Combustion chemistry: Important features of the C ₃H ₅ potential energy surface, including allyl radical, propargyl + H ₂, allene + H, and eight transition states

The C ₃H ₅ potential energy surface (PES) encompasses molecules of great significance to hydrocarbon combustion, including the resonantly stabilized free radicals propargyl (plus H ₂) and allyl. In this work, we investigate the interconversions that take place on this PES using high level coupled cluster methodology. Accurate geometries are obtained using coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)] combined with Dunning's correlation consistent quadruple-ζ basis set cc-pVQZ. The energies for these stationary points are then refined by a systematic series of computations, within the focal point scheme, using the cc-pVXZ (X = D, T, Q, 5, 6) basis sets and correlation treatments as extensive as coupled cluster with full single, double, and triple excitation and perturbative quadruple excitations [CCSDT(Q)]. Our benchmarks provide a zero-point vibrational energy (ZPVE) corrected barrier of 10.0 kcal mol ^-1 for conversion of allene + H to propargyl + H ₂. We also find that the barrier for H addition to a terminal carbon atom in allene leading to propenyl is 1.8 kcal mol ^-1 lower than that for the addition to a central atom to form the allyl radical.