title: Dynamic NMR study of the kinetics of complexation of Tl(+) ion with calix[4]crown-6.
abstract: The complexation and exchange kinetics and mechanism for the dissociation and conformational change of thallium ion complex of calix[4]crown-6 were studied in CD(3)CN/CDCl(3) (4:1 v/v) solution by dynamic (1)H NMR. The results show the formation of a 1:1 complex with cone conformation. From variable temperature dynamic NMR analysis in the range 223-293 K, two coalescence temperatures at 228 and 243 K were ascertained. The activation parameters for the dissociation process, E(a) (kJ/mol), DeltaS(++) (J/mol.K), and DeltaH(++) (kJ/mol) are 11.0, -133.2, and 10.1 for the bimolecular regime and 21.5, -112.8, and 20.6 for the unimolecular regime, respectively. In addition, the dynamic (1)H NMR spectroscopy shows that the exchange of Tl(+) between the two crown sides of the complexed ligand proceeds through an intramolecular tunneling. An Arrhenius convex curve was observed for intramolecular exchange. This phenomenon is explained in terms of two conformer state formations differentiated by hydrogen bond association.
authors.affiliation: Department of Chemistry, Faculty of Basic Science, Tarbiat Modares University, P.O. Box14115-175, Tehran, Iran.
authors.forename: Hedayat
authors.initials: H
authors.lastname: Haddadi
authors.forename: Naader
authors.initials: N
authors.lastname: Alizadeh
authors.forename: Mojtaba
authors.initials: M
authors.lastname: Shamsipur
authors.forename: Zouhair
authors.initials: Z
authors.lastname: Asfari