Investigation of H-bonding of p-(3-carboxymethyl-1-adamantyl)calix[6]arene by IR spectroscopy

The IR spectra of calix[6]arenes with adamantylacetic acid (1) and adamantyl (2) substituents along the upper rim of the molecule were studied. It was shown that the formation of an intramolecular H-bond between carboxyl groups on the upper rim of the calix[6]arene molecules with adamantylacetic acid fragments does not weaken the cyclic cooperative H-bond at the lower rim of the macrocycle, which is confirmed by DFT calculations. The calculation has shown that molecules 1 and 2 take on the conformation of a compressed cone. The reactivity of adamantylcalix[6]arenes changes due to the association of carboxyl groups. The formation of dimeric complexes due to H-bonds of carboxyl groups along the upper rim of molecule 1 does not lead to a noticeable redistribution of the charges of hydroxyl groups on the lower rim. Heating a sample of 1 to a temperature of 180 degrees C destroys a small part of the H-bonds between neighboring carboxyl groups, as evidenced by the appearance of a weak band at 3535 cm(-1). However, after cooling to room temperature, the H-bonds between the carboxyl groups are restored. Heating up to 355 degrees C-leads to irreversible changes in the H-bonding system, but even at this temperature, structure destruction has not been observed. (C) 2021 Elsevier B.V. All rights reserved.