“17O NMR as a Tool in Discrete Metal Oxide Cluster Chemistry”
in Annual Reports on NMR Spectroscopy. Link.
C. André Ohlin, William H. Casey
Abstract: This chapter covers recent developments in 17O NMR spectroscopy as applied to discrete metal oxide clusters, particularly in the context of their use as models in geochemistry and catalysis. Dynamic 17O NMR methods based on the McConnell–Bloch equations are explored in depth, and recent advances are reviewed. High-pressure NMR methods are also discussed and reviewed, as are recent developments in thBinding Geometries of Silicate Species on Ferrihydrite Surfacese use of density functional theory in the computation of 17O NMR shifts in polyoxometalates. The emphasis of the chapter is on the new developments that promise to reinvigorate 17O NMR as a central tool in the study of aqueous chemical kinetics, with the most urgent challenges being understanding the rates of isotopic substitution into bridging oxygens in clusters.
“Solution, solid-state, and computational analysis of agostic interactions in a coherent set of low-coordinate rhodium(III) and iridium(III) complexes”
in Chem. — Eur. J., Accepted. Link.
Knighton, Richard C.; Emerson-King, Jack; Rourke, Jonathan P.; Ohlin, C. André; Chaplin, Adrian B
Abstract: A homologous family of low-coordinate complexes of the formulation trans-[M(2,2′-biphenyl)(PR3)2][BArF4] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1H and 31P NMR spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3>PCy3>PiPr3>PPh3. In addition to these structure–property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH2Cl2) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M−H−C distances.
Ice and Cryosalt Formation in Saline Microporous Clay Gels
ACS Earth and Space Chemistry. https://pubs.acs.org/doi/10.1021/acsearthspacechem.7b00134
Hydrated clay minerals that are common to Earth’s atmosphere and terrestrial and aquatic environments can form gels that host saline solutions. Using cryogenic electron microscopy and vibration spectroscopy, we show that saline gels of montmorillonite frozen at < –90 °C host elongated hexagonal ice (Ih) microcrystals embedded in a network of honeycomb micropores. Freezing segregates salts into walls of aggregated clay nanoparticles sharing face-to-face contacts. Above ∼ –50 °C, clay gels that are sufficiently dense (≫10 g/L) and flexible (Na-exchanged montmorillonite) also host the cryosalt mineral hydrohalite (NaCl·2H2O), either co-existing or entirely replacing Ih in the gels. Hydrohalite does not form in gels of low-density (<10 g/L) or rigid (Ca-exchange montmorillonite) clay particles. These results suggest that hydrohalite forms in expandable clay gels that are sufficiently dense and flexible to retain saline solutions within their walls, possibly through interparticle capillary and hydration forces. These forces effectively oppose water diffusion to growing ice microcrystals within micropores, thus prolonging the lifetime of hydrohalite within these hydrated clay gels. Our findings tie the fate of ice and cryosalt nucleation and growth to the water-retention capability of expandable clay gels.
“Computational Prediction of Mg-Isotope Fractionation Between Aqueous [Mg(OH2)6]2+ and Brucite”
in Geochim. Cosmochim. Acta., Accepted. Link.
Christopher A. Colla, W. H. Casey, C. André Ohlin
Abstract: The fractionation factor in the magnesium-isotope fractionation between aqueous solutions of magnesium and brucite remarkably changes sign with increasing temperature, as uncovered by recent experiments. To understand this behavior, the Reduced Partition Function Ratios and isotopic fractionation factors (Δ26/24Mgbrucite-Mg(aq)) are calculated using molecular models of aqueous [Mg(OH2)6]2+ and the mineral brucite at increasing levels of density functional theory. The calculations were carried out on the [Mg(OH2)6]2+·12H2O cluster, along with different Pauling-bond-strength-conserving models of the mineral lattice of brucite. Three conclusions were reached: i) all of the calculations overestimate <Mg-O> bond distances in the aqua ion complex relative to Tutton’s salts; ii) the calculations predict that brucite at 298.15 K is always enriched in the heavy isotope, in contrast with experimental observations; iii) the temperature dependencies of Wimpenny et al. (2014) and Li et al. (2014) could only be achieved by fixing the <Mg-O> bond distances in the [Mg(OH2)6]2+·12H2O cluster to values close to those observed in crystals that trap the hydrated ion. Link.
