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• 1
Article
Language: English
In: The Journal of chemical physics, 28 October 2014, Vol.141(16), pp.164121
Description: A quantum-chemical method for modeling solid-state nuclear magnetic resonance chemical-shift tensors by calculations on large symmetry-adapted clusters of molecules is demonstrated. Four hundred sixty five principal components of the (13)C chemical-shielding tensors of 24 organic materials are analyzed. The comparison of calculations on isolated molecules with molecules in clusters demonstrates that intermolecular effects can be successfully modeled using a cluster that represents a local portion of the lattice structure, without the need to use periodic-boundary conditions (PBCs). The accuracy of calculations which model the solid state using a cluster rivals the accuracy of calculations which model the solid state using PBCs, provided the cluster preserves the symmetry properties of the crystalline space group. The size and symmetry conditions that the model cluster must satisfy to obtain significant agreement with experimental chemical-shift values are discussed. The symmetry constraints described in the paper provide a systematic approach for incorporating intermolecular effects into chemical-shielding calculations performed at a level of theory that is more advanced than the generalized gradient approximation. Specifically, NMR parameters are calculated using the hybrid exchange-correlation functional B3PW91, which is not available in periodic codes. Calculations on structures of four molecules refined with density plane waves yield chemical-shielding values that are essentially in agreement with calculations on clusters where only the hydrogen sites are optimized and are used to provide insight into the inherent sensitivity of chemical shielding to lattice structure, including the role of rovibrational effects.
Keywords: Inorganic, Organic, Physical And Analytical Chemistry ; Accuracy ; Carbon 13 ; Density Functional Method ; Molecular Clusters ; Molecules ; Nuclear Magnetic Resonance ; Organic Matter ; Sensitivity ; Simulation ; Solids ; Space Groups ; Tensors ; Chemistry ; Physics;
ISSN: 00219606
E-ISSN: 1089-7690
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• 2
Article
Language: English
In: Geochimica et Cosmochimica Acta, Sept 1, 2012, Vol.92, p.100(17)
Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2012.05.038 Byline: Rebecca L. Sanders (a)(b), Nancy M. Washton (c), Karl T. Mueller (a)(b)(c) Abstract: Clay mineral dissolution rates can continuously decrease over time as reactive sites located on edges are preferentially depleted under certain pH conditions. Changes in reactive surface area and the difficulties in quantifying this elusive variable have been cited as one key reason for the complexity in developing accurate rate equations for the dissolution of clay minerals. Recently, a solid-state nuclear magnetic resonance (NMR) method has been proposed for counting the number of reactive surface sites on a defined quantity of a clay mineral. Using this solid-state NMR proxy, changes in reactive surface area were monitored for a series of batch dissolution experiments of low-defect kaolinite KGa-1b and Ca-rich bentonite STx-1b, a montmorillonite-rich clay containing an opal-CT impurity, at 21[degrees]C and initial pH 3. Kaolinite specific surface area as determined from BET gas isotherm data did not change within error during 80days of dissolution whereas bentonite specific surface area decreased rapidly to about 50% of the original value as interlayer cation concentrations changed. The solid-state NMR proxy revealed decreases in the number of reactive surface sites per gram of kaolinite and bentonite as a function of dissolution time, presumed to be from the preferential dissolution of reactive sites on edges at initial pH 3. This depletion of reactive edge sites can be tied to a concomitant decrease in the rates of release of Si and Al into solution. The quantity of reactive sites can be used to estimate the dissolution rates of kaolinite and bentonite as well as estimate trends in dissolution rates of other clay minerals. These results further highlight the need to quantify the number of reactive sites present on a per gram basis as well as characterize their depletion with time to develop and use dissolution rate models for clay minerals and other heterogeneous materials in the environment. Author Affiliation: (a) Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA (b) Center for Environmental Kinetics Analysis, Pennsylvania State University, University Park, PA 16802, USA (c) Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA Article History: Received 5 August 2011; Accepted 27 May 2012 Article Note: (miscellaneous) Associate editor: Donald L. Sparks
Keywords: Bentonite -- Analysis
ISSN: 0016-7037
Source: Cengage Learning, Inc.
