CONFORMATIONAL ISOMERISM AND SELF-ASSOCIATION OF CALIXARENE BUILDING-BLOCKS IN NONPOLAR 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 l ength (oligomers) have been studied in solution by Fourier transform i nfrared spectrometry (FT-IR). Different intramolecular OH ... X intera ctions have been established in the calixarene building blocks, i.e. O H ... pi, OH ... OH, OH ... OCH3 and a bifurcated OH ... OH/OCH3 or OH ... OH/OH one. Concentration-dependent measurements show that the ''b iphenol'' and ''triphenol'' oligomers form cyclic dimers even at very low concentration (< 10(-5) M). The presence of a molecular conformati on where the terminal hydroxyl groups form part of a linear (cooperati ve) H-bond seems to promote cyclic self-association. Comparing p-tert- butylcalix[4]arene with p-tert-butylcalix[6]arene reveals that fixatio n of the skeleton strengthens the circular H-bond. Intramolecular OH . .. OCH3 or OH ... pi 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 per formed to support conformational assignment.