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 ).