Structural evolution of LiNn+ (n = 2, 4, 6, 8, and 10) clusters: mass spectrometry and theoretical calculations

Abstract

Mixed nitrogen-lithium cluster cations LiN_n⁺ were generated by laser vaporization and analyzed by time-of-flight mass spectrometry. It is found that LiN₈⁺ has the highest ion abundance among the LiN_n⁺ ions in the mass spectrum. Density functional calculations were conducted to search for the stable structures of the Li–N clusters. The theoretical results show that the most stable isomers of LiN_n⁺ clusters are in the form of Li⁺(N₂)_n/2, and the order of their calculated binding energies is consistent with that of Li–N₂ bond lengths. The most stable structures of LiN_n⁺ evolve from one-dimensional linear type (C_∞v, n = 2; D_∞h, n = 4), to two-dimensional branch type (D_3h, n = 6), then to three-dimensional tetrahedral (T_d, n = 8) and square pyramid (C_4v, n = 10) types. Further natural bond orbital analyses show that electrons are transferred from the lone pair on N_α of every N₂ unit to the empty orbitals of lithium atom in LiN_2–8⁺, while in LiN₁₀⁺, electrons are transferred from the bonding orbital of the Li–N_α bonds to the antibonding orbital of the other Li–N_α bonds. In both cases, the N₂ units become dipoles and strongly interact with Li⁺. The average second-order perturbation stabilization energy for LiN₈⁺ is the highest among the observed LiN_n⁺ clusters. For neutral LiN_2–8 clusters, the most stable isomers were also formed by a Li atom and n/2 number of N₂ units, while that of LiN₁₀ is in the form of Li⁺(N₂)₃(η¹-N₄).