J. Phys. Chem. 1993,97, 1896-1900
1896
Rate Constants for the Reaction of CH(a4E) with NO, N2, N20, CO, COz, and H2O Zhengtin Hod and Kyle D. Bayed Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024 Received: September 18, 1992; In Final Form:December 7, 1992
Rate constants for the reaction of the metastable state of methyne, CH(a42), with six molecules are reported. The CH(a42) is made by multiphoton dissociation of bromoform in the presence of oxygen atoms and methane. The decay of chemi-ions is used to monitor the time-dependence of the CH(a42) concentration. The rate constant for the reaction of CH(a42) with NO is (4.2 0.7) X cm3 molecule-' s-'. Carbon monoxide interacts more slowly with CH(a42),with a pressure independent rate constant of (8 f 2) X l&13 cm3molecule-' s-l. Only upper limits could be set for rate constants of CH(a42) reacting with N2, N20,C02, and H20. Comparisons of rate constants for the isoelectronic CH(a42), N(4S), and CH(X211) suggest that for reactions with closed shell molecules, CH(a4Z)behaves more likeN(4S) than like CH(X211). For the open shell molecules NO and 02, both CH(a42)) and CH(X211) react rapidly. Methyne is another example of a molecule in which the high-spin state is less reactive than the low-spin state.
Introduction The first electronically excited state of methyne, CH(a4Z), lies just 0.74eV above the ground state, CH(X*II).' Only recently has CH(a42) been detected directly in the gas phase by laser magnetic resonance (LMR).Z Since the reaction of oxygen atoms with acetylene gave good LMR signals, and since acetylene is a common intermediate in flames,3 it is probable that CH(a4Z) is formed during the combustion of most hydrocarbons. Rate constants will be needed to evaluate the importance of CH(a42) to flame chemistry. Evidence has been presented that the metastable CH(a42), as well as ground state CH(XZII),can react with oxygen atoms to form ~hemi-ions:~
0 + CH ---c HCO'
+ e-
(1) At room temperature, the contribution of CH(a4Z) to ion formation can be identified by adding an excess of methane, which reacts rapidly with CH(X211) but not with CH(a42).5*6 The rate of chemi-ion formation has been used as a surrogate for the time-dependentCH(a4Z)concentration.6 A rateconstant for the reaction of CH(a42) with 0 2 was measured and upper limits were set for CH(a4Z) reacting with CH4, Hz and D2.6The present study continuesthese measurementsusing other reactants.
Description of Experiments The apparatus was the same as used in our previous report.6 The CH(a4Z) was formed by the multiphoton dissociation of CHBrj at 193 nm. Two parallel gold-coated electrodes formed the ends of a cylindrical photolysis cell (diameter 4.5 cm, spacing 1.95 cm). Approximately 10-20mJ of radiation from an excimer laser (Lambda Physik 102EMG) was brought to a focus midway between the electrodes using a quartz lens of 20 cm focal length. A uniform electric field pushed the positive ions toward one electrode which contained a conical pinhole of 0.2 mm smallest diameter. Those ions passing through the pinhole entered a quadrupole mass spectrometer set to transmit only the mass 29 ions. Ions passing through the quadrupole were accelerated to a Daly doorknob7(-1 7 kV), and the resulting secondary electron pulses were detected by a scintillator (New England Nuclear, Pilot B) coupled to a photomultiplier (EM1 9524B). Amplified pulses were sent through a lower-level discriminator and then Permanentaddress: Chemistry Department, Shandong University, Jinan, Shandong, PRC. +
0022-3654/93/2097-1896$04.00/0
stored as a function of time after the laser pulse (IBM XT computer with an Ortec ACE-MCS board). The total signals and background signals, taken with the oxygen atom-generating discharge turned off, were as shown previously.6 The chemi-ion signals were taken to be the difference between the total signals and the background signals. These chemi-ion signals showed good exponential decays starting about 100 ps after the laser flash. Decay rates were calculated by nonlinear least squares, using Poisson statistics to calculate the weighting factors. Rate constants were calculated from the slopesof plots of decay rates vs partial pressures of reactants using weighted least squares. Pseudo-first-order kinetics were assumed. The error limits reported for rate constants are 95% confidence limits based on the scatter of points (Student t ) , combined with an estimated 15% systematic error in determining absolute reactant concentrations. Two gas flows were combined before entering the cell. One stream contained CHBrj, CH4, and other reactants, diluted in He. The second stream was a mixture of 1% CO2 in He, which passed through