The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
Reactlon of CH, with 0, 02,and NO
that the Stief study dealt with the same initial H2C0 wall-loss problems that we experienced (and in fact solved them in precisely the same manner). Thus if any significance is to be placed on the difference in calculated temperature dependencies from the two studies, it would have to be attributed to difficulties in handling such minute quantities of reactant necessitated by the rapidity of the reaction. The conclusions drawn by Stief e t al. regarding the stratospheric implications of this fast reaction rate are strongly supported by the results of the present study. At 230 K (-30 km), for example, the value we obtain for kl is 2.3 X lo3 times larger than that for k3. C1+ CHI
2HC1+ CH3
(3)
Since CH4has an approximate mixing ratio (V/V) of lo4 at midstratospheric altitudes, formaldehyde could be significant (210% as effective) in the removal of active Similar chlorine for mixing ratios in excess of 4 X conclusion can be drawn from comparison with removal of C1 by HOz above 40 km. I t is clear from the present results that further studies on the reactivity of other photooxidation products, as well as direct stratospheric measurements of their concentrations, are necessary to accurately assess the effect of stratospheric C1X injection on the ozone budget.
Summary Rate constants reported herein for the C1 + C H 2 0 reaction by the FPRF technique permit calculation of a simple Arrhenius expression over the temperature range 223-323 K. Results indicate a very rapid reaction with a near-zero activation energy. The role of this reaction in
2057
converting active C1 into the HC1 stratospheric sink could be significant for CH20concentrations presently calculated for the midstratosphere. The role of formaldehyde, as well as other hydrocarbon oxidation products in moderating the C10, catalytic cycle for O3 destruction, demands further study. Acknowledgment. This work was supported in part by the Upper Atmospheric Research Office of the National Aeronautics and Space Administration and the Fluorocarbon Research Program of the Manufacturing Chemists Association. Supplementary Material Available: Supplementary Table I contains the first-order decay rate data from which the kl values are calculated (3 pages). Ordering informations is available on any current masthead page.
References and Notes "Halocarbons: Effects on Stratospheric Ozone", Natlonal Academy of Sciences, 1976, and references contained therein. "Chlorofluoromethanes and the Stratosphere", NASA RP 1010 (1977). R. G. Manning and M. J. Kurylo, J . Phys. Chem. 81, 291 (1977). W. L. Chameides and R. J. Cicerone, J . Geophys. Res., 83, 947 (1978). (a) F. S.Lee and F. S.Rowland, J . Phys. Chem., 81, 684 (1977); (b) G. Poulet, G. LeBras, and J. Combourieu, ibid., 81, 2303 (1977). H. Niki, P. D. Maker, L. P. Breitenbach, and C. M. Savage, Chem. Phys. Leff., 57, 596 (1978). (a) L. J. Stief, J. V. Michael, W. A. Payne, D. F. Nava, D. M. Butler, and R. S.Stolarski, Geophys. Res. Lett., 5, 829 (1978); (b) J. V. Michael, D. F. Nava, W. A. Payne, and L. J. Stief, NASA Technical Memorandum 79675 (Nov 1978). M. J. Kurylo and W. Braun, Chem. Phys. Leff., 37, 232 (1976). M. J. Kurylo, N. C. Peterson, and W. Braun, J . Chem. Phys., 53, 2776 (1970). A. M. Bass, private communication. NASA Laboratory Measurements Committee Report (1978) updating recommendations in NASA RP 1010.
