Molecular Rotor Inside a Phosphonate Cavitand: Role of Supramolecular Interactions

A supramolecular rotor is designed based on a phosphonate cavitand center dot center dot center dot methanol/ethanol complex. Density functional theory (DFT) calculations reveal a very small barrier for rotation of methanol inside the cavity. The origin for such almost barrierless rotation is traced to the low-energy cost associated with breaking the weak O-H center dot center dot center dot O interactions in the ground-state complex and substantial stabilization of the transition state (TS) due to C-H center dot center dot center dot pi interactions. Available crystal structures of the complexes are indicative such supramolecular rotation. Further experimental signatures are predicted through NMR calculations to detect such rotations at liquid N-2 temperature.