Noncovalent Interactions Accompanying Encapsulation of Resorcinol within Azacalix[4]pyridine Macrocycle

Electronic structure and noncovalent interactions within the inclusion complexes of resorcinol and calix[4]pyridine (CXP[4]) or azacalix[4]pyridine (NCXP[4]) macrocycles have been analyzed by employing hybrid M06-2X density functional theory. It has been demonstrated that substitution of a heteroatom (-NH-) the bridging position of the CXP[4] alters the shape of the cavity from a box-shaped to funnel-like one. Penetration of resorcinol guest within the CXP[4] cavity renders a butterfly like structure to the complex, whereas the N-CXP[4] complex reveals distorted geometry with the guest being nearer to one of the pyridine rings at the upper rim of the host. Underlying hydrogen bonding, pi..pi and characterized using the Quantum Theory of Atoms in Molecules (QTAIM) and Noncovalent Interactions Gradient (NCI-RDG) methods. The coexistence of multiple intermolecular interactions is envisaged through the frequency shifts of the characteristic -NH -OHvibrations in their calculated vibrational spectra. The guest protons confined to the host cavity exhibit shielding, while those facilitating hydrogen bonding engender the downfield signals in their calculated H-1 NMR spectra.