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• 3
Article
Language: English
In: Geochimica et Cosmochimica Acta, 2011, Vol.75(2), pp.401-415
Description: The dissolution–precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH) solutions with pH 8.2–9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at or silicon sites, respectively. The dissolution mechanism varies with Δ such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by . However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated reaction rate on pH, and a poorly-constrained pH dependence of the postulated rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.
Keywords: Geology
ISSN: 0016-7037
E-ISSN: 1872-9533
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• 4
Article
Language: English
In: The Journal of Chemical Physics, 14 February 2017, Vol.146(6)
Description: We demonstrate a modification of Grimme’s two-parameter empirical dispersion force field (referred to as the PW91-D2* method), in which the damping function has been optimized to yield geometries that result in predictions of the principal values of 17 O quadrupolar-coupling tensors that are systematically in close agreement with experiment. The predictions of 17 O quadrupolar-coupling tensors using PW91-D2*-refined structures yield a root-mean-square deviation (RMSD) (0.28 MHz) for twenty-two crystalline systems that is smaller than the RMSD for predictions based on X-ray diffraction structures (0.58 MHz) or on structures refined with PW91 (0.53 MHz). In addition, 13 C, 15 N, and 17 O chemical-shift tensors and 35 Cl quadrupolar-coupling tensors determined with PW91-D2*-refined structures are compared to the experiment. Errors in the prediction of chemical-shift tensors and quadrupolar-coupling tensors are, in these cases, substantially lowered, as compared to predictions based on PW91-refined structures. With this PW91-D2*-based method, analysis of 42 17 O chemical-shift-tensor principal components gives a RMSD of only 18.3 ppm, whereas calculations on unrefined X-ray structures give a RMSD of 39.6 ppm and calculations of PW91-refined structures give an RMSD of 24.3 ppm. A similar analysis of 35 Cl quadrupolar-coupling tensor principal components gives a RMSD of 1.45 MHz for the unrefined X-ray structures, 1.62 MHz for PW91-refined structures, and 0.59 MHz for the PW91-D2*-refined structures.
Keywords: Articles
ISSN: 0021-9606
E-ISSN: 1089-7690
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• 5
Article
Language: English
In: The Journal of Chemical Physics, 21 September 2016, Vol.145(11)
Description: Pulsed field gradient nuclear magnetic resonance spectroscopy and dielectric relaxation spectroscopy have been utilized to investigate lithium dynamics within poly(ethylene oxide) (PEO)-based lithium sulfonate ionomers of varying ion content. The ion content is set by the fraction of sulfonated phthalates and the molecular weight of the PEO spacer, both of which can be varied independently. The molecular level dynamics of the ionomers are dominated by either Vogel-Fulcher-Tammann or Arrhenius behavior depending on ion content, spacer length, temperature, and degree of ionic aggregation. In these ionomers the main determinants of the self-diffusion of lithium and the observed conductivities are the ion content and ionic states of the lithium ion, which are profoundly affected by the interactions of the lithium ions with the ether oxygens of the polymer. Since many lithium ions move by segmental polymer motion in the ion pair state, their diffusion is significantly larger than that estimated from conductivity using the Nernst-Einstein equation.
