, of its absorption37 nearly coincides with the short wavelength cut-off of the lamp emission, so that photolysis of the trifluoroiodomethane is undoubtedly negligible. Nitric oxide having a stated minimum purity of 98.5% was purchased from the Matheson Co. It was purified by first freezing onto silica gel at liquid nitrogen temperature and then allowing a slow temperature increase of the condensed gas to occur. The initial and final (1) United States Atomic Energy Commission Document No. COO3416-7. (2) J. R. Dacey and D. M. Young, J. Chem. Phys., 23,1302 (1955). (3) G. 0. Pritchard, H. 0. Pritchard, and A. F. Trotman-Dickenson, Chem. Ind., 564 (1955). (4) G. 0. Pritchard, H. 0. Pritchard. H. I. Schiff, and A. F. TrotmanDickenson, Trans. Faraday SOC., 52, 849 (1956). (5) J. R. Dacey, R. F. Mann, and G. 0. Pritchard, Can. J. Chem., 43, 3215 (1965,). (6) E-Chung Wu and 0. K. Rice, J. Phys. Chem., 72, 542 (1968). (7) S-L. Chong and S. Toby, J. Phys. Chem., 74, 2801 (1970). (8) L. Herk, M. Feid, and M. Szwarc, J. Amer. Chem. Soc., 83, 2998 (1961) . (9) A. P. Stefani, L. Herk, and M. Szwarc, J. Amer. Chem. SOC., 83, 4732 (1961). V. A. Ginsburg, E. S. Vlasova, N. M. Vasil'eva, N. S. Mirzabekova, S. P. Makarov, A. I. Shchekotikhin, and A. Ya. Yakubovich, Dokl. Akad. NaukSSSR, 149,97 (1963). W. G. Alcock and E. Whittle, Trans. Faraday Soc., 62, 664 (1966). 0.Dobis, J. M. Pearson, and M. Szwarc, J. Amer. Chem. Soc., 90, 278 (1968). R. N. Haszeldine, J. Chem. SOC.,2075 (1953). J. Banus, J. Chem. Soc., 3755 (1953). J. Jander and R. N. Haszeldine, Naturwissenschaften, 40, 579 (1953). J. Jander and R. N. Haszeidine. J. Chem. SOC.,912 (1954). D. A. Barr and R. N. Haszeidine, J. Chem. SOC.,1881 (1955). J . Heickien, J. Phys. Chem., 70, 112 (1966). 0.Ruff and M. Giese, Berichte, 69,600 (1936). 0. Ruff and M. Giese, Berichte, 69,684 (1936). S. W. Charles as quoted by B. G. Tucker and E. Whittle, Trans. Faraday SOC., 61,490 (1965). A. Ya. Yakubovich, S. P. Makarov, V. A. Ginsburg, N. F. Privezentseva, and L. L. Martynova, Dokl. Akad. Nauk SSSR, 141, 125 (1961 ) . J. Mason, J. Chem. SOC., 4537 (1963). A. H. Dinwoodie and R. N. Haszeidine, J. Chem. Soc., 1681 (1965). R. E. Banks, R. N. Haszeldine, and M. J. Stevenson, J. Chem. SOC. C, 901 (1966). R. N. Haszeidine and A. E. Tipping, J. Chem. SOC.C, 1236 (1966). F. C. Kohout and F. W. Lampe, J. Chem. Phys., 46,4075 (1967). A. Maschke, B. S. Shapiro, and F. W. Lampe, J. Amer. Chem. SOC..86. 1929 (1964). H-S. Tan and F . ~W. Lampe, J. Phys. Chem., 76, 3303 (1972). W. D. Blackley and R. R. Reinhard, J. Amer. Chem. SOC., 87, 802 (1965). S. P. Makarov, A. F. Videiko, V. A. Toboiin, and M. Englin, Zh. Obshch. Khim., 37, 1528 (1967). A. V. Melhikova, M. K. Baranaev, and S. P. Makarov, Zh. Obshch. Khim., 40, 382 (1970). R. N. Haszeldine, R. E. Banks, and B. Justin, J. Chem. SOC. C, 2777 119711. R . N.'Haszeidine, R. E. Banks, and T. Myerscough, J. Chem. SOC. C, 1951 (1971). D. P. Babb and J. M. Shreeve, Intra-Sci. Chem. Rep., 55 (1971). E. Kamaratosand F. W. Lampe:,J. Phys. Chem., 74, 2267 (1970). J. G. Caivert and J. N. Pitts, Photochemistry," Wiley, New York, N. Y., 1966, p 527.
