ISSN:
1573-1111
Keywords:
Crown ether-ammonium cation complexation
;
formation constants (K) and enthalpy changes (ΔH)
;
direct1H NMR method
;
titration calorimetric method
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract A direct1H NMR method for determining logK and ΔH values for crown ether-ammonium cation complexation using milligrams of sample was tested and evaluated for accuracy and precision by comparing the results with those obtained using a titration calorimetric method. LogK values for the interactions of a non-chiral crown ether, diketopyridino-18-crown-6 (K2P18C6), with α-phenylethylammonium (PhEt+) perchlorate in 50%–50% and 90%–10% (v/v) mixtures of deuterated methanol (CD3OD) and deuterated chloroform (CDCl3) at four temperatures and, with α-(1-naphthyl)ethylammonium (NapEt+) perchlorate in 50%CD3OD-50%CDCl3 (v/v) at 25°C were determined by a direct1H NMR method. Values of ΔH for the interactions of K2P18C6 with PhEt+ in the two solvents were calculated from the temperature dependence of logK. LogK values for the interactions of a chiral crown ether, dimethyldiketopyridino-18-crown-6 (M2K2P18C6), with (R) and (S) enantiomers of NapEt+ in pure CD3OD at 25.0°C were also determined by the NMR method. The results were compared with those determined by a calorimetric method at 25.0°C in 50%-50% and 90%–10% (v/v) mixtures of plain methanol and chloroform, in 100% plain methanol, and in a 50%-50% mixture of partially deuterated methanol (with deuterium substitution on the methanol OH group, CH3OD) and deuterated chloroform. The log K values determined by both methods were found to be in good agreement, but the standard deviations associated with the NMR logK values were two to three times greater. The agreement of the ΔH values determined by the two methods was poor, differing by approximately 10 kJ/mol with the NMR method giving more negative values. The standard deviations associated with the NMR ΔH values were approximately ten times greater than those for the calorimetric values. Ion-pairing was observed for the interaction of perchlorate ion with both free and bound PhEt+ in 50%methanol-50%chloroform mixture. It is concluded that the NMR procedure is satisfactory for the determination of logK, but not ΔH values.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01053868
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