A Combined Quantum Chemistry and RRKM Calculation Predicts the O(1D) + C2H6 Reaction Can Produce Water Molecule in a Collision-Free Crossed Molecular Beam Environment

Abstract

The O(1D) + C2H6 reaction has been studied using a combined quantum chemistry and RRKM calculation to find the rate constants for the elementary reaction channels. The calculations of the relative branching ratios for various products formed through the insertion mechanism have been carried out. The calculations gave 60, 8, 4, 1, and 27% for the CH3, OH, H, H2, and H2O formation channels, respectively. These calculated results for the first four species are in good agreement with the available experimental results of 70, 25, 3, and 2%, while water molecules were not detected in the experiments. It is noted that the calculations underestimated the branching ratio for OH formation channel and predicted that a large quantity of water molecules would be formed. In addition to the calculation for the insertion mechanism, the abstraction mechanism pathway via a weak O−C2H6 complex has been examined as well. The calculation gave a low energy barrier of 2.2 kcal/mol for this mechanism, supporting the conclusion that OH formation through this abstraction channel is not negligible. The contribution by this mechanism should compensate the underestimation of the calculated value for the OH product shown above, compared with the experimental result obtained when only insertion is considered in the computation. Furthermore, the result of the large branching ratio for H2O formation shows that H2O is an important minor product of the O(1D) + C2H6 reaction. The water molecule is formed mainly by the OH and a β H, suggesting that H2O will be a non-negligible product in the reactions of O(1D) + simple alkane derivatives which have a β H. Formation of water molecules in the molecular beam collision-free environment can be verified in future experiments. Reaction rates of all elementary reactions to produce the various products described above are reported and discussed.