RESUMO
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.
RESUMO
Thermal vibrational relaxation in HCN mixtures with Kr has been observed with the laser-schlieren technique. The experiments cover the temperatures 750-2900 K and a large pressure range of 13-420 Torr in 5% and 20% HCNKr mixtures. Relaxation is extremely fast but appears to occur in two well-separated stages that are assigned to the vibrational transitions (000)-->(010) and (000)-->(100) with perhaps some lesser contribution from (000)-->(001). This interpretation is strongly supported by a comparison of net density changes to thermodynamic calculations. The first and faster process shows near constant relaxation times whereas the latter slower stage has a slight decrease of these with T. Relaxation times in pure HCN obtained by neglecting the small contribution of krypton are as follows: (a) Ptau(HCN-HCN)=27 exp(1.473T(13)) ns atm (000)-->(010); (b) Ptau(HCN-HCN)=11 exp(32.6T(13)) ns atm (000)-->(100). Probabilities suggested by these results are around 0.05 for the fast step and 0.0035 to 0.005 for the slow process. These results are close to those found by laser fluorescence measurements for deactivation of levels involving excitation of the C-H stretch (001) at 3312 cm(-1). These results are also consistent with the notion of a dominance of the fast stage by T,R-V transfer (thermal relaxation) occurring in a weakly bound complex. However, the slow step most likely occurs through a V-V process (03 (1)0)-->(100), DeltaE=27.7 cm(-1), after multiple excitation of the (010) mode. These are the first thermal measurements of relaxation in HCN and the first to see energy transfer involving the low-frequency modes.
