On the Aromaticity of Puckered Ions C₄H₄²⁺ and B₄H₄²⁻: Deciphering the Origin of Nonplanarity

Abstract

Density functional theory and Møller‐Plesset perturbation theory techniques have been employed to delve into the nonplanarity and aromaticity of C4H42+and B4H42−. C4H42+ is a well‐known two‐electron aromatic system with puckered four‐membered ring, while analogues of B4H42− (two‐electron aromatic system with puckered four‐membered ring) have been recently synthesized and their aromatic behavior has been argued. The present work aims at presenting a computational insight into the origin of nonplanarity of C4H42+ and B4H42−, as well as characterizing their aromatic behavior given that aromaticity in a nonplanar structure is not so common. Our results suggest an important role of the second‐order Jahn‐Teller effect (SOJTE) underlying the nonplanarity of C4H42+ and B4H42−. The aromatic behavior of the studied systems has been characterized by various parameters including the nucleus‐independent chemical shift (NICS) scan, σ‐π energy separation analysis, energy decomposition analysis, study of magnetic susceptibility, and bond‐order analysis.