Conformational isomerism and self-association of calixarene building blocks in non-polar solution studied by Fourier transform infrared spectrometry

In order to establish the conformational equilibrium of calix[6]arene derivatives, a series of phenolic building blocks differing in chain length (oligomers) have been studied in solution by Fourier transform infrared spectrometry (FT-IR). Different intramolecular OH ¿ X interactions have been established in the calixarene building blocks, i.e. OH ¿ ¿, OH ¿ OH, OH ¿ OCH3 and a bifurcated OH ¿ OH/OCH3 or OH ¿ OH/OH one. Concentration-dependent measurements show that the ¿biphenol¿ and ¿triphenol¿ oligomers form cyclic dimers even at very low concentration (<10¿5 M). The presence of a molecular conformation where the terminal hydroxyl groups form part of a linear (cooperative) H-bond seems to promote cyclic self-association. Comparing p-tert-butylcalix[4]arene with p-tert-butylcalix[6]arene reveals that fixation of the skeleton strengthens the circular H-bond. Intramolecular OH ¿ OCH3 or OH ¿ ¿ interaction prevents the formation of associates for the building blocks with only one terminal OH group at relatively high concentration (> 10¿2 M). Preliminary MM3 calculations were performed to support conformational assignment.