Joint experimental and theoretical studies of the surprising stability of the aryl pentazole upon noncovalent binding to β-cyclodextrin

Abstract

Herein, binding of the β-cyclodextrin (β-CD) host to the unstable aryl pentazole guest has been confirmed experimentally and theoretically. After the confinement of aryl pentazole, electron density reorganization was studied by M06-2X dispersion-corrected DFT and further reflected in the characteristic shift in the NMR spectra. Among the host–guest complexes, the inclusion complex is favored with the phenyl ring expectedly encapsulated within the cavity through noncovalent interactions such as hydrogen bonding, C–H⋯π, and the special C_sp²–H⋯H–C_sp³ bonding discovered by the NBO, QTAIM, and NCI-RDG theories. The host–guest binding renders the enhancement of the nitrogen-ring aromatic character; this has been analyzed by employing nucleus-independent chemical shift (NICS)-based profiles. The non-covalent interaction largely enhances the thermal stability of the guest through a change of the decomposition reaction path whether the guest is encapsulated or not by the host. By comparison of the structures of B1–B4, the enhancement could be assigned to the ion-type transition structure stabilized by the C–H bonds of the host.