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

Mixed nitrogen-lithium cluster cations LiN ⁺ 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 ⁺ 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 ⁺ clusters are in the form of Li⁺(N₂) , and the order of their calculated binding energies is consistent with that of Li-N₂ bond lengths. The most stable structures of LiN ⁺ 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 ⁺ 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).