Theoretical Investigation of the Mechanisms and Kinetics of the Bimolecular and Unimolecular Reactions Involving in the C4H6 Species.

A theoretical study of the mechanisms and kinetics for the C4H6 system was carried out using ab initio molecular orbital theory based on the CCSD(T)/CBS//B3LYP/6-311++G(3df,2p) method in conjunction with statistical theoretical variable reaction coordinate transition-state theory and RRKM/ME calculations. The calculated results indicate that buta-1,3-diene, but-1-yne, and C4H5 + H can be the major products of the C3 + C1 reaction, while CCH2 + C2H4 and C4H5 + H play an important role in the C2 + C2 reaction. In contrast, the C4H6 fragmentation giving rise to C3 + C1 and C4H5 + H becomes the key reaction paths under any temperature and pressure. The rate constants for the system have been calculated in the 300-2000 K temperature range at various pressures for which the C2 + C2 → C4H6 high-P limit rate constant, 10.24 × 1014T-0.51 cm3/mol/s, agrees well with the measured value of Hidaka et al., 9.64 × 1014T-0.5 cm3/mol/s. Also, the high-P limit rate constants of the channels but-2-yne → 2-C4H5 + H and C3 + C1 → C4H6, being 1.7 × 1014 exp(-351.5 kJ·mol-1/RT) s-1 and 5.07 × 1013 exp(0.694 kJ·mol-1/RT) cm3/mol/s, are in good agreement with the available literature data 5 × 1014 exp(-365.3 kJ·mol-1/RT) s-1 and 4.09 × 1013 exp(1.08 kJ·mol-1/RT) cm3/mol/s reported by Hidaka et al. and Knyazev and Slagle, respectively. Moreover, the 298 K/50 Torr branching ratios for the formation of buta-1,2-diene (0.43) and but-1-yne (0.57) as well as the total rate constant 5.18 × 1013 cm3/mol/s of the channels C3 + C1 → buta-1,2-diene and C3 + C1 → but-1-yne are in excellent accord with the laboratory values given by Fahr and Nayak, being 0.4, 0.6, and (9.03 ± 1.8) × 1013 cm3/mol/s, respectively. Last but not least, the rate constants and branching ratios for the C4H6 dissociation processes in the present study also agree closely with the theoretically and experimentally reported data.