A molecular orbital study on H and H₂ elimination pathways from methane, ethane, and propane

Decomposition pathways for propane, as well as methane and ethane for comparison, in its ground electronic state were studied using density functional and high accuracy ab initio calculations. The reaction pathways were characterized by locating the transition states and following the intrinsic reaction coordinate. Atomic hydrogen as well as molecular hydrogen elimination pathways were investigated, including three deuterated propane species for comparison with experiment. The methyl and ethyl groups in propane are found to stabilize transition states and radical/biradical intermediates along the reaction pathways. For propane, 2,2-elimination of an hydrogen molecule is found to be the dominant molecular elimination pathway, in agreement with recent photochemical experiments. We find 1,1-elimination to be the next important molecular elimination pathway, followed by 1,2-elimination, which disagrees with the experimental result favoring 1,2- over 1,1-elimination.