An Experimental and Theoretical Study of the Thermal Decomposition of C₄H₆ Isomers

The chemistry of small unsaturated hydrocarbons, such as 1,3-butadiene (1,3-C₄H₆), 1,2-butadiene (1,2-C₄H₆), 2-butyne (2-C₄H₆), and 1-butyne (1-C₄H₆), is of central importance to the modeling of combustion systems. These species are important intermediates in combustion processes, and yet their high-temperature chemistry remains poorly understood, with various dissociation and isomerization pathways proposed in the literature. Here we investigate the thermal decompositions of 1,3-C₄H₆, 1,2-C₄H₆, 2-C₄H₆, and 1-C₄H₆ inside a diaphragmless shock tube, at postshock total pressures of 26-261 Torr and temperatures ranging from 1428 to 2354 K, using laser schlieren densitometry. The experimental work was complemented by high-level ab initio calculations, which collectively provide strong evidence that formally direct dissociation is the major channel for pyrolysis of 1,3-C₄H₆ and 2-C₄H₆; these paths have not been previously reported but are critical to reconciling the current work and disparate literature reports. The reaction mechanism presented here simulates the current experiments and experimental data from the literature very well. Pressure- and temperature-dependent rate coefficients are given for the isomerization, formally direct, and direct dissociation paths.