Keywords: Articles
ISSN: 0021-9606
E-ISSN: 1089-7690
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• 6
Article
Language: English
In: The Journal of Chemical Physics, 14 June 2015, Vol.142(22)
Description: A combination of molecular dynamics simulations and pulsed field gradient nuclear magnetic resonance spectroscopy is used to investigate the role of exogenous electric fields on the solvation structure and dynamics of alkali ions in dimethyl sulfoxide (DMSO) and as a function of temperature. Good agreement was obtained, for select alkali ions in the absence of an electric field, between calculated and experimentally determined diffusion coefficients normalized to that of pure DMSO. Our results indicate that temperatures of up to 400 K and external electric fields of up to 1 V nm −1 have minimal effects on the solvation structure of the smaller alkali cations (Li + and Na + ) due to their relatively strong ion-solvent interactions, whereas the solvation structures of the larger alkali cations (K + , Rb + , and Cs + ) are significantly affected. In addition, although the DMSO exchange dynamics in the first solvation shell differ markedly for the two groups, the drift velocities and mobilities are not significantly affected by the nature of the alkali ion. Overall, although exogenous electric fields induce a drift displacement, their presence does not significantly affect the random diffusive displacement of the alkali ions in DMSO. System temperature is found to have generally a stronger influence on dynamical properties, such as the DMSO exchange dynamics and the ion mobilities, than the presence of electric fields.
Keywords: Articles
ISSN: 0021-9606
E-ISSN: 1089-7690
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• 7
Article
Language: English
In: The Journal of Chemical Physics, 21 November 2015, Vol.143(19)
Description: The 15 N chemical shift tensor is shown to be extremely sensitive to lattice structure and a powerful metric for monitoring density functional theory refinements of crystal structures. These refinements include lattice effects and are applied here to five crystal structures. All structures improve based on a better agreement between experimental and calculated 15 N tensors, with an average improvement of 47.0 ppm. Structural improvement is further indicated by a decrease in forces on the atoms by 2–3 orders of magnitude and a greater similarity in atom positions to neutron diffraction structures. These refinements change bond lengths by more than the diffraction errors including adjustments to X–Y and X–H bonds (X, Y = C, N, and O) of 0.028 ± 0.002 Å and 0.144 ± 0.036 Å, respectively. The acquisition of 15 N tensors at natural abundance is challenging and this limitation is overcome by improved 1 H decoupling in the FIREMAT method. This decoupling dramatically narrows linewidths, improves signal-to-noise by up to 317%, and significantly improves the accuracy of measured tensors. A total of 39 tensors are measured with shifts distributed over a range of more than 400 ppm. Overall, experimental 15 N tensors are at least 5 times more sensitive to crystal structure than 13 C tensors due to nitrogen’s greater polarizability and larger range of chemical shifts.
Keywords: Articles
ISSN: 0021-9606
E-ISSN: 1089-7690
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• 8
Article
Language: English
In: The Journal of Chemical Physics, 07 January 2012, Vol.136(1)
Description: Nuclear magnetic resonance spectroscopy has been utilized to investigate the dynamics of poly(ethylene oxide)-based lithium sulfonate ionomer samples that have low glass transition temperatures. 1 H and 7 Li spin-lattice relaxation times (T 1 ) of the bulk polymer and lithium ions, respectively, were measured and analyzed in samples with a range of ion contents. The temperature dependence of T 1 values along with the presence of minima in T 1 as a function of temperature enabled correlation times and activation energies to be obtained for both the segmental motion of the polymer backbone and the hopping motion of lithium cations. Similar activation energies for motion of both the polymer and lithium ions in the samples with lower ion content indicate that the polymer segmental motion and lithium ion hopping motion are correlated in these samples, even though lithium hopping is about ten times slower than the segmental motion. A divergent trend is observed for correlation times and activation energies of the highest ion content sample with 100% lithium sulfonation due to the presence of ionic aggregation. Details of the polymer and cation dynamics on the nanosecond timescale are discussed and complement the findings of X-ray scattering and quasi-elastic neutron scattering experiments.
