Aromaticity of Four-Membered-Ring 6π-Electron Systems: N2S2 and Li2C4H4

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Abstract
N2S2 is a four-membered-ring system with 6π electrons. While earlier proposals considered N2S2 to be aromatic, recent electronic structure calculations claimed that N2S2 is a singlet diradical. Our careful reexamination does not support this assertion. N2S2 is closed shell and aromatic since it satisfies all three generally accepted criteria for aromaticity: energetic (stability), structural (planarity with equal bond lengths), and magnetic (negative nucleus-independent chemical shift due to the π electrons). These characteristics as well as the electronic structure of N2S2 are compared with those for an isoelectronic π system, Li2C4H4, motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N2S2 and Li2C4H4 are both essentially 2π-electron aromatic systems with a formal N−S (C−C) bond order of 1.25 even though they both have 6π electrons. This is because four of the six π electrons occupy the nonbonding π HOMOs and only two electrons participate effectively in the aromatic stabilization. However, wave function analysis shows relatively large LUMO occupation numbers; this antibonding effect can be said to reduce the aromatic character by approximately 7% and 4% for N2S2 and Li2C4H4, respectively.

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