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 a dsorption 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 affini ty than their ortho isomers. The adsorption results are interpreted using i nfrared spectroscopy, free energy calculations, and ab initio calculations using the small molecule ethyl propionate (EP) as an analogue of the hydrog en bond accepting site of the acrylic ester sorbent. Free energy calculatio ns show that p-MHB and p-HAP bind to EP primarily via an intermolecular hyd rogen bond to the carbonyl oxygen of EP. In contrast, o-MHB and o-HAP posse ss strong intramolecular hydrogen bonds that are retained upon complexation with EP. As a consequence, complexation between the ortho isomers and EP i nvolves 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 bec ause 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 hydroge n-bonding interactions prevent intermolecular (i.e., adsorptive) hydrogen b onding for the ortho isomers and account, at least in part, for the observe d para/ortho selectivity.