Silicate Binding and Precipitation on Iron Oxyhydroxides
Environmental Science and Technology. https://pubs.acs.org/doi/10.1021/acs.est.7b04098
Silica-bearing waters in nature often alter the reactivity of mineral surfaces via deposition of Si complexes and solids. In this work, Fourier transform infrared (FTIR) spectroscopy was used to identify hydroxo groups at goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) surfaces that are targeted by ligand exchange reactions with monomeric silicate species. Measurements of samples first reacted in aqueous solutions then dried under N2(g) enabled resolution of the signature O–H stretching bands of singly (−OH), doubly (μ–OH), and triply coordinated (μ3–OH) groups. Samples reacted with Si for 3 and 30 d at pH 4 and 7 revealed that −OH groups were preferentially exchanged by silicate and that μ–OH and μ3–OH groups were not exchanged. Based on knowledge of the disposition of −OH groups on the major crystallographic faces of goethite and lepidocrocite, and the response of these groups to ligand exchange prior oligomerization, our work points to the predominance of rows of mononuclear monodentate silicate species, each separated by at least one −OH group. These species are the attachment sites from which oligomerization and polymerization reactions occur, starting at loadings exceeding ∼1 Si/nm2 and corresponding to soluble Si concentrations that can be as low as ∼0.7 mM after 30 d reaction time. Only above such loadings can reaction products grow away from rows of −OH groups and form hydrogen bonds with nonexchangeable μ–OH and μ3–OH groups. These findings have important repercussions for our understanding of the fate of waterborne silicate ions exposed to minerals.
Binding Geometries of Silicate Species on Ferrihydrite Surfaces
Silicate sorption on ferrihydrite surfaces, as monomers, oligomers, and polymers, strongly affects ferrihydrite crystallinity, thermodynamic stability, and surface reactivity. How these silicate species bind on ferrihydrite surfaces is, however, not well understood. We have determined silicate binding geometries using a combination of X-ray absorption spectroscopy (XAS), differential atomic pair distribution function (d-PDF) analysis, and density functional theory (DFT) calculations. Silicon K-edge absorption pre-edges and DFT-predicted energies indicate that silicate forms monomeric monodentate–mononuclear (MM) complexes at low silicate sorption loadings. With increasing silicate loading, the pre-edge peak shifts to higher energies, suggesting changes in the silicate binding geometry toward multidentate complexation. The d-PDF analysis determines the Si–Fe interatomic distance to be ∼3.25 Å for the high-loading samples. The DFT calculations indicate that such distance corresponds to an oligomer in the bidentate–binuclear (BB) binding geometry. The transition of the silicate sorption geometry accompanied by polymerization can affect stability of ferrihydrite and its adsorption and redox reactivity and increase the degree of Si isotopic fractionation upon silicate sorption on Fe oxides. MM monomeric complexes and BB oligomeric complexes should be used for surface complexation models predicting silicate sorption on Fe oxide surfaces.
“Cohesive Vibrational and Structural Depiction of Intercalated Water in Montmorillonite”
ACS Earth and Space Chemistry https://pubs.acs.org/doi/10.1021/acsearthspacechem.7b00103
Abstract: The vibrational spectral profiles of Na- and Ca-montmorillonite (MMT) of controlled water layer populations (nW) was extracted by chemometric analysis of new Fourier transform infrared (FTIR) spectroscopy data and validated by mixed-layer modeling of previously published X-ray diffraction data. These efforts resolved FTIR spectral profiles of 0W, 1W, and 2W interlayers, which can now be used to explore the distinct hydration states of MMT. These spectral profiles reflect water populations organized around interlayer cations (Na+, Ca2+), interacting with siloxane groups of the basal face of the interlayer, and with other bound and “free” water molecules. This cohesive description of water-bearing clays provides the link needed to relate vibrational to structural attributes of these geochemically important materials.
