Reactivity of niobium clusters with nitrogen and deuterium

Absolute rate coefficients an reported for reactions of Nb-n clusters (n=2-20) with D-2 and N-2 at 280, 300, and 370 K. Most clusters are highly reactive but there are conspicuous exceptions at n=8, 10, and 16 for both D-2 and N-2. The origin of this trend in reactivity with cluster size and the reason why D-2 and N-2 show similar trends are discussed. Density functional theory (DFT) electronic structure calculations have been used to investigate the details of the reactions for the smallest clusters Nb-2 and Nb-8 with H-2 and N-2. The steric and electronic requirements for dissociation of H-2 and N-2 are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H-2 or N-2 requires charge transfer by transit of an avoided crossing between neutral and ionic potentials. This idea is extended to larger clusters by using a simple charge transfer model that predicts an inverse correlation between reactivity and an appropriately defined effective ionization potential. Such a correlation is observed and indicates that the effective ionization potential is the dominant influence on reactivity. (C) 1995 American Institute of Physics, [S0021-9606(98)02013-3].