Kinetic and mechanistic study of the pressure and temperature dependence of the reaction CH3O + NO

Abstract

New kinetic measurements for the CH₃O + NO reaction have been performed using two different techniques. The discharge flow (DF) technique has been used to investigate the 0.5–5 Torr and 248–473 K pressure and temperature ranges and pulsed laser photolysis (PLP) has been used for the 30–500 Torr and 284–364 K ranges. These new results represent an extension of the pressure and temperature ranges investigated previously. This reaction is known to present two reaction pathways, the association pathway yielding CH₃ONO and the disproportionation pathway yielding CH₂O+HNO. Based on literature and present experimental data, using the results of ab initio calculations, a multichannel RRKM analysis was developed to interpret the experimental results. This analysis has shown that the disproportionation reaction occurs simultaneously by both a direct hydrogen abstraction reaction, and via the formation of energized CH₃ONO* complex in competition with the association reaction. The RRKM analysis, fitted to present and previous data, has yielded a second-order limiting low-pressure value of 2.5 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ at 298 K, with a complex temperature dependence. The limiting high-pressure rate constant derived in the same way is k_∞ = (3.4 ± 0.4) × 10⁻¹¹(T/298)^−0.75. The model allows the prediction of CH₃O loss rate constants and of the branching ratios in the 1–760 Torr and 220–600 K ranges. For a convenient presentation of the overall rate constant, an analytical expression using the conventional Troe expression with a temperature-dependent addition constant, has been fitted to the results of the RRKM analysis.