Theoretical determination of lowest structures of all-metal aromatic clusters M_4L_2 (M = Al, Ga, In, Tl; L = Li, Na, K, Rb, Cs)

摘要

The researches of all-metal aromatic clusters have been a thermic theme in inorganic aromaticity domain both experimentally and theoretically since the Al_4L~- (L = Li, Na, Cu) clusters were created by laser vaporization. In systemic determination of the lowest structures of 20 gaseous all-metal aromatic clusters M_4L_2 (M = Al, Ga, In, Tl; L = Li, Na, K, Rb, Cs), the isomer energy differences of four lowlying structures of each cluster were evaluated at high-quality quantum chemistry levels. Single point calculations at the coupled cluster level were performed at geometries optimized at the MP2, B3LYP, and B3PW91 levels, and harmonic frequency calculations and zero point energy corrections were implemented following optimizations at the B3LYP and B3PW91 levels. In addition to Li- and Na-containing species, theoretical investigations came down to those new clusters including K, Rb, and Cs. For many clusters, the most convincing theoretical evidences indicate that the lowest structures are a square bipyramidal isomer rather than an edge-caped square pyramidal species. A few discrepancies were addressed at the MP2, B3LYP, and B3PW91 levels in comparison with the coupled cluster results. These findings are significant because some clusters were generated by laser vaporization and served as theoretical prototypes to test the new means for assessing aromaticity.