Aromatic Surprise

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Science  11 Jan 2008:
Vol. 319, Issue 5860, pp. 137
DOI: 10.1126/science.319.5860.137d

Unsaturated hydrocarbon rings are constructively classed by their number of electrons available for pi-bonding: A multiple of four confers instability in a planar configuration (as in cyclobutadiene), whereas even nonmultiples of four (as in benzene and naphthalene) confer exceptional stability, termed aromaticity. Cyclic analogs in which the carbons are replaced by heavier atoms can be subject to greater degrees of spin-orbit coupling, and it is thus unclear how straightforwardly the aromaticity picture extrapolates.

Ugrinov et al. explored this question in analyzing an unusual compound they prepared and crystallographically characterized—a dianionic parallelogram with arsenic (As) and tellurium (TE) centers at opposite corners and charge-balancing potassium cations sequestered by crown ethers above and below the plane. A simplistic analysis would suggest that four lone pairs (one each per As and Te center) should sum to an anti-aromatic configuration. However, calculations using density functional theory suggest that the experimental geometry is consistent with a spin triplet ground state and that the net electronic configuration is in fact closer to an aromatic system. — JSY

J. Am. Chem. Soc. 10.1021/ja075513l (2007).

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