Usnic Acid and the Intramolecular Hydrogen Bond. A Computational Experiment for the Organic Laboratory

Abstract

A computational experiment is described for the organic chemistry laboratory that allows students to estimate the relative strengths of the intramolecular hydrogen bonds of usnic and isousnic acids, two related lichen secondary metabolites. Students first extract and purify usnic acid from common lichens and obtain 1H NMR and IR spectra. The 1H NMR spectrum reveals three downfield singlets at 11.1, 13.3, and 18.9 ppm, which correspond to three intramolecular hydrogen bonds. The IR spectrum additionally reveals three carbonyl stretching frequencies, which represent the hydrogen bond acceptors in usnic acid. The AM1 semi-empirical method is then employed to calculate (i) the heats of hydrogen bond formation, (ii) the atomic charges on the bridging hydrogens, (iii) the bond lengths of the acceptor carbonyl groups, and (iv) carbonyl IR stretching frequencies. The computed properties are then correlated to NMR and IR spectroscopic properties to reinforce fundamental concepts of the hydrogen bond.