Article,
Fragmentation surface of triplet ketene
Faraday Discussions, (1998)148EP Times Cited:14 Cited References Count:100.
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 harmonic vibrational frequencies of the triplet ketene reactant, the B-3(1) CH2 + (1)Sigma(+) 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 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 correlation treatments as extensive as the coupled-cluster method through triple excitations CCSDT or CCSD(T). The final theoretical curve is highly anharmonic in the transition-state region, displaying a classical barrier of 1045 cm(-1), a critical C-C distance of 2.257 Angstrom and a barrier frequency of 321i cm(-1). Effective barrier frequencies in the 100i cm(-1) range which result from 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 ab initio predictions on unravelling the intricacies of the fragmentation dynamics are discussed.
