J . Phys. Chem. 1989, 93, 6402-6407
6402
Kinetic Study of the Reactions of CN with Ethane and Propane Wayne P. Hess,+J. L. Durant, Jr.,* and Frank P. Tully Combustion Research Facility, Sandia National Laboratories, Livermore. California 94551 -0969 (Received: February 13, 1989; In Final Form: April 17, 1989)
Absolute rate coefficients for the reactions of CN with ethane, propane, and five of their deuterated isotopes are determined over the temperature range 294-736 K by using the laser photolysis/CW laser-indu~-fluorescencetechnique. The measured rate coefficients for the reactions of CN with CZH6 ( k , ) and C2D6(k,) are well described by the three-parameter modified Arrhenius expressions k l ( T ) = (7.40 X 10-15)T'.29 exp(+512 cal mol-l/RT) cm3molecule-I s-I and k2(T) = (8.32 X 10-'6)T1.57. exp(+460 cal mol-'/RT) cm3 molecule-' s-'. The rate coefficient for the reaction of CN with the half-deuterated ethane compound, CH3CD3( k 3 ) ,is well described by (1/2)(kl + k z ) . We measure rate coefficients for CN reactions with C3Hs (k4), C3D8( k 5 ) ,C3H6D2( k 6 ) , and C3D6H2(k7)and derive site-specificrate constants for abstraction of primary and secondary hydrogens. These rate coefficients, on a per hydrogen atom basis, are, in the units cm3 molecule-I s-I, kHp(T)= (3.58 X 10-15)T','4 exp(+566 cal mol-I/RT), kDp(7') = (2.63 X 10-16)T1.47 exp(+568 cal mol-'/RT), kHs(T)=(3.30 X 10-'3)p,56 exp(+649 cal mol-I/RT), and kDs(T) = (2.96 X 10-'3)p.52exp(+656 cal mol-'/RT). The k4 through k7 are well represented by the sum of the appropriate three-parameter expressions for H- and D-atom abstraction from primary and secondary sites. We characterize the primary- and secondary-site isotope effects for CN + alkanes and compare these results to those for OH + alkanes.
Introduction The reactions of C N radicals are important in a variety of environments, ranging from astrophysics1 to NO, formation and destruction in combustion.2 The C N radical reacts very rapidly with many compound^,^ and nearly gas-kinetic rate coefficients have been measured for its reactions with unsaturated hydrocarbons and some large alkanes at room t e m p e r a t ~ r e . ~Un~ fortunately, a better understanding of CN-radical reactivity is hampered by sparse experimental data. The reaction of C N with ethane is thought to proceed through H-atom abstraction with the following t h e r m o c h e m i ~ t r y : ~ ~ ~ C N + C,H6 --* H C N CzHs AH = -26 kcal/mol
+
-
H N C + CzHS AH = -15 kcal/mol (1) The reaction of CN with propane can proceed through abstraction of primary or secondary H atoms: C N CH3CHZCH3 H C N + CHzCH2CH3 AH = -26 kcal/mol
+
CN
+ -
HNC
+ CH2CH2CH3
CH3CHZCH3 HNC
-
HCN
AH = -15 kcal/mol
(2)
+ CH3CHCH3 AH = -29 kcal/mol
+ CH3CHCH3
AH = -18 kcal/mol
(3)
Reactions of C N with ethane and propane were first studied by Bullock and Cooper using pulsed r a d i o l y ~ i s . They ~ ~ ~ measured the rate coefficient for C N + ethane over the limited temperature range 300-41 5 K and arrived at a temperature-independent value of (2.4 f 0.2) X lo-'' cm3 molecule-' S S ' . They also measured C N + propane at room temperature and obtained a value of k = ( 5 . 3 f 0.8) X lo-" cm3 molecule-'^-^. Recent work at room temperature by Lichtin and L i d is in general agreement with Bullock and Cooper's findings for ethane. Most recently, Jackson and co-workers4 have studied the reactions of C Y with a wide variety of alkanes and substituted alkanes at room temperature and concluded that long-range attractive forces are important in governing the magnitude of the rate constants. I n an attempt to produce a precise, self-consistent data base on CN reactions, and to investigate fundamental kinetics, we have recently extended the powerful laser photolysis/CW laser-induced-fluorescence technique to probe C N kinetics.1° We use this technique to determine rate coefficients for the reactions of
'Sandia National Laboratories Postdoctoral Research Associate. *Author to whom correspondence should be addressed
0022-3654/89/2093-6402$0 1.50/0
C N with CzH6 (ki)? C2D6 (k2), CH3CD3 (k3), C3Hs (h). C~DB ( k s ) ,CH3CD2CH3( k 6 ) ,and CD3CH2CD3(k,) from 294 to 736 K. The detailed kinetic measurements allow us to determine temperature-dependent, site-specific rate coefficients for the C N propane reaction and to characterize the temperature dependence of the kinetic isotope effect for both primary- and secondary-site H-atom abstraction.
