Solvent isotope effects on the complexation and exchange kinetics of Tl(I), K~+ and Na~+ with 18-crown-6 by competitive NMR spectroscopy: Competition between homo- and heterobimolecular cations exchange

摘要

The thermodynamic properties of complexation and exchange kinetics of thallium by 18-crown-6 have been studied by thallium NMR spectroscopy. Effects of solvent isotope, counterion (ClO_4~- and NO _3~-) and presence of competitive cations, such as Na ~+ and K~+, on the exchange characteristics of the system have been considered. The obvious relationships between the effects of D _2O-H_2O solvent isotope on the thermodynamic properties and activation parameters of complexation have been investigated. In the absence of competitor cations, the mechanism of thallium exchange is unimolecular decomplexation and in the presence of competitor cations, homobimolecular cation exchange is the predominant mechanism at low concentrations of the ligand. At higher concentrations of the ligand, the measured rate constants show that the complexation/decomplexation process obeys a heterobimolecular cation interchange mechanism. The rate constants ratios (k_(D2O)/k_(H2O) < 1) for unimolecular mechanisms also show an inverse solvent isotope effect.