Do calix[n]arenes really exist as discrete monomers in solution? Comments based on mass spectrometry

Abstract

Mass spectrometric measurements have provided unequivocal evidence that calix[n]arenes (2_n), which are so far believed to exist as discrete monomers in solution because of strong intramolecular hydrogen bonding, aggregate partially to form dimers and trimers through intermolecular hydrogen bonds. This trend is particularly conspicuous in 2₈ and next in 2₇ and only a very weak dimer peak is observable for 2₆. In contrast, such an aggregate peak is not detected for 2₄ and 2₅. The results are consistent with the prediction offered on the basis of theoretical calculations that 2₄ and 2₅ can adopt a stable C₄- and C₅-symmetrical ring structure, respectively, as well as a stable intramolecular hydrogen-bonding array, whereas 2₆, 2₇ and 2₈ feature a pinched ring conformation as their most stable structure which destabilizes the intramolecular hydrogen-bonding array. The new finding reasonably explains several curious aspects of the behaviour of 2_n such as very poor solubility of 2₈ and gelation in organic fluids.