+
Experimental Section The experimental technique is a modification of the laser photolysis/CW laser-induced-fluorescence (LP/CWLIF) method developed for OH-radical reaction kinetics",'* and described previously by Durant and Tully.lo Briefly, C N radicals form following 193-nmexcimer-laser photolysis of C2N2in a heatable quartz flow cell. The CN-radical concentration is then probed, in real time, by exciting the P(7) or the P( 11) line in the (0,O) band of the CN(B+X) transition with a C W dye laser operating near 388 nm. The nonresonant (0,l) fluorescence is detected with a photomultiplier tube and photon-counting electronics. In contrast to pulsed-laser/pulsed-laserkinetic experiments, the LP/CWLIF technique collects an entire radical-concentration profile following each photolysis pulse. Figure 1 displays a schematic diagram of the experimental apparatus. The 10-15-mW output of a C W dye laser operating with Exalite 392E dye13 is attenuated by neutral density filters to 1.5 mW and directed into the reaction cell by a multimode UV (1) Bauschlicher, C. W., Jr.; Langhoff, S.R.; Taylor, P. R. Astrophys. J . 1988, 332, 531. (2) Thorne, L. R.; Branch, M. C.; Chandler, D. W.; Kee, R. J.; Miller, J. A. Twenty First Symposium (International) on Combustion; The Combustion
Institute: Pittsburgh, PA, 1986; p 965. (3) de Juan, J.; Smith, I. W. M.; Veyret, B. J . Phys. Chem. 1987,91,69. (4) Sayah, N.; Li, X.; Caballero, J. F.; Jackson, W. M. J. Photochem. Photobrol., A: Chem. 1988, 45, 177. (5) Lichtin, D. A,; Lin, M. C. Chem. Phys. 1985, 96, 473. (6) Bullock, G. E.; Cooper, R. J. Chem. SOC.,Faraday Trans. 1 1971,67, 3258. (7) McMillan, D. F.; Golden, D. M. Annu. Rev.Phys. Chem. 1982, 33, 493.
(8) Maki, A. G.; Sams, R. L. J. Chem. Phys. 1981, 75, 4178. (9) Bullock, G. E.; Cooper, R. J. Chem. SOC.,Faraday Trans. 1 1972,68, 2185.
(10) Durant, J. L., Jr.; Tully, F. P. Chem. Phys. Lett. 1989, 154, 568. ( I 1 ) Tully, F. P.; Goldsmith, J. E. M. Chem. Phys. Lett. 1985, 116, 345.
(12) Tully, F. P.; Droege, A. T.; Koszykowski, M. L.; Melius, C. F. J. Phys. Chem. 1986, 90, 691. (13) Tully, F. P.; Durant, J. L., Jr. Appl. Opt. 1988, 27, 2096.
0 1989 American Chemical Society
Reactions of C N with Ethane and Propane
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6403 2000
REAGENTS IN
I
1750
I
1500
HEATABLE
""E
1
-I
P
i
br
/
/
I
/
OBJECTIVE
I
0.0
0.5
I
I
1.0
I
1.5
[PROPANE] x
2.0
I
2.5
1
3.0
(molecule ~ m - ~ )
Figure 2. Pseudo-first-order rate coefficient k'as a function of propane concentration at T = 475 K. 0,C,D,; 0,C3Hs.
