Theoretical and experimental studies of unimolecular reactions relevant to combustion and the atmosphere
The pyrolysis of methyl nitrite (1 torr) in the presence of nitrogen dioxide (1 torr) was studied at 458K over the pressure range 0-760 torr of carbon tetrafluoride. The only detectable products were methyl nitrate and formaldehyde. The decomposition can be described by the following simple mechanism: CH₃ONO + M → CH₃O + NO + M (1); CH₃O + NO₂ → CH₃ONO₂ (2); CH₃O + NO₂ → HCHO + HONO (3). Step (1) was found to be stongly pressure dependent with a P½ value of 760 torr. The rate constant for decomposition k₁ was found from RRKM modelling to be given by the following expression: log (k₁/s⁻¹) = 15.89 - 8879/T. The ratio k₃/k₂ was determined over the pressure range and was also found to be pressure dependent. This was attributed to the pressure dependence of step (2). Estimates were also made for the ratio of disproportionation to combination for the reaction between nitric oxide and the methoxy radical. This ratio was also found to be strongly pressure dependent. The pyrolysis of perfluoroazo-2-propane, PAP, (25 torr) was studied over the temperature range 450-514K. The products were nitrogen and perfluorohexane, PFH, which were produced in equal amounts. The production of nitrogen was found to be first order with respect to the azo compound. First order kinetics were observed even for extents of reaction exceeding 60%. No surface effects were observed. The reaction was pressure independent. i-C₃F₇N²i-C₃F₇ → i-C₃F₇N₂ + i-C₃F₇ (4). The rate constant for decomposition, k₄, was found to be given by the following expression: log(k₄) = 16.74 - 9856/T. The pyrolysis of formaldehyde (4-10 torr) was studied using a static system over the temperature range 705-773K and 150-760 torr of carbon dioxide. Methane (4-10 torr) was used as an inert marker. Preliminary experiments showed that methane did not decompose under these experimental conditions. The only measurable products were hydrogen and carbon monoxide. No pressure dependence was observed, even at the highest temperatures studied. The rate of formation of products was found to be 1.04 ± 0.05 with respect to formaldehyde. From this the reaction was taken to be first order. The addition of small concentrations of toluene was found to markedly reduce the rate of formation of products. There did not appear to be any surface effects, indicating that the reaction was homogeneous.