Jozef Hodyl PhD Thesis - Theses - Flinders University

3.1 where K A is the association, or guest binding constant that is sought.K A = association constantK D = dissociation constantIf the system undergoes slow exchange of the guest between the free and boundstates the resonances of the host, the guest and the host-guest complex are observedas separate entities allowing the equilibrium constant to be evaluated, directly, fromthe integration of the resonances’ intensity. 7 More commonly, though, the rate ofexchange between the associated and dissociated forms of the host-guest complex israpid on an NMR timescale and only a single set of resonances for the host and guestare seen where the chemical shift values are the average of the values for theassociated and dissociated species, weighted by the prevailing mole fraction of each.Thus, for example,δ G = δ G χ G + δ HG χ HGIt is then the case that when the chemical shift values associated with themonitored species, which in this work was the guest, are plotted against theincreasing concentration of the host, a titration curve is produced, the curvature ofwhich diminishes with increasing magnitude of K A . Cursory inspection of thephysical appearance of the titration curve, which may vary from virtually a flat line,to a steep climb with an angular bend at its zenith, plateauing into linearity, gives, tothe experienced eye, a rough estimate of the strength of the host-guest interaction.An accurate, quantitative determination of the host-guest binding constant can becalculated by a curve fitting program that displays the experimental data incomparison with the theoretical curve for calculated values of K and δ HG . A53