Reactivity of lanthanoid mono-cations with ammonia: A combined inductively coupled plasma mass spectrometry and computational investigation

The behavior of La⁺, Sm⁺, Eu⁺ and Gd ⁺ with NH₃(g) and ND₃(g) was studied to understand gas phase chemical reactions used for separations in the reaction cell of a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS). For Ln⁺ = La⁺ and Gd⁺, the primary reaction channel is the formation of the LnNH⁺ protonated nitride leading to H ₂ elimination. The LnNH(NH₃)_1-5⁺ ammonia complexes of the Ln protonated nitride are further generated. Sm ⁺ and Eu⁺ are less reactive: the protonated nitride is not detected, and only small amounts of Ln(NH₃)_0-6 ⁺ are observed. Quantum chemical calculations at the DFT, MP2, CCSD(T) and CASPT2 levels of theory were employed to explore the potential energy surfaces. For the La⁺ and Gd⁺ ions of f-block elements, the reaction pathways are composed of three steps: first the formation of LnNH₃⁺, then the isomerization to HLnNH ₂⁺, and finally the loss of H₂ associated with the formation of an LnN triple bond in the final product LnNH⁺. On the other hand, the isomerization leading to triple bond formation with H ₂ loss did not proceed for Sm⁺ and Eu⁺ ions.