Investigations of the Cobalt Hexamine Uranyl Carbonate System: Understanding the Influence of Charge and Hydrogen Bonding on the Modification of Vibrational Modes in Uranyl Compounds.

Hydrogen bonding networks within hexavalent uranium materials are complex and may influence the overall physical and chemical properties of the system. This is particularly true if hydrogen bonding takes places between the donor and the oxo group associated with the uranyl cation (UO22+). In the current study, we evaluate the impact of charge-assisted hydrogen bonding on the vibrational modes of the uranyl cation using uranyl tricarbonate [UO2(CO3)3]4- interactions with [Co(NH3)6]3+ as the model system. Herein, we report the synthesis and structural characterization of five novel compounds, [Co(NH3)6]Cl(CO3) (Co_Cl_CO3), [Co(NH3)6]4[UO2(CO3)3]3(H2O)11.67 (Co4U3), [Co(NH3)6]3[UO2(CO3)3]2Cl (H2O)7.5 (Co3U2_Cl), [Co(NH3)6]2[UO2(CO3)3]Cl2 (Co2U_Cl), and [Co(NH3)6]2[UO2(CO3)3]CO3 (Co2U_CO3), which contain differences in the crystalline packing and extended hydrogen bonding networks. We show that these slight changes in the supramolecular assembly and hydrogen bonding networks result in the modification of modes as observed by infrared and Raman spectroscopy. We use density functional theory calculations to assign the vibrational modes and provide an understanding about how uranyl bond perturbation and changes in hydrogen bonding interactions can impact the resulting spectroscopic signals.

Adsorption of small organic acids and polyphenols on hematite surfaces: Density Functional Theory + thermodynamics analysis.

HYPOTHESIS: The interactions of organic molecules with mineral surfaces are influenced by several factors such as adsorbate speciation, surface atomic and electronic structure, and environmental conditions. When coupled with thermodynamic techniques, energetics from atomistic modeling can provide a molecular-level picture of which factors determine reactivity. This is paramount for evaluating the chemical processes which control the fate of these species in the environment. EXPERIMENTS: Inner-sphere adsorption of oxalate and pyrocatechol on (001), (110), and (012) α-Fe2O3 surfaces was modeled using Density Functional Theory (DFT). Unique bidentate binding modes were sampled along each facet to study how different adsorbate and surface factors govern site preference. Adsorption energetics were then calculated using a DFT + thermodynamics approach which combines DFT energies with tabulated data and Nernst-based corrective terms to incorporate different experimental parameters. FINDINGS: Instead of a universal trend, each facet displays a unique factor that dominates site preference based on either strain (001), functional groups (110), or topography (012). Adsorption energies predict favorable inner-sphere adsorption for both molecules but opposite energetic trends with varying pH. Additionally, vibrational analysis was conducted for each system and compared to experimental IR data. The work presented here provides an effective, computational methodology to study numerous adsorption processes occurring at the surface-aqueous interface.

Density functional theory and thermodynamics analysis of MAl12 Keggin substitution reactions: Insights into ion incorporation and experimental confirmation.

Polyaluminum cations, such as the MAl12 Keggin, undergo atomic substitutions at the heteroatom site (M), where nanoclusters with M = Al3+, Ga3+, and Ge4+ have been experimentally studied. The identity of the heteroatom M has been shown to influence the structural and electronic properties of the nanocluster and the kinetics of ligand exchange reactions. To date, only three ε-analogs have been identified, and there is a need for a predictive model to guide experiment to the discovery of new MAl12 species. Here, we present a density functional theory (DFT) and thermodynamics approach to predicting favorable heteroatom substitution reactions, alongside structural analyses on hypothetical ε-MAl12 nanocluster models. We delineate trends in energetics and geometry based on heteroatom cation properties, finding that Al3+-O bond lengths are related to heteroatom cation size, charge, and speciation. Our analyses also enable us to identify potentially isolable new ε-MAl12 species, such as FeAl12 7+. Based upon these results, we evaluated the Al3+/Zn2+/Cr3+ system and determined that substitution of Cr3+ is unfavorable in the heteroatom site but is preferred for Zn2+, in agreement with the experimental structures. Complimentary experimental studies resulted in the isolation of Cr3+-substituted δ-Keggin species where Cr3+ substitution occurs only in the octahedral positions. The isolated structures Na[AlO4Al9.6Cr2.4(OH)24(H2O)12](2,6-NDS)4(H2O)22 (δ-CrnAl13-n-1) and Na[AlO4Al9.5Cr2.5(OH)24(H2O)12](2,7-NDS)4(H2O)18.5 (δ-CrnAl13-n-2) are the first pieces of evidence of mixed Al3+/Cr3+ Keggin-type nanoclusters that prefer substitution at the octahedral sites. The δ-CrnAl13-n-2 structure also exhibits a unique placement of the bound Na+ cation, which may indicate that Cr3+ substitution can alter the surface reactivity of Keggin-type species.

