J. Juselius et al., Magnetic-shielding calculations on Al-4(2-) and Analogues. A new family ofaromatic molecules?, J PHYS CH A, 105(43), 2001, pp. 9939-9944
The molecular structures, the nuclear magnetic shieldings, and the aromatic
ring-current shieldings (ARCS) have been calculated for Al-4(2-), Al4Li-,
and Al4Cu- at the Hartree-Fock (HF) level, the second-order Moller-Plesset
(MP2) level, the coupled-cluster singles and doubles (CCSD) level, and the
coupled-cluster singles and doubles level augmented by a perturbative corre
ction for triple excitations (CCSD(T)). The ARCS calculations show that the
square-shaped Al-4(2-) ring sustains a very large diatropic ring current i
n an external magnetic field. Because the induced ring current is one measu
re of the molecular aromaticity, the Al-4(2-) ring can be considered aromat
ic. 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-4h symmetr
y. The ARCS calculations indicate that they are aromatic, too. New neutral
Al-4(2-) analogues such as Si2B2, Si2Al2, and Si2Ga2 are proposed. The mole
cular structure and ARCS calculations on the neutral analogues yield planar
ring structures with large diatropic ring-current susceptibilities.