Jozef Hodyl PhD Thesis - Theses - Flinders University

It can be seen here that a water molecule is hydrogen bonded to the carboxylateoxygen atom of the o-hydroxybenzoate guest. This suggests that in aqueousconditions with receptors 29 and 94 there exists in a similar way the potential forH 2 O molecules to mediate hydrogen bond bridges between the guest hydroxy groupsand the host aromatic rings, as shown in Figure 6.3 (a) and (b). If, for instance,water molecules were to form a hydrogen bond bridging network within the hostcavity as shown by (a) and (b), the observed increase in binding strength for orthosubstituted hydroxybenzoates in water could be explained. 229(a) (b) (c)Figure 6.3Representation of potential hydrogen bonding schemes that may be contributing to thedifference in guest selectivity experienced in H 2 O as opposed to DMSO and MeOH.Water molecules, having two hydrogen bond donor atoms, are able to formhydrogen bond bridges from guest carboxylate oxygens and hydroxy hydrogenswhereas, methanol is only able to bridge to an aromatic ring from guest hydroxyhydrogen atoms (Figure 6.3 (c)) over a more limited distance, making it a lesseffective mediator.To verify whether the trend observed in aqueous conditions was linked to nonclassicalhydrogen bonding a series of test experiments was conducted with receptor94 using a similar set of guest molecules, but eliminating the potential for nonclassicalhydrogen bonding interactions by replacing the hydroxy group with a101