Molybdenum Oxide Cluster Ions in the Gas Phase:  Structure and Reactivity with Small Molecules

The combination of the Knudsen cell with a standard cubic ICR trapping cell produces a number of cluster ions of molybdenum oxides Mo x O y + (x = 1−5, y = 1−15). Ionization of molybdenum trioxide vapors by electron impact yields Mo x O y + with high oxygen-to-metal ratios. Collisions with vacuum gas lead to reduction of oxygen-saturated molybdenum oxide cluster ions and to fragmentation of Mo x O y + ions with x > 3, whereas dimers and trimers are relatively stable. Time and temperature dependencies of Mo x O y + concentrations suggested that Mo4O12 + and Mo5O15 + are the primary products of MoO3 vaporization and other ions are the products of their fragmentation. A simple pair-potential model was used to calculate energy-optimized geometric structures of the clusters. The model identifies the most abundant clusters as having the lowest calculated energy per atom. The six-ring Mo3O9 cluster was found to be the most stable species, and molybdenum oxides with four and five metal atoms also include the six atom ring where one or two oxygen atoms are substituted for the MoO3 group. Ion−molecular reactions of molybdenum oxide cluster ions with some small molecules have been studied. Mo x O y + ions readily oxidize the CO molecule to CO2. Reactions of Mo x O y + ions with cyclopropane occur through the activation of the C−C bond of cyclopropane. In general, different molybdenum oxides exhibit the same reactivity toward c-C3H6. Dimer and trimer ions undergo the ligand-exchange reactions with ammonia, and molybdenum oxide trimers Mo3O8 + and Mo3O9 + dehydrogenate the NH3 molecule, yielding a complex with nitrogen.