Reactions of small carbon cluster ions with HCN: a structural probe

The results of a detailed study of the primary and secondary reactions of carbon cluster ions, C _n ⁺ (3≤n≤20), with HCN are presented and discussed. The experiments were performed in a Fourier transform (ICR) mass spectrometer, using direct laser vaporization of graphite to form the carbon cluster ions. Evidence for two structural forms of then=7, 8 and 9 cluster ions is obtained from their differing reactivity with HCN. The C ₇ ⁺ ion is anomalous in its behavior in many respects, which is interpreted by an isomerization mechanism. The HCN reactions offer a contrast to the reactions with nonpolar neutrals studied previously. All HCN reactions produced ions of the type C_nX⁺ (primary product) or C_nXY⁺ (secondary product) where X, Y=H, CN or HCN. Fragmentation of the original carbon cluster was not observed, while radiative association is an important reaction channel. Low-energy collision-induced dissociation studies of the product ions support the mechanism of insertion into the H-CN bond and formation of covalent bonds at the carbene site for the primary reactions. In most secondary reactions however, the HCN associates weakly with the ion, rather than binding covalently.