John H. Kiefer

The Homogeneous Pyrolysis of Acetylene II: The High Temperature Radical Chain Mechanism

Abstract A comprehensive kinetic/thermochemical model for the high-temperature radical chain decomposition of acetylene and the polyacetylenes is presented. This mechanism is tested against new shock tube data: time-of-flight mass spectra in C4H2/H2, mixtures over 1600-2100K and laser schlieren experiments in C2H2; covering 2700-3500 K. Some earlier time-of-flight measurements on C2H2, are also modeled. The paper includes a very extensive consideration of species thermochemistry, and the model is in full accord with the current consensus, having Δƒ H0(C2H) = 135·5kcal/mol. Entropies are computed from recent determinations of molecular parameters for C,H and from new high-level ab initio calculations for C4H reported here.Thetime-of-flight measurementssupport Δƒ H0(C2H2)= 111 kcal/mol or 28·24kcal/mol for the [C,-(C1)] group. Rate constants for the key C-H fission reactions of the acetylenes are taken from RRKM calculations calibrated against the direct measurements of dissociation in C2;H2, and C4H2:. Alt...

Shock Tube and Theory Investigation of Cyclohexane and 1-Hexene Decomposition

The decomposition of cyclohexane (c-C(6)H(12)) was studied in a shock tube using the laser-schlieren technique over the temperature range 1300-2000 K and for 25-200 Torr in mixtures of 2%, 4%, 10%, and 20% cyclohexane in Kr. Vibrational relaxation of the cyclohexane was also examined in 10 experiments covering 1100-1600 K for pressures below 20 Torr, and relaxation was found to be too fast to allow resolution of incubation times. The dissociation of 1-hexene (1- C(6)H(12)), apparently the sole initial product of cyclohexane decomposition, was also studied over 1220-1700 K for 50 and 200 Torr using 2% and 3% 1-hexene in Kr. On heating, cyclohexane simply isomerizes to 1-hexene, and this then dissociates almost entirely by a more rapid C-C scission to allyl and n-propyl radicals. This two-step reaction results in an initial small density gradient from the slight endothermicity of the isomerization. The gradient then rises strongly as the product 1-hexene dissociates. For the lower temperatures, this behavior is fully resolved here. For the higher pressures, 1-hexene decomposition generates negative gradients (exothermic reaction) as the radicals formed begin to recombine. Cyclohexane also generates such gradients, but these are now much smaller because the radical pool is depleted by abstraction from the reactant. A complete mechanism for the 1-hexene decomposition and for that of cyclohexane involving 79 reactions and 30 species is used in the final modeling of the gradients. Rate constants and RRKM fit parameters for the initial reactions are provided for the entire range of conditions. The possibility of direct reaction to allyl and n-propyl radicals, without stabilization of the intermediate 1-hexene, is examined down to pressures as low as 25 Torr, without a clear resolution of the issue. High-pressure limit rate constants from RRKM extrapolation are k(infinity)(c-C(6)H(12) --> 1-C(6)H(12)) = (8.76 x 10(17)) exp((-91.94 kcal/mol)/RT) s(-1) (T = 1300-2000 K) and k(infinity)(1-C(6)H(12) --> (*)C(3)H(7) + (*)C(3)H(5)) = (1.46 x 10(16)) exp((-69.12 kcal/mol)/RT) s(-1) (T = 1200-1700 K). This high-pressure rate for cyclohexane is entirely consistent with the notion that the isomerization involves initial C-C fission to a diradical. These extrapolated high-pressure rates are in good agreement with much of the literature.

Effect of VV Transfer on the Rate of Diatomic Dissociation

The effect of VV transfer processes on the rate of thermal dissociation of diatomic molecules is considered within the steady‐state approximation for a single‐quantum step master equation. An accurate rate law is obtained for the truncated harmonic oscillator through replacement of the agents for VV transfer by the vibrational energy. Approximate numerical solutions for a Morse oscillator model of the representative species O2, H2, and HCl are in quite satisfactory agreement with the measured rates and provide at least qualitative explanations for nearly all the unusual features of the measurements. The rate enhancement effect of VV transfer is found to be slight; the primary, and quite general consequence of VV transfer is rather a depression of the rate, especially at high temperatures, which then appears as a considerable reduction in activation energy. This effect is shown to arise from a violation of detailed balance for the VV processes whenever the vibrational energy is depleted by rapid dissociation.

