| Abstract |
Gas-phase reactions of Ta2+ and TaO2+ with oxidants, including thermodynamically facile O-atom donor N2O and ineffective donor CO, as well as intermediate donors C2H4O (ethylene oxide), H2O, O-2, CO2, NO, and CH2O, were studied by Fourier transform ion cyclotron resonance mass spectrometry. All oxidants reacted with Ta2+ by electron transfer yielding Ta+, in accord with the high second ionization energy of Ta (ca. 16 eV). TaO2+ was also produced with N2O, H2O, O-2, and CO2, oxidants with ionization energies above 12 eV; CO reacted only by electron transfer. The following charge separation products were also observed: TaN+ and TaO+ with N2O; and TaO+ with O-2, CO2, and CH2O. TaOH2+, formed with H2O, reacted with a second H2O by proton transfer. TaO2+ abstracted an electron from N2O, H2O, O-2, CO2, and CO. Oxidation of TaO2+ by N2O was also observed to produce TaO22+; on the basis of density functional theory (DFT) results, this species is a dioxide, \O-Ta-O\(2+). TaO22+ reacted by electron transfer with N2O, CO2, and CO to give TaO2+. Additionally, it was found that TaO22+ oxidizes CO to CO2 and that it acts as a catalyst in the oxidation of CO by N2O. TaO22+ also activates H-2 to form TaO2H2+. On the basis of the rates of electron transfer from N2O, CO2, and CO to Ta2+, TaO2+, and TaO22+, the following estimates were made for the second ionization energies of Ta, TaO, and TaO2: IE[Ta+] = 15.8 +/- 0.3 eV, IE[TaO+] = 16.0 +/- 0.5 eV, and IE[TaO2+] = 16.9 +/- 0.4 eV. These IEs, together with recently reported bond dissociation energies, D[Ta+-O] and D[OTa+-O], result in the following bond energies: D[Ta2+-O] = 657 +/- 58 kJ mol(-1) and D[OTa2+-O] = 500 +/- 63 kJ mol(-1), the first of which is in good agreement with the value obtained by DFT. |