a microwave discharge 20 cm upstream of the reaction cell. Mixtures were made manometrically except for the 1% COZin He (99.995%), which was a commercial mixture (Matheson). Total flows were measured by timing pressuredrops in 12 1 bulbs and were in the range 0.5 1 s-I at 2 Torr to 1.3 I s-I at 8 Torr. The replacement time for the gas within the photolysis cell ranged from 60 ms at 2 Torr to 25 ms at 8 Torr. The laser usually operated at a repetition rate of 40 Hz; operation at 10 Hz gave identical decay times. All experiments were done at 294 f 2 K. In the absence of added reactant, the decay of the signal depended on the total prwureand the oxygen atom concentration. Higher total pressures resulted in slower decay rates, because of the slower rate of diffusionaway from the region near the pinhole. At a given total pressure, increasing the COz flow through the discharge resulted in slightly faster decays; however, the presence of the large gold surfaces prevented quantitative determination of the oxygen atom concentrations. The bromoform (Aldrich 99%) and water were degassed by freeze-pumpthaw cyclesbefore being evaporated into the mixing bulb. Nitric oxide (Matheson 99%) passed through a dry ice trapbeforeuse. Othergases wereusedasreceived: He(Matheson 99.995%);CH4 (Matheson 99.97%); Nz (Air Products99.999%); N20 (Liquid Carbonic 99%);CO (Matheson 99.5%);CO2 (Liquid Air 99.8%). Q 1993 American Chemical Society
Rate Constants for CH(a42) Reactions
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1897 12 I
0
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I 0
Nitric Oxide / mTorr Figure 1. Decay rates of the chemi-ion signal in the presence of various amounts of nitric oxide. Total pressure was 2 Torr, CHBr3 3 mTorr, CHI 1 IO mTorr, CO2 18 mTorr, and applied voltage 157 V. Different symbols represent runs on two different days.
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Time / ms
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Total Pressure /Torr
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150
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Carbon Monoxide / mTorr
'on
r 4 t0 1
50
Figure 3. Decay rates of the mass 29 signal at 8 Torr as a function of partial pressure of CO. The filled squares are the decays shown in Figure c
l
f
Figure4. Rateconstantsfor theremovalofCH(a4Z) by COasa function of total pressure. The filled square is the slope of the line in Figure 3.
Figure 2. Chemi-ion signal at mass 29 as a function of time after the laser pulse for various partial pressures of carbon monoxide: none; 46 mTorr; 0 104 mTorr; 0 164 mTorr; A 230 mTorr. The CHBr3 partial pressure was 2.7 mTorr, CH4 104 mTorr, CO2 18 mTorr, and the total pressure increased from 7.84 to 8.00 Torr as the CO was added. The applied voltage was 291 V, dwell time 30 ps, and signals from 5000 laser pulses were summed for each run. Solid lines are weighted least-squares fits. o
Observations The addition of small partial pressures of nitric oxide increased the decay rate of the mass 29 chemi-ion signal. The decay rates increased linearly with the NO concentration (Figure 1). Since these small concentrations of NO should not affect the oxygen atom concentration,the slope of the line in Figure 1 gives directly a rateconstant for reaction 2 of (4.2 f 0.7) X lo-" cm3molecule-l S-'.
CH(a4Z)
+NO
-
products
(2) Carbon monoxide also increased the decay rate of the chemiion signal, but much less efficiently than NO (Figure 2). Since we suspected a pressure dependent rate constant, measurements with CO were made at different total pressures from 2 to 8 Torr. At each pressure the decay rates were observed to increase linearly with CO concentration (Figure 3), so the slopes were taken to be the rate constants. No significant variation of rate constant with total pressure was observed (Figure4). All thevalues shown in Figure 4 were averaged to give a rate constant for reaction 3 of (8 i 2) X lP'3 cm3 molecule-' s-I.
CH(a4Z) + C O
-
products
(3) Addition of N2 had no effect on the mass 29 chemi-ion decay rate at 2 or 4.5 Torr total pressure (Figure 5). A linear leastsquares treatment of this nitrogen data yields a rate constant of (1.5 f 4.3) X cm3molecule-l s-I. Since the observed value
40
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200
n .
0
"
"
400
600
. 800
1000
Nitrogen I mTorr Figure 5. Decay rates of the mass 29 signal in the presence of various amountsof nitrogen. Partial pressures: CHBr3 2.8 mTorr; CHI 90mTorr; COz 18 mTorr. Appliedvoltage was 291 V. Totalpressures: m2.1 Torr; 0 4.5 Torr.
is less than theconfidencelimit, only the upper limit is meaningful. The rate constant for the removal of CH(a42) by N2 is
