of observed and calculated values strongly supports our excitation mechanism.
The rate law for the production of HCN over the temperature range 1850-2900 K was established by recording the time-dependent infrared emission from this species at 3.0 pm in the reflected shock zone. Four mixtures of C1CN and H2 dilute in argon, differing in the ratio of initial reactant concentrations and initial shock pressures, were studied in order to determine the various order dependencies. The formation of the product was in all experiments observed to be nonlinear with respect to reaction time. The data were fit to the equation 1 -/HCN//HCN,max = expH[ClCN]o0'6[H2]00'1[Ar]0'4í2), where k = 102L8±°•06 exp(-70.3 ± 0.6/flT) cm3 mol"1 s"2. The units for the activation energy are kcal mol"1. Experiments in which the reflected shock zone was analyzed with a time-of-flight mass spectrometer revealed the products to be HCN, HC1, and C2N2. Computer calculated profiles of HCN using a 14 step atomic mechanism with available literature rate constants failed to reproduce the experimental profiles.
The reaction of an equimolar mixture of 28N2 and 30N2 (4% each) diluted by a mixture of Ne (80%) and Kr (12%) was studied over the temperature and density range 4700-5400 K and 2.5-2. X 10"6 mol cm"3, respectively. The reflected shock zone was sampled by a time-of-flight mass spectrometer at 20-µ8 intervals during typical observation periods of 0.5 ms. The product profiles were fit to the equation (1 -2/29) = exp(-fc'tz), where /29 is the mole fraction of 29N2. The time dependence z was determined to be 2.7 and the rate constants were best represented by the expression k = iO"1,24±0•54 exp(-138 ± 10/RT) µß"2•7. The inert gas dependence was assumed to be first order as reported from a previous single-pulse shock tube investigation. The rate constants from the single-pulse study were recalculated using z = 2.7 and were combined with the results herein. The resulting Arrhenius parameters span the extended temperature range 3200-5400 K: log A = 20.66 ± 0.08; E = 140 ± 1 kcal mol'1. The units of A are cm3 mol"1 s"2,7. The nonlinear time dependence for product formation confirms the earlier proposition of a mechanism which consists of a multistep sequence of reactions.
The rate of exchange of carbon monoxide with carbon dioxide was studied over the temperature range 3060-4115 K by analyzing the gas from the reflected shock zone at 20-µ8 intervals with a time-of-flight mass spectrometer. Separate mixtures containing 2% C180-2% C02 and 2% 13CO-2% C02 each diluted with inert gas were sampled dynamically in order to measure the time dependence of a major product, m/e 46 or m/e 45. The time dependence was determined to be nonlinear for both mixtures. However, the rate of formation of m/e 46 exceeded that of m/e 45 over the range covered. Computer simulation of the respective product profiles using the appropriate atomic mechanism failed to account for the total amount of exchange conversion observed. A molecular mechanism involving excitation of reactants prior to the transition state leading to exchange is proposed. Two molecular channels involving three and four
The reaction of equimolar mixtures of hydrogen and bromine diluted by inert gases was studied in the reflected shock zone over a temperature and total density range of 1400-2000 K and 1.5 X lO-6^^X 10"6 *mol cm"3, respectively. Infrared emission from HBr passing through a narrow interference filter centered at 3.60 pm was recorded during observation periods typically of 500-pa duration. Conversion of the emission intensity traces to concentration-time data revealed nonlinear product growth for the low-temperature runs and near-linear product profiles at the higher temperatures. The individual experimental profiles were matched with the corresponding model calculations which employed a modern set of rate constants for the various elementary reactions comprising the atomic mechanism. The average percent deviation of 62 experiments from the calculated profiles was 5.4%.
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