An Energetic and\nTopological Approach to Understanding\nthe Interplay of Noncovalent Interactions in a Series of Crystalline\nSpiropyrrolizine Compounds

Synthesis of quinoline-containing spiropyrrolizine was\nachieved\nvia a 1,3-dipolar cycloaddition reaction of azomethine ylide (generated\nin situ from ninhydrin and l-proline) and (<i>E</i>)-2-styrylquinoline. The synthesized compounds were characterized\nby <sup>1</sup>H NMR, <sup>13</sup>C NMR, HRMS, and single-crystal\nXRD analysis. The XRD data revealed that the solid-state structures\nof the compounds belong to the monoclinic system of the space group <i>P</i>2<sub>1</sub>/<i>c</i> and are stabilized through\nvarious weak noncovalent interactions such as C–H···O,\nC–H···π, and π···π\ninteractions. The noncovalent interactions are characterized and quantified\nthrough Hirshfeld surface analysis. Moreover, the interaction energies\nof the intermolecular noncovalent interactions are calculated through\nPIXEL calculation. The PIXEL calculation provides precise interaction\nenergy with an energy decomposition scheme. Energy Framework calculations\nhave also been performed to delve deeper into understanding the intermolecular\ninteractions. The intermolecular interactions are further characterized\nusing Bader’s theory of “atoms in molecules”\n(QTAIM) and the “noncovalent” (NCI) interaction plot\nindex. The nature and strength of noncovalent interactions are analyzed\nfrom the topological parameters at (3, −1) bond critical points\n(BCPs).