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 ≈ 10(11) 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) × 10(3) K/T, which is in excellent agreement with the theoretical prediction.