All possible conformations of unmodified calix[n]arenes (1n: n = 4-7) were systematically generated by CONFLEX3 using corresponding model compounds 2n: the numbers of the trial conformations and the final conformations are 110 and 5 for 24, 645 and 23 for 25, 4438 and 133 for 26 and 31620 and 809 for 27. These initial structures for 1n, were optimized by MM3(92): the numbers of the energy minima are 4 for 14,10 for 15, 90 for 1 6 and 651 for 17. The most stable structures screened out of these conformational isomers were compared with those predicted by 1H NMR spectroscopic studies and with those determined by X-ray crystallographic studies. It was shown that (i) the small-ring calix[n]arenes (14 and 15) 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 (16 and 17) 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 (14 and 1 6) and odd-numbered calix[n]arenes (15 and 17) 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.
|Number of pages||12|
|Journal||Journal of the Chemical Society, Perkin Transactions 2|
|Publication status||Published - 1995|
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