Structural and Thermodynamic Properties of the CmIII Ion Solvated by

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Structural and Thermodynamic Properties of the CmIII Ion Solvated by Water and Methanol Morgan P. Kelley,*,†,‡ Ping Yang,*,‡ Sue B. Clark,† and Aurora E. Clark*,† †

Department of Chemistry, Washington State University, Pullman, Washington 99164, United States Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87544, United States

‡

S Supporting Information *

ABSTRACT: The geometric and electronic structures of the 9-coordinate Cm3+ ion solvated with both water and methanol are systematically investigated in the gas phase at each possible solvent-shell composition and configuration using density functional theory and second-order Møller− Plesset perturbation theory. Ab initio molecular dynamics simulations are employed to assess the effects of second and third solvent shells on the gasphase structure. The ion−solvent dissociation energy for methanol is greater than that of water, potentially because of increased charge donation to the ion made possible by the electron-rich methyl group. Further, the ion− solvent dissociation energy and the ion−solvent distance are shown to be dependent on the solvent-shell composition. This has implications for solvent exchange, which is generally the rate-limiting step in complexation reactions utilized in the separation of curium from complex metal mixtures that derive from the advanced nuclear fuel cycle.

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INTRODUCTION Curium (Cm) is an important component of spent nuclear fuel and other high-level radioactive wastes; as an element with long-lived α-particle emitting isotopes, Cm is responsible for many of the thermal and radiotoxicological problems associated with the long-term storage of nuclear waste.1−3 The most stable oxidation state is CmIII, leading to behavior that is often generalized to other late actinides.4 Major research efforts have focused on the separation of Cm3+ and other trivalent actinides from the trivalent lanthanides using multistage solvent extraction, and thus numerous studies have been conducted regarding solvation of the Cm3+ ion.4−15 Experimental results, obtained primarily using high-energy X-ray scattering, agree that the hydrated Cm3+ ion has an average coordination number of 9, in a tricapped trigonal-prismatic geometry. Theory has played an important role in the interpretation of this experimental data.6,16−20 Studies using density functional theory (DFT) have noted that the use of generalized gradient functionals (GGAs) predicts that the 8-coordinate structure is thermodynamically favored, while B3LYP yields the correct energetically stabilized tricapped trigonal-prismatic 9-coordinate geometry.5,17,19 Both sets of functionals yield reasonable Cm−OH2 bond lengths. Studies employing Møller−Plesset perturbation theory also reproduce the correct thermodynamic trends and geometric parameters in good agreement with experimental data.4,5,18 Classical and ab initio molecular dynamics (CMD and AIMD, respectively) have investigated both the structure and dynamics of the first and extended solvation shells.6,17,19 CMD simulations have predicted a mean residence time of water in the first solvation shell of the Cm3+ © 2016 American Chemical Society

ion of at least 0.6 ns; this quantity is difficult to obtain using AIMD because practical computational cost limits these simulations to tens of picoseconds. As demonstrated in more recent work, there is significant interest in the effects of more complex solutions upon speciation of Cm3+. Atta-Fynn and coworkers20 used AIMD simulations to investigate the effect of counterions on the hydration structure of the Cm3+ ion; in the presence of Cl−, a hydration number of 8 was dominant, while when perchlorate (a “non-complexing counterion”1) was used, the 9-coordinate geometry was more frequently observed. Along these lines, the current work focuses on speciation of the Cm3+ ion in binary solutions. In particular, we note that mixed solvents are prevalent in U.S. Department of Energy waste sites and in the nuclear fuel cycle. Binary water/methanol mixtures are frequently used in a variety of tasks, from enhancing droplet formation and the ionization efficiency in electrospray ionization mass spectrometry21 to separations.22,23 Trumm and co-workers24 utilized a 1:1 water/methanol mixture as the aqueous phase in a solvent extraction scheme in order to increase the solubility of bis(triazinyl)pyridine ligands (extractants showing a preference for AnIII species over LnIII species but only sparingly soluble in water) to study their complexation with CmIII and EuIII. While significant work has been done on the interaction of the water/methanol mixture with ions,25−36 singly charged main-group elements, such as Na+ or Cl−, have been the focus of the bulk of prior studies. Herein we extend the current understanding of Cm3+ solvation Received: March 1, 2016 Published: April 27, 2016 4992

DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999

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Inorganic Chemistry

Figure 1. Average solvent dissociation energy by system, for both methanol (●) and water (▲). Variance is the standard deviation of different solvent molecules from all structures of the same water/methanol composition.

in water to mixed water/methanol environments. To that end, a combination of gas-phase density functional theory (DFT) and second-order Møller−Plesset perturbation theory (MP2) calculations, along with solution-phase AIMD simulations, is used to probe the solvent binding energy and solvation structure of Cm3+ and its first solvation shell at every possible water/methanol composition.

