NITROGEN-FIXATION BY NITROGENASES - A QUANTUM-CHEMICAL STUDY

The mechanism of ammonia formation by nitrogenase has been studied usi ng the hybrid density functional method B3LYP with large basis sets. M ost of the results were obtained with a simple iron dimer model, but a few calculations were also done for larger models, the largest one co ntaining eight iron atoms. The model clusters were in general chosen t o have a net neutral ionic charge with the iron atoms in the low Fe(II ) oxidation state with ferromagnetically coupled spins. In a key resul t we find that placing a hydrogen atom on a bridging sulfur dramatical ly changes the affinity of the cluster for N-2. In the dimer model wit hout this hydrogen atom, N-2 forms only a weak end-on bond to one of t he iron atoms, but with the hydrogen atom present N-2 becomes strongly activated in a bridging coordination. The effect of the hydrogen atom can be described as a local promotion effect reducing the Fe-2(II,II) system to an Fe-2(I,II) set. A very similar promotion effect is seen also for the larger clusters. Another interesting effect noted for the Fes cluster is that a cavity between the cubanes in the cluster can b e opened by reducing the cluster with two hydrogen atoms on bridging s ulfurs. Coupled electron and proton transfer and energetic aspects are emphasized. Most steps of ammonia formation are shown to lead to H at om addition energies in the range 50-60 kcal/mol, which is argued to b e optimal for the function of nitrogenase.