Nitrogen dioxide reactions with atomic lanthanide cations and their monoxides: gas-phase kinetics at room temperature

Abstract

Results of experimental investigations are reported for the gas-phase kinetics of chemical reactions between nitrogen dioxide (NO₂) and 14 different atomic cations of the lanthanide series, Ln⁺ (Ln = La–Lu, excluding Pm), and their monoxides, LnO⁺. Measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer in helium buffer-gas at a pressure of 0.35 ± 0.01 Torr and at 295 ± 2 K. The atomic lanthanide cations were produced at ca. 5500 K in an ICP source and allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction with NO₂. The atomic ions were observed to react rapidly with NO₂ with large rate coefficients, k > 2 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, and almost exclusively by oxygen-atom abstraction to produce lanthanide-oxide LnO⁺ cations. In contrast to results of previous studies with many other molecules, the reaction efficiency exhibits essentially no dependence upon the energy required to promote an electron to achieve a d¹s¹ excited electronic configuration, in which two non-f electrons are available to Ln⁺ for chemical bonding. Apparently the radical character of NO₂ (X ²A₁) leads to the efficient formation of LnO⁺ by the end-on abstraction of an oxygen atom by Ln⁺. In the reactions with La⁺, Ce⁺, Pr⁺ and Gd⁺ an additional minor channel (less than 2%) leads to the formation of NO⁺. The LnO⁺ product ions participate in various secondary and higher order reactions with NO₂ resulting in the formation of ions of the type LnO_x(NO)_y(NO₂)_z⁺ with x = 1–2, y = 0–2, and z = 0–2, as well as the ions NO⁺ and NO₂⁺.