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 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. [4] Here, one site, for example, 17 O in bulk water, is selectively excited with a 1808 pulse and magnetization is transferred through chemical exchange to another site, e.g. 17 O in UO 2 2+ : dM z;a dt ¼ À ðM z;a ðtÞ À M 1 z;a Þ T 1;a þ k b M z;b ðtÞ À k a M z;a ðtÞ ð1ÞWith 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 M z;b ðt 0 Þ ¼ ÀM 1 z;b and M z;a ðt 0 Þ ¼ M 1 z;a which allows for the analytical solution of the set of differential equations. In practice, long, shaped pulses are required to provide for a truly selective 1808 pulse during the life-time of which some relaxation of b will occur, so that M z;b ðt 0 Þ ! ÀM 1 z;b . The system was thus solved for M z;b ðt 0 Þ ¼ ÀM 0 z;b and M z;a ðt 0 Þ ¼ M 0 z;a . The activation volume (DV°) is defined by Equat...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.