The rate calculations in this work were based on PESs calculated with single-state multireference Møller-Plesset perturbation theory (MRMP) as implemented in the GAMESS US program suite. CASSCF wavefunctions served as the basis of the perturbation expansion and were optimized with an active space of 10 electrons in 9 orbitals. The natural orbitals we chose to include in the active space are illustrated in Fig. SI.1. These give the flexibility necessary for an accurate description of the bond stretches, in particular of the C-S bond, which is important for the reaction mechanism. For all electronic-structure calculations the Karlsruhe valence triple-zeta basis with a set of polarization functions (TZVP) was employed, which was found sufficient in accordance with Ref. 45.