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A discrepancy has been reported in the literature between experimentalmeasurements and density functional theory (DFT) calculations, on the one hand,and simulations based on empirical potential energy functions on the otherhand, regarding the solvation shell of the aqueous $\mathrm{Fe}^{3+}$ ion; theformer studies reporting a coordination number of 6.0 corresponding to a welldefined octahedral solvation shell, while the latter studies report a highervalue, typically around 6.5. The simulations presented here were conducted forlong time intervals and a wide range in ion concentration, using variouspotential energy functions as well as DFT calculations for several of thegenerated configurations. The results indicate that the solvation shell canundergo abrupt transitions between two well-separated states: an octahedralstate (OH) with 6-fold coordination and a high degree of sphericity, $\phi^6$,and a capped trigonal prism (CTP) state with 7-fold coordination andsignificantly smaller $\phi^6$. In dilute $\mathrm{FeCl_3}$ solutions, with ionconcentration on the order of 0.1 $\mathrm{mol/dm^3}$($[\mathrm{Fe}^{3+}]/[\mathrm{H_2O}]$ ratio of 1:600), the lifetime of the twostates is similar, on the order of a ns, but the lifetime of the OH stateincreases with ion concentration, while that of the CTP state remains roughlyconstant. When a uniform negative background is used instead of explicitcounterions, as is often done in DFT calculations, the lifetime of the OH stateincreases strongly, quadrupling as the ion concentration is increased to 0.9$\mathrm{mol/dm^3}$. Experimental measurements of lower concentrationsolutions, as well as corresponding simulations based on electronic structurecalculations spanning at least several nanoseconds, are needed to test theexistence of the predicted CTP state.

Evidence of sharp transitions between octahedral and capped trigonal prism states of the solvation shell of Fe$^{+3}$(aq)