Jozef Hodyl PhD Thesis - Theses - Flinders University

This phenomenon correlates well with the chemical shifts obtained with receptor26. 186 A difference from the studies of the complexation of both these histidinateswith receptor 26, is that with the latter the logK values for both complexes are ofequal magnitude, logK (4.2 ± 0.2), whereas the logK values obtained for thecomplexes with receptor 49, differ marginally, D-histidinate, logK (3.54 ± 0.09) andL-histidinate, logK (3.37 ± 0.04) . This indicates slight thermodynamic selectivitytoward the D-enantiomer with receptor 49. It is noteworthy that the H A resonance forL-histidinate is broader than the one for D-histidinate showing that the less tightlybound guest is also more labile.3.2.71 H NMR titrations with hydroxy substituted benzoatesTo ascertain whether hydroxy substitution on the aromatic rings of the guest specieswould affect the inclusion capabilities of [Cd(TracHP12)](ClO 4 ) 2 , 49, a small groupof hydroxy substituted benzoate guests was also studied. It is known thattheformation of hydrogen bonds between the hydroxy hydrogen atoms of the hostpendant-arms and the carboxylic oxygen atoms of the guest is the primary mode ofbinding experienced in these host-guest complexes. Therefore, this study wasconducted to determine whether non-classical hydrogen bonding interactionsbetween the guest hydroxy groups and the pendant arm aromatic moieties might be afactor in determining the magnitude of the binding strength of the guest molecule.This would be of interest since, a trend that showed binding constants with o-hydroxybenzoate > m-hydroxybenzoate > p-hydroxybenzoate was observed inrelated work by the Wainwright group using receptor 29 (page 98) in 20% aqueousdioxane.A generic structure representing the sodium salt of a hydroxy substitutedbenzoate guest species is shown in Figure 3.11.63