“Accumulation of counterions and coions evaluated by cryogenic XPS as a new tool for describing the structure of electric double layer at the silica/water interface”
Phys.Chem.Chem.Phys., 2017, 19, 29047-29052 DOI: 10.1039/C7CP06439J
Jiri Škvarla, Mária Kanuchová, Andrey Shchukarev, Ivan Brezáni, and Juraj Škvarla.
We introduce a new method of evaluating the structure of electric double layer (EDL) at the native solid/liquid interface using cryogenic X-ray photoelectron spectroscopy technique. This method isbased on evaluating the atomic concentration ratio of counterions and co-ions of supporting electrolyte at the close-to-in situ state surface of colloid particles by the cryo-XPS and comparing it withanalogous ratio predicted by EDL models. For silica colloids in aqueous KCl solutions at pH 6 to 8 it has been found that the latter ratio is higher than unity, as expected for the negatively charged surface of silica, but does not correspond with the prediction of the basic Gouy–Chapman EDL model for the ideal interface. However, it agrees with that deduced from experiments on electrolytic coagulation kinetics of analogous silica colloids by applying a simple EDL model of swellable ion-permeable (Donnanian) polyelectrolyte gel layer. It turns out that the traditional Stern layer-based concept of EDL at solid/liquid interfaces is not justified for metal oxides at least in KCl solutions.
“PNacPNacE: (E = Ga, In, Tl) – monomeric group 13 metal(I) heterocycles stabilized by a sterically demanding bis(iminophosphoranyl)methanide”
in Dalton Trans., 2017, 46, 16872-16877.
Christian P. Sindlinger, Samuel R. Lawrence, Shravan Acharya, C. André Ohlin, Andreas Stasch
The salt metathesis reaction of the sterically demanding bis(iminophosphoranyl)methanide alkali metal complexes LM (L – = HC(Ph 2 P=NDip) 2- , Dip = 2,6- i Pr 2 C 6 H 3 ; M = Li, Na, K) with “GaI”, InBr or TlBr afforded the monomeric group 13 metal(I) complexes LE:, E = Ga (1), In (2) and Tl (3), and small quantities of LGaI 2 4 in case of Ga, respectively. The molecular structures of LE: 1-3 from X-ray single crystal diffraction show them to contain puckered six-membered rings with N,N’-chelating methanide ligands and two-coordinated metal(I) centres. Reduction reactions of LAlI 2 5, prepared by iodination of LAlMe 2 , were not successful and no aluminium(I) congener could be prepared so far. DFT studies on LE:, E = Al–Tl, were carried out and support the formulation as an anionic, N,N’-chelating methanide ligand coordinating to group 13 metal(I) cations. The HOMOs of the molecules for E = Al-In show a dominant contribution from a metal-based lone pair that is high in s-character.
“Cryo-XPS: probing intact interfaces in nature and life”
Andrey Shchukarev and Madeleine Ramstedt
Surface and Interface Analysis, 2017, 49, 349-356 DOI: 10.1002/sia.6025
Experimental studies of solid-aqueous solution interfaces are of great importance for reaching better chemical understanding of interfacial phenomena at themolecular level. This perspective article presents a recently developed approach for investigation of intact interfaces, based on fast freezing of centrifuged wet pastes followed by traditional XPS measurements at liquid nitrogen temperatures. Sample preparation and handling protocols, applicable to any suspension or gel, are discussed in detail. For mineral suspensions, cryogenic XPS is an important complement to traditional analyses of supernatant solutions and dry solids that is capable of revealing novel insights of the electrical double layer in terms of structure and composition. It can be used to study changes in the biochemistry of bacterial cell walls as influenced by external stimuli, and interfacial features related to biocompatibility of implant materials. Herein we review how the technique has been applied to minerals in electrolyte solutions, intact bacterial surfaces, and biomaterial interfaces in biologically relevant media, and highlight some future requirements for development of interface analysis methodologies.
