| The kinetic energy dependence of the reactions of Fen +, Crn+ (n = 2--18) with O2 and Fen+, Crn + (n = 1--18) with CO2 is studied in a guided ion beam mass spectrometer. In the reactions with O2, a variety of MmO2+, MmO +, and Mm+ where M = Fe or Cr and m ≤ n, product ions are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. In both systems reaction efficiencies are near unity for all but the smallest clusters. The energy dependence of the products is analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to iron cluster ions. For the CO2 reactions, the primary product ions are MnO+, which then decompose by sequential loss of iron atoms as the kinetic energy is increased. Simple collision-induced dissociation to form the Mn--1+ product ions is also observed. Large cluster ions, n ≥ 9, form the MnCO2+ adduct at low kinetic energies. The cross section for the primary reaction, Mn+ + CO2 → MnO+ + CO, exhibits an interesting bimodal energy behavior in both metal systems that is discussed in some detail. Mn+-O bond energies from the CO2 reaction system are measured and found to compare well with measurements obtained from studies of the O2 systems. The trends in this thermochemistry are discussed and compared to bulk phase values. |
