Thermal stability of iron-sulfur clusters.

The thermal decomposition of free cationic iron-sulfur clusters FeS (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric FeS (y = x) or near stoichiometric FeS (y = x ± 1) clusters is observed. In particular, FeS represents the most abundant product up to 600 K, FeS and FeS are preferably formed between 600 K and 800 K, and FeS clearly dominates the cluster distribution above 800 K. These temperature dependent fragment distributions suggest a sequential fragmentation mechanism, which involves the loss of sulfur and iron atoms as well as FeS units, and indicate the particular stability of FeS. The potential fragmentation pathways are discussed based on first principles calculations and a mechanism involving the isomerization of the cluster prior to fragmentation is proposed. The fragmentation behavior of the iron-sulfur clusters is in marked contrast to the previously reported thermal dissociation of analogous iron-oxide clusters, which resulted in the release of O molecules only, without loss of metal atoms and without any tendency to form particular prominent and stable FeO clusters at high temperatures.