Absolute rate coefficients are reported for reactions of Nbₙ clusters(n=2–20) with D₂ and N₂ at 280, 300, and 370 K. Most clusters are highly reactive but there are conspicuous exceptions at n=8, 10, and 16 for both D₂ and N₂. The origin of this trend in reactivity with cluster size and the reason why D₂ and N₂ 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 Nb₂ and Nb₈ with H₂ and N₂. The steric and electronic requirements for dissociation of H₂ and N₂ are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H₂ or N₂ 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.
The Journal of Chemical Physics108, no. 13 (1 April 1998): 5476–5490.