Although integral to remote marine atmospheric sulfur chemistry, the reaction between methylsulfinyl radical and ozone may pose trouble to theoretical treatments. Motivated by a previously published theoretical treatment that found an unphysically large barrier for this reaction, we performed preliminary second order Møller-Plesset perturbation theory computations with an unrestricted Hartree--Fock reference. These computations exhibited egregious spin contamination, making restricted open-shell Hartree-Fock (ROHF) the preferred reference for subsequent wavefunction-based computations. Along with preliminary complete active space self-consistent field theory (CASSCF) computations, this result also legitimized concerns about multireference character of the system. Both the ground state of ozone and the breaking and forming of bonds can be intractable to single-reference computational methods. To address these concerns in more detail, we characterized the potential energy surface using density functional theory (DFT), but also employed CASSCF and second-order perturbation theory (CASPT2) to search for stationary points. DFT computations reveal an adduct and a transition state with a low energetic barrier of around 1 kcal mol-1. This adduct and transition state evidenced by density functional theory do not appear in CASSCF and CASPT2 scans, which instead indicate that the reaction is barrierless. The MP2 computations described in the literature provide a qualitatively different picture than the one described here, likely due to the use of a poor reference wavefunction. Using multireference methods and density functional theory, our computational investigation shows that the reaction proceeds through either a small barrier or no barrier at all. On this basis, we expect the reaction of methylsulfinyl radical and ozone to proceed rapidly to the products CH3, SO2, and O2, in accord with the published experimental studies.

Computational treatment of the methylsulfinyl radical-ozone reaction

2019 Spring National Meeting

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