Prof. Wesley Allen, University of Georgia

Mechanisms of Hydrocarbon Reactions: Combustion Archetypes, Methodological Breakthroughs, and a Tunneling Marvel

Physical Chemistry Seminar Series

Recent high-level ab initio theoretical investigations of hydrocarbon chemistry are presented in four parts.
(1) The celebrated C2H5 + O2 reaction is an archetype for hydrocarbon oxidation, and the critical step in the process is the concerted elimination of HO2 from the ethylperoxy intermediate (C2H5O2). A serious disparity between theory and experiment persisted for almost two decades in the reaction barrier for this step, bringing the overall reaction mechanism into doubt. We have resolved this problem by converging on the ab initio limit via groundbreaking focal-point computations of the highest possible quality, including the incorporation of quadruple excitations via CCSDT(Q) theory.
(2) The pyrolysis of aromatic fuels generates substantial quantities of diacetylene. Our theory, used in conjunction with exhaustive experiments, has established the pathway for benzene pyrolysis, which proceeds via a retro-Diels-Alder fragmentation of ortho-benzyne to diacetylene plus acetylene. The enthalpy of formation of diacetylene has been pinpointed for the first time, exemplifying our general thermochemical scheme that employs a hierarchy of reaction types to increasingly balance theoretical errors: isogyric < isodesmic < hypohomodesmotic < homodesmotic < hyperhomodesmotic.
(3) A state-specific and rigorously size-extensive multireference coupled cluster theory (Mk MRCC) has been developed into a powerful tool for chemical research. The effectiveness of our Mk-MRCC methods is demonstrated by extensive computations on numerous benchmark problems. The Mk-MRCC methodology is particularly effective for treating singlet diradicals encountered often in hydrocarbon combustion.
(4) Research published in June 2008 in Nature is described, wherein hydroxymethylene was isolated and characterized for the first time by means of a novel synthetic route and definitive electronic structure computations. In an unanticipated process described as a Houdini feat by the popular press, matrix-isolated H-C-OH at 11 K disappears with a half-life of ca. 2 hr. by tunneling under a prodigious 30 kcal mol-1 barrier. Theoretical computations are presented that prove the occurrence of this remarkable tunneling mechanism.