1337
Reaction of Trifluoromethyl Radicals with Nitric Oxide .
I
I
I
I
1
-
In Figure 2 is shown the ion intensities of m / e 119, 285, and 166 as a function of time in the initial stages of the photolysis. The ion at m / e 166 arises from the hexafluoroazomethane; the dependence of intensity on time indicates that only about 5% of the reactant was consumed over the time of the experiment. Assuming the mechanism 1-5 to be operative, the trifluoromethyl radicals to be at all times at steady-state concentration, and the amount of perfluorohexamethyltetrazine to be negligible leads to the result shown by
I
/ _ I
70C
Photolysis Time in Minutes
Figure 1. Dependence of ion intensities on time of photolysis of pure hexafluoroazomethane at 11.4 Torr: 0, m/e 285; 0,m/e 119; 0 , m / e 235; 0, m/e 216; ., m/e 304; +, m / e 230; a, m/e 318.
10% of the gas distilling from the silica gel were discarded and the entire procedure repeated. All photolyses were carried out at a temperature of 56 3".
*
Results and Discussion 1. Photolysis of Pure Hexafluoroazomethane. When 11.4 Torr of pure hexafluoroazomethane is photolyzed a t 56" one clearly observes, as may be seen in Figure 1, a simultaneous growth in the intensities of ions having m f e values of 119, 216, 230, 235, 285, 304, and 318. The relative intensities of these ions are independent of photolysis time, indicating that they arise from electron impact ionization of a common molecule or of different molecules produced in simultaneous reactions. Mass spectrometric and stoichiometric considerations, within the framework of the known products of the photolysis,2-7 lead to the following conclusions concerning the identification of these ions: (1) m / e 119 is C2F5+ arising from electron impact ionization and dissociation of C#6; (2) m l e 304 is (CF&NN(CF&+, the parent ion of perfluorotetramethylhydrazine, while m f e 285, 235, and 216 are, respectively, (CF:&NN(CF3)CF2+, (CF&NN(CFa)+, and (CF&NN=CFz+, all fragment ions produced from perfluorotetramethylhydrazine; (3) m / e 318 and 230 are observed only at very low intensities and we conclude that they are (CF&NN(CF3)NCF3+ and CFsNN(CF3)N=CF2+, respectively, which are expected fragment ions of perfluorohexamethyltetrazine. The most predominant ion in the mass spectra of all the product molecules of the photolysis is expected to be CF3f a t m f e 69.26,38 However, this ion is also the most predominant one in the mass spectrum of the reactant hexafluoroazomethane39 and so the contributions of the products to the intensity a t m / e 69 are completely obscured. The very low intensities of the ions a t m / e 230 and 318 are in accord with previous observations that under our conditions perfluorotetramethylhydrazine is found in much greater abundance than is the perfluorohexamethyltetrazine.2.4 The mass spectrometric results are thus in general accord with the accepted mechanism depicted by eq 1-5.
