John H. Kiefer

The decomposition of cyclohexane (c-C₆H₁₂) 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- C₆H₁₂), 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_∞(c-C₆H₁₂ → 1-C₆H₁₂) = (8.76 × 10¹⁷) exp((−91.94 kcal/mol)/RT) s⁻¹ (T = 1300−2000 K) and k_∞(1-C₆H₁₂ → ^•C₃H₇ + ^•C₃H₅) = (1.46 × 10¹⁶) 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.

Vibrational relaxation has been seen in shock waves in propane, isobutene, isobutane, neopentane, and toluene dilute in krypton with the laser–schlieren technique. These experiments cover about 600–2200 K and post-shock pressures from 5 to 29 Torr. The process cannot be resolved in any for T<600 K, or in any for large molecule fraction. All the ultrasonic measurements of relaxation in these at room temperature show characteristic times in the 1–5 ns atm range, corresponding to fewer than five collisions, whereas the relaxation times in the shock waves range from 20 to 200 ns atm, with a clearly defined negative or “inverted” temperature dependence. It would seem the observed slowdown of relaxation with increasing T is simply a consequence of the much increased energy transfer required at high temperature in such large polyatomics when this is combined with a collision efficiency, here interpreted as 〈ΔE〉down, already so large it cannot much increase. The simple method for the extraction of a 〈ΔE〉down from relaxation data offered here by consideration of the energy relaxation equation for Evib=0 appears to be original and should prove quite useful in connecting thermal relaxation data to values obtained from spectroscopy and master-equation analyses. Here it is found that the derived 〈ΔE〉down extrapolate well to room temperature ultrasonic measurements, showing a slight increase with temperature.

We report CASSCF/cc‐PVDZ calculations of the potential surface for the title Jahn–Teller molecule. Energies are calculated along all normal coordinates that can be affected by Jahn–Teller distortion. Both the E′₂ in‐plane bend and stretch modes are found to be involved, but the bend contribution is very small and its contribution to the entropy is negligible. The in‐plane stretch modes strongly dominate, affording a maximum stabilization energy of 4.73 kcal/mol. Harmonic frequencies are calculated for the other modes in the D_5h configuration, and these are scaled in accordance with a comparison of experimental frequencies with similar calculations on cyclopentadiene. Energy levels for nuclear motion in the Jahn–Teller modes are then determined using the calculated surface, and these are combined with the scaled harmonic frequencies to obtain complete thermodynamic functions for the radical. The results are then fit to both the NASA polynomial form and a compact power series in inverse temperature for modeling applications. Because of changes in degeneracy and symmetry number, the entropy is actually lower than previous estimates. Nonetheless, the effect of the pseudorotation on the entropy is noticeable over the entire span of temperatures. This work is evidently the first to present both energy levels and thermodynamic functions for a Jahn–Teller molecule derived from a complete and consistent theoretical treatment. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 834–845, 2001