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

Abstract

N(2)S(2) is a four-membered-ring system with 6pi electrons. While earlier proposals considered N(2)S(2) to be aromatic, recent electronic structure calculations claimed that N(2)S(2) is a singlet diradical. Our careful reexamination does not support this assertion. N(2)S(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 pi electrons). These characteristics as well as the electronic structure of N(2)S(2) are compared with those for an isoelectronic pi system, Li(2)C(4)H(4), motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N(2)S(2) and Li(2)C(4)H(4) are both essentially 2pi-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6pi electrons. This is because four of the six pi electrons occupy the nonbonding pi 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(2)S(2) and Li(2)C(4)H(4), respectively.

Electronic Structures and Molecular Properties of Chalcogen Nitrides Se2N2 and SeSN2

“…However, large LUMO occupation numbers indicated the presence of a strong antibonding effect which caused a 7% reduction to the aromatic character of S2N2. The conclusions of Jung et al 25 are for the most part in agreement with our recent high level ab initio analysis of the electronic structure of S2N2, as we also found it to be a primarily aromatic system. 16 Our detailed wave function analysis showed that S2N2 possesses 6% diradical character which could be attributed to the nitrogen atoms.…”

“…16 Our detailed wave function analysis showed that S2N2 possesses 6% diradical character which could be attributed to the nitrogen atoms. This value is in good agreement with the antibonding effect discussed by Jung et al 25 However, they do not directly relate the antibonding effect to the diradical character in their conclusions.…”

“…We show here that the chalcogen-nitrogen ring systems S2N2, Se2N2 and SeSN2 are all mainly aromatic in nature and have only a small amount (68%) singlet diradical character. However, contrary to the conclusions drawn by Jung et al, 25 the diradical character is extremely significant to take into account in theoretical calculations.…”

“…[20][21][22][23] This opinion, however, has been questioned by Gerratt et al 24 who used spin-coupled VB theory calculations to show that while the structure is a singlet diradical in nature, the diradical character is solely assigned to the sulfur atoms, 3. More recently Jung et al, 25 using different ab initio and DFT methods, have shown that S2N2 is a closed shell 2π-electron aromatic with an insignificant amount of diradical character. However, large LUMO occupation numbers indicated the presence of a strong antibonding effect which caused a 7% reduction to the aromatic character of S2N2.…”

A Critical Assessment of the Performance of Magnetic and Electronic Indices of Aromaticity

“…However, simple total electronic counts lead sometimes to incorrect results [14,[53][54][55]. In addition, electron counting does not provide a quantitative value, so comparisons of aromaticity from different compounds are not possible.…”