Aromaticity of Four-Membered-Ring 6π-Electron Systems: N_2S_2 and Li_2C_4H_4

摘要

N2S2 is a four-membered-ring system with 6 electrons. While earlier proposals considered N_2S_2 to be aromatic, recent electronic structure calculations claimed that N_2S_2 is a singlet diradical. Our careful reexamination does not support this assertion. N_2S_2 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 N_2S_2 are compared with those for an isoelectronic π system, Li_2C_4H_4, motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N2S2 and Li2C4H4 are both essentially 2-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6 electrons. This is because four of the six electrons occupy the nonbonding 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 N_2S_2 and Li_2C_4H_4, respectively.