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[Cm(H 2O)x (OHCH3)y ]3 + ⇒ [Cm(H 2O)x − 1(OHCH3)y ]3 + + H 2O

(1)

[Cm(H 2O)x (OHCH3)y ]3 + ⇒ [Cm(H 2O)x (OHCH3)y − 1]3 + + OHCH3

(2)

The lowest-energy geometry at each solvent configuration was chosen to conduct natural population and natural bond order analyses45 using Gaussian0946 so as to understand trends in the electronic structure as a function of the first shell composition. AIMD. Carr−Parinello dynamics,47 as implemented in NWChem,48 were performed for several model systems. Gas-phase AIMD simulations of only the Cm3+ ion and the nine solvent molecules comprising its first solvation shell were performed using the lowestenergy DFT-optimized structure as the initial geometry. Solutionphase simulation cells using periodic boundary conditions were created by placing 75 solvent molecules around the DFT-optimized first solvation shell of the Cm3+ ion using the Packmol49 program. The energy of these simulation cells was then minimized prior to simulation using the conjugate gradient method.50 All AIMD simulations were performed using pseudopotential plane-wave basis sets with the PBE51 GGA exchange-correlation functional. The Cm3+ ion was simulated using a Troullier−Martins pseudopotential52 with 78 electrons in the core region and a valence region consisting of 6s26p65f76d0. An energy cutoff of 1.63268 keV and a fake mass of 650 au with a time step of 0.157 fs were used, and trajectories were dumped every 16 timesteps. All simulations were equilibrated for 2 ps, after which data were collected for at least 10 ps at 298.15 K in the NVT ensemble using a Nose−Hoover thermostat.53,54 All hydrogen atoms were replaced by deuterium. Analysis by Intermolecular Network Theory (INT). Trajectories from the AIMD simulations were analyzed by INT55 using the ChemNetworks package.56 Star graphs were created between the ion and the solvent molecules; the ion and the solvent molecules were transformed into vertices of the graph, and edges were formed between the vertices if the distance between the ion and the oxygen atom of the solvent molecule was less than 3 Å (chosen to allow for thermal fluctuations slightly beyond the experimentally determined distance of 2.48 Å for the first hydration shell and the DFT distance of

COMPUTATIONAL METHODS

Gas-Phase Calculations. The average geometry of the aqueous 9coordinate Cm3+ ion is well established as being tricapped trigonal prismatic, consisting of six axial water molecules (⟨rCm−O⟩ = 2.47 Å) and three equatorial water molecules (⟨rCm−O⟩ = 2.48 Å).5 In the average Cm(H2O)93+ structure, the six axial water molecules are identical with one another, as are the three equatorial water molecules. Substitution of a single methanol molecule into the coordination sphere presents two unique placement sites: axial or equatorial. The replacement of a second water molecule presents six unique sites, based on both the axial/equatorial positioning and the placement of the previous methanol. To generate initial structures for the entire range of water/methanol coordinated Cm3+ speciesfrom [Cm(H2O)9]3+ to [Cm(OHCH3)9]3+methanol was systematically substituted for water at geometrically unique sites, resulting in a total of 74 structures (see the Supporting Information). DFT was then used to optimize each of these structures, using the hybrid B3LYP functional37,38 with an integral internal screening threshold of 10−16 and a numeric integration grid of 10−8. Frequency analyses were performed on all optimized geometries to ensure that all structures were local minima with no imaginary vibrations. The aug-cc-pVDZ basis sets were used for the water and methanol molecules (oxygen, hydrogen, and carbon),39,40 while the Stuttgart relativistic small-core basis set41,42 (which simulates the core n = 1−4 shells as a pseudopotential to include scalar relativistic effects) was used for the Cm3+ ion along with its associated basis set. Subsequent single-point MP2 calculations,43,44 shown by Wiebke et 18 al. to more accurately predict solvation of the Cm3+ ion, were employed to determine the solvent binding energy (ΔE) of each solvent molecule according to the following reactions: 4993

DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999

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Inorganic Chemistry

Figure 2. Average solvent dissociation energy versus ion−solvent distance for methanol (●) and water (▲). Systems with a greater methanol content are darker, with molecules more likely to be both lower in dissociation energy and further from the ion. Variance is the standard deviation (of both the dissociation energy and ion−solvent distance) of different solvent molecules from all structures of the same water/methanol composition. 2.52 Å for the [Cm(CH3OH)9]3+ structure). Although these graphs (or networks) could not be analyzed for dynamic properties because of the short time scale accessible by the AIMD simulations, they can be very effectively analyzed for the structure of the solvation shell by the PageRank algorithm,57 as demonstrated on a variety of ions including La3+ by Mooney et al.58 In this analysis, the polyhedral arrangement of the solvent molecules about the ion is represented by a network with unique connectivity for any given polyhedral arrangement. Analyzing this network via PageRank produces a mathematically unique PageRank (PR) value; each PR corresponds to a specific solvation geometry.59,60 For example, the PR of an ion in a tricapped trigonalprismatic solvation environment is 0.148144. For each snapshot of simulation data, the calculated PR is compared to a library of PR values for different polyhedral arrangements; when the calculated PR is matched with the PR of known polyhedra, the solvation structure can be identified on a snapshot-by-snapshot basis.

nearly homogeneous) can be attributed to differences in the axial and equatorial positions of the solvent molecules. Additionally, methanol has a shorter bond distance to the Cm3+ ion than water. This is the case in both the DFToptimized structures and the gas-phase AIMD simulations of Cm3+ with its first solvation shell at every solvent composition. In the former, methanol molecules are, on average, 0.09 ± 0.03 Å closer to the ion than water molecules despite the greater bulk of the alcohol. The AIMD simulations of Cm3+ and its first solvation shell also indicate that methanol molecules are slightly closer to the ion in each mixed-solvent system, yet the average ion−solvent distance is slightly greater in the [Cm(CH3OH)9]3+ structure than in the [Cm(H2O)9]3+ structure (Table 1). This observation may be attributed to the larger size

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Table 1. Average Cm3+−Solvent Distances for Water and Methanol Molecules from Gas-Phase AIMD Simulations of the First Solvation Shell (Å)a

DISCUSSION General Trends Based on the Solvent-Shell Composition. Single-point MP2 energy calculations demonstrate that the introduction of methanol into the first solvation shell of the hydrated Cm3+ ion does have an effect on the solvent−ion binding energy as defined by eqs 1 and 2. When averaged across all potential solvent placements for each solvent-shell configuration, increasing the amount of methanol in the first solvation shell of the Cm3+ ion results in decreased ion−solvent dissociation energy for both water and methanol (Figure 1). This is likely due to an increase in the ion−solvent distances with the inclusion of methanol; the correlation between the ion−solvent distance and the solvent dissociation energy is especially clear in Figure 2, which shows that, on average, systems with shorter ion−solvent distances also have higher solvent dissociation energies. The high variance in the ion− solvent distances for some systems (most notably those that are

structure

Cm3+−OH2

3+

[Cm(CH3OH)9] [Cm(H2O)1(CH3OH)8]3+ [Cm(H2O)2(CH3OH)7]3+ [Cm(H2O)3(CH3OH)6]3+ [Cm(H2O)4(CH3OH)5]3+ [Cm(H2O)5(CH3OH)4]3+ [Cm(H2O)6(CH3OH)3]3+ [Cm(H2O)7(CH3OH)2]3+ [Cm(H2O)8(CH3OH)1]3+ [Cm(H2O)9]3+

2.67 2.56 2.56 2.53 2.55 2.53 2.53 2.52 2.51

± ± ± ± ± ± ± ± ±

0.02 0.05 0.08 0.06 0.04 0.03 0.04 0.02 0.02

Cm3+−OHCH3 2.52 2.51 2.50 2.47 2.46 2.46 2.47 2.45 2.44

± ± ± ± ± ± ± ± ±

0.08 0.11 0.03 0.08 0.05 0.05 0.04 0.02 0.02

a

High-energy X-ray scattering experiments have estimated the average Cm3+−O distance to be 2.48(1) Å in the [Cm(H2O)9]3+ structure.5 No data are available for Cm3+ in methanol. 4994

DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999

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Figure 3. Average natural charge on methanol (●) and water (▲) molecules in the lowest-energy optimized structure at each solvent-shell configuration. Variance is the standard deviation of solvent molecules in the same structure.

of the methanol molecules that sterically push the water molecules further and further away from the ion as the number of methanol molecules in the first solvation shell increases, as demonstrated in [Cm(H2O)1(CH3OH)8]3+, where the water molecule is, on average, 0.1 Å further from the ion. A similar, although smaller, effect exists for methanol: as more methanol molecules are substituted into the first solvation shell, the average Cm3+−methanol distance increases. These changes in the ion−solvent distances are correlated with the decreasing ion−solvent dissociation energy observed upon increased methanol in the first solvent shell. Natural Population Analysis. The trends in the solvent binding energy and ion−solvent distance have also been examined in the context of migration of the electron density from the individual solvent molecules to the ion upon solvation. A decrease of the natural charge, q, on the Cm3+ ion to 1.77 e− in the [Cm(H2O)9]3+ structure and 1.87 e− in the [Cm(CH3OH)9]3+ structure is observed. When the solvation shell is of mixed composition, however, the average charge donated by methanol molecules is significantly greater than that donated by water molecules. The greater charge (and hence increased electron donation) on methanol molecules is illustrated in Figure 3. Additionally, as methanol is added to the first solvation shell, the average charge donated to the ion by each solvent molecule decreases. Together these observations help to explain the trends observed in Figure 1, where methanol has a higher ion−solvent dissociation energy than water and where the average ion−solvent dissociation energy decreases as methanol is added to the first solvation shell. A strong linear correlation also exists between the average charge on the solvent molecules and the average distance between the solvent molecules and the ion for both water and methanol. The linear trend lines on the distance versus charge plot, shown in Figure 4, produce R2 correlation values of 0.89 and 0.90 for methanol and water, respectively.

Figure 4. Average natural charge on methanol (●) and water (▲) molecules plotted versus the average ion−solvent distance (taken from Table 1), including lines of best fit (dashed lines). Variance is the standard deviation of solvent molecules within structures of the same composition.

The donation of electrons to the Cm ion in the [Cm(H2O)9]3+ structure is slightly lower than that to similarly sized 9-coordinate aqueous lanthanides, a potentialy exploitable difference for separations because solvent displacement is a key step in ion complexation. According to electronic structure calculations by Kuta and Clark61 using the B3LYP functionals and the same basis sets employed in this work, only La has a higher effective charge (greater than 1.8 e−) than seen in the Cm ion here (1.77 e−). Gd (4f7) has an effective charge of less than 1.7 e− in the [Gd(H2O)9]3+ structure, approximately 0.1 e− lower than Cm (5f7). Like all of the lanthanides except for La and Lu, no Cm−OH2 bonding orbitals were observed in natural bond order analysis. As the atomic number increases across the actinide series, the energy level of the 6d orbital 4995

DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999

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Figure 5. Average solvent dissociation energy for methanol and water molecules from the Cm3+ ion, separated by the solvent position in the tricapped trigonal-prismatic geometry formed around the ion. Axial (black) and equatorial (gray) solvent molecules are shown for methanol (●) and water (▲). Variance is the standard deviation of different structures of the same water/methanol composition; axial/equatorial placements in solvent-shell compositions with less than three structures therefore have no variance values.

shell configuration for each water/methanol composition was used as a starting point for AIMD simulations of the Cm3+ ion and its first solvation shell. The polyhedral geometry of the first solvation shell of the Cm3+ ion was analyzed by INT as described above; every simulation frame of each solvent composition exhibited the tricapped trigonal-prismatic geometry seen in both the DFT optimizations and experiments. Yet the inclusion of methanol in the solvation shell does have an impact on its structure. The DFT-optimized [Cm(H2O)9]3+ structure has a clear delineation between the axial and equatorial positions of the water molecules, with average ion−O distances of 2.57 and 2.54 Å for the equatorial and axial water molecules, respectively. This differentiation between the axial and equatorial positions becomes steadily less clear as methanol is substituted for water in the first solvation shell; in the [Cm(CH3OH)9]3+ structure, the ion−O distances are indistinguishable at 2.58 ± 0.08 Å for equatorial methanol molecules and 2.57 ± 0.07 Å for axial methanol molecules. While the average dissociation energy for water molecules is dependent on their position in the tricapped trigonal prism, the dissociation energy of methanol ceases to be once at least two methanol molecules have been added to the solvation shell (Figure 5). The DFT and MP2 calculations, along with the first solventshell AIMD simulations discussed so far, necessarily leave out solvent−solvent interactions inherent in solution. To investigate the effect of second and third solvation shells on the structure of the first solvation shell, AIMD simulations were performed with the DFT-optimized first solvation shell surrounded by 75 solvent molecules (either all water or all methanol) in a periodic system. As in the gas-phase calculations, solution-phase AIMD simulations of all first solvent-shell compositions ([Cm(H 2 O) 9 ] 3 + , [Cm(H 2 O) 8 (CH 3 OH)] 3+ , [Cm(H 2 O) 4 (CH 3 OH) 5 ] 3+ , [Cm-

