8599
J. Phys. Chem. 1995, 99, 8599-8603
Kinetics of Phenyl Radical Reactions with Selected Cycloalkanes and Carbon Tetrachloride T. Yu and M. C. Lin* Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received: December 12, 1994; In Final Form: March 10, 1995@
The kinetics of C6H5 reactions with c-CjHIO, c-CgH12,c-C7H14,c-CgH16,and C C 4 have been studied by means of the cavity-ring-down technique in the temperature range 297-523 K. The rates of these reactions were monitored by measuring either the decay of C6H5 at 504.8 nm or the formation of C6H502at 496.4 nm (or 510 nm) when a small, constant amount of 0 2 was added to the slow-flowing reaction mixtures. Our absolute rate constants determined at several specific temperatures compare reasonably well with those measured in solution by either a direct probing (as for CC14) or an indirect, relative rate method (as for the cycloalkanes). The effects of temperature on the reactions with CC4, c-C~HIO, and c - C ~ H Ihave Z been investigated and our results give rise to the following Arrhenius equations: kCc4 = 10-".70i0.05exp[(-1379 f 56)/T]; k c . ~ 5 = ~,o 10-".35i0.0s exp[(-2039 f 66)/T]; k C . ~ 6 ~=, 210-11.10*0.24 exp[(-1913 f 191)/T], where kx's are given in cm3/molecule*s,was used to units of cm3/molecule*s. Our averaged value of kCcl4 at 333 K, 3.40 x evaluate many rate constants for C6H5 reactions with hydrocarbons (RH) of interest to combustion using previously determined kRH/kCCI4 ratios in solution.
Introduction The reactivity of phenyl radical (C&) toward organic molecules has been studied extensively in solution; most of these studies were carried out with relative rate measurement^.'-^ The most commonly used reference reactions in these relative rate measurements are
For reactions in solution, Scaiano and StewartI5 reported absolute rate constants for C6H5 reactions with 17 substrates at 298 K in Freon 113. These rate constants were measured either by directly probing optically active reaction intermediates formed in the reactions of interest or by indirectly probing the optically active product of the phenyl reaction with a "probe" molecule (such as diphenyl methanol and P-methylstyrene). For the C6H5 C C 4 reaction, they reported a value of (1.3 & 0.1) x cm3/molecule.s, which agrees closely with our preliminary value, (1.2 & 0.4) x cm3/molecule*s,measured in the gas phase at 298 K. In this article, we report in detail the final result of the widely used reference process, C6H5 Cc14, and the absolute rate constants for phenyl reactions with several selected cycloalkanes, C ~ H I O - C ~ HThe I ~ . rate constants for these cycloalkane reactions relative to that of the C c 4 reaction have been measured in solution at 333 K.' Comparison of all known gaseous reactions of C6H5 with those studied in solution will be made.
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because c - C ~ H Iand Z CC4 are good organic solvents and the products of these reactions, C6H6 and C6H5C1, can be readily measured by gashquid chromatography. The selection of a reference process depends entirely on the substrate reaction of interest. If one is interested in the kinetics of the C6H5 0 2 and C6H5 BrCCl3 reactions, for example, C-CgH12 would be a good choice, because the product of reaction 1, C6H6, will not interfere with the products of the 0 2 and BrCC13 reactions. For studies on a series of H-abstraction reactions,
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+
C6H5
+ RH - C6H6+ R
(3)
where R = alkyls, H, etc., on the other hand, CC4 would be an ideal reference molecule and has been used e~tensively.'-~ Prior to our recent series of studies on the kinetics and mechanisms of C6H5 reactions in the gas there has been only limited success in measuring the absolute rate constants for phenyl reactions. In 1989, Preidel and ZellnerI4 reported the rate constants for C&5 reactions with NO and NO2 in the gas phase using a conventional multipass absorption method with a continuous-wave Arf laser operating at 488 nm, at which no known transition or absorption of C6Hj had been shown to exist. In view of their failure to measure many of the reactions for which we have successfully obtained reasonable values of absolute rate constants, for example, C6H5 CzH2," C2H4,I2O2,I3 and CC4,10the rate constants reported by them, including those for NO and N02, now appear to be questionable.
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Experimental Section The cavity-ring-down (CRD) technique developed for kinetic studies of free-radical reactions has been described in detail in our earlier publication^.^-'^ This novel multipass-absorption technique lies in the measurement of the decay time of a short pulse of photons ( r 5 10 ns) injected inside a highly reflective optical cavity (0.5 m in length). In the absence of absorption, the nanosecond photon pulse may be lengthened to about 30 p s by multiple reflections if the cavity is well-aligned with a pair of mirrors with 99.99% reflectivity. The presence of absorption by the species of interest will shorten the photon decay time from tco= 30 p s to the range of 10 ns 1 tc 130 ,us, depending on the concentrationof the species and the magnitude of its extinction coefficient at the wavelength probed according to the following relati~nship:~,~
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@
Abstract published in Advance ACS Abstracts, April 15, 1995.
