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 LiN(8)(+) 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(+)(N(2))(n/2), and the order of their calculated binding energies is consistent with that of Li–N(2) 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 N(2) unit to the empty orbitals of lithium atom in LiN(2–8)(+), while in LiN(10)(+), 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 N(2) units become dipoles and strongly interact with Li(+). The average second-order perturbation stabilization energy for LiN(8)(+) 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 N(2) units, while that of LiN(10) is in the form of Li(+)(N(2))(3)(η(1)-N(4)).