FT-ICR Study on Hydrogenation of Niobium Cluster Cations Nb_n⁺(n = 2-15) in Seeded Supersonic Jet and Multiple-Collision-Induced Dissociation of Nb_nH_m⁺ Hydrides

Hydrogenation of niobium cluster cations Nb_n⁺ (n = 2-15) in a seeded supersonic jet of H₂/He and multiple-collision-induced dissociation (MCID) of the resulting Nb_nH_m⁺ hydrides have been studied using an FT-ICR mass spectrometer. The nascent Nb_nH_m⁺ hydrides trapped in the FT-ICR cell have broad m distributions with no apparent prevalence of odd or even m. A pulse of argon applied to the trapped clusters causes a dramatic squeezing of the initial m distribution (through the collision-induced removal of weakly bound H₂ molecules), favoring several particular hydrides for each cluster size n, e.g., Nb₇H₈⁺, Nb₇H₁₁⁺, and Nb₇H₁₂⁺ for n = 7. The maximum m values of these stable hydrides are close to the stoichiometric composition of NbH₂ for the clusters with n<13, and approach that of NbH at larger n. The hydrides observed in our experiments are different from the products of the Nb_n⁺ + H₂ reactions performed in the FT-ICR cell at room temperature, which show only even and strongly n-dependent m values. The MCID of the Nb_nH_m⁺ clusters occurs through the sequential desorption of H₂ molecules yielding Nb_nH⁺ and Nb_n⁺ as final dissociation products for odd and even m, respectively. Based on the experiments on the MCID of Nb₁₂H₁₈⁺, an explanation is suggested for different reactivities of the Nb₁₂⁺ clusters towards H₂ in the ICR and fast-flow-reactor experiments.