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The nature of interactions between the neutral, charged and substituted cubane, within cucurbituril host were analyzed employing the dispersion corrected density functional theory. The structural comparison between the inclusion complexes of CB7 and CB8 shows a best fit for the CB7. The computed Gibbs free energy for the formation of inclusion complexes by the neutral guests were close to the experimental estimate. The dicationic guest within CB has the highest strain energy with least strain on the host. The conceptual DFT based analysis ECT analysis shows a charge transfer from the host to guest in neutral systems, while for the dicationic guest an electron transfer from guest to host has been noticed, which was further conformed from the quantitative MESP values. The computed NICS values on the cubane, are not affected by the introduction of charge/substituents implying that cubane part act as a spacer group while the functional groups present on the cubane dictates the charge transfer. In AIM analysis, the ρ value is least for the complex cubane@CB7 and highest for the cubane functionalized dicationic complex@CB complexes. The NCI-RDG analysis for the inclusion complex with dicarboxylic guest, the spike at higher density region undergoes a shift, reflecting the increase in repulsive energy. In the inclusion complexes of CB7 the patch escalation in the NCI isosurface occurs evenly in CB7 than on the CB8 inclusion complexes, signifying the fit-induced extra stability of the CB7 inclusion complexes. EDA analysis shows that the Pauli repulsive energy increase with the increase in the size of the guest molecule and among the guest the dicationic has the highest repulsive energy. The presence of higher amount of electrostatic interactions in the dicationic complex has remunerated the dispersion contribution in the dicationic complexes.

We have carried out a density functional theory study on the structures of DMSO clusters and analysed the structure and their stability using molecular electrostatic potential and quantum theory of atoms-in-molecules (QTAIM). The ground state geometry of the DMSO clusters, prefer to exist in ouroboros shape. Pair wise interaction energy calculation show the interaction between methyl groups of adjacent DMSO molecules and destabilization is created by the methyl groups which are away from each other. Molecular electrostatic potential analysis shows the existence of σ-hole on the odd numbered clusters, which helps in their highly directional growth. QTAIM analysis show the existence of two intermolecular hydrogen bonds, of type S O ⋯ H C hydrogen bonds and methyl C H ⋯ H C dihydrogen bonds. The computed ρ and Laplacian values were all positive for the intermolecular bonds, supporting the existence of noncovalent interactions. The computed ellipticity for the dihydrogen bonds have values > 2, which confirms the delocalization of electron, are mainly due to the hydrogen-hydrogen interactions of methyl groups. A plot of total hydrogen bonding energy vs the observed total local electron density shows linearity with correlation coefficient of near unity, which indicates the cooperative effects of intermolecular dihydrogen H·H bonds.

We have carried out combined experimental and density functional study on the complexation of acetaminophen and with pillar [5]arene. UV–Vis and NMR studies indicates the stoichiometry between acetaminophen and pillar [5]arene to be 1:1. The computed binding energy and the formation energy show the inclusion complex is stable, and its formation can occur at room temperature. The HOMO-LUMO gap for the inclusion complex is lower than the pristine pillar [5]arene and acetaminophen, which indicates that complex formation can be kinetically controllable. MESP and NBO analysis support the occurrence of charge transfer between the guest and host molecule. AIM analysis establishes the intermolecular bonds in the inclusion complex can be classified under a category with partially noncovalent and with a partial electrostatic in character. NCI isosurface displays blue patches due to H-bonding and green patches due to electrostatic interaction and is evenly distributed inside the pillar [5]arene, that could be responsible for the stabilization of the inclusion complex.