Temperature Dependence of the Reactions of Methylene with Oxygen Atoms, Oxygen, and Nitric Oxide C. Vlnckier" and W. Debruyn Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan ZOOF, 3030 Heverlee, Belgium (Received December 29, 1978; Revised Manuscript Received March 13, 1979) Publication costs assisted by the Nationaal Fonds voor Wetenschappelijk Onderzoek Belgium
Molecular beam sampling and subsequent mass spectrometric analysis have been used as detection techniques for methylene radicals produced in the oxidation process of acetylene with oxygen atoms in a fast flow reactor. Reactions of CH2with oxygen atoms, molecular oxygen, and nitric oxide are investigated in the temperature region between 295 and 600 K. For reaction 2, CH2 + 0, an activation energy of about 0 kcal mol-l has been found while for reaction 3, CH2 + 02,the value E3 = 1.5 f 0.3 kcal mol-' is derived. Reaction 4, CH2 + NO, shows non-Arrhenius behavior and has a negative activation energy, E4 = -1.1 f 0.4 kcal mol-'. In addition, complete Arrhenius expressions for the rate constants of reactions 3 and 4 are given and an attempt is made to determine the possible reaction products of these reactions.
Introduction The demand for accurate kinetic data of elementary reactions in the gas phase increased considerably since computer modeling techniques have been introduced in the field of air and (or) stratospheric pollution, combustion, etc. While extensive compilation and data evaluation programs of many elementary reactions exist,l only a limited number of studies on the methylene radical have been carried out. Since this radical is a major intermediate 0022-3654/79/2083-2057$0 1.OO/O
in the oxidation process of acetylene and other hydroc a r b o n ~ , ~its- ~kinetic behavior is important primarily because it may lead to the formation of highly unsaturated hydrocarbons in flames. In the past, reactions of methylene with various hydrocarbons have been studied and, in general, it undergoes an addition reaction to double bonds5 and an insertion into or an abstraction from single bonds.6 While most of the kinetic parameters were deduced from the formation rate of stable products, only in @ 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. IS, 1979
C.Vinckier
a few experiments CH2was directly detected by means of a spectroscopic7 or mass spectrometric t e ~ h n i q u e . ~More -~ recently in flash photolysis studies of ketene, rate constants of singlet and triplet methylene with several compounds such as C2H2,02,NO, CO, and Hz have been determined a t room t e m p e r a t ~ r e . ~ ~ ~ In this work, reactions of triplet methylene with oxygen atoms, molecular oxygen, and nitric oxide are investigated in a temperature interval between 300 and 600 K. Methylene is produced in the oxidation process of acetylene with oxygen atoms in a fast flow reactor and profiles of CH2 are followed by means of the molecular beam sampling technique and subsequent mass spectrometric analysis. Arrhenius parameters for these reactions are deduced and the formation of some reaction products is discussed. The reaction of CH, + NO has some fundamental importance since it is characterized by non-Arrhenius behavior.
Experimental Section The experimental setup was the same as used in the investigation of the room temperature reactions of methylene.12 A brief description of the apparatus is given below. Methylene was produced in the oxidation process of acetylene with oxygen atoms in a fast flow reactor (1.6 cm i.d.). Oxygen atoms were generated by flowing a 2% oxygen mixture in helium through a microwave discharge (2450 MHz, 75 W). In some cases they were produced in a NO + N titration reaction. A mixture of 1% acetylene in helium was introduced through a coaxial inlet (0.3 cm 0.d.) which was movable along the axis of the flow reactor. Typical experimental conditions were a total reactor pressure of 2.2 torr and a linear flow velocity of 23.5 m at room temperature. The temperature was variable between 295 and 