EVALUATION OF A DIRECT H-1-NMR METHOD FOR DETERMINING LOG K AND DELTA-H VALUES FOR CROWN-ETHER ALKYLAMMONIUM CATION COMPLEXATION

A direct H-1 NMR method for determining log K and DELTA-H values for c rown ether-ammonium cation complexation using milligrams of sample was tested and evaluated for accuracy and precision by comparing the resu lts with those obtained using a titration calorimetric method. Log K v alues for the interactions of a non-chiral crown ether, diketopyridino -18-crown-6 (K2P18C6), with alpha-phenylethylammonium (PhEt+) perchlor ate in 50%-50% and 90%-10% (v/v) mixtures of deuterated methanol (CD3O D) and deuterated chloroform (CDCl3) at four temperatures and, with al pha-(1-naphthyl)ethylammonium (NapEt+) perchlorate in 50%CD3OD-50%CDCl 3 (v/v) at 25-degrees-C were determined by a direct H-1 NMR method. Va lues of DELTA-H for the interactions of K2P18C6 with PhEt+ in the two solvents were calculated from the temperature dependence of log K. Log K values for the interactions of a chiral crown ether, dimethyldiketo pyridino-18-crown-6 (M2K2P18C6), with (R) and (S) enantiomers of NapEt + in pure CD3OD at 25.0-degrees-C were also determined by the NMR meth od. The results were compared with those determined by a calorimetric method at 25.0-degrees-C in 50%-50% and 90%-10% (v/v) mixtures of plai n methanol and chloroform, in 100% plain methanol, and in a 50%-50% mi xture of partially deuterated methanol (with deuterium substitution on the methanol OH group, CH3OD) and deuterated chloroform. The log K va lues determined by both methods were found to be in good agreement, bu t the standard deviations associated with the NMR log K values were tw o to three times greater. The agreement of the DELTA-H values determin ed by the two methods was poor, differing by approximately 10 kJ/mol w ith the NMR method giving more negative values. The standard deviation s associated with the NMR DELTA-H values were approximately ten times greater than those for the calorimetric values. Ion-pairing was observ ed for the interaction of perchlorate ion with both free and bound PhE t+ in 50%methanol-50%chloroform mixture. It is concluded that the NMR procedure is satisfactory for the determination of log K, but not DELT A-H values.