High-frequency vibrations of the simplest benzylic amide [2]catenane

Infrared and Raman spectroscopies along with molecular orbital calculations are used to study - for the first time - the vibrational motions of a topologically complicated chemical system, namely a [2]catenane of the benzylic amide type. Because of the intrinsic line width of the spectra, comparison of experiments and theory is only partially successful. The insight given by the simulations-which show that the C=O normal modes are delocalized while the N-H modes are localized - is, however, useful and makes us propose a simple model that explains both the larger line width of the spectra of the catenane with respect to those of the parent macrocycle and the great sensitivity of the infrared spectrum of the catenane to the environment. Examples of such sensitivity are the frequency shifts observed upon going from KBr to CsI, features not present in the case of the macrocycle.