title: Optimized spin crossings and transition states for short-range electron transfer in transition metal dimers.
abstract: Electron-transfer reactions in eight mixed-valence manganese dimers are studied using B3LYP. One of the dimers is a model of the active site of manganese catalase, while another represents a basic building block of the oxygen-evolving complex in photosystem II. The adiabatic reactions are characterized by fully optimized transition states where the single imaginary frequency represents the electron-transfer coordinate. When there is antiferromagnetic coupling between different high-spin centers, electron transfer must be accompanied by a spin transition. Spin transitions are characterized by minimum-energy crossing points between spin surfaces. Three reaction mechanisms have been investigated. First, a single-step reaction where spin flip is concerted with electron transfer. Second, an initial transition to a center with intermediate spin that can be followed by electron transfer. Third, an initial transition to a ferromagnetic state from which the electron can be transferred adiabatically. The complexes prefer the third route with rate-determining barriers ranging from 5.7 kcal/mol to 17.2 kcal/mol for different complexes. The origins of these differences are discussed in terms of oxidation states and ligand environments. Many DFT functionals overestimate charge-transfer interactions, but for the present complexes, the error should be limited because of short Mn-Mn distances.
authors.affiliation: Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden. marc@physto.se
authors.forename: Marcus
authors.initials: M
authors.lastname: Lundberg
authors.forename: Per E M
authors.initials: PE
authors.lastname: Siegbahn
mesh_terms: D019281:Dimerization; D004583:Electrons; D008670:Metals / Q000737:chemistry*; D008958:Models, Molecular
chemical_list: D008670:Metals