Intramolecular Versus Intermolecular Hydrogen Bonding in the Adsorption of Aromatic Alcohols Onto an Acrylic Ester Sorbent
K L. Mardis, B J. Brune, P Vishwanath, B Giorgis, G F. Payne, M K. Gilson
Intramolecular hydrogen bonds influence the intermolecular binding constantsof adsorbate/adsorbent and guest/host systems. Understanding the competition between the two types of hydrogen bonds may result in the ability to design separation systems with enhanced selectivities. In this study, a series of four aromatic alcohols, 2-phenyl-ethanol, 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 are found to prefer intramolecularly hydrogen bonded conformations, while 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 intra-molecular 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 are found to favor the intramolecularly hydrogen bonded species, while the dihedral energy term and entropic term favor conformers without an intramolecular hydrogen bond. The balance determines the most stable conformer. Upon complexation with EP, the calculations predict that all four alcohols bind weakly as compared to para-methoxyphenol. This ranking is in good agreement with experimental adsorption measurements. The small calculated )G 's of -.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.
, Brune, B.
, Vishwanath, P.
, Giorgis, B.
, Payne, G.
and Gilson, M.
Intramolecular Versus Intermolecular Hydrogen Bonding in the Adsorption of Aromatic Alcohols Onto an Acrylic Ester Sorbent, Journal of the American Chemical Society
(Accessed June 6, 2023)