An Energetic and Topological Approach to Understanding the Interplay of Noncovalent Interactions in a Series of Crystalline Spiropyrrolizine Compounds

Synthesis of quinoline-containing spiropyrrolizine was achieved via a 1,3-dipolar cycloaddition reaction of azomethine ylide (generated in situ from ninhydrin and l-proline) and (E)-2-styrylquinoline. The synthesized compounds were characterized by ¹H NMR, ¹³C NMR, HRMS, and single-crystal XRD analysis. The XRD data revealed that the solid-state structures of the compounds belong to the monoclinic system of the space group P2₁/c and are stabilized through various weak noncovalent interactions such as C-H···O, C-H···π, and π···π interactions. The noncovalent interactions are characterized and quantified through Hirshfeld surface analysis. Moreover, the interaction energies of the intermolecular noncovalent interactions are calculated through PIXEL calculation. The PIXEL calculation provides precise interaction energy with an energy decomposition scheme. Energy Framework calculations have also been performed to delve deeper into understanding the intermolecular interactions. The intermolecular interactions are further characterized using Bader's theory of "atoms in molecules" (QTAIM) and the "noncovalent" (NCI) interaction plot index. The nature and strength of noncovalent interactions are analyzed from the topological parameters at (3, -1) bond critical points (BCPs).