increases, while that of the 5f orbital decreases; later in the series (starting around Am), the 5f orbital energy level has significant energetic overlap with the 2p band of O (the 6d orbital in Cm is at a much higher level).62 While electron donation may be expected in the 5f orbitals of Cm due to energetic overlap, the radial extent of the 5f orbital is significantly contracted (Cm starts the latter half of the actinide series); the much more diffuse 6d orbital has significantly greater geometric overlap with the coordinating molecules.63 Additionally, Dinescu and Clark64 have observed electrostatic repulsion between the lone pairs of coordinating water molecules and the single 4f electron of the Ce3+ ion; water molecules aligned with the occupied 4f orbital were further away from the ion than other water molecules. The electronic structure of the Cm3+ ion, [Rn]5f7, features no unoccupied 5f orbitals; each is half-filled and would experience the lone-pair repulsion observed in Ce3+. Because of both the repulsion between oxygen lone pairs and the occupied 5f orbitals of Cm3+ and the greater geometric extent of the unoccupied 6d, 7s, and 7p orbitals,65 coordinating solvent molecules do not donate electrons to the 5f orbital. Regardless of whether the coordinating solvent molecule is water or methanol, the donated electrons are distributed approximately 57% in 6d, 27% in 7p, and 16% in 7s, with a negligible amount donated to the 5f orbital. Solvation Shell Organization. Experiments suggest that the structure of the first coordination sphere of the 9coordinate hydrated Cm3+ ion is tricapped trigonal prismatic, both in the solid state (X-ray diffraction) and in solution (highenergy X-ray scattering and extended X-ray absorption fine structure spectroscopy).5 The DFT-optimized structures are in excellent agreement with these experiments for all solvent-shell configurations and compositions. To further probe the structure of the first solvation shell, the lowest-energy solvation 4996

DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999

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Inorganic Chemistry (H2O)5(CH3OH)4]3+, and [Cm(CH3OH)9]3+) maintain the tricapped trigonal-prismatic structure over the entire course of the simulation (≥10 ps), in secondary solvation environments of both methanol and water. As in the DFT-optimized structures and the AIMD simulations of only the first solvation shell, AIMD simulations of the ion in solution (approximated by 75 additional solvent molecules outside the first solvation shell in a periodic system) showed the tricapped trigonal-prismatic geometry. However, the solvent molecules in the first solvation shell exhibit considerably more dynamic motion over the course of the simulations than the gas-phase AIMD calculations. With the dynamic movement inherent in the first solvation shell, the ion−solvent distances (averaged over the course of the simulation and disregarding the first picosecond) in the first solvation shell are less defined than those in the DFToptimized structures or the AIMD simulations of only the first solvation shell. Differences in the bond lengths between the AIMD simulations and the DFT-optimized structures can be attributed to the functional used: DFT calculations used B3LYP, while PBE was used in the AIMD simulations. Gasphase DFT-optimized structures using the PBE functional also have ion−solvent distances 0.01 Å shorter than those of the corresponding B3LYP-optimized structures; a comparison between the PBE and B3LYP structures can be found in the Supporting Information. Despite the differences between the functionals used in the DFT optimizations and AIMD simulations, the overall trends in the ion−solvent distances are the same. The average ion−solvent distances in the periodic systems increase slightly with the number of methanol molecules in the first solvation shell. This is not dependent on the composition of the second solvation shell, occurring when the second shell is both entirely water and entirely methanol. For example, the average Cm3+−OH2 distance in the periodic system is 2.43 ± 0.04 Å when the first solvation shell has nine water molecules but increases to 2.48 ± 0.06 Å when the first solvation shell has four water and five methanol molecules. Interestingly, the preferred isomers in DFT data are not observed in the AIMD simulations. For example, the [Cm(H2O)8(CH3OH)]3+ structure is energetically preferred in the DFT calculations when the methanol molecule is in an axial position rather than in an equatorial position. Despite starting in the lowest-energy gas-phase configuration, the methanol molecule moved in and out of an equatorial position during the simulation as all solvent molecules dynamically shifted positions within the tricapped trigonal prism. The energetic preferences for specific isomers at a given solvent composition are not maintained in the AIMD simulations, which have a more realistic depiction of the bulk solvent. These preferences (Figure 5) were only 2−3 kcal/mol in the gas phase and would easily be overcome by thermal effects in solution and with competing interactions between the first and second solvation shells. Finally, previous work with singly charged ions in water/ methanol solutions suggests that the composition of these solutions will have an impact on the ion−solvent dynamics.66 Although this is an important question of interest, it requires time scales inaccessible by AIMD and is instead being pursued using classical simulations in ongoing work.

