Gas-phase reactions of transition-metal ions with molecular oxygen: Room-temperature kinetics and periodicities in reactivity

An Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer has been employed in a systematic survey of room- temperature reactions Of O-2 with 29 transition-metal ions. The atomic ions are produced at ca. 5500 K in an ICP source and are allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients were measured for the reactions of first-row atomic ions from Sc+ to Zn+, of second-row atomic ions from Y+ to Cd+ (excluding Tc+) and of third-row atomic ions from La+ to Hg+. Both O-atom abstraction and 02 addition were observed as primary reaction channels. Periodicities in reactivity are identified and are compared with periodicities in the O-atom affinity of the atomic ion. PO-atom abstraction was observed to be efficient when exothermic and inefficient when endothermic. NbO+, MoO+, TaO+, and WO+ reacted further in a second O-atom transfer reaction with O-2. ReO2+ produced by direct addition Of O-2 to Re+ reacted further by both O-atom abstraction and O-2 addition. Sequential O-2 addition was observed to produce the higher oxides: MO3+ (M = Ti, V, Y, Zr, Hf, Os), MO4+ (M = Cr, Fe, Co, Ni, Cu, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, It, Pt, Au), MO5+ (M = V, Re, OS), MO6+ (M = Nb, W, Re) and MO7+ (M = V). Novel termolecular reactions second-order in oxygen were identified for Os+ and ZrO+ at high oxygen concentrations to produce O-2(+) and OsO+ from Os+ and a neutral metal oxide from Zr+.