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
Publication costs assisted by the National Science FoundationA mixture containing 3% each of the reactants CzDz and HC1 in an Ne-Ar diluent was studied over the temperature range 1650-2600 K utilizing a shock tube coupled to a time-of-flight mass spectrometer. Plots of the mole fractions f of the exchange products, DC1 and CzHD, revealed two distinct regions of growth: (a) an initial low conversion region characterized by an induction period ti; and (b) a region of accelerated exchange during which exchange products were formed with a quadratic dependence of the reaction time. These two regions labeled a and b were combined using two empirical equations, 1 -fa/feq,a = exp-[-k,[M]t], where t 5 ti, and 1 -fb/feq,b = exp[-kb[M](t -ti)2], in order to represent the entire reaction profile at any given temperature within the interval investigated. The Arrhenius parameters for k , and k b were determined to be 1011.15*0.30 exp(-19990 f 2850/RT) and 1016.40*0.41 exp(-31480 f 4200/RT), respectively, for DC1 and 1011.69*0.29 exp(-19150 f 2740/RT) and 1015.24*0.34 exp(-17620 f 3480/RT) forCzHD. The units for k , are cm3 mol-l sec-l and cm3 mol-l s e c 2 for kb. Activation energies are reported in cal mol-l. Comparison with the profiles obtained for acetylene pyrolysis strongly suggests that the mechanism for the exchange is atomic. Furthermore, the exchange experiments indicate that the initial step in the pyrolysis of acetylene is the disproportionation reaction, 2C2H2 -CZH + CzH3.
The exchange reaction, 13C160 + 12C180 ^13C180 + 12C160 (k\,k-\), was investigated in order to assess the importance of reactant vibrational excitation and its effect upon the isotopic switching rate. A shock tube equipped to record infrared emission was used to select two reaction environments; one in which the vibrational equilibration of CO was established in a very short time after shock arrival (8% CO, 11% H2, 80% Ne, 1% Ar) and one in which equilibration was about six times slower (8% CO, 20% Kr, 72% Ar). The rate of exchange was studied with a shock tube connected to a time-of-flight mass spectrometer over the temperature range 2900-4650 K. The time dependence for product formation was observed to be linear and there was no discernible difference in the exchange rate constants from either mixture. The rate constants from this work were combined with the tabular data of Lifshitz reported in a previous single pulse shock tube study which employed an argon diluent over the temperature range of 2000-2650 K. The results are represented by one Arrhenius line from 2000 to 4650 K; namely &b = 1012-58±°•07 exp(-55,470 ± 980/RT) cm3 mol-1 sec""1. The observation times spanned by the shock tube experiments were all in the region where vibrational equilibration was attained prior to the production of measurable amounts of exchange products. It was necessary to assume that the rate constants obtained from the Kr-Ar mixture could be extrapolated along the combined Arrhenius line to lower temperatures (~2100 K) in order to propose that the rate of self-exchange is not solely dependent upon the vibrational energy content of the reactants. However, this indirect conclusion is supported directly by experiments which utilized mercury photosensitization to populate = 2-9 levels in carbon monoxide and which failed to induce exchange in a 12C180, 13C160, Kr, Ar mixture at room temperature.
greater than that of Figure 10. Thus, in general, for a given j?/T, tr computed via eq C2 will be slightly larger than that computed via the Wilson-Kivelson equations.Lastly, we note that there is no indication in these results on VO(acac)2 in toluene of any substantial departure of the spectral densities from Debye-type spectral densities (i.e., the definitions of µ and in eq C2 such as was found for PD-Tempone in toluene.12 This is, perhaps, consistent with the slow-motional result that ESR spectra of the vanadyl complexes are fit by Brownian diffusion while PD-Tempone is fit by jump diffusion.
The rate of isotopic exchange of equimolar mixtures of 3202 + 3602 dilute in Ne-1% Ar was studied over the temperature range 2625-3700 K. The reacting gas was analyzed from the reflected shock zone at 20-ps intervals with a time-of-flight mass spectrometer. Each experimental product profile obtained was compared to the corresponding computer-simulated profile calculated from an atomic mechanism by using previously published rate constants. It was observed that the growth of the exchange product exceeded that predicted by the atomic mechanism at the lower temperatures of this study. However, these differences diminished as the temperature increased. Static analysis of the gas mixtures investigated revealed that H2 or D2 if present were at a level of less than 2,5 ppm. Computer simulation of product profiles demonstrated that this impurity level was insufficient to affect the observed rate of product formation. It is proposed that contributions from molecular channels are operative at the lower temperatures while atomic pathways dominate at the higher temperatures.
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