Fragmentation surface of triplet ketene

The photofragmentation of ketene to triplet methylene and carbon monoxide i s a paradigm for unimolecular dissociation over an exit channel barrier. Th e geometric structures, quadratic force fields, and harmonic vibrational fr equencies of the triplet ketene reactant, the B-3(1) CH2 + (1)Sigma(+) CO p roducts, and both in-plane (C-s(II)) and out-of-plane (C-s(I)) transition s tates have been determined at the TZ(2d1f,2p) coupled-cluster singles and d oubles (CCSD) level of theory. An unusual, shallow minimum at long range [R (C-C)= 4.0 Angstrom] 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 corre lation treatments as extensive as the coupled-cluster method through triple excitations [CCSDT or CCSD(T)]. The final theoretical curve is highly anha rmonic in the transition-state region, displaying a classical barrier of 10 45 cm(-1), a critical C-C distance of 2.257 Angstrom and a barrier frequenc y of 321i cm(-1). Effective barrier frequencies in the 100i cm(-1) range wh ich result from RRKM modelling with tunnelling corrections of the observed steplike structure in the triplet ketene dissociation rate constant are thu s shown to be physically untenable. Various implications of such ab initio predictions on unravelling the intricacies of the fragmentation dynamics ar e discussed.