Gas-Phase Reactions of Atomic Lanthanide Cations with Sulfur Hexafluoride: Periodicity in Reactivity

Abstract

Room-temperature reactions of sulfur hexafluoride (SF6) have been surveyed systematically with atomic lanthanide cations (Ln+, excluding Pm+) in the gas phase, using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and product distributions were measured in helium at a pressure of 0.35 Torr and temperature of 295 K. All the Ln+ cations were observed to react efficiently (k/kc > 0.24) and predominantly by single and multiple F atom and fluoride abstraction to produce both LnFn+ and SFn+(where n = 1, 2, 3). The observed periodic trend in reaction efficiency along the lanthanide row matches the periodic trend in the electron-promotion energy of the Ln+ cation. A remarkable Arrhenius-like correlation is observed for the dependence of reactivity on promotion energy: the early and late lanthanide cations exhibit effective temperatures of 45 500 and 14 000 K, respectively. SFn+ product ions are observed to be unreactive with SF6, whereas up to two molecules of SF6 have been observed to add to LnFn+ product ions under the experimental operating conditions of the ICP/SIFT tandem mass spectrometer.