Intramolecular versus Intermolecular Hydrogen Bonding in the Adsorption of Aromatic Alcohols onto an Acrylic Ester Sorbent

Abstract

Intramolecular hydrogen bonds influence intermolecular binding in adsorption and molecular recognition, but the interplay between intra- and intermolecular hydrogen bonding is poorly understood. In this study, a series of four aromatic alcohols, 2-phenylethanol, 3-phenyl-1-propanol, 2-phenoxyethanol, and 3-phenoxy-1-propanol, are examined to determine the effect of intramolecular hydrogen bond formation on the binding to ethyl propionate (EP), an analogue of an acrylic ester separation resin. A combination of infrared spectroscopy, molecular modeling, and ab initio calculations are used to investigate the conformational preferences of the alcohols and the alcohol:EP complexes in hexane. Without EP, 2-phenylethanol and 2-phenoxyethanol prefer intramolecularly hydrogen-bonded conformations, whereas 3-phenyl-1-propanol overwhelmingly favors a conformer without an intramolecular hydrogen bond. For 3-phenoxy-1-propanol, there is a smaller preference for conformers without an intramolecular hydrogen bond. These results agree qualitatively with the experimentally measured IR spectra. The conformational preferences are explained by examining the energy components of low-energy conformers. Electrostatic interactions favor the intramolecularly hydrogen-bonded species, whereas the dihedral energy term and entropic term favor conformers without an intramolecular hydrogen bond. The balance determines the most stable conformer. The calculations predict that all four alcohols bind EP weakly compared with para-methoxyphenol. This ranking is in good agreement with experimental adsorption measurements. The small calculated ΔG° values of ≈ −0.9 to −2.4 kJ/mol for the alcohols is explained in terms of hydrogen bond donating ability, entropy, and the competition between inter- and intramolecular hydrogen bonds.