A quantum-mechanical description of macrocyclic ring rotation in benzylic amide.

Catenanes can undergo rotation of one ring through the cavity of the other. Since macroscopic and molecular properties must clearly vary with the relative positions and orientations of the interlocked components, a complete understanding of the way that the rings rotate is of considerable importance. Here we show that low-dimensional quantum-mechanical modeling can yield rate constants and barriers similar to those obtained by temperature-dependent nuclear magnetic resonance experiments. Data from both non-hydrogen bond disrupting (e.g. CDCl3) and hydrogen bond disrupting (e.g. [D6]DMSO) solvents are well reproduced demonstrating the validity of the model. The successful simulation of the rates of circumrotations by entirely harmonic transition 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 extendable, in principle, to the calculation of properties dependent upon circumrotational activity.

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