A quantum-mechanical description of macrocyclic ring rotation in benzylic amide [2]catenanes

Catenanes can undergo rotation of one ring through the cavity of the other. Since macroscopic and molecular properties must clearly vary with the rela tive positions and orientations of the interlocked components, a complete u nderstanding of the way that the rings rotate is of considerable importance . Here we show that low-dimensional quantum-mechanical modeling can yield r ate constants and barriers similar to those obtained by temperature-depende nt nuclear magnetic resonance experiments. Data from both non-hydrogen bond disrupting (e.g. CDCl3) and hydrogen bond disrupting (e.g. [D-6]DMSO) solv ents are well reproduced demonstrating the validity of the model. The succe ssful simulation of the rates of circumrotations by entirely harmonic trans ition state theory originates from the description of the anharmonic levels of the systems through an effective harmonic frequency, not very different from twice the zero point energy. The nature of the model makes it extenda ble, in principle, to the calculation of properties dependent upon circumro tational activity.