Fragmentation surface of triplet ketene

Abstract

The photofragmentation of ketene to triplet methylene and carbon monoxide is a paradigm for unimolecular dissociation over an exit channel barrier. The geometric structures, quadratic force fields, and harmonicvibrational frequencies of the triplet ketene reactant, the ³B₁ CH₂+¹Σ⁺ CO products, and both in-plane (C_s^II) and out-of-plane (C_s^I) transition states have been determined at the TZ(2d1f,2p) coupled-cluster singles and doubles (CCSD) level of theory. An unusual, shallow minimum at long range [R(C–C)=4.0 Å] has also been discovered and characterized. A rigorous mapping and analytic parametrization has been performed of the TZ(2d1f,2p) CCSD intrinsic reaction paths connecting the C_s^II transition state to both the reactant and products. Final potential-energy functions along the entire reaction path have been determined with the aid of [(C,O)/H] atomic-orbital basis sets as large as [6s5p4d3f2g1h/5s4p3d2f1g] and electron correlation treatments as extensive as the coupled-cluster method through triple excitations [CCSDT or CCSD(T)]. The final theoretical curve is highlyanharmonic in the transition-state region, displaying a classical barrier of1045 cm^-1, a critical C–C distance of 2.257 Å, and a barrier frequency of321i cm^-1. Effective barrier frequencies in the 100i cm^-1 range which resultfrom RRKM modelling with tunnelling corrections of the observed steplike structure in the triplet ketene dissociation rate constant are thus shown to be physically untenable. Various implications of such abinitio predictions on unravelling the intricacies of the fragmentation dynamics are discussed.