J. Phys. Chem. 1994,98, 6324-6326
6324
Formation and Reactions of Ethylidyne Zhengli Hod and Kyle D. Bayes. Department of Chemistry dt Biochemistry, University of California, Los Angeles, Los Angeles, California 90024 Received: March 3, 1994.
W e have attempted to form ethylidyne (CCH3) by the multiphoton dissociation of l,l,l-tribromoethane. A neutral intermediate is formed that will react with a ground-state oxygen atom to form a chemi-ion with a mass-to-charge ratio of 43. By analogy with the known behavior of CH, we propose that the,observed chemiions are formed by reaction 1, CCH3 O(3P) CHsCO+ e-. Using the rate of formation of chemi-ions as a surrogate for the ethylidyne concentration, rate constants for CCH3 could be measured. Ethylidyne reacts rapidly with 0 2 and NO, with rate constants comparable to those observed for CH(a42-). The ethylidyne does not react with N20 or N2, again similar to what is observed for CH(a42-) and in sharp contrast to the behavior of ground-state CH(XZII). Guided by its kinetic behavior and by previous ab initio calculations, we conclude that the ethylidyne is in its metastable quartet state, CCH3(g4Az).
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+
Introduction Ethylidyne (CCHs), also called methylcarbyne, has not yet been identified as a free molecule in the gas phase. Ethylidyne is recognized as a bound structure on surfaces, being formed by the dissociation and rearrangement of ethylene absorbed on some metals.' Some years ago Vinckier et a1.2 speculated that the primary chemi-ionobserved at a mass-to-chargeratio of 43 during the oxidation of 2-butyne might be the result of reaction 1. CCH,
+ O(3P)
-
+
CH3CO+ e-
(1)
This reaction would be the methyl analog of the Calcote reaction, CH
+ O('P)
-
HCO'
+ e-
(2)
which is thought to be the dominant source of chemi-ions during hydrocarbon combustion.3 Reaction 2 can occur with either ground-stateCH(X2II) or with metastableCH(a4Z-)." Ab initio calcuations have probed the energetics and geometries of ethylidyne and the vinyl radical, both on the ground doublet and metastable quartet surfaces.' The CH radical can be generated in the gas phase by multiphoton dissociation of bromoform at 193 nm.53 We have attempted to form ethylidyne by an analogous process, photolyzing 1,l ,I-tribromoethane.
Description of Experiments The apparatus has been described in previous reports.5~~ Briefly, twogas streams were mixed just beforeentering the region between two parallel gold-coated electrodes. Onestream contained oxygen atoms, formed upstream by a microwave discharge in 1% COz in He. Partial pressures of oxygen atoms were in the range of 1-3 mTorr. The second gas stream contained l,l,l-tribromoethane (- 5 mTorr), made by the synthesisof Stengle and Taylor? plus additional reactants diluted in He. Gas mixtures were made up manometrically (MKS Baratron) in 12-L bulbs, and absolute flows were calculated by following the pressure drops in the reservoir bulbs. A pulsed excimer laser (20 mJ at 193nm) was focused midway between the two gold electrodes, just in front of a small pinhole in one of the electrodes. A uniform electric field between the two f Present address: Department of Biochemistry and Biophysics, Iowa State University, Ames, IA 5001 1. * Abstract published in Advance ACS Absfracts, June 1, 1994.
0022-3654/94/2098-6324304.50/0
+
electrodes (240 V, 1.95-cm separation) forced the positive ions toward theelectrode containingthe pinhole. Ionspassing through the pinhole entered a quadrupole mass spectrometerset to transmit only one mass-to-charge ratio, normally 43 amu. The transmitted ions were counted on a multichannel analyzer as a function of time after the laser pulse. Signals from many laser shots, typically 5000, were added to give one data set. After subtracting a background data set, taken for identical conditions except with no oxygen atoms present, the numbers were fit to an exponential decay using weighted least squares, assuming Poisson statistics. Using the applied electric field and typical ion mobilities,"J one can calculate that positiveions formed between the electrodes will migrate to the electrode within about 5 ps. Transport of the ions through the quadrupole mass spectrometer requires another 25 ps. The early channels were flooded with counts, probably due to multiphoton ionization by the intense laser pulse. However, simple exponential decays were observed at mass 43 for times longer than about 100 ps. The 95% confidence limits for the decay rates were calculated using Student's r distribution. Rate constant uncertainties are determined from 95%confidence limits based on the scatter of points, plus an additional estimated uncertainty of 15% in absolute concentrations.
