Solidly Aromatic

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Science  11 Nov 2011:
Vol. 334, Issue 6057, pp. 739
DOI: 10.1126/science.334.6057.739-b

Chemists have long been intrigued by the unusual stability conferred by the pi-bonding arrangements in benzene and related unsaturated carbon rings and heterocycles. The phenomenon, termed aromaticity, involves delocalization of a specific number of contiguous electrons (even non–multiples of four) in a plane. In recent years, researchers have increasingly sought to coax heavier congeners of carbon, such as silicon and germanium, into aromatic motifs. Because these elements tend to favor non-planar bonding geometries, bulky substituents must be appended to push molecular frameworks toward planarity. Kuhn et al. explore a preexisting compound with a complementary means of imposing planarity: the constraints of a crystal lattice. Specifically, they probe a Zintl phase of the crystalline lithium silicide Li12Si7. The lattice contains alternating layers of planar five-membered silicon rings and compensating lithium ions, and previous theoretical analyses have predicted aromatic stabilization in these rings. A hallmark of aromaticity is the impact of magnetically induced ring currents on the chemical shifts of nearby nuclei (pushed upfield near the ring's interior; downfield on its periphery) in nuclear magnetic resonance (NMR) spectra. The authors therefore probed samples with solid-state Li NMR spectroscopy, and indeed observed a telltale upfield shift associated with the Li ions centered between the rings.

Angew. Chem. Int. Ed. 50, 10.1002/anie.201105081 (2011).

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