X-ray Diffraction, Spectroscopy, Optical Properties, NPA, NBO, FMO, and Hirshfeld Surface Analyses of Two Newly Synthesized Piperidinium Ionic Liquids

The present study elaborates on the synthesis, crystal structure, and computational studies of two new ionic liquids. In the crystal structure, [C5H12N][C21H14ClN2O2S] (4a), the anions form chains along the a-axis direction through C-H center dot center dot center dot pi(ring) interactions. These are connected into layers that run approximately parallel to the ac plane by a variety of hydrogen bonds. In the compound structure, [C5H12N][C18H15N2O2S] (4b), the two ions are primarily associated by an N-H center dot center dot center dot N hydrogen bond. In the crystal structure, layers parallel to the bc plane are formed by pairs of C-H center dot center dot center dot O and N-H center dot center dot center dot S hydrogen bonds and by C-H center dot center dot center dot pi(ring) interactions. A theoretical study reveals that 4a has lower energy than 4b and is more stable. The NBO and DOS studies further confine the liquids' structural reactivity and electronic properties. The quantum theory of atoms in a molecule (QTAIM) analysis reveals the important non-covalent interactions among the fragments and charge transfer. The global reactivity descriptors indicate their molecular reactivity relationship with the presence of functional groups. The remarkable polarizability (alpha o) and hyperpolarizability (beta o) values indicate their optical and nonlinear optical (NLO) properties. Furthermore, the analysis performed by CrystalExplorer shows the intermolecular interactions and reactive sites between cations and anions in ionic liquids. The 2D fingerprint plots and Hirshfeld surfaces indicate the major interactions of crystals with neighboring elements in crystal packing. For both compounds, the H center dot center dot center dot H interactions are significantly higher than the other element interactions.