Article pubs.acs.org/JPCA
Proton Transfer in Th(IV) Hydrate Clusters: A Link to Hydrolysis of Th(OH)22+ to Th(OH)3+ in Aqueous Solution Philip X. Rutkowski,† Maria del Carmen Michelini,*,‡ and John K. Gibson*,† †
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Dipartimento di Chimica, Università della Calabria, 87030 Arcavacata di Rende, Italy
‡
S Supporting Information *
ABSTRACT: Gas-phase reactions of thorium hydroxide cations with water were studied in an ion trap and by density functional theory. The Th(OH)22+ ion adds five inner-shell water molecules. Addition of outer-shell water molecules to produce the Th(OH)22+·(H2O)6−8 yields Th(OH)3+·(H2O)0−3 by intracluster proton transfer and elimination of a protonated water cluster, (H3O)+(H2O)2. Facile hydrolysis of Th(IV) in these small hydrate clusters correlates with solution hydrolysis of Th(OH)22+(aq) to Th(OH)3+(aq). The Th(OH)3+ ion adds up to three inner-shell water molecules. For the other studied Th(IV) singly charged ions, ThO(OH)+ exothermically hydrolyzes directly to Th(OH)3+ by addition of a water molecule, ThO(O2)+ hydrolyzes to Th(OH)3+ via nonthermalized Th(OH)2(O2)+, and Th(OH)2(O2)+ hydrolyzes to Th(OH)3+·(H2O) by a sequence that requires exothermic hydration prior to hydrolysis. Computed structures and energetics are in accord with the experimental observations.
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solution species.14 Thorium hydrolysis has been studied by electrospray ionization mass spectrometry (ESI-MS) as an analytical tool with a goal of identifying solution hydrolysis species by transferring them from solution to gas.15 The mechanisms of thorium hydrolysis in solution remains largely unexplored due to experimental constraints, but thorium hydration and hydrolysis have been the subject of theoretical studies.16 In the present work we present an experimental and computational study of the gas-phase hydration and hydrolysis of singly charged and doubly charged Th(IV) cations. The results for the hydrolysis of Th(OH)22+·(H2O)n (n = 6−8) by intracluster proton transfer between inner-shell and outer-shell water molecules are particularly significant in providing insights into hydrolysis in bulk solution.16a,17
INTRODUCTION The study of hydration and hydrolysis of metal ions in the gas phase by experiment and theory has been an active field that provides insights into the chemistry of metal ions in aqueous solutions.1 Early work in the field focused on the hydration of singly charged metal cations, M+.2 There are relatively few reports of gas-phase hydration of ligated metal ions, such as VO+ 3 and MgNO3+.4 Although producing doubly charged hydrates presents challenges,5 Kebarle and co-workers extended this line of inquiry to doubly charged metal ions and reported intracluster hydrolysis of M 2 + ·(H 2 O) n to produce MOH+·(H2O)n−2 and H3O+.6 Beyer et al. proposed a saltbridge mechanism that facilitates these hydrolysis processes.7 Hydration and hydrolysis of doubly charged cations has recently been examined by infrared spectroscopy,8 and by collision induced dissociation.9 Metz and co-workers reported proton transfer in doubly charged metal clusters in which the metal ion is coordinated by acetonitrile and water.10 Williams and co-workers demonstrated that water clusters containing triply charged cations could be isolated and investigated provided that the clusters are adequately large, M3+·(H2O)n with n > 15, such that they can be considered as nanodroplets.11 Studies of trivalent lanthanide ions, Ln3+, in such nanodroplets are providing important new insights into bulk solution chemistry.12 Stabilization of quadruply charged metal ions in water clusters presents a substantial challenge due to the high fourth ionization energy of most metal ions, and the propensity for M4+ ions to hydrolyze at moderate pH ≈ 7. The solution chemistry of Th4+ is particularly complex due to the propensity for hydrolysis to produce monomeric hydroxides Th(OH)n(4−n)+ (n = 1−4),13 as well as polymeric and colloidal © 2013 American Chemical Society
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EXPERIMENTAL SECTION The experimental approach summarized here has been described in detail previously.18 A stock aqueous solution of 0.46 M Th(ClO4)4 in 1.2 M HClO4 was diluted with water to prepare the 0.46 mM Th(ClO4)4 in 1.2 mM HClO4 (pH = 2.9) solution used for the experiments. The experiments were performed using an Agilent 6340 quadrupole ion trap mass spectrometer (QIT/MS) with and electrospray ionization (ESI) source. In high resolution mode, the instrument has a detection range of 50−2200 m/z and a peak width of ∼0.25 m/ z (fwhm). The thorium solution was injected into the ESI capillary at a flow rate of 1 μL·min−1. High-purity nitrogen gas for nebulization and drying in the ion transfer capillary was Received: September 28, 2012 Revised: December 13, 2012 Published: January 4, 2013 451
dx.doi.org/10.1021/jp309658x | J. Phys. Chem. A 2013, 117, 451−459
The Journal of Physical Chemistry A
Article
structures. For each optimized stationary point, analytical frequencies were calculated to confirm that the optimized structure is a local minimum on the potential energy surface of the system, and to evaluate the zero-point vibrational energy (ZPVE) corrections to the electronic energies. All the reported energies include the ZPVE correction at 0 K (ΔE0). In addition to the ΔE0 values, the Gibbs free energy at 298 K is reported for each of the studied reactions. The accuracy of the ΔG298 values is necessarily somewhat limited by the use of the harmonic oscillator approximation to treat nuclear motion. With the exception of the species containing dioxygen, all the studied cations have singlet spin states. The species containing dioxygen are doublet-spin-state superoxo complexes in which the unpaired spin density is localized on the oxygen atoms. Singlet spin optimizations were performed within the restricted Kohn−Sham formalism, whereas open-shell structures were computed using the unrestricted approach. All the studied species contain the formally closed-shell Th(IV) cation; therefore, spin−orbit corrections are not expected to be important and are not treated explicitly in this study. Moreover, these contributions have been shown to remain invariant with the addition of water molecules in similar hydration processes, and therefore are not expected to significantly affect the sequential hydration energies.24e It should be remarked that all of the computations were performed for a pressure of 1 atm. In reality, the water pressure under these experimental conditions is approximately 10−6 Torr. Furthermore, the “pressures” of other reactants and products, both ions and neutrals, are very low (≪10−6 Torr) and are unknown. Accordingly, the computed reaction entropies and free energies may deviate from the actual values under these experimental conditions. However, the values for ΔE0 are not pressure dependent and the key conclusions, which are based on ΔE0 values, are not affected by the uncertainties in ΔG298.
supplied from the boil-off of a liquid nitrogen Dewar. The instrumental parameters used to obtain optimal mass spectra for singly charged and doubly charged thorium ions are provided as Supporting Information. The background water pressure in the ion trap is estimated as ∼10−6 Torr;19 reproducibility of hydration rates confirms that the background water pressure in the trap remains constant to within
