Understanding Binding of Cyano-Adamantyl Derivatives to Pillar[6]arene Macrocycle from Density Functional Theory

The ωB97X-based density functional theory has been employed to characterize molecular interactions between adamantane carbonitrile (ACN) or adamantane methyl carbonitrile (AMCN) and the ethylated pillar[6]arene (EtP[6]) molecular receptor. The inclusion complexes in 1:1 stoichiometry are stabilized through noncovalent interactions such as hydrogen bonding, C-H···π and dipole-dipole interactions. Gibbs free energies accompanying the encapsulation of ACN or AMCN within EtP[6] revealed that the formation of complex is spontaneous and thermodynamically favorable. Underlying interactions are unraveled through quantum theory of atoms in molecules and molecular electrostatic potential topography. Structural changes consequent to guest encapsulation have been rationalized through characteristic infrared and NMR spectra. The frequency downshifts for -C≡N stretching of the guest accompanying the complexation has been attributed to hydrogen bonding and C-H···π interactions. The methylene vibrations of ACN reveal the frequency shifts in opposite directions consequent to distinct binding features with EtP[6] host. The selective binding of AMCN further brings about a significant distortion of the host cavity. Calculated ¹H NMR spectra of ACN and AMCN complexes show shielded signals for the adamantyl protons in consonance with experiment.