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.