Correlation between adsorption and small molecule hydrogen bonding

Previous studies indicate that phenolic solutes adsorb from hexane onto an acrylic ester sorbent (XAD-7, Rohm and Haas) through a hydrogen-bonding mec hanism. Because of experimental Limitations, adsorption studies typically p rovide only thermodynamic energies but no direct mechanistic information. T o overcome this limitation, we used ethyl propionate as a small molecule an alogue of the acrylic ester sorbent and studied solution-phase hydrogen bon ding between ethyl propionate and various phenolic solutes. Consistent with a hydrogen-bonding mechanism, FTIR spectra of hexane solutions containing phenols showed that the hydroxyl stretching peak was broadened and displace d to lower wavenumbers in the presence of ethyl propionate. Molecular model ing showed hydrogen bonding between the,phenolic hydroxyl and the carbonyl oxygen of ethyl propionate. Qualitatively, small molecule hydrogen bonding studies provide evidence that the poor adsorption of 2,6-disubstituted phen ols is due to steric limitations. For 2,6-dimethylphenol and 2,6-di-tert-bu tylphenol, IR spectra showed suppressed hydrogen bond formation with ethyl propionate, whereas molecular modeling showed that the hydrogen bonds that did form were lengthened and distorted compared to those of phenol. The sma ll molecule binding studies also provided qualitative evidence that the poo r adsorption of a-methoxyphenol and 2-chlorophenol is due to competing intr amolecular hydrogen bonds. Quantitatively, it was observed that for a serie s of phenolic solutes the adsorption affinity cross-correlates to the IR fr equency shift observed for hydrogen bonding to ethyl propionate. Correlatio ns between adsorption affinities and computed binding energies were limited because of the insensitivity of computed binding energies to substituent e ffects. These studies indicate that the small molecule analogue provides a convenient system for studying hydrogen-bonding interactions that affect ad sorption onto the polymeric adsorbent.