methanol molecules are added to the solvation shell, the dissociation energy of both water and methanol from the ion decreases by approximately 5 kcal/mol over the entire solventshell composition range. This is potentially due to the increasing ion−solvent distance with increasing methanol content because the ion−solvent distance correlates strongly with the solvent dissociation energy. Additionally, methanol molecules require more energy to remove from the ion than water molecules in every composition of the first solvation shell; they are also closer to the ion than water molecules in optimized gas-phase structures of the same composition, likely because of their slightly greater charge donation to the ion made possible by the electron-rich methyl group. The tricapped trigonal-prismatic structure seen experimentally is reproduced here in the gas-phase DFT optimizations, in the gas-phase AIMD simulations of Cm3+ with its first solvation shell, and in the solution-phase AIMD simulations. The solution-phase AIMD simulations help to provide a link between the solid-state X-ray diffraction experiments used to determine the tricapped trigonal-prismatic structure and solution. Methanol does distort the structure slightly; as more methanol molecules are added to the solvation shell, the distinction between the axial and equatorial positions becomes blurred. The thermal motion of the solvent molecules and the competing interactions between the first and second solvation shells in the solution-phase simulations make the difference between the Cm3+−OH2 and Cm3+−OHCH3 distances statistically insignificant, washing out structural preferences observed in the gas phase. Interestingly, the composition of the second solvation shell has no effect on the structure of the first shell at the time scales analyzed. Yet, previous results of singly charged ions in water/ methanol mixtures suggest it will have a significant impact on the dynamics. CMD simulations are required to reach the time scales necessary to study ion−solvent dynamics, particularly with the highly charged Cm3+ ion, which is a topic of ongoing study.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00477. Methodology for substituting methanol into the first solvation shell of the CmIII ion, comparison of the [Cm(H2O)4(CH3OH)5]3+ structure using B3LYP and PBE, and listings of xyz coordinates for all optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.P.K. acknowledges support from the G. T. Seaborg Institute, Los Alamos National Laboratory, and a Department of Energy Office of Science Graduate Student Research Fellowship. P.Y. was supported by the U.S. Department of Energy, Office of

CONCLUSIONS The introduction of methanol into the first solvation shell of the hydrated Cm3+ ion creates clear differences in the gas-phase dissociation energies of solvating water molecules. As more 4997

DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999

Article

Inorganic Chemistry

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Science, Basic Energy Sciences, Chemical Sciences, Biosciences, and Geosciences Division, Heavy Element Chemistry Program, at Los Alamos National Laboratory under Contract DE-AC5206NA25396 (operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy). S.B.C. acknowledges support from the U.S. Department of Energy, Office of Science, Heavy Elements Program (Grant DE-SC-000-4102). A.E.C. acknowledges support from the U.S. Department of Energy, Office of Science, Heavy Elements Program (Grant DE-SC-000-1815). Calculations were performed using the Molecular Science Computing Facilities at William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy, Biological and Environmental Research, and located at Pacific Northwest National Laboratory.

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DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999

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DOI: 10.1021/acs.inorgchem.6b00477 Inorg. Chem. 2016, 55, 4992−4999