Unimolecular reaction mechanisms involving C 3H 4, C 4H 4, and C 6H 6 hydrocarbon species

Abstract

The unimolecular rearrangement processes of unsaturated aliphatic hydrocarbons representative of intermediates in rich hydrocarbon flames have been investigated using the quantum chemical BAC-MP4 method. In particular, we have investigated the unimolecular reaction mechanisms involving (1) allene-cyclopropene-propyne rearrangement, (2) vinylacetylene pyrolysis leading to acetylene and diacetylene, and (3) various C 6 H 6 compounds, including 1,5-hexadiyne, 1,2,4,5-hexatetraene, and 1,2-hexadien-5-yne leading to 3,4-dimethylenecyclobutene, fulvene, benzene, 2-ethynyl-1,3-butadiene, and other C 6 H 6 species. Rate constants for various reaction steps give good agreement with available experimental measurements. The reaction mechanisms are also consistent with various deuterium isotope labeling experiments. The results indicate that many different types of reaction mechanisms occur, including concerted pathways involving carbene and vinylidene intermediates and insertion reactions of vinylidenes into C−H bonds and C−C multiple bonds. New decomposition pathways are presented for vinylacetylene pyrolysis, for conversion of 1,2-hexadien-5-yne to fulvene, and for conversion of fulvene to benzene.