Formation of Nanoscale [Ge4 O16 Al48 (OH)108 (H2 O)24 ]20+ from Condensation of ϵ-GeAl12 8+ Keggin Polycations*.

Keggin-type polyaluminum cations belong to a unique class of compounds with their large positive charge, hydroxo bridges, and divergent isomerization/oligomerization. Previous reports indicated that oligomerization of this species can only occur through one isomer (δ), but herein we report the isolation of largest Keggin-type cluster that occurs through self-condensation of four ϵ-isomers ϵ-GeAl12 8+ to form [Ge4 O16 Al48 (OH)108 (H2 O)24 ]20+ cluster (Ge4 Al48 ). The cluster was crystallized and structurally characterized by single-crystal X-ray diffraction (SCXRD) and the elemental composition was confirmed by ICP-MS and SEM-EDS. Additional dynamic light scattering experiments confirms the presence of the Ge4 Al48 in thermally aged solutions. DFT calculations reveal that a single atom Ge substitution in tetrahedral site of ϵ-isomer is the key for the formation of Ge4 Al48 because it activates deprotonation at key surface sites that control the self-condensation process.

Density Functional Theory and Thermodynamics Modeling of Inner-Sphere Oxyanion Adsorption on the Hydroxylated α-Al2O3(001) Surface.

The inner-sphere adsorption of AsO43-, PO43-, and SO42- on the hydroxylated α-Al2O3(001) surface was modeled with the goal of adapting a density functional theory (DFT) and thermodynamics framework for calculating the adsorption energetics. While DFT is a reliable method for predicting various properties of solids, including crystalline materials comprised of hundreds (or even thousands) of atoms, adding aqueous energetics in heterogeneous systems poses steep challenges for modeling. This is in part due to the fact that environmentally relevant variations in the chemical surroundings cannot be captured atomistically without increasing the system size beyond tractable limits. The DFT + thermodynamics approach to this conundrum is to combine the DFT total energies with tabulated solution-phase data and Nernst-based corrective terms to incorporate experimentally tunable parameters such as concentration. Central to this approach is the design of thermodynamic cycles that partition the overall reaction (here, inner-sphere adsorption proceeding via ligand exchange) into elementary steps that can either be fully calculated or for which tabulated data are available. The ultimate goal is to develop a modeling framework that takes into account subtleties of the substrate (such as adsorption-induced surface relaxation) and energies associated with the aqueous environment such that adsorption at mineral-water interfaces can be reliably predicted, allowing for comparisons in the denticity and protonation state of the adsorbing species. Based on the relative amount of experimental information available for AsO43-, PO43-, and SO42- adsorbates and the well-characterized hydroxylated α-Al2O3(001) surface, these systems are chosen to form a basis for assessing the model predictions. We discuss how the DFT + thermodynamics results are in line with the experimental information about the oxyanion sorption behavior. Additionally, a vibrational analysis was conducted for the charge-neutral oxyanion complexes and is compared to the available experimental findings to discern the inner-sphere adsorption phonon modes. The DFT + thermodynamics framework used here is readily extendable to other chemical processes at solid-liquid interfaces, and we discuss future directions for modeling surface processes at mineral-water and environmental interfaces.

Ga3+ Incorporation into Al13 Keggin Polyoxometalates and the Formation of δ-(GaAl12)7+ and (Ga2.5Al28.5)19+ Polycations.

Keggin-type polyaluminum species (ε-Al13, δ-Al13, Al26, Al30, Al32) can form upon partial hydrolysis of Al3+-bearing solutions and are important species for water purification and contaminant transport. While the structural features for the major Al3+ polyaluminum species have been delineated, much less is known regarding heteroatom substitution and resultant structures other than the previously identified ε-GaAl127+ and ε-GeAl128+ cations. Single-atom substitution within polyaluminum species can change the surface reactivity within water treatment scenarios; thus, it is important to understand heteroatom incorporation within this system. The present work describes the synthesis and characterization of two novel Ga3+-substituted Keggin-type polyaluminum species. Na[GaO4Al12(OH)24(H2O)12](2,6-NDS)4(H2O)20.5 (δ-GaAl12) and [Ga2O8Al28.5Ga0.5(OH)58(H2O)27(SO4)2](SO4)4Cl7(H2O)8.5 (Ga2.5Al28.5) were crystallized from a thermally aged, partially hydrolyzed Ga3+/Al3+ solution. Structural refinement from single-crystal X-ray diffraction indicated fully occupied Ga3+ within tetrahedral site(s) of both isolated species. Partial substitution was observed for octahedral sites for the larger Ga2.5Al28.5 cluster. The chemical compositions of both clusters were confirmed by inductively coupled plasma mass spectrometry (ICP-MS). Density functional theory (DFT) calculations corroborated the structural refinement, with the energetics of Ga3+ substitution suggesting preferential substitution within tetrahedral sites for both species. Additional theoretical work suggests that the rotated trimer in δ-GaAl12 is highly reactive, which can serve as the driving force in the formation of the Ga2.5Al28.5 cluster.