Keywords: Articles
ISSN: 0021-9606
E-ISSN: 1089-7690
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• 9
Article
Language: English
In: The Journal of Chemical Physics, 21 May 2013, Vol.138(19)
Description: Polymer backbone dynamics of single ion conducting poly(ethylene oxide) (PEO)-based ionomer samples with low glass transition temperatures (T g ) have been investigated using solid-state nuclear magnetic resonance. Experiments detecting 13 C with 1 H decoupling under magic angle spinning (MAS) conditions identified the different components of the polymer backbone (PEO spacer and isophthalate groups) and their relative mobilities for a suite of lithium- and sodium-containing ionomer samples with varying cation contents. Variable temperature (203–373 K) 1 H- 13 C cross-polarization MAS (CP-MAS) experiments also provided qualitative assessment of the differences in the motions of the polymer backbone components as a function of cation content and identity. Each of the main backbone components exhibit distinct motions, following the trends expected for motional characteristics based on earlier Quasi Elastic Neutron Scattering and 1 H spin-lattice relaxation rate measurements. Previous 1 H and 7 Li spin-lattice relaxation measurements focused on both the polymer backbone and cation motion on the nanosecond timescale. The studies presented here assess the slower timescale motion of the polymer backbone allowing for a more comprehensive understanding of the polymer dynamics. The temperature dependences of 13 C linewidths were used to both qualitatively and quantitatively examine the effects of cation content and identity on PEO spacer mobility. Variable contact time 1 H- 13 C CP-MAS experiments were used to further assess the motions of the polymer backbone on the microsecond timescale. The motion of the PEO spacer, reported via the rate of magnetization transfer from 1 H to 13 C nuclei, becomes similar for ${\rm T} \mathbin{\lower.3ex\hbox{\buildrel〉\over{\smash{\scriptstyle\sim}\vphantom{_x}}}} {\rm 1}{\rm.1}$ T ≳ 1.1 T g in all ionic samples, indicating that at similar elevated reduced temperatures the motions of the polymer backbones on the microsecond timescale become insensitive to ion interactions. These results present an improved picture, beyond those of previous findings, for the dependence of backbone dynamics on cation density (and here, cation identity as well) in these amorphous PEO-based ionomer systems.
Keywords: Articles
ISSN: 0021-9606
E-ISSN: 1089-7690
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• 10
Article
Language: English
In: Geochimica et cosmochimica acta, 2012, Vol.92, pp.100-116
Description: Clay mineral dissolution rates can continuously decrease over time as reactive sites located on edges are preferentially depleted under certain pH conditions. Changes in reactive surface area and the difficulties in quantifying this elusive variable have been cited as one key reason for the complexity in developing accurate rate equations for the dissolution of clay minerals. Recently, a solid-state nuclear magnetic resonance (NMR) method has been proposed for counting the number of reactive surface sites on a defined quantity of a clay mineral. Using this solid-state NMR proxy, changes in reactive surface area were monitored for a series of batch dissolution experiments of low-defect kaolinite KGa-1b and Ca-rich bentonite STx-1b, a montmorillonite-rich clay containing an opal-CT impurity, at 21°C and initial pH 3. Kaolinite specific surface area as determined from BET gas isotherm data did not change within error during 80days of dissolution whereas bentonite specific surface area decreased rapidly to about 50% of the original value as interlayer cation concentrations changed. The solid-state NMR proxy revealed decreases in the number of reactive surface sites per gram of kaolinite and bentonite as a function of dissolution time, presumed to be from the preferential dissolution of reactive sites on edges at initial pH 3. This depletion of reactive edge sites can be tied to a concomitant decrease in the rates of release of Si and Al into solution. The quantity of reactive sites can be used to estimate the dissolution rates of kaolinite and bentonite as well as estimate trends in dissolution rates of other clay minerals. These results further highlight the need to quantify the number of reactive sites present on a per gram basis as well as characterize their depletion with time to develop and use dissolution rate models for clay minerals and other heterogeneous materials in the environment. ; p. 100-116.
Keywords: Models ; Clay ; Bentonite ; Aluminum ; Active Sites ; Kaolinite ; Equations ; Silicon ; Ph ; Nuclear Magnetic Resonance Spectroscopy ; Surface Area
ISSN: 0016-7037
Source: AGRIS (Food and Agriculture Organization of the United Nations)
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