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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···π(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···N hydrogen bond. In the crystal structure, layers parallel to the bc plane are formed by pairs of C—H···O and N—H···S hydrogen bonds and by C—H···π(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 (αo) and hyperpolarizability (βo) 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···H interactions are significantly higher than the other element interactions.

The asymmetric unit of the title compound, C25H21N3O2S, comprises four molecules. Their conformations differ primarily in the orientations of the styryl and the N-phenylcarboxamido groups. In the crystal, intermolecular N—H...N, C—H...O and C—H...S hydrogen-bonding contacts as well a C—H...π(ring) interactions lead to the formation of a layer structure parallel to (010). Hirshfeld surface analysis revealed that H...H interactions represent the main contributions to the crystal packing.

In the title molecule, C21H20N2O3S, the styryl and ester substituents are displaced to opposite sides of the plane of the pyridine ring. In the crystal, C—H...O hydrogen bonds form chains extending parallel to the a-axis direction, which pack with partial intercalation of the styryl and ester substituents. A Hirshfeld surface analysis indicates that the most significant contributions to the crystal packing are from H...H (43.6%), C...H/H...C (15.6%), O...H/H...O (14.9%) and N...H/H...N (11.2%) contacts.

The title molecule, C29H29N3O4S, adopts a conformation with the two phenyl substituents disposed on opposite sides of the mean plane of the isoquinoline unit. In the crystal, corrugated layers of molecules are formed by N—H...O, C—H...N and C—H...S hydrogen bonds together with C—H...π(ring) interactions. These layers are connected by C—H...O contacts. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H...H (45.2%), C...H/H...C (20.2%), O...H/H...O (15.8%) and N...H/H...N (11.0%) interactions.

In the title molecule, C28H25Cl2N3O3S, the heterocyclic portion of the tetrahydroisoquinoline unit is planar while the cyclohexene ring adopts a twist-boat conformation. The two 4-chlorophenyl groups extend away from one side of this unit while the hydroxyl and acetyl groups extend away from the opposite side and form an intramolecular O—H...O hydrogen bond. The crystal packing consists of layers parallel to the bc plane. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions to the crystal packing are from H...H (37.3%), Cl...H/H...Cl (17.6%), O...H/H...O (11.1%), C...H/H...C (10.9%) and N...H/H...N (9.7%) interactions.