600 K by means of heating tapes strapped around the reactor and was measured with a chromelalumel thermocouple inside and at the tip of the movable inlet. The pressure in the reactor was measured at the end of the kinetic zone with a Datametrics capacitance manometer. The stability of the flows was better than 5% over a 10-h period except for nitric oxide where it was about 10%. All reagents, prepared as mixtures in helium, purity 99.998%, were used directly from the cylinders. Concentration profiles of atoms, radicals, and stable species were obtained by molecular beam sampling and subsequent mass spectrometric analysis. Gas sampling occurs through a quartz probe with an orifice diameter of 0.03 cm, 1 cm height, and an apex angle of 100'. The molecular beam section consists of only one chamber evacuated with a 300 L s-l oil diffusion pump and a torr is maintained during the expressure of 3 X periments. A skimmer orifice (0.2 cm diameter), located 4.3 cm downstream of the sampling orifice, allows a fraction of the molecular beam to enter a second chamber evacuated with a 600 L s-l oil diffusion pump in combination with a CCT 100 liquid nitrogen trap (Vacuum Generators). In the absence of a molecular beam, a pressure of 1 x torr is reached while a 2-torr nitrogen torr. An beam results in a chamber pressure of 6 X Extranuclear quadrupole mass spectrometer is mounted inside this chamber with its electron impact ionizer located 8.3 cm downstream of the sampling orifice. The main characteristics of the whole instrument can be summarized as follows: (1)The overall effects of mass discrimination in the molecular beam are found to be small and only minor corrections have to be applied. (2) Traces of methane in helium at 2-torr reactor pressure and at 295 K gave a sensitivity of (3.2 X X pV X (particles ~ m - ~ ) - ' or 210 pV/ppm under normal conditions. (3) The de-
and
w. Debruyn
5 OC
4 00
-> -
300
I
c. N
7
N
m" 2 00
100
0
1
2
3
103/T( K )
Figure 1. The acetylene signal in the fast flow reactor at 2.2 torr as a function of the inverse of the temperature T .
tection limit for most of the hydrocarbon radicals lies around 6 X lo9 particles ~ m - ~(4). When a signal, SM,T, of component M was followed as a function of the temperature T i n the reactor at constant pressure, it was found that SM,T
= KMIMl
ToTO/
(1)
where [MI represents the concentration of M expressed in particles ~ m - To ~ ,is room temperature, and KM is a calibration factor which has to be determined for each component. This relationship is illustrated in Figure 1for M = C2H2,where SM,T as a function of 1/T results in a straight line going through origin. This behavior is expected when sampling occurs under effusive flow conditions and when the calibration factor is independent of temperature. Since [MIT = [MIT,,(To/T), eq I leads to SM,T
= KM[MlT
(11)
In order to avoid fragmentation of stable components, radicals or atoms were measured at electron energies of a few electronvolts above the ionization potential; for CH2 the value of 13.5 eV was selected. A statistical analysis of the results was made by using the method of least squares and 95% confidence limits were determined with the Student's t method.
Results and Discussion Activation Energy of the Reaction CH2 + 0. When acetylene is not present in great excess, the formation and disappearance of CH2 in the oxidation process of acetylene with oxygen atoms is governed by the following reaction ~equence:~~J~ C2Hz + 0 CH2 + CO (1) CH2 + 0 CO + 2H t 2) which leads, under steady-state conditions, to [CH2I,, = kl[C2H,I/k2 (111)
-
+
It should be noticed that reaction 1 is sufficiently exothermic to form both triplet ground state and singlet
The Journal of Physical Chemistry, Vol. 83,No. 16, 7979 2059
Reaction of CHz with 0, OZ,and NO
c
I
9
15
25
20
30
35
1 03/ T K
Figure 2. Logarithm of the product of the methylene signal and Tas a function of 1/T. Total pressure 2.2 torr. [CZH,Ip = 9.9 X l o i 3 molecules cm3; [O],, = 1.6 X 1014 molecules cm- .
