Ab initio prediction for product stereo-specificity in the CH3CHI + O2 reaction: formation of syn- vs anti-CH3CHOO

Context

The stereo-specific production of syn- and anti-CH₃CHOO conformers from the CH₃CHI + O₂ reaction has been investigated by ab initio quantum-chemical and statistical theory studies. The results of the studies clearly indicate that the [syn]:[anti] product ratio depends on both temperature and pressure of the reaction system, and is kinetically, rather than thermodynamically, controlled. Most experimental data measured near room temperature at 2–10 Torr He pressure agree with the predicted results in terms of either the absolute rate constants for syn- and anti-CH₃CHOO production and/or the [syn]:[anti] product ratio. If the stereo-specificity of syn- and anti-CH₃CHOO formation were controlled thermodynamically, one would predict [syn]:[anti] = 241:1 independent of pressure at 298 K, instead of (80 ± 10):(20 ± 10) measured experimentally or 86:14 predicted theoretically at 5-Torr He pressure.

Methods

All calculations were performed using Gaussian 16 software. Geometry, frequency, and IRC analysis calculations were conducted at the B3LYP/Aug-cc-PVTZ level of theory. The potential energy surface of the system was computed at the CCSD(T)/Aug-cc-PVTZ//B3LYP/Aug-cc-PVTZ level. The rate constants for individual product channels in the reaction, including the direct production of IO + CH₃CHO and the collisional deactivation of the excited CH₃CHIO₂* intermediate formed by the association of CH₃CHI with O₂, were predicted by statistical theory calculations using the Variflex code.