Conformational landscape of a chiral crown ether: a vibrational circular dichroism spectroscopy and computational study

Herein we report a study of the most stable conformers of the chiral crown ether (all-S)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18c6H4), which constitutes a key building block for materials in asymmetric catalysis and enantiomeric separation. Infrared and vibrational circular dichroism spectroscopies are employed to characterize the vibrational modes of the 18c6H4 molecule in a low polarity solvent (CDCl3). The investigation covers a broad range of wavenumbers (800–4000 cm−1) and focuses particularly on the O–H, C–H, CO and C–O stretching modes and on the C–O–H bending mode, which yield the most intense IR and VCD signals. The experimental results are combined with a computational conformational survey and structure optimization at the B3LYP/6-311++G(2d,2p) level. The conformational landscape of 18c6H4 is found to be governed by bowl-like structures sustained by various types of H-bonding arrangements of the carboxylic side chains with each other and with the central crown ether ring.

Graphical abstractLowest energy conformer of the (all-S)-18c6H4 and details of the IR–VCD spectra.Figure optionsDownload full-size imageDownload as PowerPoint slide