excited state methylene. However, the production of lCH2 will be less favorable since it violates the spin-conservation rule in reaction 1, However, even if reaction 1would partly yield T H 2 ,its deactivation process with helium is very fast under our experimental conditions7 and it can be calculated that at t = 0.2 ms the ratio [1CH2]/[3CH2]drops below Additionally the reactivity of T H 2 and 3CH2 with molecular hydrogen allows us to distinguish between both states7-’l and, on this basis, it was experimentally shown that methylene present in the CzH2-0 system is in the ground triplet state.15 To check the validity of eq 111, the CH2 signal was followed as a function of [CzH2]and it was found indeed that, at a reaction time of 5.2 ms, [CH218, [CzH211when [C2Hz]varied between 3.3 X 1013 and 2.3 X 1014molecules ~ m - A ~ .similar behavior for CH2 as a function of acetylene has already been reported in the literature.2 It is clear, however, that eq I11 only holds as long as [C2H2]remains practically constant or hl[O] t I 0.1. When the oxygen atom concentration was varied between 3.3 X 10l2 and 3.3 X 1013 atoms ~ m - the ~, methylene signal remained almost unaffected. This indicates that the destruction of CH2 occurs uniquely in a reaction with oxygen atoms so that loss processes on the reactor wall may be neglected. Also reactions with other compounds formed in the system are unimportant since their concentrations are about a factor of 100 lower than [O]. Additionally, measurements of the radial concentration profile of CH2 revealed that its concentration at the center of the reactor ( r = 0 cm) was only 10% higher than at 0.2 cm from the reactor wall ( r = 0.6 cm), so that the radial CH2profile can be considered as flat which again is a strong indication that loss processes of CH2 on the reactor walls were unimportant. As products of reaction 2, CO and atomic hydrogen were unambiguously established since their rate of formation deduced from their concentration growth profile was equal to the atomic oxygen removal rate. After inserting the appropriate temperature dependences of reaction rate constants and concentrations in eq I11 and by combining it with eq 11, one obtains
-
n
with C a constant equal to KcH,[C,H,] ToTO(h~,l/h~,2), where [C2H2ITo is the concentration of C2H2measured at room temperature To, and ko,l and ho,2are the preexponential factors of the rat,e constants of reaction 1 and 2, respectively. When the CH2 signal was followed as a function of the temperature with [C2Hz]To= 9.9 X 1013molecules , seen cm-3 and [ o ] T o= 1.65 x ioi4 molecules ~ m -it~was
- 30
I
I
15
20
I
I
I
I
25
I
30
35
l8iTi
K) Figure 3. Logarithm of the rate constant kl of the reaction C2H2 0 as a function of l / T i n two mixtures. Total pressure 2.2 torr. ( 0 ) I n excess of acetylene: [CzHz] = 5.2 X loi4 molecules c m 3 . (X) I n excess of oxygen atoms: [O?,, = 3.1 X 1014 atoms ~ m - ~ .
+
TABLE I: Activation Energies E , for the Reaction C,H, t 0 E ,I kcal temp region, mol-’ K 3.2 3 3.15 3 3.7 3.2
230-450 243-673 273-729 1300-1700 290-600
method flow tube flow tube flow tube recommended value flames flow tube
ref 16
17 18 19 20 this work
that methylene increases markedly when T varies between 290 and 600 K which indicates already that E 2 will be smaller than El. When [C2H21T,,remains constant, eq IV can be written in logarithmic form as
(El - E,) RT The left-hand side of eq V plotted vs. 1/T gives a straight line in Figure 2 and the slope yields directly (E, - E,) = (3.3 f 0.4) kcal mol-l. In order to evaluate E2 the value of El should first be determined. The values of hl at In (ScH,,,T) = In C -
various temperatures were deduced from the pseudofirst-order decay either of acetylene in an excess of oxygen atoms or oxygen atoms in an excess of acetylene. Figure 3 shows In kl as a function of 1/T and from its slope one can derive the value of El = (3.2 f 0.2) kcal mol-l. Several authors have also reported values of El (Table I). By comparing El with ( E , - E2) one may conclude that the activation energy of reaction 2 is rather insignificant and is equal to (-0.1 f 0.2) kcal mol-l. In view of the fact that E2is very small and is moreover deduced from the difference between two large numbers with about f10% uncertainty on each, a kinetic interpretation of a negative activation energy of 0.1 kcal mol-l seems inappropriate. Since the rate constant of reaction 2 at room temperature molecule-l cm3 s-l,12a very was already (1.3 f 0.3) X small activation energy had to be expected, and as a good approximation E2may be set equal to zero. There are no values of E, in the literature to compare with but a similar reaction such as CH3 + 0 shows also very little temperature dependence.21 It should be emphasized that, in this method for the determination of hz or E2, only relative values of the CH2 concentration had to be known and no calibration factor for CH2 intervened. It is clear that the value of h2 itself plays a crucial role in the determination of the preexponential factors of the reactions investigated in this work. The “approach to steady-state” method was
2060
The Journal of Physical Chemistty, Vol. 83,
No. 16, 7979
C. Vinckier and W. Debruyn
TABLE 11: Rate Constants k, for the Reaction CH, t 0, in the Temperature Region between 295 and 600 K
T,K
0
15
30
45
60
[021/[01
Figure 4. The left-hand side of eq VI1 plotted vs. [ O , ] / [ O ] . Total pressure 2.2 torr. The temperatures Tare (0)514 K, (X) 363 K, and (0) 326 K. The appropriate concentrations are given in Table 11.