“Electrochemical Response of Bound Electrolyte Ions at Oriented Hematite Surfaces: A Local Electrochemical Impedance Spectroscopy Study”
J. Phys. Chem. C. https://pubs.acs.org/doi/10.1021/acs.jpcc.7b08157
The electrochemical response of millimeter-sized hematite (α-Fe2O3) electrode surfaces to bound ions of NaCl, NH4Cl, and NaHCO3 salts was monitored by alternating current scanning electrochemical microscopy (AC-SECM). Local electrochemical impedance spectroscopy (LEIS) measurements along 100 μm lines on the (001) and (012) faces of hematite were used to extract capacitance and resistance parameters affected by bound inorganic ions. Equivalent circuit modeling was used to suggest that (1) double layer capacitances are affected by the spatial distribution of ions, and that (2) compact plane capacitance and resistance are affected by the closeness of association of ions to surface hydroxo groups. This study confirms the sensitivity of the technique to electrolyte ion binding, and provides new and key insight into the micrometer-scale electrochemical properties of iron oxides exposed to environmentally relevant conditions.
“Cobinding of Pharmaceutical Compounds at Mineral Surfaces: Mechanistic Modeling of Binding and Cobinding of Nalidixic Acid and Niflumic Acid at Goethite Surfaces”
in Environmental Science and Technology https://pubs.acs.org/doi/abs/10.1021/acs.est.7b02900
Although emerging contaminants rarely exist individually in environmental contaminated systems, only limited information on their adsorption mechanisms in multicomponent solutions is currently available. To address this shortcoming, this work examines for the first time the accuracy of a surface complexation model in predicting the cooperative adsorption of nalidixic acid (NA) and niflumic acid (NFA) at goethite (α-FeOOH) surfaces. Our model adequately predicts cobinding of an outer-sphere (OS) complex of NFA onto NA bound to goethite through metal-bonded (MB), hydrogen-bonded (HB), or OS complexes. More positive charge is introduced in the system via sodium interactions in order to describe the NFA adsorption at high NaCl concentrations in both single and binary systems. Our model confidently predicts multilayers of NA on goethite as well as NFA binding on goethite-bound NA over a large range of pH and salinity values as well as NA and NFA loadings. These findings have strong implications in the assessment and prediction of contaminant fate in multicomponent contaminated systems by invoking a nontraditional form of ligand–ligand interaction in this field of study.
“Co-Binding of Pharmaceutical Compounds at Mineral Surfaces: Molecular Investigations of Dimer Formation at Goethite/Water Interfaces”
in Environmental Science and Technology, http://pubs.acs.org/doi/abs/10.1021/acs.est.7b02835
Jing Xu, Rémi Marsac, Dominique Costa , Wei Cheng, Feng Wu, Jean-François Boily, and Khalil Hanna
Abstract: The emergence of antibiotic and anti-inflammatory agents in aquatic and terrestrial systems is becoming a serious threat to human and animal health worldwide. Because pharmaceutical compounds rarely exist individually in nature, interactions between various compounds can have unforeseen effects on their binding to mineral surfaces. This work demonstrates this important possibility for the case of two typical antibiotic and anti-inflammatory agents (nalidixic acid (NA) and niflumic acid (NFA)) bound at goethite (α-FeOOH) used as a model mineral surface. Our multidisciplinary study, which makes use of batch sorption experiments, vibration spectroscopy and periodic density functional theory calculations, reveals enhanced binding of the otherwise weakly bound NFA caused by unforeseen intermolecular interactions with mineral-bound NA. This enhancement is ascribed to the formation of a NFA–NA dimer whose energetically favored formation (−0.5 eV compared to free molecules) is predominantly driven by van der Waals interactions. A parallel set of efforts also showed that no cobinding occurred with sulfamethoxazole (SMX) because of the lack of molecular interactions with coexisting contaminants. As such, this article raises the importance of recognizing drug cobinding, and lack of cobinding, for predicting and developing policies on the fate of complex mixtures of antibiotics and anti-inflammatory agents in nature.
