Iodization Threshold in Size-Dependent Reactions of Lead Clusters Pb with Iodomethane.

Utilizing a magnetron-sputtering (MagS) source in tandem with a multiple-ion laminar flow tube (MIFT) reactor and a customized triple quadrupole mass spectrometer (TQMS), we have prepared clean Pb ( = 1-13) clusters and measured their reactivity with iodomethane under high carrier gas pressures. Strong size dependences are found for the reactivity of these cationic Pb clusters with CHI. For the Pb with ≤ 4, iodinated clusters PbI were found to be the dominant products, in strong contrast to > 4 where no such products were seen. Quantum chemical studies show that with an increasing number of Pb atoms, the Pb-Pb interatomic interactions become stronger compared with the Pb-I bonding in PbI clusters. Furthermore, the reactions of Pb with CHI have fairly small transition state energy barriers, in contrast to those for Pb clusters which have magnitudes that will prevent reactions under the ambient conditions.