Multiple field carbon-13 NMR relaxation of calix[4]arene in solution

Abstract

Carbon-13 relaxation times and nuclear Overhauser enhancement values were determined for the carbons of calix[4]arene in both CDCl3 and DMSO-d6 solvents at magnetic field strengths of 5.9, 9.4, and 14.1 T at 30 °C. The relaxation data for the CDCl3 solution were well described by an isotropic motional model indicating motions in the extreme narrowing limit with a rotational correlation time of 32–38 ps. The effective shielding anisotropies for the substituted aromatic carbons were found to be in the range 150–170 ppm while the unsubstituted aromatic carbons had values in the range 180–200 ppm. In DMSO-d6 solution, the dipolar relaxation is out of the extreme narrowing limit and it was possible to apply a Lipari–Szabo treatment of the relaxation data. This model demonstrated through the value of the generalized order parameter, S2, a slight increase in the range of motion for the aliphatic CH2 carbon over the unsubstituted aromatic carbons. It was not possible, however, to determine reliable values for the effective shielding anisotropies for the aromatic carbons. An earlier report of carbon-13 relaxation for calix[4]arene in CDCl3 had indicated distinctly different relaxation times for the two unsubstituted aromatic carbons. We found virtually identical relaxation times for both carbons at all three magnetic fields used in this study. Copyright © 1999 John Wiley & Sons, Ltd.