Jozef Hodyl PhD Thesis - Theses - Flinders University

Na +Figure 3.11 Generic structure depicting the sodium salt of a typical hydroxybenzoate guest species.H A and H B protons are shown for when the hydroxy group is in the para position.This set of 1 H NMR titration experiments was conducted in DMSO-d 6 by amethodology identical to that specified in the preceding binding constant study. The1 H NMR monitored titration experiments were run at a constant guest concentrationof 10 -3 M for benzoate, p-hydroxybenzoate, o-hydroxybenzoate, and 2,6-dihydroxybenzoate. This concentration was increased to 1.7 x 10 -3 for m-hydroxybenzoate due to difficulty in monitoring the chemical shifts of theresonances for this guest species. Benzoate and all the hydroxybenzoate based guestspecies exhibited chemical shifts, ∆δ max., of between 0.1 and 0.2 ppm and in all,excluding p-hydroxybenzoate, where the H B protons were followed, the chemicalshift monitored was from the resonance of the proton para to the carboxylate group.The chemical shifts for the resonance of this proton were always well removed fromthe host resonances.Each transference of the hydroxy group from para to ortho was expected to bringthe hydroxy group into a more favourable proximity and angular alignment for nonclassicalhydrogen bonding to the π-electron systems of the aromatic groups of thehost pendant-arms. This is depicted in Figure 3.12. It is well known that the strengthof hydrogen bonds is distance and angle dependent. 201 From the depiction of thehost-guest complexes represented in Figure 3.12, which is based on X-raydetermined structures, such as that in Figure 1.10, it is obvious that the potential for64