COMBINED NMR-SPECTROSCOPY AND MOLECULAR MECHANICS STUDIES ON THE STABLE STRUCTURES OF CALIX[N] ARENES

All possible conformations of unmodified calix[n]arenes (1(n): n = 4-7) were systematically generated by CONFLEX3 using corresponding model compounds 2,: the numbers of the trial conformations and the final conformations are 110 and 5 for 2(4), 645 and 23 for 2(5), 4438 and 133 for 2(6) and 31620 and 809 for 2(7). These initial structures for 1(n) were optimized by MM3(92): the numbers of the energy minima are 4 for 1(4), 10 for 1(5), 90 for 1(6) and 651 for 1(7). The most stable structures screened out of these conformational isomers were compared with those predicted by H-1 NMR spectroscopic studies and with those determined by X-ray crystallographic studies. It was shown that (i) the small-ring calixl[n]arenes (1(4) and 1(5)) keep the high molecular symmetry because stable intramolecular hydrogen bonds among the OH groups can be formed in the regular cone conformer whereas the large-ring calix[n]arenes (1(6) and 1(7)) lose the molecular symmetry and tend to adopt a 'pinched' conformation because the formation of the stable intramolecular hydrogen bonds inevitably induces ring deformation, (ii) there exists no special torsional difference between even-numbered calix[n]arenes (1(4) and 1(6)) and odd-numbered calix[n]arenes (1(5) and 1(7)) and (iii) the structures obtained from X-ray analysis and NMR spectroscopic studies can be generated as the most stable structures in the computational studies. The results have enabled us to give reasonable rationales to several controversial problems that have been troubling calixarene chemists.