Pisa Composite Schemes Meet Master Equation Modeling for Accurate Pressure-Dependent Kinetics: HCI Elimination from...

We present an integrated computational protocol for the accurate prediction of temperature- and pressure-dependent kinetics of unimolecular reactions and apply it to the competing HCl elimination channels of 2-chloropropene yielding propyne and allene. Reaction pathways and stationary points are characterized within the Pisa Composite Schemes (PCS) framework by combining PCS2-optimized geometries with PCS3 single-point energies augmented by TQ_λ contributions, thus achieving subchemical-accuracy performance (better than 0.25 kcal mol^-1) at a feasible computational cost. A key element of the workflow is the availability of analytical gradients for composite methods, enabled by an interoperable tool based on generalized internal coordinates, which renders composite-level geometry optimizations and IRC calculations straightforward and robust. Vibrational effects are refined beyond the harmonic approximation by including anharmonic corrections through second-order vibrational perturbation theory. Final pressure-dependent rate constants and branching ratios are obtained by solving the 1-D master equation with the Master Equation System Solver software. In the high-pressure limit, the computed activation barrier for the dominant propyne-forming channel is ΔH₀^⧧ = 65.54 kcal mol^-1, and the corresponding Arrhenius activation energy agrees with shock-tube measurements within 0.3%. Overall, the proposed framework offers a robust and systematically improvable strategy for predictive pressure-dependent kinetics, enabling quantitative support to detailed combustion modeling and shock-tube kinetic analyses at an affordable computational cost.