A PM3-SRP + analytic function potential energy surface model for O( ³P) reactions with alkanes. Application to O( ³P) + ethane

The PM3 semiempirical electronic structure theory is reparametrized with specific reaction parameters (SRPs) to develop a potential energy surface (PES) for O( ³P) processing of alkanes. The results of high-level ab initio calculations for the O( ³P) + C ₂H ₆ primary reactions, yielding OH + C ₂H ₅, C ₂H ₅O + H, and CH ₃O + CH ₃, 11 ensuing secondary and unimolecular dissociation reactions involving products of these primary reactions, and additional reactions were used to develop two PM3-SRP models for the PES. The ab initio results used for this fitting were taken from previous multiconfiguration calculations and additional PMP2/cc-pVTZ calculations reported here. Even though these two PM3-SRP models are unable to quantitatively represent the many reactions that occur in high-energy collisions of O( ³P) with alkanes, they are vast improvements over the PES of PM3 theory. These models are used in direct dynamics classical trajectory simulations of the O( ³P) + C ₂H ₆ reaction at a 5 eV collision energy. The results of the simulations show that the products of the three primary reactions are highly excited and are able to undergo a large number of ensuing secondary and unimolecular dissociation reactions, and long-time trajectory integrations are required to study these many product channels. The large internal excitations of the primary reactions' products agree with results of a previous MSINDO direct dynamics trajectory study. Reaction cross sections calculated for the primary reaction channels are also in good agreement with the MSINDO results. Velocity scattering angles, calculated for products of the secondary and unimolecular dissociation channels, provide detailed information concerning the molecular dynamics of these products. They are formed directly and also via long-lived intermediates.