Gas-phase reactions of atomic lanthanide cations with methyl chloride — Periodicities in reactivity

Abstract

Room temperature reactions of lanthanide atomic cations (excluding Pm+) with CH3Cl are surveyed systematically in the gas phase using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Reaction rate coefficients are reported along with product distributions in He at 0.35 Torr (1 Torr = 133.3224 Pa) and 295 K. Cl atom transfer is the predominant reaction channel observed with all 14 lanthanide cations, but minor CH3Cl addition also occurs with the late lanthanide cations Dy+, Ho+, Er+, Tm+, and Yb+. The reaction efficiency for Cl atom transfer is shown to be governed by the energy required to promote an electron to achieve a d1s1excited 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 C—Cl bond insertion mechanism of the type proposed previously for insertion reactions of Ln+cations with hydrocarbons and methyl fluoride. Direct Cl atom abstraction by a harpoonlike mechanism was excluded because of an observed noncorrelation 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 (1.4 ± 0.2) × 104and (4.5 ± 0.3) × 103 K, respectively. A rapid second Cl atom transfer occurs with LaCl+, CeCl+, GdCl+, TbCl+, and LuCl+, but there was no evidence for a third chlorine atom abstraction with any of the LnCl2+cations. Both LnCl+and LnCl2+add up to five methyl chloride molecules under the experimental operating conditions of the ICP/SIFT tandem mass spectrometer.Key words: lanthanide cations, Cl atom transfer, electron promotion, methyl chloride.