Refractive index change and curvature in shock waves by angled beam refraction

Observations of front geometry and refractive index jump across shock waves in rare gases have been made with a new and particularly simple technique. The technique involves determination of the angular deflection of a narrow laser beam intersecting the shock front at a shallow angle. Measured refractive index jumps in rare gases are in excellent agreement with those calculated using Snell’s law and ideal shock theory. The apparent shock curvature is in close accord with deBoer’s theory for loading pressures below 20 Torr, but above this presure there is evidence of an indentation near tube center.

Experimental and theoretical investigation of the self-reaction of phenyl radicals.

A combination of experiment and theory is applied to the self-reaction kinetics of phenyl radicals. The dissociation of phenyl iodide is observed with both time-of-flight mass spectrometry, TOF-MS, and laser schlieren, LS, diagnostics coupled to a diaphragmless shock tube for temperatures ranging from 1276 to 1853 K. The LS experiments were performed at pressures of 22 +/- 2, 54 +/- 7, and 122 +/- 6 Torr, and the TOF-MS experiments were performed at pressures in the range 500-700 Torr. These observations are sensitive to both the dissociation of phenyl iodide and to the subsequent self-reaction of the phenyl radicals. The experimental observations indicate that both these reactions are more complicated than previously assumed. The phenyl iodide dissociation yields approximately 6% C(6)H(4) + HI in addition to the major and commonly assumed C(6)H(5) + I channel. The self-reaction of phenyl radicals does not proceed solely by recombination, but also through disproportionation to benzene + o-/m-/p-benzynes, with comparable rate coefficients for both. The various channels in the self-reaction of phenyl radicals are studied with ab initio transition state theory based master equation calculations. These calculations elucidate the complex nature of the C(6)H(5) self-reaction and are consistent with the experimental observations. The theoretical predictions are used as a guide in the development of a model for the phenyl iodide pyrolysis that accurately reproduces the observed laser schlieren profiles over the full range of the observations.

Thermal decomposition of propargyl bromide and the subsequent formation of benzene

Mixtures of 3% C 3 H 3 Br, 3% C 3 H 3 Br + 5% H 2 , and 3% C 3 H 3 Br + 5% D 2 , all three containing neon diluent, were analyzed behind reflected shock waves by time-of-flight mass spectrometry to investigate the role of propargyl radical (C 3 H 3 ) as precursor to benzene formation at high temperatures. The first mixture yields significant concentrations of benzene over the range 1310-1470 K; benzene yield is observed to increase twofold in the second mixture at comparable temperatures. The third mixture reveals a temporal ratio of [HBr]/([DBr] + [Br]) ∼ 1, which is interpreted as evidence of an equal contribution from each of the two initiation reactions: (1) C 3 H 3 Br → c-C 3 H 2 + HBr, where c-C 3 H 2 is singlet cyclopropenylidene; and (2) C 3 H 3 Br → C 3 H 3 + Br. In the first two mixtures, the major products are C 2 H 2 , C 4 H 2 , C 6 H 2 , C 6 H 2 and HBr. A 2% propyne + neon mixture was also studied over the temperature range 1750-2620 K. The reaction profiles for C 3 H 4 , C 2 H 2 , C 4 H 2 , and C 6 H 6 are modeled satisfactorily with a mechanism in which the dominant channel for propyne dissociation produces c-C 3 H 2 + H 2 . Reaction pathways involving c-C 3 H 2 insertion into C-H bonds are presented along with an analysis for the important disproportionation step, c-C 3 H 2 + C 3 H 4 → 2C 3 H 3 . Inclusion of six steps describing the reactions of bromine containing species to the core propyne decomposition mechanism results in satisfactory fits to the C 3 H 3 Br and C 3 H 3 Br + H 2 experiments. It is concluded that benzene production in these mixures is best explained by a sequence of reactions initiated by the dimerization of propargyl radicals

Vibrational relaxation, dissociation, and dissociation incubation times in norbornene

