Reaction of Methylidyne with Ethane: The C-C Insertion Is Unimportant

High-accuracy coupled-cluster calculations in combination with the E,J-resolved master-equation analysis are used to study the reaction mechanism and kinetics of methylidyne with ethane. This reaction plays an important role in the combustion of hydrocarbon fuels and in interstellar chemistry. Two distinct mechanisms, the C-C and the C-H insertions of CH in C₂H₆, are characterized. The C-C insertion pathway is identified to have a large barrier of 34.5 kcal mol^-1 and hence plays no significant role in kinetics. The C-H insertion pathway is found to have no barrier, leading to a highly vibrationally excited n-C₃H₇ radical, which rapidly dissociates (within 50 ps) to yield CH₃ + C₂H₄ and H + C₃H₆ in a roughly 7:3 ratio. These findings are in good agreement with an experimental result that indicates that about 20% of the reaction goes to H + C₃H₆. The reaction of the electronically excited quartet state of the CH radical with C₂H₆ is examined for the first time and found to proceed as a direct H-abstraction via a small barrier of 0.4 kcal mol^-1 to yield triplet CH₂ and C₂H₅. The reaction on the quartet state surface is negligibly slow at low temperatures characteristic of interstellar environments but becomes important at high combustion temperatures.