Gas-phase reactions of atomic lanthanide cations with methyl fluoride: periodicities reactivity

Room temperature reactions of atomic lanthanide cations (excluding Pm+) with CH3F have been surveyed systematically in the gas phase using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35 Torr and 295 K. F-atom transfer was the predominant reaction channel and exhibits increasing efficiency with increasing exothermicity. Minor CH3F addition was observed with the late lanthanide cations that react slowly. The reaction efficiency for F-atom transfer appears to be governed by the energy required to promote an electron to achieve a d1s1 excited electronic configuration in which two non-f electrons are available for bonding: it decreases as the promotion energy increases and the periodic trend in reaction efficiency along the lanthanide series matches the periodic trend in the corresponding electron-promotion energy. This behaviour is consistent with a CF bond insertion mechanism of the type proposed previously for insertion reactions of Ln+ cations with hydrocarbons. Direct F-atom abstraction by a harpoon-like mechanism was excluded because of an observed non-correlation of reaction efficiency with IE(Ln+). A remarkable Arrhenius-like correlation is observed for the dependence of reactivity on promotion energy: the early and late lanthanide cations exhibit characteristic temperatures of 18,000 and 4400 K, respectively. A rapid second F-atom transfer occurs with LaF+, CeF+, GdF+, TbF+ and LuF+, but there was no evidence for a third fluorine-atom abstraction with any of the LnF2+ cations. Both LnF+ and LnF2+ can add methyl fluoride molecules under the experimental operating conditions of the ICP/SIFT tandem mass spectrometer.