Jozef Hodyl PhD Thesis - Theses - Flinders University

naphthoate (pK a = 3.60) and 2-naphthoate (pK a = 4.17). This has been corroboratedpreviously by guest inclusion studies with hydroxybenzoate guest species wherebenzoate (pK a =4.19) and p-hydroxybenzoate (pK a =4.54) both show modest percentinclusion values although their pK a values are high. So, some other factor must beresponsible for augmenting their binding values. For instance, the contribution of πelectrons from the adjoining aromatic ring increases the potential for forminghydrogen bond bridges between the guest and host aromatic groups. 231-233 There isalso the possibility of hydrophobic effects and π-π interactions between guest andhost species if the respective aromatic groups come within bonding distance of oneanother. Summation of these modes of binding appears to be reflected in increasedpercent inclusion values for 1-naphthoate and 2-naphthoate with the differenceresulting from one being more basic than the other. It is known from crystalstructures of Na 4 [calix[4]arene sulfonate]•13.5H 2 O that water molecules are able tobridge, through hydrogen bonding, between two aromatic π-acceptor groups. 231Indications of how these bridging π ... H-O-H ... π hydrogen bonding interactions mightoccur between a naphthoate guest and the host species are given in Figure 6.5.Figure 6.5Representation of the possible hydrogen bonding interactions enhancing the bindingbetween naphthoate guests and Si-GPS-[Cd(Trac)](ClO 4 ) 2 , 94.111