Intramolecular proton transfer in calixphyrin derivatives

Calix[4]phyrins are convenient models for porphyrin metabolism intermediates. They also attract attention as complexation agents in macromolecular chemistry. For the biological function and chemical properties, their flexibility plays an important role. In this study, we explore the inner hydrogen motion previously detected in some calix[4]phyrins as a slow chemical exchange in the NMR spectra (J. Am. Chem. Soc. 2004, 126, 13714). The potential energy surface of this motion is defined by two generalized coordinates and modeled at DFT levels. The transitional barriers thus obtained agree reasonably well with those calculated from dynamic (1)H NMR measurements. The transitions over both the sp(2) and sp(3) links are found to be possible. During the transfer, an intermediate is formed where the two hydrogen atoms are attached to the neighboring nitrogens. Surprisingly, the barriers channeling the sp(2) and sp(3) paths in significantly depend on the dispersion and other interactions between remote side chains. The results confirm the possibility of a fine-tuning of the calixphyrin core properties by distant substituents.