Size-selective dehydrogenation of benzene by gas-phase niobium cluster ions (Nb+x)

The gas-phase reaction of positively charged niobium cluster ions Nb+x with normal and perdeuterobenzene has been studied using a fast-flow reactor and direct ion sampling techniques. Chemisorption of benzene onto the ion is facile, with the rate being essentially independent of x. However, both the resulting product distributions, consisting of species NbxC6H+m and NbxC6D+m with m=0,2,4,6, and the relative product yields not only display a dramatic dependence on cluster size, but are also remarkably similar to those previously observed for neutral Nbx using gentle, low-fluence, single-photon 6.42 eV photoionization. Direct observation of products with m≤6 indicates that both chemical dehydrogenation of benzene and desorption of hydrogen from the cluster ions must occur in the fast-flow reactor. Further, the relative yield of dehydrogenated products is found to increase substantially with increase in cluster ‘‘temperature.’’ Complete dehydrogenation of benzene to m=0 is also found to occur for niobium cluster oxides NbxO+ containing 5–14 Nb atoms. The present results are discussed in terms of the mechanism of the dehydrogenation process, as well as with respect to their relevance in determining ‘‘true’’ parent product distributions formed in reactions of the corresponding neutral clusters.