Gas-Phase Reaction Studies of Dipositive Hafnium and Hafnium Oxide Ions: Generation of the Peroxide HfO22+

Fourier transform ion cyclotron resonance mass spectrometry was used to characterize the gas-phase reactivity of Hf dipositive ions, Hf2+ and HfO2+, toward several oxidants: thermodynamically facile O-atom donor N2O, ineffective donor CO, and intermediate donors O-2, CO2, NO, and CH2O. The Hf2+ ion exhibited electron transfer with N2O, O-2, NO, and CH2O, reflecting the high ionization energy of Hf+. The HfO2+ ion was produced by O-atom transfer to Hf2+. from N2O, O-2, and CO2, and the HfO22+ ion by O-atom transfer to HfO2+ from N2O; these reactions were fairly efficient. Density functional theory revealed the structure of HfO22+ as a peroxide. The HfO22+ ion reacted by electron transfer with N2O, CO2, and CO to give HfO2+. Estimates were made for the second ionization energies of Hf (14.5 +/- 0.5 eV), HfO (14.3 +/- 0.5 eV), and HfO2 (16.2 +/- 0.5 eV), and also for the bond dissociation energies, D[Hf2+ - O] = 686 +/- 69 kJ mol(-1) and D[OHf2+-O] = 186 +/- 98 kJ mol(-1). The computed bond dissociation energies, 751 and 270 kJ mol(-1), respectively, are within these experimental ranges. Additionally, it was found that HfO22+ oxidized CO to CO2 and is thus a catalyst in the oxidation of CO by N2O and that Hf2+ activates methane to produce a carbene, HfCH22+.