used to derive k 2 under conditions where reaction 2 was the dominant loss process for methylene radicals. The only reaction which might possibly interfere is 3CH, C2H2. Recent literature data22confirm, however, that its rate constant is about a factor of 100 lower than k2 if reaction 1yields uniquely CH, as a primary product. Also, the fact that the stoichiometric coefficient is 1.8,,, even when [C2H2]/[O]= 50, proves that the removal of CH2 with C2H2is a slow process. Finally when CH, was followed as a function of the acetylene concentration, it was found that eq I11 remains perfectly valid for [C2H2]/ [ 01 ratios up to 40. This definitely shows that the rate constant of the reaction CH2 + C2H2is at least a factor of 40 smaller than k, so that this reaction remains unimportant under our experimental conditions. Activation Energy of the Reaction CH2 + 02. When various amounts of molecular oxygen are added, an additional destruction path for CH, has to be taken into account: CH, + 0, products (3)
+
-
and a term has to be added in the denominator of eq 111:
In the absence of molecular oxygen, [CH2],s,ois equal to kl[C2H2]/h2so that eq VI can be rewritten as
To deduce the values k3/k2, the left-hand side of eq VI1 should be plotted as a function of [O,] / [O] as is shown in Figure 4 for temperatures of 326,363, and 516 K. As only ratios of signals or concentrations are plotted, it is clear that no temperature corrections have to be taken into account. Since the rate constant k 2 is known, k3 can be calculated directly from the slopes of curves at various temperatures. In Table 11, the values of k3 are given at different temperatures and under a wide variety of mixture compositions. Two values of k3 at room temperature which are available in the literature were added to Table 11. They have been determined with an entirely different experimental technique, namely, where flash photolysis of CH2-CO is used as a source of CH,. When a graph is made of In k3 as a function of 1/T, Figure 5, the activation energy, E3 = (1.5 f 0.3) kcal mol-l, could be derived. In CO/O2 flames containing acetylene, somewhat higher values for E3 have been reported: 2.35 kcal mol-l between
10IZk, 10'4[C,H,]~ 10'3[o]~ (molecule" (molecules (atoms cm3 s - ' ) cm-,) )
295
1.5 1.7 1.5' 1.2&
4.9 5
1.4 0.83
326 356 363 368 404 445 441 488 513 514 546 565 513
2.3 2.0 3.0 2.8 3.5 3.3 3.0 3.7 3.8 4 5.2 7.6 6.8
1.3 3.6 1.1 1.0 1.9 0.86 0.46 0.42 0.40 0.40 0.38 0.36 0.61
6.3 2.9 4.6 4.5 4.8 3.3 4.4 4.1 3.8 3.4 3.1 3.5 2.4
-255 -
The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
Reaction of CH, with 0, 02, and NO
2061
TABLE 111: Rate Constants for the Reaction CH, t NO in the Temperature Region between 295 and 600 K 1014. 1013. 1014. p, [ C , H , ] T ~ [ O ] T ~ [ N O ] T ~ torr
1011.