where $J is the quantum yield of (1); 10 is the light flux incident on the photolysis cell; a is the effective absorption coefficient of hexafluoroazomethane; imle is the ion intensity at the m / e indicated; and the mass spectrometric C i d ibration constants are defined by p119i119 = [C2F6] and 8285i285 = [(CF3)2NN(CF&]. The subscript zero on the derivatives and on the concentration of hexafluoroazomethane indicates initial rates and initial concentration. The quantity (+loa) was found to be 3.41 f 0.3 X sec-1 from replicate measurements of the initial rates of nitric oxide consumption when -1% of nitric oxide was added to the photolysis cell. This calibration procedure is valid because the intensity-concentration proportionality constant for m / e 30 from nitric oxide can be measured id!rectly and in the initial stages of the reaction trifluoroinitrosomethane is the sole product. The proportionality constant Pll9 can also be determined directly using CzFe, and, therefore, the data of Figure 2 may be treated by l(9) to yield the proportionality constant b285 that relates ion intensity to concentration of perfluorotetramethylhydrazine . In terms of the mechanism 1-5 we may write for the photolysis of pure hexafluoroazomethane
~ ~ [ C F ~ I [ C F ~ N Z C(11) F~IO where in (11) the term ~ ~ [ C F ~ ] [ C F ~ N is ~ Cequated F ~ ] O to kd[CF3][(CF3)2N2CF3] because of our assumptions of a steady state for [CFs] and a negligible rate of formation of perfluorohexamethyltetrazine. The steady-state concentration, [CFs], may be eliminated from (10) and (11) to yield (12), which permits calculation of the ratio ( k 2 / k&'2)-'from data such as shown in Figure 2. Thus, wehave from three replicate experiments at 56", of which a typical one is shown in Figure 2
2.7
* 0.3 x lo-"
cm3/2 molecule-'/2 sec-l
(l2)
The value of kz/k31/2 shown in (12) is in satisfactory agreement with that of 1.8 f 0.6 X 10-11 6111312 mole(38) s. s. Dubov and A. M. Khokhlova, Zh. Obshch. Khim., 34, 586
(1964). (39) P. 6.Ayscough,J. Chem. Phys., 24,944 (1956).
The Journal of Physical Chemistry, Vol. 77, No. 1 7 , 1973
Wok-Seng Tan and F. W. Lampe
1338
00
70
E
2 ea
.- 50-
.- 40r
c ._
.-30f
;20-
I
0
'
Photolysis Time in Minutes
Figure 2. Intensity-time dependence of ions from C2F6 and (CF3)2N2(CF3)2 in initial stages of photolysis: 0, m l e 285; 0, m / e 119; (3, m / e 166 (from CF3N2CF3).
c u k 1 I 2 sec-l, reported by Pritchard, Pritchard, Schiff, and Trotman-Dicken~on.~ This agreement confirms the general validity of our experimental method. Combining the value of ( k ~ l k 3 ~from ' ~ ) (12) with that of 123 = 3.88 X 10-l1 cm3 molecule-I sec-l reported by Ayscough39 leads to the result k2 = 1.7 0.2 X 10-16 cm3 molecule-1 sec-I. 2. Photolysis of Hexafluoroazomethane-Nitric Oxide Mixtures. When mixtures of hexafluoroazomethane and nitric oxide (1-10 mol 70)were photolyzed, the only product formed in the initial stages was CFsNO, as indicated by the rapid increase with time of the peak height at m l e 99 (CF3NO+). As shown in Figure 3, which refers to photolysis of 12 Torr of a mixture containing 10% nitric oxide, the intensity of CF3NO+ increases to a maximum a t a photolysis time of about 20 min. At about the same time as the maximum in [CFaNO], there is observed a simultaneous formation of ions with m l e values of 237, 218, 168, 149, and 130, of which only m l e 237, 218, and 130 are shown in Figure 3. The absence of a time dependence of the relative intensities of m l e 130 and 218, as shown in the inset of Figure 3, suggests that these ions arise from electron impact on a common molecule. Due to the very low peak heights of m l e 237, 168, and 149, the intensities of these ions were not used in quantitative calculations; however, on the basis of the simultaneous increase in intensities a t m l e 237, 218, 168, 149, and 130, there is little doubt that they all arise from the expected22-26931 product (CF3)2NOCF3 (mol wt = 237 amu). The ions in the mass spectrum are then taken to be (CF3)2NOCF3+ ( m / e 2371, (CF3)2NOCF2+ ( m l e 218), (CF&NO+ ( m / e 168), (CF3)(CF2)NO+ ( m l e 149), and (CF3)(CF)NO+ ( m l e 130), respectively, although not much significance should be attached to the actual structures written for the fragment ions. It is concluded, therefore, that the common precursor to the above set of ions is perfluorotrimethylhydroxylamine, and that a t low concentrations of nitric oxide the kinetic behavior of trifluoromethyl radicals is analogous to that of methyl28 and ethyl radicals.29 These conclusions and facts upon which they are based are in accord with previous studies22-26931 of the thermal and photochemical reactions of trifluoronitrosomethane. We The Journalof Physical Chemistry, Voi. 77, No. 11, 1973
Photolysis Time in Minutes
Figure 3. Dependence of ion intensities of product ions in photolysis of CF3N2CF3 (90%)-NO (10%) mixture at 12.2 Torr: 0, m l e 99 X IO; 0,m / e 130 X 1; 0, m l e 218 X 5; ., m l e 237
x 5.
