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

The\nkinetics of the homogeneous gas-phase thermal isomerization\nof cyclopropane to propene has been studied theoretically to clarify\nexisting discrepancies regarding the interpretation of its mechanism.\nHigh-level <i>ab initio</i> and density functional theory\ncalculations were used to determine the branching ratios of the biradical\nand carbene reaction channels over wide temperature and pressure ranges.\nFor this, relevant molecular and thermochemical properties of the\nproposed intermediates and related transition states were computed\nand compared with literature values. The Arrhenius equation, derived\nbetween 400 and 1400 K in the high-pressure limit at the CCSD(T)/6-311++G(3df,3pd)//CCSD/6-311++G(d,p)\nlevel of theory, is given by log<sub>10</sub>(<i>k</i><sub>overall,∞</sub>/s<sup>–1</sup>) = (15.60 ± 0.06)\n– (65.70 ± 0.17) kcal mol<sup>–1</sup> (2.303 RT)<sup>−1</sup>. This expression is in very good agreement with the\navailable experimental data. According to these results, the biradical\npathway is the predominant mechanism, while the carbene pathway contributes\n1–2% at higher temperatures. The G4//B3LYP/6-311++G(3df,3pd)\nand G4//M06-L/6-311++G(3df,3pd) levels showed comparable Arrhenius\nparameters. Low-pressure limit rate coefficients and falloff curves\nwere also estimated to evaluate the effect of pressure on the reaction.\nAdditionally, the possibility of a concerted path is considered, but\ncalculations showed unstable wave functions, suggesting that this\nmechanism would not be plausible.