Spin density contours indicate that coordinated carvacrol, in its neutral protonated form, does not show a radical character (oxidized state) that could explain the reduction of one Fe III to engender a mixed-valence Fe II /Fe III system (Figure ). Accumulated charges confirm the closed-shell nature of carvacrol moiety, which is barely charged (+0.08e; Figure ), similar to that calculated for the coordinated water molecule (+0.09e). On the other hand, time-dependent DFT calculations predict low-lying singlet electronic transitions in the range of 2.2-2.6 eV from the carvacrol system to the Fe-d orbitals (Figure ), supporting its electron-donor character compared to the tri-metallic cluster of MIL-100(Fe). Spontaneous one-electron carvacrol oxidation and MIL-100(Fe) reduction was hypothesized to origin upon deprotonation of the relatively acid hydroxy group of carvacrol when coordinated. Theoretical calculations show that proton transfer to other entities is energetically feasible (for example, proton transfer to a coordinating fluoride and water exchange is calculated (< 10 kcal/mol; Figure ). Upon carvacrol deprotonation, the excess of charge in carvacrol moiety is partially withdrawn by the oxo-centred cluster, especially in one of the iron atoms, thus supporting the formation of a mixed-valence Fe II /Fe III tri-metallic system (Figure ). This leads to the formation of a carvacrol radical, which is confirmed by the spin density contours (Figure ).