Kinetics and Mechanism of the Thermal Isomerization of Cyclopropane to Propene: A Comprehensive Theoretical Study

The kinetics of the homogeneous gas-phase thermal isomerization of cyclopropane to propene has been studied theoretically to clarify existing discrepancies regarding the interpretation of its mechanism. High-level <i>ab initio</i> and density functional theory calculations were used to determine the branching ratios of the biradical and carbene reaction channels over wide temperature and pressure ranges. For this, relevant molecular and thermochemical properties of the proposed intermediates and related transition states were computed and compared with literature values. The Arrhenius equation, derived between 400 and 1400 K in the high-pressure limit at the CCSD(T)/6-311++G(3df,3pd)//CCSD/6-311++G(d,p) level of theory, is given by log₁₀(<i>k</i>_overall,∞/s^-1) = (15.60 ± 0.06) - (65.70 ± 0.17) kcal mol^-1 (2.303 RT)^-1. This expression is in very good agreement with the available experimental data. According to these results, the biradical pathway is the predominant mechanism, while the carbene pathway contributes 1-2% at higher temperatures. The G4//B3LYP/6-311++G(3df,3pd) and G4//M06-L/6-311++G(3df,3pd) levels showed comparable Arrhenius parameters. Low-pressure limit rate coefficients and falloff curves were also estimated to evaluate the effect of pressure on the reaction. Additionally, the possibility of a concerted path is considered, but calculations showed unstable wave functions, suggesting that this mechanism would not be plausible.