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WIREs Comput Mol Sci
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Spin‐forbidden reactions: computational insight into mechanisms and kinetics

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Many chemical reactions involve one or more changes in the total electronic spin of the reacting system as part of one or more elementary steps. Computational and theoretical methods that can be used to understand such reaction steps are described, and a number of recent examples are highlighted. A particularly strong focus is given to general rules that govern multistep reactions of this type. The two most important rules are (1) that spin‐state change without change in atom connectivity, or spin crossover, is facile and rapid, at least when it is exothermic; and (2) that reactions involving spin‐state change and changes in atom connectivity tend to prefer stepwise mechanisms in which spin crossover steps alternate with spin‐allowed bond‐making and breaking steps. WIREs Comput Mol Sci 2014, 4:1–14. doi: 10.1002/wcms.1154 This article is categorized under: Structure and Mechanism > Molecular Structures
Proton‐coupled electron transfer in oxidation of a Co(II) complex by TEMPO.
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Intersection of potential energy surfaces of two states A and B typically occurs near the minimum of the high‐lying surface.
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Potential energy surfaces and kinetic scheme for addition of ligands L to triplet iron tetracarbonyl.
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Computed potential energy surfaces for spin‐state change of LNi(C2H5)Br, and for addition of methyl radical.
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Potential energy surfaces for oxidation of dihydroanthracene by triplet and quintet iron oxo species. ‘X’ denotes a trifluoroacetate ligand.
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Schematic Eyring plot for analogous spin‐forbidden and spin‐allowed rate constants, corresponding to the same energy barrier. The spin‐forbidden reaction is slower by a factor κ at all temperatures, due to the probability of surface hopping.
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Competing spin‐forbidden and spin‐allowed reaction steps. Reaction through the MECP dominates in case (i), whereas adiabatic reaction over the transition state is favored in case (ii).
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Potential energy surfaces for decomposition of triplet methoxy cation 3CH3O+ into formyl cation HCO+ and hydrogen. Two MECPs A and B with different structures correspond to different decomposition mechanisms.
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Schematic potential energy surfaces for (i) an adiabatic reaction occurring only on the electronic ground state, (ii) a nonadiabatic reaction involving one change in electronic state, and (iii) a reaction involving two changes in electronic state.
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Schematic potential energy surface topologies for bimolecular reactions A + B → C + D, favoring stepwise spin‐state change then bond‐making or breaking reaction [cases (a) and (b)] or concerted spin‐state and bond‐making/breaking (right hand side). MECPs are highlighted with large dots.
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