have assumed in the above that contributions of the perfluorodimethylnitroxide radical to the observed mass spectra are negligible. Although stable, this radical is expected to react sufficiently fast with the trifluoromethyl radicals30-35 uia (8) to keep its stationary concentration below our mass-spectrometric detection level. The dependence of the intensity of m / e 30 (NO+) on photolysis time for two different electron energies is shown in Figure 4. At an ionizing energy of 75 eV the intensity of m l e 30 parallels that of m l e 99 (CF3NO+), shown in Figure 3, in that a maximum a t -20 min is observed. However, for a nominal ionizing energy of 1F eV, at which NO+ arises essentially exclusively from electron impact on nitric oxide, the intensity of m l e 30 decreases to a steady-state minimum at -20 min. Since no product other than trifluoronitrosomethane (Figure 3) is observed prior to the maximum in the intensity of m / e 99 (CF3NO+), we may write for this time interval the material balance [NO], = [NO] + [CF,NO] (13), where ~ O ] isO the initial concentration of nitric oxide. It is thus apparent from Figure 4 that trifluoronitrosomethane contributes an intense m / e 30 peak upon impact of 75-eV electrons. The minimum in the intensity of m l e 30 a t 16 eV indicates that the nitric oxide concentration has been so reduced, that a t this time, and subsequent to it, nitric oxide contributes only negligibly to the intensity of m l e 30. Thus, we conclude that a t the maximum in m l e 99 (CF3NO+), shown in Figure 3, the intensity of m l e 30 (NO+) observed a t 75 eV, and shown in Figure 4, is due entirely to formation of NO+ by electron impact on trifluoronitrosomethane. For 75-eV electrons, then, we interpret the maxima in m / e 30 and 99 to signify the time a t which depletion of nitric oxide via (6) is essentially complete.
Reaction of Trifluoromethyl Radicals with Nitric Oxide
1339
Lo
c 3 L
60 n L
a c c
r g 40
r“ x0
a“ O‘
Ib 20 $0 40 i o Pnotolysls Time in Minutes
60
’
20
Figure 4. Dependence of intensity of m l e 30 (NO+) at 75 and 16 eV. Ionizing energy on time of photolysis of CF3N2CF3 (90%)-NO (lO%)lat 12.2 Torr: A, i30 X 0.1 at 75 eV; 0, i30 X
2
2.5 at 16 eV.
