STRUCTURE AND DYNAMICS OF P-SULFONATOCALIX[4]ARENE AND ITS HYDRATION SHELL - NUCLEAR MAGNETIC-RELAXATION RESULTS

The longitudinal C-13 spin-lattice relaxation times and {H-1}-C-13 nuc lear Overhauser factors of the aliphatic and aromatic carbon atoms of calix[4]arene-p-sulfonic acid (1a) and its pentasodium salt (1b) were measured for a solution in D2O and a temperature range from 270 to 347 K; those of the calixarene (1a) were measured also for a solution in [H-2(4)]methanol and from 203 to 324 K. The results show that the extr eme narrowing condition is not fulfilled for the C-13 relaxation data. For the aqueous solutions the longitudinal H-2 spin-lattice relaxatio n times of D2O were determined as well. The observed relaxation times were fitted to the theoretical expressions for rotational motions of t he molecules. The reorientational molecular dynamics showed Arrhenius behavior in all cases, and the corresponding activation parameters wer e evaluated. The rotational motion of the calixarene molecules is esse ntially isotropic in the investigated solutions. The order of the reor ientation rates is methanolic solution of 1a > aqueous solution of 1b > aqueous solution of 1a. The differences in the motional behavior are an indication of differing intermolecular interaction mechanisms betw een solvent and solutes and, in the case of the aqueous solutions, a d ifferent dynamical behavior of the water around the calixarene molecul es. A scaling of the spectral densities with the generalized order par ameter S-2 of the model-free approach by Lipari and Szabo has to be ap plied for quantitative interpretation of the relaxation data. This is caused by the existence of a very fast initial decay of the reorientat ional correlation function of the calixarene molecules, stemming from fast librational and/or internal motions in addition to the overall ro tational diffusion process.