Spin‐forbidden reactions are frequently encountered when first row transition metal‐oxo species are involved. One possible approach to describe these challenging reactions is the so‐called minimum energy crossing point (MECP) between the diabatic reactants and products potential energy surfaces (PESs). Alternatively, inclusion of spin‐orbit coupling (SOC) effects allows to locate a saddle point on a single adiabatic PES (TS SOC). The TS SOC approach is rarely applied yet because of its high computational cost. Recently evidence for a TS SOC impact on sizably lowering the activation barrier in O2 addition to a Au(I)‐H complex reaction (Chem. Sci. 2016, 7, 7034‐7039) or even on predicting a qualitatively different reaction mechanism in Hg chemistry (Angew. Chem. Int. Ed. 2016, 55, 11503‐11506) has been put forward. Using both approaches we provide here a systematic analysis of three prototypical transition metal‐oxo spin‐forbidden processes, involving Cr, Fe and Mn, to investigate their implications on reactivity. Results show that the spin‐forbidden reactivity of first row transition metal‐oxo species, where the spin‐orbit coupling is relatively weak, can be safely described by a MECP approach. However, for the Mn‐oxo reactivity, the TS SOC approach allows to straightforwardly explore detailed features of the adiabatic potential energy surface which in principle could affect the overall reaction rate in cases where the involved diabatic PESs are tricky.
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