When 10 Torr of hexafluoroazomethane was photolyzed in the presence of a much lower concentration of nitric oxide, namely, 0.96%, [CFsNO], the corresponding intensities of mle 30 (NO+) and 99 (CF3NOf) att‘ain maxim a at about 2 min of photolysis time. The intensity a t m / e 99 (CF3NO+) is, however, too weak a t this low concentration of nitric oxide for use in quantitative calculations. Hence, as shown in Figure 5, the maximum in [CFsNO] and subsequent decrease of [CF3NO] due to formation of perfluorotrimethylhydroxylamine via (7) and (8) and to leakage into the mass spectrometer are manifested (for 75-eV ionizing energy) by the maximum and subsequent decrease in the intensity of m / e 30 (NO+). As in the experiments with 10% of nitric oxide, it can be seen from Figure 5 that a t the time of occurrence of the maximum in [CF3NO] (and in i30), [(CF3)2NOCF3], as reflected by the intensity of mle 130, increases rapidly, reaching a broad maximum after about 8 min of photolysis. Near the beginning of this broad maximum in [(CF3)2NOCF3] there is observed an onset of ions at m / e 304, 285, 235, 216, 119, and 114, of which only those a t mle 304 and 1.19 are shown in Figure 5. The ions at mle 304, 285, 235, 216, and 114 arise from electron impact on perfluorotetrarnethylhydrazine, while the ion at mle 119 is from hexafluoroethane, as described in the previous section. It is interesting to note in Figure 5 that after the maximum in the intensity of mle 130, the intensity of mle 30 shows a small but real increase. This observation suggests that, as the reaction proceeds, products are being formed which upon electron impact yield significant amounts of NO+. A careful search of the mass spectrum subsequent to the maximum in [(CF3)2NOCF3] has not revealed the nature of these products. However, Haszeldine and Tipping26 have reported that both the compounds I and I1 yield significant amounts of NO+ on electron-Cmpact;-We believe that in our system I1 is much more likely to be formed than I because it can arise simply by association of the radical product of (2) with nitric oxide, the latter being present in the latter stages of the reaction at a steady-state concentration that is about 10% of it’s initial value. The actual consumption of nitric oxide in this experiment may be seen in Figure 6, where the intensity of
4 6 0 1 0 1 2 Photolysis Time in Minutes
1
4
Figure 5. Time dependence of ion intensities in photolysis 01
CF3NzCF3 (99%)-NO (1%) mixture at 10.4 Torr: 0 , m / e 304 X 10; A, m / e 119 X 10; v m / e 130 X 2.5; 17, m l e 30 X 1; 0 m / e 166 X 10.
mle 30 at 16-eV ionizing energy is plotted as a function of‘ time.
I
I1
The absence of an ion at mle 198 indicates that within experimental error no dimerization of trifluoronitro. somethane occurs. This is in contrast to the mass-spectro. metric observations with nitrosomethane40 and nitrosoethane29 but is in agreement with previous work on trifluoronitrosomethane.13-17 The rearranged “dimer,” (CF3)2NONO, is known to be formed by the successive addition of a trifluoromethyl radical and a nitric oxide molecule to trifluoronitrosomethane;26 however, under our conditions of low concentrations of nitric oxide such a sequence is very improbable. As may be seen in Figure 5, the concentration of hexafluoroazomethane does not vary much during the photolysis. It is interesting to note, however, that, as shown by the time dependence of & 3 , an increased conversion rate of the hexafluoroazomethane occurs simultaneously with the onset of formation of perfluorotetramethylhydrazine ( m l e 304). This clearly shows the increased consumption rate of hexafluoroazomethane that begins when the [NO] and [CF3NO] have been reduced to such low values that (2) can occur. All these observations discussed above confirm clearly that the reaction mechanism is shown by eq 1-8. In addition, we suggest that our results support the minor occurrence of
3. Rate Constant f o r Addition of Trifluoromethyl Radical to Trifluoronitrosomethane. Assuming that under our (40) F. A. Thomassy and F. W. Lampe, J. Pbys. Cbem., 74, 1188 (1970).
The Journal of Physical Chemistry, Vol. 77,
No. 11, ‘197:3
Hiok-Seng Tan and F. W. Lampe
1340
0.96% to less than 0.096% in 2 min of photolysis. The subsequent decrease of [CFBNO] and the increase of [(CF&NOCF3] indicate that [NO] becomes so small by 2 min that the reactions of trifluoromethyl radicals can no longer be inhibited and thus for this system [NO] has effectively become zero. The rate of depletion of [CF3NO], after the maximum, is then described by d[CF,NOJ/dt = h,[CF,NO][CF,] + /Z[CF,NOl (15) where X is the first-order leak-rate constant through the pinhole of the photolysis cell. The steady-state concentration of trifluoromethyl radicals is given by W 3 I
=
1/(k,[CF3NO] I
I
I
I
2 4 6 Photolysis Time in Minutes
)
I
I
I
I
I
2k3
8
Figure 6. Depletion of NO in photolysis of 10.4 Torr of CFsNaCF3 (99%)-NO (1%) mixture. -
+ k2[CF3N,CF3])2 + 4 ~ l o a k 3 [ C F 3 N , C ?-
I
I
The relationship between [CF3NO] and the ion intensity at mle 30, after the maximum in JCF3NO1, is described by [CF,NOl = P3oi30
[5. , ,
O
1.50
,
I I
I
1
40 80 120 160 200 Time in Seconds ( t ’ l
Figure 7. (a) Stationary concentration of CF3 radicals as a function of photolysis time after consumption of NO. (b) Kinetic descrbtion of CFqNO depletion after consumDtion of NO.