Observations Ion signals could be observed at many masses soon after the laser pulse: However, most of these signals decayed rapidly, with time constants of lo5 s-I, as expected for ions formed during the 204s laser pulse. Only the mass 43 (C2H3O+) and mass 29 (HCO+) ions showed significantly slower decays. Typical data sets for the mass 43 ion, with and without oxygen atoms, are shown in Figure 1. The decay of the mass 29 ion has been treated in previous papersss6 and will not be discussed here. When the twodata sets in Figure 1 aresubtracted, thedifference signal shows a good exponential decay (upper trace in Figure 2). The slope of such lines, 4.1 X 103 s-' in this case, was the primary measure of the kinetics in this system. Such slow decays for mass 43 were only observed when both oxygen atoms and l , l , l tribromoethane were present and the laser was focused in the vicinity of the pinhole. In general, the mass 43 signals were weaker than those we reported previouslys.6 for mass 29 chemiions, which used photolysis of CHBr3 as the source of CH. The photolysis of l,l,l-tribromoethane can and probably does form products other than C2H3 (+3 Br). Addition of 0 2 or NO accelerated the decay of the mass 43 signals (Figure 2). These reactants increased the rate of decay,
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0 1994 American Chemical Society
Formation and Reactions of Ethylidyne
The Journal of Physical Chemistry, Vol. 98, No. 25, 1994 6325
t
102 7
1
u
lo;:0
.
*
'
0.2
'
0.4
'
'
0.6
0.8
time I ms Figure 1. Counts at mass 43 as a function of time after the laser pulse, with (m) and without ( 0 )oxygen atoms. Total pressure was 3.9 Torr with no added 02.Counts were summed for SO00 flashes.
0
0.5
1
1.5
2
No /mrr
Figure 4. Decay rates of the mass 43 signal as a function of the partial pressure of added NO. Total pressure was 3.7 Torr. The signals during these experiments were weaker than normal due to a depleted reservoir of CBroCH,.
1
I lo;!O
'
0:2
'
0:4
016
'
'
0!0
time I ms Figure 2. Dccay of the mass 43 signal as a function of time for 02 partial pressures of 0 (m), 3.75 mTorr ( O ) , and 7.92mTorr ( 0 ) .The difference between the two data sets shown in Figure 1 are used for the zero 02 decay.
9 d
2
-
3"
0
"
2
"
4
'
6
"
0
'
0 2 /rdorr Figure 3. Decay rates of the mass 43 signal as a function of the partial pressure of added 02.Total pressures were 3.94.1 Torr.
without reducing the extrapolated zero-time amplitude of the ion signal. This shows that the added reactants are removing a neutral precursor, i.e. C ~ H Jrather , than removing the mass 43 ions by ion-molecule reactions. The decay rates were linear in the added gas concentration, as shown in Figures 3 and 4. Addition of large amounts of N20 or N2 did not increase the mass 43 decay rates (Figure 5 ) .
Discussion We attribute the slowly decaying mass 43 signal to reaction 1 for the following reasons.
0
100
a00
300
N20 or N2 /mTorr Figure 5. Decay rates of the mass 43 signal as a function of the partial pressure of added NzO (A)or N2 (0). Total pressure varied from 3.3 to 3.7 for NzO and from 3.7 to 3.8 Torr for N2.
(1) Any ions formed directly by the laser pulse should be removed rapidly by the applied electric field. In the absence of added reactants, the mass 43 signal was observed to decay with a time constant of several hundred microseconds. Therefore, these ions are being formed by the reaction of neutral particles. (2) The slowly decaying mass 43 ions were only observed when ground-state oxygen atoms were present. If a mass 43 chemi-ion is formed by reaction of an oxygen atom (mass 16), the other reactant must have mass 27 (e.g. C2H3). (3) The slowly decaying mass 43 signal was only observed when photolyzing l,l,l-tribromoethane. This precursor could form CCH3 by simple bond cleavage. Photolysis of methyl vinyl ketone, a known source of vinyl radicals," gave no slowly decaying signal at mass 43. (4) Addition of molecules that are expected to react with ethylidyne, such as 0 2 and NO, .increased the rate of decay of the mass 43 signal. The decay rates were linear in reactant concentration (Figure 3 and 4), as expected for pseudo-firstorder kinetics. The recent ab initio calculations by Schaefer and co-workers7 on the relative energies of the ethylidyne-vinyl radical system are very relevant to the current experiments. A summary of their findings is given in Figure 6, where the energy scale has been adjusted to the known heat of formation of the vinyl radical (70 kcal mol-') and a ground-state oxygen atom (59.5 kcal mol-1).12 The dashed lines indicate the calculated energy barriers for isomerization on the lowest doublet and quartet surfaces. Using the known12 heat of formation of CHsCO+, one can calculate that, for reaction 1 to be exoergic, the heat of formation
Hou and Bayes
6326 The Journal of Physical Chemistry, Vol. 98, No. 25, 1994 1
TABLE 1: Com risen of Rate Coastants for Reactions of CH(a4E), CH(X26, and tbe hecursor to the Mass 43 Chemi-ion, Here Denoted as CCH3 reactant 0 2
NO N20
$1
\
CCH, 1.4 i 0.5 2.8 A 1.6
C0.02
CH(a4Z) 2.6 4.2
CH(X211) 5.1