“Atomistic simulations of cation hydration in sodium and calcium montmorillonite nanopores”
in The Journal of Chemical Physics, http://aip.scitation.org/doi/10.1063/1.4992001
Guomin Yang, Ivars Neretnieks, and Michael Holmboe
During the last four decades, numerous studies have been directed to the swelling smectite-rich clays in the context of high-level radioactive waste applications and waste-liners for contaminated sites. The swelling properties of clay mineral particles arise due to hydration of the interlayer cations and the diffuse double layers formed near the negatively charged montmorillonite (MMT) surfaces. To accurately study the cation hydration in the interlayer nanopores of MMT, solvent-solute and solvent-clay surface interactions (i.e., the solvation effects and the shape effects) on the atomic level should be taken into account, in contrast to many recent electric double layer based methodologies using continuum models. Therefore, in this research we employed fully atomistic simulations using classical molecular dynamics (MD) simulations, the software package GROMACS along with the CLAYFF forcefield and the SPC/E water model. We present the ion distributions and the deformation of the hydrated coordination structures, i.e., the hydration shells of Na+ and Ca2+ in the interlayer, respectively, for MMT in the first-layer, the second-layer, the third-layer, the fourth-layer, and the fifth-layer (1W, 2W, 3W, 4W, and 5W) hydrate states. Our MD simulations show that Na+ in Na-MMT nanopores have an affinity to the ditrigonal cavities of the clay layers and form transient inner-sphere complexes at about 3.8 Å from clay midplane at water contents less than the 5W hydration state. However, these phenomena are not observed in Ca-MMT regardless of swelling states. For Na-MMT, each Na+ is coordinated to four water molecules and one oxygen atom of the clay basal-plane in the first hydration shell at the 1W hydration state, and with five to six water molecules in the first hydration shell within a radius of 3.1 Å at all higher water contents. In Ca-MMT, however each Ca2+ is coordinated to approximately seven water molecules in the first hydration shell at the 1W hydration state and about eight water molecules in the first hydration shell within a radius of 3.3 Å at all higher hydration states. Moreover, the MD results show that the complete hydration shells are nearly spherical with an orthogonal coordination sphere. They could only be formed when the basal spacing d001 ≥ 18.7 Å, i.e., approximately, the interlayer separation h ≥ 10 Å. Comparison between DFT and MD simulations shows that DFT failed to reproduce the outer-sphere complexes in the Stern-layer (within ∼5.0 Å from the clay basal-plane), observed in the MD simulations.
“Surface chemistry of carbon dioxide revisited”
in Surface Chemistry, https://doi.org/10.1016/j.surfrep.2016.09.001
Taifan, William; Boily, Jean-François; Baltrusaitis, Jonas.
This review discusses modern developments in CO2 surface chemistry by focusing on the work published since the original review by H.J. Freund and M.W. Roberts two decades ago (Surface Science Reports 25 (1996) 225–273). It includes relevant fundamentals pertaining to the topics covered in that earlier review, such as conventional metal and metal oxide surfaces and CO2 interactions thereon. While UHV spectroscopy has routinely been applied for CO2 gas–solid interface analysis, the present work goes further by describing surface–CO2 interactions under elevated CO2 pressure on non-oxide surfaces, such as zeolites, sulfides, carbides and nitrides. Furthermore, it describes additional salient in situ techniques relevant to the resolution of the interfacial chemistry of CO2, notably infrared spectroscopy and state-of-the-art theoretical methods, currently used in the resolution of solid and soluble carbonate species in liquid–water vapor, liquid–solid and liquid–liquid interfaces. These techniques are directly relevant to fundamental, natural and technological settings, such as heterogeneous and environmental catalysis and CO2 sequestration.
“Diverse composites of metal-complexes and PEDOT facilitated by metal-free vapour phase polymerization”
in Reactive and Functional Polymers, 2017, 116, 101-106.