conditions the radical-combination reaction, ( 8 ) , is very rapid, the rate-determining step for the formation of perfluorotrimethylhydroxylamine is (7). The specific reaction rate of (7) may be evaluated by kinetic analysis of the photolyzing mixture at reaction times after the maximum in [CF3NO]; that is, after essentially complete depletion of nitric oxide, provided it may be assumed that the produced perfluorotrimethylhydroxylamine does not contribute a mass peak a t mle 30 and provided we may take [NO] = 0. With 16-eV electron impact, the mass peak at mle 30 decreases rapidly to about 10% of its initial intensity at about the time [CF3NO] reaches a maximum, as shown in Figure 6. Subsequent to this maximum, the decrease in intensity of the peak at mle 30 with 75-eV ionizing energy is so steady that with little doubt the (CF&NOCF3 molecule does not contribute significantly to m / e 30. As shown in Figure 6, [NO] is reduced from The Journal of Physical Chemistry, Vol. 77, No. l l, 7973
(17)
where p30 is a proportionality constant and is0 is the ion intensity a t mle 30 for 75-eV ionizing energy. As described in the preceding sections, the values of k2 and q5100c are known and k3 has been reported in the literature.39 Then with an initial guess that k7 is the same as for the corresponding reaction in the nitrosoethane system,29 the steady-state [CFs] can be computed from (16) as a function of [CF3NO], and hence as a function of time, using (17) and the experimental data of the ion intensity at mle 30 with 75-eV ionizing energy, as shown in Figure 4. When this is done, it is found that over an appreciable range of time, beginning about 40 sec past the [CF3NO] maximum, the computed steady-state [CFs] is an approximately linear function of time. Hence, we can write for this time range
+
[CF,] = cz bt’ (18) where t’ is the reaction time less some convenient time to, (t’ = t - t o ) , and a and b are empirical constants determined by a plot of [CF3] us. t’ as shown in Figure 7a. Substituting (18) into (15) and integrating leads to
where [CF3NO]o is the trifluoronitrosomethane concentration a t to. In a manner completely analogous to that described previously for the nitrosoethane case,29 a plot of the left-hand side of (19) us. t’ should be linear with a A). Such a plot slope of k 7 b / 2 and an intercept of ( h a ifor an initial pressure of nitric oxide of 0.1 Torr is shown in Figure 7b. Using the first values of a and b as determined by computation of [CFB] from (16) a first-improved k7 can be computed. Then, with the improved k7, the process can be repeated until successive iterations give no significant change in k 7 . The linearity of the plot according to (19), shown in Figure 7b, attests to the validity and internal consistency of the method. The average value obtained from two replicate experiments at 56” is k7 = 9.7 f 0.7 X 10-14 cm3 molecule-1 sec-1. As discussed in the determination of the specific rate of the corresponding ni-
Reaction of Excited Oxygen Atoms with
1341
N20
trosoethane reaction,Zg k7 determined by this procedure is actually a lower limit. The lower limit found for 1127 indicates that (7) is a very fast reaction and that therefore trifluoronitrosomethane is an effective scavenger for trifluoromethyl radicals. The scavenging sequence in the trifluoromethyl radical-nitric oxide system is clearly seen in Figures 5 and 6. Thus, the behavior of m / e 30 and 130 with time shows that (6) occurs exclusively until the nitric oxide has been so depleted and the trifluoronitrosomethane so abundant that (7) replaces (6) as the exclusive fate of the trifluoromethyl radicals. The onset of m / e 119 and 304 a t about 7 min when the trifluoronitrosomethane has been consumed shows that only after this time can the uninhibited photochemi-
cal decomposition mechanism via eq 2-5 take place. The almost complete inhibition of reactions 2-5 by (7) indicates; that trifluoronitrosomethane is a more effective scavenger for trifluoromethyl radicals than is nitrosomethane for methyl radicals28 or nitrosoethane for ethyl radicals.29
Acknowledgment. This work was supported in part by the United States Atomic Energy Commission under Contract No. AT(l1-1)-3416 and in part by the Petroleum Research Fund, administered by the American Chemical Society. We also thank the National Science Foundation for providing funds to assist in the original purchase of the time-of-flight mass spectrometer.
