Self-exchange of carbon monoxide behind reflected shock waves

Abstract

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.