Reactivity of niobium clusters with nitrogen and deuterium

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Abstract
Absolute rate coefficients are reported for reactions of Nb\u2099 clusters(n=2\u201320) with D\u2082 and N\u2082 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\u2082 and N\u2082. The origin of this trend in reactivity with cluster size and the reason why D\u2082 and N\u2082 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\u2082 and Nb\u2088 with H\u2082 and N\u2082. The steric and electronic requirements for dissociation of H\u2082 and N\u2082 are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H\u2082 or N\u2082 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.Peer reviewed: YesNRC publication: Ye