Titration calorimetry was used to measure equilibrium constants and standard molar enthalpies for the reactions of phenethylamine, ephedrines, and related substances with a-and /3-cyclodextrin. Changes in the chemical shifts A6 of both the ligand and cyclodextrin protons were measured with NMR. The thermodynamic results have been examined in terms of structural features of the ligand that affect these interactions such as the separation of the charge at an amino group and the aromatic ring, steric effects, the presence of additional functional groups (amino, hydroxy, methoxy, and methyl) attached to the aromatic ring, the presence and location of hydroxy group(s) on the ligand, changes in the chirality of the ligand, and the flexibility of the organic molecules attached to the aromatic ring. It was found that the values of thermodynamic quantities for these reactions in phosphate and acetate buffers were different. This difference is attributable to the presence of a hydrophobic alkyl group in the neutral acetic acid molecule and its interaction with the cyclodextrins. Also, there are significant differences in the thermodynamic quantities for the reactions of the chiral isomers of ephedrine and pseudoephedrine in their reactions with /3-cyclodextrin. A plot of the standard molar enthalpy vs the standard molar entropy for the reactions of these chiral isomers with a-and /3-cyclodextrin is linear; the relative order of the ephedrines and pseudoephedrines in the enthalpy-entropy plot is the same for the reactions of these substances with both a-and /3-cyclodextrin. NMR studies demonstrated that the magnitude of the upfield shifts of the cyclodextrin's H3 and H5 protons, A<5(H3) and A<5(H5), and their relative ratio, A<5(H5)/A<5(H3), can be used, respectively, as a measure of the complex stability and the depth of inclusion of the ligand into the cavity. The equilibrium constants determined by titration calorimetry correlate well with the changes in chemical shifts Ad determined by NMR.

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The line-broadening of 81Br NMR signals observed in the presence of CyDs was applied to evaluate their binding abilities toward inorganic anions in aqueous solutions. The apparent association constants of α -CyD with anions have been found to decrease in the order SCN− &gt; ClO4− &gt; I− &gt; NO3− &gt; Br−   F− &gt; Cl−.

Keywords Thallium(I)-selective electrode, polymer membrane, ring-enlarged dibenzo-crown-6 ether From an environmental viewpoint, estimations of thallium-ion concentration have become of major interest due to their high toxicity. Ion-selective electrodes are one of the most convenient and reliable analytical tools for estimating metal-ion concentrations. There have been only a few reports concerning liquid-membrane thallium(I)-selective electrodes (Tl+-ISEs).'_4 They need to be improved with regard to both their low selectivities against alkali metal ions and pH dependence.In a previous paper, we described a poly(vinyl chloride) (PVC) membrane Tl+-ISEs based on bis(crown ether)s containing the benzo-l5-crown-5 moiety.5 These crown ether-based ISEs exhibited good slope and linearity of calibration plots, and the electrode response was stable over a wide pH range. The Tl+ selectivity against K+ for one of the bis(15-crown-5)s (Bis-15), however, was fairly poor (log KTiK=-0.52 at least).Recently, we found that ring-enlarged dibenzo-20-crown-6 (DB-20) and -22-crown-6 (DB-22) show higher selectivities to Tl+ than does symmetrical dibenzo-l8-crown-6 (DB-18) in solvent extraction experiments.6 It is therefore of interest to apply these crown ethers to neutral carriers for Tl+-ISEs, selectivities may be expected.since their high TP ExperimentalThe crown ethers employed in this work are shown in Fig. 1. The preparation of the PVC membranes {i.e., PVC, 50 mg; crown ether, 5 mg; o-nitrophenyl octyl ether (NPOE), 125 mg; sodium tetrakis [3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB),1 mg} was carried out as described previously.'The EMF measurements were carried out at 25±0.1 ° C with an electrochemical cell of Ag-AgCI/ 4 M KCl/ 0.1 M NH4NO3/sample solution/PVC membrane/ 10.3 M T1NO3/AgCI-Ag after conditioning for 24 h by soaking into a 1 X 10-3 M T1NO3 solution. Results and DiscussionEach electrode based on DB-18, DB-20 or DB-22 Fig. 1 Structures of the crown ethers.