Thermochemistry of small cationic iron–sulfur clusters

The kinetic energy dependences of the reactions of $Fen+$ with COS $(n=2–6)$ and $CS2$ $(n=2–5)$ are studied in a guided-ion beam tandem mass-spectrometer. The main products arise from sulfur transfer and subsequent losses of Fe atoms. In the case of $CS2,$ this reactant also formally replaces one Fe atom of the cluster to form $Fen−1CS2+$ with losses of further Fe atoms at elevated energies. In addition, the kinetic energy dependences of the reactions of $FenS+$ $(n=2–4)$ with Xe and $CS2$ are studied. The former system yields collision-induced dissociations, whereas the latter reagent effects sulfur transfer accompanied by subsequent losses of Fe atoms. Analyses of the cross sections for endothermic reactions yield the bond energies $D0(Fen+–S),$ $n=2–5,$ $D0(SFen−1+–Fe),$ $n=2–5,$ $D0(SFen+−S),$ $n=1–3,$ and $D0(S2Fen−1+–Fe),$ $n=2, 3,$ as well as the ionization energy $IE(Fe2S2).$ These values are derived with explicit consideration of the lifetimes of the energized reaction intermediates. The binding between sulfur and the cluster core strengthens as the cluster size increases, which is rationalized by simple structural arguments.