DO CALIX[N]ARENES REALLY EXIST AS DISCRETE MONOMERS IN SOLUTION - COMMENTS BASED ON MASS-SPECTROMETRY

Mass spectrometric measurements have provided unequivocal evidence tha t calix[n]arenes (2(n)), which are so far believed to exist as discret e monomers in solution because of strong intramolecular hydrogen bondi ng, aggregate partially to form dimers and trimers through intermolecu lar hydrogen bonds, This trend is particularly conspicuous in 2(8) and next in 2(7) and only a very weak dimer peak is observable for 2(6). Tn contrast, such an aggregate peak is not detected for 2(4) and 2(5). The results are consistent with the prediction offered on the basis o f theoretical calculations that 2(4) and 2(5) can adopt a stable C-4- and C-5-symmetrical ring structure, respectively, as well as a stable intramolecular hydrogen-bonding array, whereas 2(6), 2(7) and 2(8) fea ture a pinched ring conformation as their most stable structure which destabilizes the intramolecular hydrogen-bonding array, The new findin g reasonably explains several curious aspects of the behaviour of 2(n) such as very poor solubility of 2(8) and gelation in organic fluids.