T,K
kqa
293
2.0 2.2 1.6 1.4 1.6'
5.3 5.3 5.3 5.3
1.7 2.2 1.8 2.8
5.1 4.7 4.7 4.7 4.7 5 4.5 4.2 4.2 4.2 1.9 4 3.9 2.4 3.9 1.8 3.5 3.5 3.5 1.6
1.6 2.6 3.3 .90
2.2 4.2 2.2 2.2
1.5
1.08
1
0
I
I
1
I
I
10
5
15
I
1 20
302 328 330 1
25
I
331
[N 01 [OI
1.1
Flgure 0. The left-hand side of eq VI11 plotted vs. [NO]/[O]. The temperatures Tare (X}514 K, (0)331 K, and (0) 293 K. The appropriate concentrations and total pressures are given in Table 111.
346 366 368
ethylene, and methane. Whether CH2 disappears in a reaction with O2 or oxygen atoms depends primarily on the [02]/[0]ratio. Indeed, a rate constant of h3 = (2.2L;i) X exp[(-1.5 f 0.3) kcal mol-'/RT] molecule-' cm3 s-l can be derived from Figure 5, which leads to a value of about 1.5 X molecule-' cm3 s-l at 2000 K. Since in can exceed easily a some lean flames the ratio [02]/[0] factor of 10, reaction 3 becomes a considerable removal process of CH2 under those conditions. Activation Energy of the Reaction CH2 + NO. In the same way as the reaction between methylene and O2 was investigated, one may add various amounts of nitric oxide to determine the rate parameters of reaction 4 and an CH2 NO products (4)
370 387 400
+
-
equation analogous to eq VI1 can be written:
1.9 0.96 1.5 0.95 1.4
407 445 453 479 496 50 3 514 548 555 588
1.5 1.2
1 0.88 1.3 0.91 1.4 1.4 0.77 0.88 0.58 0.48 0.67 0.55 0.60 0.55 0.67 0.90 1.2 0.99 0.83 0.64 0.54
1.1
1.1 2.2 0.89 1.7 0.66 2.3 3.6 3.8 1.7 3.1 0.37 3.3 1.8 0.34 1.7 3.1 3.3 1.7 1.7 1.7 0.38 3.0
1.5 3.1 3.1 3.1
1.1 1.4 2.8 2.8 2.8
' Molecules-' cm3 s-'.
2.2 2.2 3.4 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.4
1.5
1.5 0.99 1.6 1.8
1.1 1.1 1.8 1.3 1.8 1.3 1.3 1.3 1.3
4.4 0.9 2.2 2.2 2.2 2.2
1.3
1.1
1.1
1.3
1.3
1
1.1
Molecules crn'j.