0022-365419512099-8599$09.00/0
Utc = l/rco
+ B[A],e-"'
(1)
or ln(l/tc - lit,") = C - k't'
0 1995 American Chemical Society
(11)
Yu and Lin
8600 J. Phys. Chem., Vol. 99, No. 21, 1995 where C = In B[A]o; [AI0 is the initial concentration of the absorbing species, B is a constant related to the length of the absorbing medium, the index of refraction of the medium, cavity length, etc., and k' is the pseudo-first-order decay constant of the species probed at t' after its generati~n.~,' The measurement of k' over a wide range of excess molecular reactant concentrations allows us to determine the second-order rate constant (k") from the slope of a standard K vs concentration plot; viz. k' = k"
+ k"[X]
r
-3.80
-4.30
k
.6.30
(111)
where k" is the decay constant of C6H5 in the absence of a molecular reactant due to diffusion and other losses such as C6H5 C6H5, etc. [XI is the concentration of the molecular reactant X. In this experiment, two lasers were employed for kinetic measurement as described above. The phenyl radical was generated by the photodissociation of C~HSNO at 248 nm using a Lambda Physik LPX 100 excimer laser with ca. 5 x 10l6 photons per beam per pulse. Nitrosobenzene was placed on a sealed, fritted glass disk and was carried into the reactor via the mixing tube with the Ar carrier gas. The concentration of C6H5NO was estimated to be less than 1 x lOI5 molecules/ cm3. On the basis of our measured extinction coefficient of C&NO at 248 nm, the concentration of C6H5 generated by photolysis with unfocused laser beams is approximately 1 x 1OI2 molecules/cm3. A tunable, pulsed dye laser (Laser Photonics N2-pumped dye laser, fwhm E 5 ns) was used for the direct probing of the C6H5 radical by the CRD method at 504.8 nm or for the indirect detection of C6H502 at 496.4 nm or 5 10 nm (at which no C6H5 absorption peak is known to exist).I3 Both detection schemes were used in the present study with effectively the same results (vide infra). The reactor (heatable up to 700 K), the experimental layout and the details of data acquisition can be found e l ~ e w h e r e . ~ - ~ The radical source (C6H5NO) and all the molecular reactants (Cc4, C-C~HIO, C-CgH12, c - C ~ H Iand ~ , C-CgH16) were obtained from Aldrich. As a rule, the chemicals of highest stated purities were acquired. Argon was purchased from Specialty Gases with the stated purity of 99.999%.
.6.80 0
+
000
1200
t ( W 4
1500
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Figure 1. Typical pseudo-frst order decay plots for the C a s c-C& reaction under various, excess concentrations of c-C~HIZ at 333 K. 0, [C-cgHj~]= 0; A, [C-CgHlz] = 2.17 X 0,[C-C~HIZ] = 2.89 X all in units of molecules/cm3.
standard least-squares analysis yield k', the apparent first-order decay constants of C6H5 in the presence of the known concentrations of C-CgH12. The altemative method for the determination of K , as indicated above, is to use C6H502 as a probe. The rise time for the absorption by the C6H5O2 produced by the reaction of C6H5 with a small, constant amount of added 0 2 in the presence of a known, excess amount of an organic reactant (RH), such as c-CgH12, can be given byI3 llt, = l/t,"
+ B[Al0(1 - 8'')
(IV)
where
'k = k"
+ IC''~~[O~] + KIRH[F2H]
(V)
In eq V, k", as defined before, is the decay constant of C& in the absence of 0 2 and RH due to diffusion loss and recombination reactions (C6H5 NO and C6H5 C&). At long reaction -) when all of C6H5 is consumed by its reactions time (t' with 0 2 and RH, eq IV becomes
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Results As alluded to in the preceding section, the rate constants for the C6H5 reactions of interest in this study have been measured with two probing methods, by directly monitoring the decay of C6H5 or indirectly detecting the growth of C&O2 when a small, constant amount of 0 2 was added in the flowing reaction mixture. The reaction of C6H5 with 0 2 has been shown experimentally to produce only C6H502 at temperatures below 523 K.I3 Theoretically, we have shown that the production of C6H50 does not become significant until T 2 900 K, despite its e~othermicity.'~The C6H502 radical absorbs strongly and continuously over a wide spectral range in solutionI6 as well as in the gas phase in the visible region of the spectrum.I3 In an aqueous solution, Mertens and von Sonntag16 have recently measured its extinction coefficient at Amax = 490 nm to be 1.6 x lo3 dm3 mol-' cm-I. At 504.8 nm, where C6H5 has a known Accordingly, it is strong absorption peak, E C ~ EH ~ ~ ~ advantageous to monitor C6H502 in lieu of C&, provided that the added 0 2 does not react with the product of the C&I5 reaction of interest to generate optically interfering species. Figure 1 shows the typical pseudo-first-order plots for the reaction of C& with C-CgH12 under different excess molecular The linear concentration conditions, [C&]
