Magnetic-Shielding Calculations on Al₄²⁻ and Analogues. A New Family of Aromatic Molecules?

The molecular structures, the nuclear magnetic shieldings, and the aromatic ring-current shieldings (ARCS) have been calculated for Al 4 2-, Al 4 Li -, and Al 4 Cu - at the Hartree−Fock (HF) level, the second-order Møller−Plesset (MP2) level, the coupled-cluster singles and doubles (CCSD) level, and the coupled-cluster singles and doubles level augmented by a perturbative correction for triple excitations (CCSD(T)). The ARCS calculations show that the square-shaped Al 4 2- ring sustains a very large diatropic ring current in an external magnetic field. Because the induced ring current is one measure of the molecular aromaticity, the Al 4 2- ring can be considered aromatic. Molecular structure optimizations on the group IIIA analogues show that B 4 2-, Ga 4 2-, In 4 2-, and Tl 4 2- also exist and have D 4 h symmetry. The ARCS calculations indicate that they are aromatic, too. New neutral Al 4 2- analogues such as Si 2 B 2, Si 2 Al 2, and Si 2 Ga 2 are proposed. The molecular structure and ARCS calculations on the neutral analogues yield planar ring structures with large diatropic ring-current susceptibilities.