profiles by using a standard phenomenological model, [CN] = [CN], (1 4 ~ t ) -e~p(-k[alkane]t),'],'~ ~ and the measured pseudo-first-order rate coefficients, k' = k[alkane], are determined from nonlinear least-squares fits to the diffusion-loss-corrected [CN] decay profiles. CN-radical concentration profiles are obtained at six alkane concentrations and at [alkane] = 0 for each temperature studied. The k' values are plotted versus alkane concentration, as displayed in Figure 2, and the bimolecular reaction rate coefficients, ki(T), are determined from the slope of the least-squares lines fitted through the ([alkane],k') data points. The buffer-gas pressure is typically 300 Torr of He, but some rate coefficients are also measured at 127 and 400 Torr. The C2N2 precursor concentration is varied from 5 X loL2to 6 X loL3 molecules/cm3, and the fluence of the excimer laser is varied from 0.5 to 4.0 mJ/cm2. Typical initial CN-radical concentrations range from ca. 3 X 10' to 2 X lo9 molecules/cm3. The measured rate coefficients are insensitive to changes in these parameters as well as to variations in the total flow rate and the photolysis-laser repetition rate. The measured rate coefficients are, however, sensitive to the probe-laser power and increase with power at levels above 3 mW. The power dependence is slight, and only a 6% rate increase is measured when the dye-laser power is increased from 3 to 10 mW. No power dependence is observed between 0.05 and 2.0 mW, and all experiments are conducted at laser powers below 2 mW. The dependence on the probe-laser power is probably a result of the rapid reaction of C N radicals excited to the B 2Z state, or possibly, a reaction of C N radicals that are produced in the A 211 state by decay from the B 28 state. We use chemicals of the following stated minimum purities: He = 99.9999%, further purified by passing through a gettering furnace, C2N2/He = a 100 ppm mixture of 97.5% C2N2 in 99.9999% He (Matheson). All isotope samples were produced by Merck, Sharp and Dohme Isotopes Inc. except for the C2D6 sample, which was provided by Cambridge Isotope Laboratories. We determine the purity of the alkane samples by G C or GC/MS, and all compounds except C3D8are at least 99.5% pure. For these compounds, the impurities have been identified and the measured reaction-rate coefficients should be affected by less than 0.5%. The propane-d8 sample is composed of 98.3% C3D8,0.8% propene-d,, 0.4% butane-h,,, and 0.4% diethyl ether-hIo. In order to correct the measured rate coefficient for impurity contributions we require knowledge of the C N impurities rate coefficients, none of which are measured. The rate coefficient for C N + propene-h,, however, is reported by Lichtin and Lins as 2.3 X cm3 molecule-I s-], at 294 K. We assume that the C N + prop-
+
Figure 1. Schematic diagram of the experimental apparatus showing the photolysis and probe lasers; the neutral density filter, ND; the fiber optic with microscope objective lens input and output couplers; the reaction cell; the photomultiplier tube, PMT; and the multichannel scaler, MCS.
optical fiber. The dye-laser beam is coupled into the optical fiber by a microscope objective lens. The laser emission is recollimated, after traversing the fiber optic, by a second objective lens, producing a circular output beam 7 mm in diameter. The output of an ArF-excimer laser is skimmed by a pair of irises to produce a beam of 7-mm diameter and -0.7-mJ pulse energy that is directed into the reaction cell perpendicular to the probe-laser beam and the fluorescence-collectionaxis. Downward-propagating fluorescence passes through a narrow band-pass filter, centered at 420 nm, and is detected by a photomultiplier. The photomultiplier signal is sent to an amplifier/discriminator, and the resulting pulses are counted by a personal computer equipped with a multichannel-scaler card. We average 1500-3000 laser pulses to obtain CN-concentration profiles spanning 7-9 1/e-decay lifetimes.', Excimer-laser-induced window fluorescence contaminates the first few channels of the concentration profiles, and we remove this interference by subtracting the signal collected with the probe beam blocked. Multichannel-scaler dwell times range from 15 to 25 ps for reactive decays, and data is stored throughout 1024 channels. We heat the reaction cell resistively with a regulated dc power supply and measure the temperature, at flow and pressure conditions identical with those used in the kinetics experiments, with a removable, chromel-alumel thermocouple. The stabilized temperature is constant to within 1 2 K over the dimensions of the probed volume and the duration of the experiment. All reactive experiments proceed with [CN]