Reaction of Excited Oxygen Atoms with Nitrous Oxide. Rate Constants for Reaction of Ozone with Nitric Oxide and with Nitrogen Dioxide' J. A. Ghormley,
R. L. Ellsworth,2 and C. J. Hochanadel"
Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (Received December 75,1972) Publication costs assisted by the U.S. Atomic Energy Commission
The ratio of rate constants for reaction of O(lD) with 0 2 and with N2O was found to be 0.31 f 0.01, and assuming the absolute value for reaction with 0 2 to be 3.6 X 1O1O M - l sec-l, this gives 1.20 x 10l1 M-1 sec-l for reaction of O(1D) with N2O. This reaction proceeds predominantly by two paths giving either Oz or (b) 2NO. We found the ratio a / b to be 0.70 f 0.02. The rate constant for reaction of 03 (a) Nz with NO was 8.5 f 0.1 x lo6 M - l sec-l at 298"K, and for reaction of 0 3 with NO2 the rate constant was 4.7 f 0.3 x lo4 M - l sec-l a t 298°K. The O(lD) were produced by flash photolysis of O3 at low concentration in an atmosphere of 0 2 N20, and the rates of reaction of 0 3 with NO and with NOz, both produced in situ, were measured by means of kinetic spectrophotometry.
+
+
Introduction Reactions of electronically excited oxygen atoms, particularly the O(lD) which possesses 1.96-eV excess energy over the ground state O(3P), are of considerable interest in atmospheric chemistry, radiation chemistry, and other branches of chemical kinetics. We are particularly interested in the role played by O(lD) in O3 formation in the radiolysis of 0 2 , especially in the presence of other gases. Ground state 0 atoms are known to react slowly with molecules such as Hz, HzO, NH3, CH4, N20, et^.,^,^ whereas reaction with O(lD) is often close to the diffusion-controlled rate. The reaction with O(lD) may be deactivation as with Oz,5 N z , ~and COz,7 or chemical reaction as with the above list of molecules.8 The O(lD) can be produced by photolysis a t suitable wavelength of any of several oxygen-containing molecules such as 0 2 itself, 03,COz, NOz, N20, etc. In this paper we report results of a study of the reaction of O(lD) with NzO. The O(lD) atoms were produced by the flash photolysis of 0 3 a t low concentration ( - 10-610-5 M ) in mixtures of 0 2 and KzO a t a total pressure of 1 atm (-4 X 1W2 M ) . Under these conditions the system
is relatively simple, and allows evaluation of the rate constants of several important reactions. Following the initial decomposition of 0 3 by the flash, we observe, by means of kinetic spectrophotometry, three distinct processes leading to a net loss of 03.The fastest occurs with a half-time of -10 psec, the next -10 msec, and the longest -5 sec. We also observe the formation of NO2 on the 10-msec time scale, and its decay on the 5-sec time scale. The principal reactions in the sequence observed are the following OsL
O('D)
+
02('A)
(1)
(1) Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. (2) Student trainee under the Great Lakes College Association program from the College of Wooster, Wooster, Ohio 44691. (3) W. D. McGrath and J. J. McGarvey, Planet. Space Sci., 15, 427 (1967). (4) K. Schofield, Planet. Space Sci., 15,643 (1967). (5) J. F. Noxon, J. Chem. Phys., 52, 1852 (1970). (6) M. Loewenstein, J. Chem. Phys., 54, 2282 (1971). (7) L. F. Loucks and R. J. Cvetanovic, J. Chem. Phys., 57, 1682 (1972). (8) G. Paraskevopoulos and R. J. Cvetanovic, J. Amer. Chem. SOC., 91, 7572 (1969).
The Journal of Physical Chemistry, Vol. 77,No. 11, 1973