Impacts of hydrogen bonding interactions with Np(v/vi)O2Cl4 complexes: vibrational spectroscopy, redox behavior, and computational analysis.

The neptunyl (Np(v)O2+/Np(vi)O22+) cation is the dominant form of 237Np in acidic aqueous solutions and the stability of the Np(v) and Np(vi) species is driven by the specific chemical constituents present in the system. Hydrogen bonding with the oxo group may impact the stability of these species, but there is limited understanding of how these intermolecular interactions influence the behavior of both solution and solid-state species. In the current study, we systematically evaluate the interactions between the neptunyl tetrachloride species and hydrogen donors in coordination complexes and in the related aqueous solutions. Both Np(v) compounds (N2C4H12)2[Np(v)O2Cl4]Cl (Np(V)pipz) and (NOC4H10)3[Np(v)O2Cl4] (Np(V)morph) exhibit directional hydrogen bonding to the neptunyl oxo group while Np(vi) compounds (NC5H6)2[Np(vi)O2Cl4] (Np(VI)pyr) and (NOC4H10)4[Np(vi)O2Cl4]·2Cl (Np(VI)morph) assemble via halogen interactions. The Raman spectra of the solid-state phases indicate the activation of vibrational bands when there is asymmetry of the neptunyl bond, while these spectral features are not observed within the related solution phase spectra. Density functional theory calculations of the Np(V)pipz system suggest that activation of the ν3 asymmetric stretch and other combination modes lead to additional complexity within the solid-state spectra. Electrochemical analyses of complexes in the solution phases are consistent with the results of the crystallization experiments as the voltammetric potentials of Np(v)/Np(vi) complexes in the presence of protonated heterocycles differ from the potentials of pure Np(v) and may correlate with the hydrogen bonding interactions.

Actinyl-cation interactions: experimental and theoretical assessment of [Np(vi)O2Cl4]2- and [U(vi)O2Cl4]2- systems.

The interaction of the actinyl (AnO22+) oxo group with low-valent cations influences the chemical and physical properties of hexavalent actinides, but the impact of these intermolecular interactions on the actinyl bond and their occurrence in solution and solid state phases remain unclear. In this study, we explore the coordination of alkali cations (Li+, Na+, K+) with the [NpO2Cl4]2- coordination complexes using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory (DFT) calculations and compare to the related uranyl system. Three solid-state coordination compounds ([Li(12-crown-4)]2[NpO2Cl4] (LiNp), [Na(18-crown-6)H2O]2[NpO2Cl4] (NaNp), and [K(18-crown-6)]2[NpO2Cl4] (KNp) have been synthesized and characterized using single-crystal X-ray diffraction and Raman spectroscopy. Only Li+ cations interact with the neptunyl oxo in the solid-state compounds and this results in a red-shift of the NpO22+ symmetric stretch (ν1). Raman spectra of Np(vi) solutions containing lower Li+ concentrations display a single peak at ∼854 cm-1 and increasing the amount of Li+ results in the ingrowth of a second band at 807 cm-1. DFT calculations and vibrational analysis indicate the lower frequency vibrational band is the result of interactions between the Li+ cation and the neptunyl oxo. Comparison to the related uranyl system shows similar interactions occur in the solid state, but subtle differences in the actinyl-cation modes in solution phase.

Systematic Study of Aluminum Nanoclusters and Anion Adsorbates.

The interactions between aqueous aluminum (Al) nanoclusters and ions in solution influence the reactivity of nanomaterials in natural waters and are crucial to the targeted syntheses of aluminum oxides. To contribute to the fundamental understanding of how both anion and Al-nanocluster properties affect the interactions, we carry out systematic modeling studies that employ density functional theory calculations embedded in a continuum solvent model. Energetic and electronic structure analysis is applied toward delineating the interactions of a range of probe adsorbate anions with Al nanoclusters to elucidate how small molecules may react with naturally occurring nanomaterials. The study spans seven small molecules on three model Al nanoclusters. Using this ion set, we correlate the size, shape, and formal charge of the adsorbate to the trends in adsorption energies. A key finding is that the collective effects of exposed oxygen functional groups, i.e., the distribution of functional groups, dictates the electrostatic potential of the nanocluster surface, which, in turn, controls trends in anion adsorption. The computed adsorption and deprotonation trends are correlated to known synthetic routes of Al-nanocluster formation and subsequent crystallization to give insight into the potential optimization of synthetic conditions.