Dynamic NMR study of ligand exchange reactions in U(VI)-phosphonic acid systems
- Argonne National Lab., IL (United States). Chemistry Div.
The rates of hydrogen ion exchange on phosphonic acid ligands and that of phosphonate ligand exchange on selected uranyl-phosphonate complexes have been investigated by dynamic NMR spectroscopy. The spin-spin relaxation time (ln(1/T{sub 2})) for H{sup +} exchange on the fee ligands exhibits a parabolic dependence on reciprocal temperature (1/T (K{sup {minus}1})). The empirical fit parameters are correlated with the activation parameters ({Delta}H{sup *}, {Delta}S{sup *}, {Delta}C{sub p}{sup *}) by adapting the statistical mechanical framework developed by Braibanti et al. to the Eyring activated complex theory. The correlation of the apparent activation enthalpy with temperature indicates that 6--11 water molecules participate in the transition from reactants to activated complex for ligand proton exchange reactions, depending on the ligand. Mechanistic details of ligand exchange reactions of phosphonic acid complexants onto the corresponding uranium(VI) complexes are not fully developed, but the negative values for {Delta}S{sup *} imply increased order in the transition to the activated complex. Though {Delta}H{sup *} for ligand exchange covers a range of 15 kJ/mol, the free energy of activation ({Delta}G{sup *}) is nearly constant for the series of ligands, implying substantial entropy compensation in the activation process. Application of the Marcus relationship defines a correlation between {Delta}G{sup *} and the thermodynamic stability ({Delta}G{degree}) for the 1:2 uranyl complexes with methanediphosphonic acid and 1-hydroxyethane-1,1-diphosphonic acid.
- OSTI ID:
- 351567
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Vol. 103, Issue 18; Other Information: PBD: 6 May 1999
- Country of Publication:
- United States
- Language:
- English
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