Jozef Hodyl PhD Thesis - Theses - Flinders University

pK a = 2.97 pK a = 1.05Figure 3.15 Representation of the intramolecular hydrogen bonded structures of o-hydroxybenzoateand 2,6-dihydroxybenzoate.A second consequence is a weakening of the hydrogen bond acceptor capability ofthe carboxylate. For this reason the host-guest binding constants become appreciablylower in magnitude with o-hydroxybenzoate < m-hydroxybenzoate < p-hydroxybenzoate.It appears from this study that binding strength with these types of guests may bedependent on the solvent in which guest inclusion occurs. Previous studies withreceptor 29 were performed in 20% aqueous dioxane whereas this study withreceptor 49 utilised DMSO-d 6 or CD 3 OD as the solvent. Indications are that thehydrogen bond donor/acceptor abilities of the solvent have a significant bearing onthe magnitude of the binding constants obtained. Further investigations of this andthe turnaround in the present trend can be found in Chapter 6 where the silica boundhost is used under aqueous conditions.The data obtained from 1 H NMR monitored titration experiments with,[Cd(TracHP12)](ClO 4 ) 2 , 49, and a number of different anionic guest speciesdemonstrate the suitability of a three-walled cavity for guest inclusion. From theseresults it can also be said that removal of one of the aromatic groups does not affect,to a great degree, the behaviour and efficacy of the binding cavity. Binding constantsfor inclusion of some of the guest species were lower, but not significantly reduced68