Atoms
~ m - ~ ,
where [CH2],,,o now represents the CH2 steady-state concentration in the absence of NO. Although the reaction NO + 0 is a very slow process in the gas phase,' it was found that an added amount of 3 X 1014molecules cm-3 results in a decrease in 0 atom concentration of about 50%,probably due to some wall reactions. Therefore NO was fed to the system together with the C2H2flow through the coaxial inlet. The left-hand side of eq VI11 plotted vs. [NO]/[O] is shown in Figure 6 for a series of mixtures at 293, 331 and 514 K. Straight lines are obtained whose slopes are directly equal to k4/k2. It is apparent from this graph that the slopes diminish with increasing temperature and, since k2 is almost independent of temperature, k4 should decrease or reaction 4 has a negative activation energy. The obtained rate constants k4 as a function of various experimental conditions are given in Table 111. To understand this table, some additional explanation is needed. The ratio [NO]/[O] was changed either by adding NO at constant [O] or by keeping [NO] constant while changing the reaction time. In the former case [NO] is not shown in the table since it was a variable, while in the latter case NO concentrations are given with corresponding [O] at t = 1.7 ms and it is clear that the variation of [ O ] as a function of reaction time depends on the initial acetylene concentration. In most cases [O] drops at least a factor of 3 between 1.7 and 10 ms. The total pressure is also given since a few experiments were carried out a t a helium pressure different from 2.2 torr. The two literature values at 295 K were again determined in flash photolysis studies of ketene. Laufer and Bass report that k4 is independent of the helium pressure from 20 to 700 torr,8 while Pilling
- 2L.O
I
t -2701 15
I
1
20
I
1
I
25
1 0 3 / ~ (K )
i
30
1
1
35
+
Figure 7. Logarithm of the rate constant k , of the reaction CH, NO as a function of 11T. Experimental conditions are the same as in Table 111: (full circle) ref 7; (triangle) ref 8. When the reactor pressure was different from 2.2 torr it is indicated by the number in parentheses.
and Robertson found k4 to decrease from 1 X at 200 torr of argon to 5 X molecule-' cm3 s-' at 10 torrsg The variation of the pressure in our experiments between 0.9 and 4.2 torr seems too small to allow any conclusion about the pressure dependence of k4. When In k4 is plotted vs. 1/T, Figure 7, a rate constant of k4 = (2.3:;f) X exp[(+l.l f 0.4) kcal mol-l/RT] molecule-l cm3 s-' is found. The concept of a negative activation energy is mostly used in recombination reactions and has to be associated automatically with an activation energy of the reverse decomposition reaction which should be smaller than the endothermicity of the reaction. Only in a few
2062
The Journal of Physical Chemistry, Vol. 83,No. 16, 1979
cases a negative activation energy for bimolecular reactions has already been reported and most of them involve reactions of atomic oxygen with olefinsz6or a reaction between NO and a radical such as in the reactions C10 + ( E = -0.49 kcal mol-') and NH2 + NOz7( E = -1.05 kcal mol-l). While calculations based on the transition state theory showed a slight negative temperature dependence for the reaction CO + OH in the temperature region from 200 to 500 K,28the temperature dependences of the partition functions are not sufficient to explain the magnitude of the negative activation energy. An alternative explanation has been presented by Singleton and C v e t a n o ~ i cwhere ~ ~ the formation of intermediate complexes has been postulated, such as, for instance in this case [CH2.NO] with a potential barrier for the reverse reaction to form again CH2 + NO somewhat higher than the barrier to form the final reaction products. It was very difficult to gather any evidence concerning the nature of these reaction products since too many products from the C2H2-0 reaction itself interfere, such as, an isotope of C2Hz with HCN. Also the adduct CHzNO,if formed at all, must disappear in a fast reaction since its signal has the same magnitude as the 40-kV background signal of very minor traces of C 0 2 produced in this system. In view of the absence of better information on the products of reaction 4,a discussion on the importance of this reaction in the removal of NO in combustion processes is of pure speculation. Indeed, if for instance HCN were formed, it will be oxidized rapidly at flame temperature with 0 atoms and NO will appear again as final reaction product. Conclusions Molecular beam sampling and subsequent mass spectrometric analysis of the reaction products of C2Hz+ 0 revealed that this system provides an excellent source of methylene. The kinetic parameters of reactions with oxygen atoms, molecular oxygen, and nitric oxide are determined in the temperature region from 295 to 600 K. It should be pointed out that the rate constants deduced in this work are obtained by following directly the CHz profile but without having to know its absolute concentration. Although an attempt was made to determine products of the investigated reactions, the experimental technique used seems not to be suited for that purpose since too many components present in the C2Hz + 0 system itself interfere with possible products of reactions 3 and 4.
C. Vinckier and W. Debruyn
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