Reactivity Patterns of Carbon Cluster Cations with Nitrogen-Containing Molecules

Abstract

Reaction kinetics of cyclic carbon cluster ions C10+ to C24+ with acrylonitrile, four isomeric C4 nitriles, and pyrrole and pyridine are reported at 300 K. A prominent 4-fold periodicity was observed in reactivity with the five nitriles, with the highest reactivity being observed for the n = 4k + 1 series of Cn+ cluster ions. This is interpreted as reflecting enhanced reactivity for the Cn+ cluster ions which have an antiaromatic 4k electron count in the HOMO. The reactivity dropped off markedly with increasing cluster size, which is considered to indicate sharply decreasing cluster-ion/nitrile binding energy with increasing cluster size. By contrast, nearly all the cluster ions reacted rapidly with pyridine and pyrrole. A two-step mechanistic picture is discussed in which the overall reaction rate is controlled by the initial electronic interaction between the cluster ion and the neutral reactant, while subsequent bond-making/bond-breaking reactions are governed by the potential surface of the complex and are strongly affected by the available energy. Judging by reaction rates with acrylonitrile and crotononitrile, most of the carbon cluster ions generated by graphite ablation in the present work were identical with the corresponding ions previously generated by Sun et al. by ionization of perchlorohydrocarbons, although some differences in reactivity were found for ions of low reactivity.