Experimental and theoretical study on the thermal decomposition of C₃H₆ (Propene)

The mechanism of the thermal unimolecular decomposition of C₃H₆ (propene) is studied both theoretically and experimentally. The potential energy surfaces for possible reaction pathways are investigated by CBS-QB3 level of quantum chemical calculations, and RRKM/master-equation calculation is performed for the main channels. The time evolutions of H atoms are observed experimentally by using a highly sensitive detection technique (ARAS, detection limit ≈ 10¹¹ atoms cm^-3) behind reflected shock waves (0.5-1.0 ppm C₃H₆ diluted in Ar, 1450-1710 K at 2.0 atm). The objective of this study is to examine the main product channels by combining the experimental and theoretical investigations on the yield and the rates of H atom production. Present quantum chemical calculations identify reactions (1a-1d) as the candidates of product channels: C₃H₆ → aC₃H₅ (allyl radical) + H (1a), C₃H₆ → CH₃ + C₂H₃ (vinyl radical) (1b), C₃H₆ → CH₄ +:CCH₂ (singlet vinyldene radical) (1c), and C₃H₆ → C₃H₄ (allene) + H₂ (1d). The RRKM calculations reveal the branching fractions for (1a), (1b), and (1c) to be approximately 0.8, 0.2, and 0.01, respectively. Reaction (1d) and other product channels are negligible (< 0.1 %), and the pressure dependence of the branching fraction is small under the present experimental conditions. The experimental yield of H atoms (1.7-2.0) is consistent with the theoretical branching fractions considering the H-atom production from the rapid subsequent thermal decomposition of a C₃H₅ and C₂H₃. From the observed time profiles of H atoms, the rate of overall thermal decomposition of C₃H₆ can be evaluated as Ln(k₁/s^-1) = (38.05 ± 1.18) - (48.91 ± 1.85) × 10³ K/T, which is in excellent agreement with the theoretical prediction.