Competition between intra- and intermolecular hydrogen bonding: Effect on para/ortho adsorptive selectivity for substituted phenols

This study examines the molecular basis for para/ortho selectivity in the adsorption of two phenol derivatives from a nonpolar solvent onto an acrylic ester sorbent. Experimental results show that p-methyl hydroxybenzoate (p-MHB) and p-hydroxyacetophenone (p-HAP) adsorb with a 400-fold higher affinity than their ortho isomers. The adsorption results are interpreted using infrared spectroscopy, free energy calculations, and ab initio calculations using the small molecule ethyl propionate (EP) as an analogue of the hydrogen bond accepting site of the acrylic ester sorbent. Free energy calculations show that p-MHB and p-HAP bind to EP primarily via an intermolecular hydrogen bond to the carbonyl oxygen of EP. In contrast, o-MHB and o-HAP possess strong intramolecular hydrogen bonds that are retained upon complexation with EP. As a consequence, complexation between the ortho isomers and EP involves not intermolecular hydrogen bonding but other, weaker interactions. Although the free energy calculations reveal the mechanism for para/ortho selectivity, they do not reproduce the magnitude of these selectivities because of an underestimation of the para isomer binding affinities. Ab initio calculations suggest that part of this underestimation is due to a failure of the force field to adequately account for substituent effects. Overall, the results from this study indicate that competing intramolecular hydrogen-bonding interactions prevent intermolecular (i.e., adsorptive) hydrogen bonding for the ortho isomers and account, at least in part, for the observed para/ortho selectivity.