Electron Transfer of Hydrated Transition-Metal Ions and the Electronic State of Co3+(aq)
| Autor: | Kasper Planeta Kepp, Klaus A. Moltved, Mathias T. Nielsen |
|---|---|
| Rok vydání: | 2018 |
| Předmět: |
010405 organic chemistry
Chemistry State (functional analysis) 010402 general chemistry 01 natural sciences Transition metal ions 0104 chemical sciences Marcus theory Catalysis Ion Inorganic Chemistry Electron transfer Physical chemistry Physical and Theoretical Chemistry Anomaly (physics) Linear trend |
| Zdroj: | Nielsen, M T, Moltved, K A & Kepp, K P 2018, ' Electron Transfer of Hydrated Transition Metal Ions and the Electronic State of Co3+(aq) ', Inorganic Chemistry, vol. 57, no. 13, pp. 7914-7924 . https://doi.org/10.1021/acs.inorgchem.8b01011 |
| ISSN: | 1520-510X 0020-1669 |
| DOI: | 10.1021/acs.inorgchem.8b01011 |
| Popis: | Electron transfer (ET) is broadly described by Marcus-type theories, and plays a central role in many materials and catalytic systems and in biomolecules such as cytochromes. Classic ET processes are the self-exchange reactions between hydrated transition metal ions such as Fe2+(aq) + Fe3+(aq) → Fe3+(aq) + Fe2+(aq). A well-known anomaly of Marcus theory is Co2+/Co3+ exchange, which proceeds ~105 times faster than predicted. Co3+(aq) is a complex and reactive system widely thought to feature low-spin Co3+. We studied the self-exchange process systematically for Cr2+/Cr3+, V2+/V3+, Fe2+/Fe3+, Co2+/Co3+ using six distinct density functionals. We identify directly the ~105 anomaly of Co2+/Co3+ from the electronic reorganization energies without use of empirical cross relations. Furthermore, when modeling Co3+ as high-spin, the anomaly disappears, bringing all four processes on a linear trend within the uncertainty of experiment and theory. We studied both the acid-independent [Co(H2O)6]3+ species that dominates at low pH, and the acid-dependent [Co(OH)(H2O)5]2+ species that becomes important at higher pH, and use two distinct explicit second-sphere hydration models and models of perchlorate anion association. The high-spin state with weaker Co-O bonds is stabilized by vibrational energy and entropy by ~11 and ~12 kJ/mol, correcting gap estimates from absorption spectroscopy. High-spin Co3+(aq) explains the full experimental data series of the M(aq). systems. Low-spin Co3+ and high-spin Co2+ involves changes in eg occupation upon electron transfer, with associated M-O bond changes and increased reorganization energy. In contrast, with high-spin Co3+(aq) the redox-active electrons shuffle between t2g orbitals to minimize structural changes, producing a relative rate in excellent agreement with experiment. This eg occupation effect explains most of the experimental differences in rate constants, with the remaining part explained by second-sphere hydration and anion effects. Our results consistently suggest that some high-spin Co3+(aq) is active during the experiments. |
| Databáze: | OpenAIRE |
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