Ab Initio/RRKM Study of the Potential Energy Surface of Triplet Ethylene and Product Branching Ratios of the C(³P) + CH₄ Reaction

Calculations of the lowest triplet state potential energy surface for the C( 3 P) + CH 4 reaction have been performed using the CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p) method, and the microcanonical RRKM approach has been used to compute rate constants for individual reaction steps and product branching ratios. The results show that the reaction can occur by abstraction and insertion mechanisms. The abstraction pathway producing CH( 2 Π) + CH 3 ( 2 A 2 ‘ ‘) has a barrier of 26.9 kcal/mol relative to the reactants. The insertion leading to the HC−CH 3 ( 3 A‘ ‘) intermediate via a 12.2 kcal/mol barrier followed by its isomerization to H 2 C−CH 2 ( 3 A 1 ) (through a 1,2 H shift) and/or by dissociation with an H-atom loss is found to be a more favorable mechanism. At a low excess internal energy originating from the collision energy (12.2 kcal/mol), the sole reaction products are C 2 H 3 + H, where 90% of them are formed through the fragmentation of HC−CH 3 and the rest (10%) are produced via the H 2 C−CH 2 intermediate. At the higher excess internal energy (2 eV), CH + CH 3 can be formed mainly through the H-abstraction channel. The calculated C 2 H 3 + H and CH + CH 3 branching ratios at the excess internal energy of 2 eV are 69.8 and 30.2%, respectively. With further increases of the excess internal energy, the abstraction channel becomes more important, and the CH + CH 3 branching ratio increases to 68.9 and 82.8% at 3 and 4 eV, respectively. The C 2 H 2 + H 2 products can be formed only through the secondary C 2 H 3 + H hydrogen disproportionation reactions or via singlet−triplet intersystem crossing in the vicinity of the HC−CH 3 intermediate followed by fragmentation of the vibrationally hot ground-state singlet C 2 H 4 molecule. Since only the CH + CH 3 products have been characterized so far experimentally, 9 new experimental measurements are encouraged.