Acharya, Shravan S.; Eastone, Christopher D.; McCoy, Thomas M.; Spiccia, Leone; Ohlin, C. André; Winther-Jensen, Bjorn
Abstract: Oxidative polymerization for the manufacture of conducting polymers such as poly(3,4-ethylenedioxy- thiophene) has traditionally employed iron(III) salts. Demonstrated in this study is vapour phase polymerization of 3,4-ethylenedio-xythiophene using a metal-free oxidant, ammonium persulfate, leading to films with an estimated conductivity of 75 S/cm. Additionally, a route for embedding active transition metal complexes into these poly(3,4-ethylenedioxythiophene)-poly(styrene-4-sulfonate) (PEDOT/PSS) films via vapour assisted complexation is outlined. Here, the vapour pressure of solid ligands around their melting temperatures was exploited to ensure complexation to metal ions added into the oxidant mixture prior to polymerization of PEDOT. Four composite systems are discussed, viz. PEDOT/PSS embedded with tris(8-hydroxyquinolinato)cobalt(III), tris(2,2-bipyridine)cobalt(II), tris(1,10- phenanthroline)cobalt(II) and tris(8-hydroxyquinolinato)aluminium(III). Using these composites, electrochemical reduction of nitrite to ammonia with a faradaic efficiency of 61% was reported.
“Rates of water exchange in 2,2′-bipyridine and 1,10-phenanthroline complexes of Co(II) and Mn(II)”
in Australian Journal of Chemistry, 2017, 70, 751-754,
Acharya, Shravan S.; Winther-Jensen, Bjorn; Spiccia, Leone; Ohlin, C. André
Abstract: The rates and activation parameters of water exchange at pH 3.0 have been determined using variable temperature 17O NMR for four Co(II) complexes and one Mn(II) complex, viz. [Co(bpy)(H2O)4]2+, [Co(bpy)2(H2O)2]2+, [Co(phen)- (H2O)4]2+, [Co(phen)2(H2O)2]2+, and [Mn(bpy)(H2O)4]2+. Substitution of 1,10-phenanthroline or 2,2′-bipyridyl for aquo ligands leads to an increase in the rate of exchange in the manganese complexes, from k 298 (1.8 ± 0.1) · 10 7 for [Mn(H2O)6]2+ to (7.2 ± 0.3) · 10 7 s -1 for [Mn(phen)2(H2O)2]2+ , whereas the trends are more complex for the cobalt complexes. We have used the new data in conjunction with literature data for similar complexes to analyse the impact of M-OH2 distance and degree of substitution.
“27Al MQMAS of the δ-Al13-Keggin”
in Dalton Transactions, 2017, 46, 2249-2254.
Pilgrim, Corey D.; Callahan, Joseph R.; Christopher A. Colla; Ohlin, C. André; Harris E., Mason; Casey, William H.
Abstract: One-dimensional 27Al, 23Na, Magic-Angle-Spinning (MAS) NMR and 27Al Multiple-Quantum Magic-Angle-Spinning NMR (MQMAS) measurements are reported for the δ-isomer of the Al13 Keggin structure at high spinning speed and 14.1 T field. Values for the CQ and η parameters are on the same scale as those seen in other isomers of the Al13 structure. Density functional theory (DFT) calculations are performed for comparison to the experimental fits using the B3PW91/6-31+G* and PBE0/6-31+G* levels of theory, with the Polarizable Continuum Model (PCM).
“A Non-Aqueous Microwave Assisted Protocol for Accessing Molybdovanadates”
in Angewandte Chemie, International Edition, 2017, 56(29), 8568–8572.
Spillane, Samuel; Sharma, Rupali; Zavras, Athanasios; Mulder, Roger; Ohlin, C. André; Goerigk, Lars; Best, Stephen P.; O’Hair, Richard A. J.; Ritchie, Chris
Abstract: We report a novel approach for the synthesis of heterohexa- and heterodecametalates via the use of non-aqueous, microwave assisted reaction conditions. The two novel molybdovanadates have been isolated and characterized in the solid and solution states using single-crystal X-ray diffraction, FT-IR, UV/Vis, multinuclear NMR, and ESI-MS. The relative stabilities of the possible structural isomers were probed using DFT calculations for both polyoxometalate systems.