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

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 (D3h, n = 6), then to three-dimensional tetrahedral (Td, n = 8) and square pyramid (C4v, n = 10) types. Further natural bond orbital analyses show that electrons are transferred from the lone pair on Nα of every N2 unit to the empty orbitals of lithium atom in LiN2-8+, while in LiN10+, electrons are transferred from the bonding orbital of the Li-Nα bonds to the antibonding orbital of the other Li-Nα 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 LiN2-8 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).