Experimental detection and theoretical characterization of germanium-doped lithium clusters Li(n)Ge (n = 1-7).

We report a combined experimental and quantum chemical study of the small neutral and cationic germanium-doped lithium clusters Li(n)Ge(0,+) (n = 1-7). The clusters were detected by time-of-flight mass spectrometry after laser vaporization and ionization. The molecular geometries and electronic structures of the clusters were investigated using quantum chemical calculations at the DFT/B3LYP and CCSD(T) levels with the aug-cc-pVnZ basis sets. While Li3Ge(0,+) and Li4Ge+ prefer planar structures, the clusters from Li4Ge to Li7Ge and the corresponding cations (except Li4Ge+) exhibit nonplanar forms. Clusters having from 4 to 6 valence electrons prefer high spin structures, and low spin ground states are derived for the others because valence electron configurations are formed by filling the electron shells 1s/1p/2s/2p based on Pauli's and Hund's rules. Odd-even alternation is observed for both neutral and cationic clusters. Because of the closed electronic shells, the 8- and 10-electron systems are more stable than the others, and the 8-electron species (Li4Ge, Li5Ge+) are more favored than the 10-electron ones (Li6Ge, Li7Ge+). This behavior for Ge is different from C in their doped Li clusters, which can be attributed to the difference in atomic radii. The averaged binding energy plot for neutrals tends to increase slowly with the increasing number of Li atoms, while the same plot for cations shows a maximum at Li5Ge+, which is in good agreement with the mass spectrometry experiment. Atom-in-molecules (AIM) analysis suggests that Li atoms do not bond to one another but through Ge or pseudoatoms, and an essentially ionic character can be attributed to the cluster chemical bonds. An interesting finding is that the larger clusters have the smallest adiabatic ionization energies known so far (IEa approximately 3.5 eV).