Molecular Rotor Inside a Phosphonate Cavitand: Role of Supramolecular Interactions

A supramolecular rotor is designed based on a phosphonate cavitand···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···O interactions in the ground-state complex and substantial stabilization of the transition state (TS) due to C−H···π interactions. Available crystal structures of the complexes are indicative of such supramolecular rotation. Further experimental signatures are predicted through NMR calculations to detect such rotations at liquid N₂ temperature