A Low-Temperature Kinetic Study of the C(³P) + CH₃OCH₃ Reaction: Rate Constants, H Atom Product Yields, and Astrochemical Implications

Abstract

Atomic carbon in its ground electronic state, C( 3 P), is expected to be present at high abundances during the evolution of dense molecular clouds. Consequently, its reactions with other interstellar species could have a strong influence on the chemical composition of these regions. Here, we report the results of an investigation of the reaction between C( 3 P) and dimethyl ether, CH 3 OCH 3 , which was recently detected in dark cloud TMC-1. Experiments were performed to study the kinetics of this reaction using a continuous supersonic flow reactor employing pulsed laser photolysis and pulsed laser-induced fluorescence for atomic radical generation and detection, respectively. Rate constants for this process were measured between 50 and 296 K, while additional measurements of the product atomic hydrogen yields were also performed over the 75−296 K range. To better understand the experimental results, statistical rate theory was used to calculate rate constants over the same temperature range and to provide insight on the major product channels. These simulations, based on quantum chemical calculations of the ground triplet state of the C 3 H 6 O molecule, allowed us to obtain the most important features of the underlying potential energy surface. The measured rate constant increases as the temperature falls, reaching a value of k C+CHd 3 OCHd 3 = 7.5 × 10 −11 cm 3 s −1 at 50 K, while the low measured H atom yields support the theoretical prediction that the major reaction products are CH 3 + CH 3 + CO. The effects of this reaction on the abundances of interstellar CH 3 OCH 3 and related species were tested using a gas-grain model of dense interstellar clouds, employing an expression for the rate constant, k(T) = α(T/300) β , with α = 1.27 × 10 −11 and β = −1.01. These simulations predict that the C( 3 P) + CH 3 OCH 3 reaction decreases gas-phase CH 3 OCH 3 abundances by more than an order of magnitude at early and intermediate cloud ages.