Shock waves in norbornene (bicyclo [2,2,1] hept‐2‐ene, C7H10)–krypton mixtures have been examined with the laser‐schlieren technique over the very wide range of conditions, 542–1480 K, and 34–416 Torr in 0.5%, 2%, and 4% C7H10. The experiments exhibit both vibrational relaxation (542–1480 K) and the retro‐Diels–Alder dissociation, norbornene→1,3‐cyclopentadiene+ethylene (869–1480 K). Over 869–1304 K, and for pressures below 140 Torr, both relaxation and dissociation are resolved. These experiments provide the first measurements of unimolecular incubation (induction) times for the dissociation of a large polyatomic molecule. The ratio ti/τ decreases from ∼5 to 2 in 900–1300 K. Vibrational relaxation is rapid, log10 Pτ (μs atm)=0.066−6.70/T1/3, with a weak inverse temperature dependence, but is completely consistent with series excitation through the lowest‐frequency mode. Dissociation shows very strong unimolecular falloff. A Rice–Ramsperger–Kassel–Marcus (RRKM) model, parametrized to fit the observations ...

Shock tube/time-of-flight mass spectrometer for high temperature kinetic studies

A shock tube (ST) with online, time-of-flight mass spectrometric (TOF-MS) detection has been constructed for the study of elementary reactions at high temperature. The ST and TOF-MS are coupled by a differentially pumped molecular beam sampling interface, which ensures that the samples entering the TOF-MS are not contaminated by gases drawn from the cold end wall thermal boundary layer in the ST. Additionally, the interface allows a large range of postshock pressures to be used in the shock tube while maintaining high vacuum in the TOF-MS. The apparatus and the details of the sampling system are described along with an analysis in which cooling of the sampled gases and minimization of thermal boundary layer effects are discussed. The accuracy of kinetic measurements made with the apparatus has been tested by investigating the thermal unimolecular dissociation of cyclohexene to ethylene and 1,3-butadiene, a well characterized reaction for which considerable literature data that are in good agreement exist. The experiments were performed at nominal reflected shock wave pressures of 600 and 1300 Torr, and temperatures ranging from 1260 to 1430 K. The rate coefficients obtained are compared with the earlier shock tube studies and are found to be in very good agreement. As expected no significant difference is observed in the rate constant between pressures of 600 and 1300 Torr.

Monte Carlo trajectory study of Ar + H2 collisions: Master-equation simulation of a 4500 K shock wave experiment with thermal rotation

Abstract Thermally averaged rate coefficients for vibrational state changes and dissociation from individual vibrational levels in H 2 -Ar collissions at 4500 K are derived from Monte Carlo quasiclassical trajectory calculations. The rate matrix is completed by linear surprisal interpolation. Relaxation times, induction times, and steady dissociation rates simulating a shock wave experiment are calculated by a matrix-eigenvalue solution of the master equation. Rotational equilibrium is assumed, but vibrational nonequilibrium effects are included in full. The resulting steady dissociation rates are only about 30% less than at equilibrium.

Pyrolysis of cyclopentadiene: Rates for initial C−H bond fission and the decomposition of c-C5H5

The pyrolysis of cyclopentadiene has been observed in shock waves with two complementary methods: time-of-flight mass spectrometry (TOFMS) and laser-schlieren (LS) densitometry. Experiments, encompassed the ranges 100–450 Torr and 1500–2500 K. The low-temperature LS experiments show brief, early maxima indicative of chain reaction initiated by C−H fission. Precise and consistent rate constants are obtained from both experiments for this reaction, and a very good Rice-Ramsperger-Kassel-Marcus (RRKM) fit is possible with standard parameters. The result is a barrier of 84±2 kcal/mol for C−H fission and a consequent Δ f H o 298 =65.3 kcal/mol for c -C 5 H 5 radical Rate constants are also estimated for the important secondary reaction c -C 5 H 5 →C 3 H 3 +C 2 H 2 . These are also successfully RRKM-modeled with the aid of high-level density-functional calculations of the transition state for the 1,2 H-atom shift that controls the rate of this reaction. The large rates for this are shown to be a consequence of a newly calculated low barrier of 61.9 kcal/mol and a 10-fold reaction path degeneracy that arises from a facile pseudorotation in this Jahn-Teller molecule. The experiments and model reported here provide the first quantitative evidence for the H-atom shift mechanism for this reaction.