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

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 LnOx(NO)y(NO2)z+ with x = 1–2, y = 0–2, and z = 0–2, as well as the ions NO+ and NO2+.