The Pressure Dependence of Oxygen Isotope Exchange Rates Between Solution and Apical Oxygen Atoms on the [UO₂(OH)₄]²⁻ Ion

Abstract

A key concern in the use of nuclear energy is the disposal and long term storage of spent fuel. The aqueous chemistry of the uranyl(VI) ion under strongly alkaline conditions, such as those found in many above-ground waste storage tanks, remains poorly understood. To this effect, the oxygen atom exchange between the uranyl oxygen atoms and bulk solvent water has been studied by 17 O NMR spectroscopy under acidic [1] and alkaline conditions [2][3][4] using line-broadening by Clark et al. and, recently, saturation-transfer methods by Szabó, Grenthe et al., but no determination of activation volumes has been attempted. Such data are invaluable in verifying theoretical models for reaction pathways.[5] The activation volume probes the transition state and complements molecular dynamics simulations. Herein, the activation volume of the oxygen atom exchange of the uranyl oxygen atoms with bulk water has been determined by experiments at 60 8C between 0.1 and 350 MPa as a function of uranyl and hydroxide concentrations.A common method for determining exchange that involves coordinated oxygen atom species and bulk water is by line-broadening studies according to the method by Swift and Connick, [6,7] in which the broadening of a signal because of rapidly occurring dynamic processes is followed. While this is experimentally straightforward, the separation of contributing factors to the observed line width is often difficult. Such is particularly the case when there are several exchangable oxygen atoms, such as apical "yl" and equatorial waters or hydroxo ligands. Saturation-transfer measurements provides an alternative approach which, assuming that certain conditions are met-such as well-separated NMR signals and slow longitudinal relaxation-offers a more direct and precise determination of exchange rates.[ With selective inversion, the x and y components of the magnetization may be omitted and the system may be completely described by the z component of the Bloch equations. We here adopt a McConnell formalism for exchange (see Supporting Information for derivation of model equation). M i is the bulk magnetic moment, T 1 denotes the longitudinal relaxation for sites a and b, and k is the pseudo first-order rate constant.[7] For a selective 1808 pulse at t 0 on site b, the initial conditions in the ideal case become The activation volume (DV°) is defined by Equation (3) (R gas constant, T temperature, k rate coefficient, P pressure), and can be determined by measuring rates as a function of pressure.According to transition-state theory the activation volume can then be interpreted as the difference in molar volume between the transition state and the sum of the partial molar volumes of the reactants and thus offers a less ambiguous measure than activation entropy as to whether a reaction is associative or dissociative, in addition to giving an indication as to the species involved. The activation volume is one of a few properties of the transition state that can be accurately probed and therefore it is an invaluable indica...

A High‐Pressure NMR Probe for Aqueous Geochemistry

“…At ambient conditions, NMR spectroscopy has emerged as a powerful analytical technique to monitor chemical processes within the environment. [12] Furthermore, advancements in high-resolution, high-pressure probe design [13][14][15] have not only allowed for the study of geochemical reaction dynamics, [16][17][18] but also the pressure dependencies of critical micelle concentrations, [19] dissolved gas formation from catalytic reactions, [20] as well as several studies probing protein folding, aggregation, and stabilization of rare highenergy states. [21][22][23][24] Despite the tremendous scientific advancement from these studies, the current NMR probe designs cannot fully accommodate both high pressures and highresolution molecular-level data needed to begin to evaluate the geochemical models.…”

17 O NMR as a Tool in Discrete Metal Oxide Cluster Chemistry

“…In studying the exchange between the -yl oxygen and bulk water the coalescence method thus cannot be used. Harley et al obtained rates of exchange by using a saturation-transfer technique [13]. Here, by use of a selective π pulse the net magnetisation of the bulk water signal was inverted.…”

“…Uranium, with the discovery of the uranyl (per)oxo clusters, has shown that it possesses a rich range of chemistry which is not that dissimilar from the classical polyoxometalates [28]. Harley et al looked at the pressure-dependent rates of exchange of the -yl oxygen sites in the uranyl ion, [UO 2 (OH 2 ) 4 ] 2À , with bulk water [13]. Here, the absence of a paramagnetic centre precludes Swift-Connick-type exchange kinetics, the fast exchange precludes preenrichment of the molecule as is done with the polyoxoniobates, and the distance between the 17 O NMR signal of the -yl oxygen, which is found at ca 1100 ppm, and the bulk signal water, which is the shift reference at 0 ppm, rules out direct coalescence studies.…”

The actinides

“…42 In addition to work on Tp Me2 complexes of U(III) already cited, 24 45 An addition to the ranks of phosphine complexes is [(dmpe) 4 U 4 Cl 16 ]Á2CH 2 Cl 2 , which has a linear chain structure with bridging chlorides; it is used as a synthon for mononuclear [(dmpe)(dmbpy)UCl 4 ]. 47 Hydrolysis of U(VI) with a non-coordinating base is believed 48 to lead to trimers in weakly basic solution, but to monomeric [UO 2 (OH) 4 3 OH] also being isolated. 47 Hydrolysis of U(VI) with a non-coordinating base is believed 48 to lead to trimers in weakly basic solution, but to monomeric [UO 2 (OH) 4 3 OH] also being isolated.…”