Investigation of molecular dynamics of crown ethers by NMR T1 relaxation time in the presence and absence of ions

VI SUMMARY In this study, molecular motions of crown ethers and binding constants of crown ethers doped with Na+ were investigated by NMR relaxation measurement. Each solution containing 10 mg of crown ether in 10 ml of CDCI3 was prepared for NMR measurements. The samples were transferred into 5 mm-NMR tubes and degassed three times by freeze-thaw method and sealed for measurement. Sample temperature was decreased from 313 to 277 by steps of 4 °C in Ti measurements by using a variable t emperature control unit. T 1 values of the crown ethers with increasing concentration of NaC104 were also determined in order to determine association bindings. In this case, concentration of crown ethers in each sample was kept constant. The relaxation measurements were carried out on a BRUKER 400 MHz NMR spectrometer. The theory based on dipole-dipole interaction was used for the evaluation of the temperature dependence data. It was assumed that the extreme narrowing condition was valid and In Ti was plotted versus 1/T. Correlation times and activation energies were determined from the plots. The correlation times (xc) were attributed to the molecular tumbling. In order to find the complexation of the crown ethers with Na+ ion, ion concentration versus 1/Ti graphics and 'H-NMR chemical shifts were discussed in terms of structural modification. The alkali metal salt, when added to CD3CN solutions of the crown ethers, causes a low-field shift in the resonances of almost all of the protons, whereas the resonances of NCH2 protons move to high field.