DISSOCIATION KINETICS OF CALIXARENE ESTER-NA+ COMPLEXES - EFFECT OF SODIUM-ION EXCHANGE-REACTION ON NA-23 LONGITUDINAL MAGNETIZATION RECOVERY CURVES AND H-1-NMR SPECTRA

The effect of the sodium ion exchange reaction on Na-23 longitudinal magnetization recovery (LMR) curves and H-1 NMR spectra has been investigated for p-tert-butylcalix[4]arene (BCA) and its ethyl acetate derivative (BCAD) in chloroform + methanol (2:1 v/v) solutions. For BCA + NaSCN system, Na-23 LMR curves and NMR spectra were almost identical with those of solvated sodium ion in the absence of BCA. In the case of the system BCAD + NaSCN, Na-23 LMR curves at 1 < R < 1.2 (R = [NaSCN]/[BCAD]) showed non-single-exponential decays, and Na-23 NMR spectra showed non-Lorentzian line shapes because of the presence of chemical exchange between solvated sodium ions and Na+.BCAD complexes. At R = 0.4 the sodium ion exchange reaction was confirmed by the temperature dependence of the H-1 NMR spectra. The lifetimes of the Na+.BCAD complexes (A) and solvated sodium ions (B), tau-A and tau-B, for the first time have been determined between 23 and 60-degrees-C by the simulations of Na-23 LMR curves at R = 1.1 by using coupled expressions for the time evolution of the magnetization at sites A and B under the presence of chemical exchange: tau-A = 1.0 X 10(-2)-1.5 X 10(-1) s and tau-B = 1.1 X 10(-3)-2.6 X 10(-2) s. Since the lifetimes of sites A were independent of R, the predominant mechanism for the decomplexation of Na+.BCAD complex is unimolecular and is characterized by the following activation parameters for dissociation path: DELTA-H(d) = 56.3 +/- 6.6 kJ/mol, DELTA-S(d) = -39 +/- 21 J K-1 mol-1, and DELTA-G(d)(298 K) = 67.9 +/- 9.1 kJ/mol.