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Ab Initio Energies and Product Branching Ratios for the O + C3H6 Reaction

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journal contribution
posted on 20.12.2007, 00:00 by Gary D. DeBoer, James A. Dodd
Intermediate and transition-state energies have been calculated for the O + C3H6 (propene) reaction using the compound ab initio CBS-QB3 and G3 methods in combination with density functional theory. The lowest-lying triplet and singlet potential energy surfaces of the O−C3H6 system were investigated. RRKM statistical theory was used to predict product branching fractions over the 300−3000 K temperature and 0.001−760 Torr pressure ranges. The oxygen atom adds to the C3H6 terminal olefinic carbon in the primary step to generate a nascent triplet biradical, CH3CHCH2O. On the triplet surface, unimolecular dissociation of CH3CHCH2O to yield H + CH3CHCHO is favored over the entire temperature range, although the competing H2CO + CH3CH product channel becomes significant at high temperature. Rearrangement of triplet CH3CHCH2O to CH3CH2CHO (propanal) via a 1,2 H-atom shift has a barrier of 122.3 kJ mol-1, largely blocking this reaction channel and any subsequent dissociation products. Intersystem crossing of triplet CH3CHCH2O to the singlet surface, however, leads to facile rearrangement to singlet CH3CH2CHO, which dissociates via numerous product channels. Pressure was found to have little influence over the branching ratios under most conditions, suggesting that the vibrational self-relaxation rates for p ≤ 1 atm are negligible compared to the dissociation rates.