Solid-State NMR Spectroscopy Study of Molecular Dynamics of Pyridine 1-Oxide Encapsulated in...

The mixture of guests 25% pyridine 1-oxide (PNO) and 75% nitrobenzene (NB) encapsulated in a host cavity of p-tert-butylcalix[4]arene (p-tBC) is a suitable system for the study of intermolecular weak interactions, and the preferred orientation of the guest molecules within the host was derived. Variable temperature deuterium nuclear magnetic resonance line shape and spin-lattice relaxation studies were performed on three samples of 25% PNO as a guest (selectively and entirely deuterated) with 75% NB encapsulated in p-tBC as a host system. It is found that the PNO molecular motion supports the two-site jump model and is highly mobile throughout the temperature range from -130 to 20 °C. PNO reorients about its C₂ molecular symmetry axis followed by reorientation around the compound's C₄ axis of symmetry of a host. Calculation showed that besides precise molecular jumping, the guest also experiences the vibrational motion with a variation angle dispute about 15°. The correlation times for the molecular guest motion and two-dimensional exchange spectroscopy confirmed a fast exchange limit inside the cavity. These molecular dynamics follow an Arrhenius behavior motion from which the small activation energy is delivered that strongly suggests a relatively weak intermolecular interaction between the host and guest species. PNO has definite dipole orientational motion in the cavity where its aromatic covalent bond structure minimizes contact with the host p-tert-butyl groups. Experimentally found, PNO-d₅ deuterons in the single crystal collapse at 55 and 125° at ambient temperature. The first splitting of 124 kHz belongs to D₃(D₅), the second splitting of 100 kHz belongs to D₂(D₆), and the last splitting of 52 kHz belongs to D₄ of the ring of PNO inside the p-tBC.