Potential energy surface and product branching ratios for the reaction of C(3Pj) with the allyl radical:: An ab initio/RRKM study

Ab initio G2M(MP2)//B3LYP/6-311G** calculations have been carried out to investigate the potential energy surface for the C(P-3(j)) + C3H5(X(2)A(1)) reaction. The results show that C(P-3(j)) can add without a barrier either to a terminal CH2 group of the allyl radical to form a metastable intermediate INT1 or between two CH2 groups to produce a bicyclic structure INT2. INT1 immediately isomerizes to buta-1,3-dien-4-yl (INT3), which can decompose to vinylacetylene + H or to the vinyl radical + acetylene. Buta-1,3-dien-4-yl (INT3) can also rearrange to 1,2-dien-4-yl (INT4), and the latter fragments by the H atom loss to vinylacetylene or butatriene. The alternative pathway from the reactants to 1,2-dien-4-yl (INT4) involves two sequential ring openings in the bicyclic intermediate INT2. 1,2-dien-4-yl (INT4) can also rearrange to but-2-yn-1-yl (INT7), which in turn decomposes to butatriene + H. The RRKM theory has been applied to compute rate constants for unimolecular reaction steps and the product branching ratios. Vinylacetylene + H and vinyl radical + acetylene are predicted to be the major reaction products, and their branching ratios strongly depend on the initial branching between the intermediates INT1 and INT2 varying between 29.8/69.8 for 100% of INT1 to 62.4/35.7 for 100% of INT2. Butatriene + H are expected to be the minor products, while the other product channels are much less favorable according to the calculated energies and rate constants.