Magnetic Anisotropic Effects in Charged Aza[10]annulene Analogs with a Non-planar Carbon Framework

Classically, aromaticity portrays the unique stability and peculiar reactivities of cyclic planar conjugated systems with (4n+2) π electrons. Understanding the electronic environments in new chemical frameworks through experimental and theoretical validation is central to this ever-expanding theme in chemical science. Such investigations in curved π-surfaces have special significance as they can unravel the variations when the planarity requirement is slightly lifted. In this report, we discuss the synthesis, spectroscopic and theoretical studies involving a new group of cyclazine analogs having a charged aza[10]annulene periphery, centrally locked through a sp³ carbon. Magnetic anisotropic effects arising from electron delocalization through its curved π-surface were mapped through a specific set of chemical groups introduced through this sp³ carbon. The nucleus-independent chemical shift calculations revealed negative chemical shift values, indicating the aromatic nature of the aza[10] annulene rim. This is corroborated by a clockwise diatropic ring current, evident from anisotropy-induced current density analysis. Variations in the chemical shift of NMR signals in these systems were also computationally examined through isotropic chemical shielding surface analysis.