Monday, March 29, 2010

Acc. Chem. Res.: Noninvasive Testing of Art and Cultural Heritage by Mobile NMR

Noninvasive Testing of Art and Cultural Heritage by Mobile NMR
Bernhard Blmich, Federico Casanova, Juan Perlo, Federica Presciutti, Chiara Anselmi and Brenda Doherty
Acc. Chem. Res., Article ASAP
DOI: 10.1021/ar900277h
Publication Date (Web): March 26, 2010

Thursday, March 25, 2010

J. Phys. Chem B and C, vol. 114, Issues 12

Measurement of the Reactive Surface Area of Clay Minerals Using Solid-State NMR Studies of a Probe Molecule†

Rebecca L. Sanders, Nancy M. Washton and Karl T. Mueller*
J. Phys. Chem. C, 2010, 114 (12), pp 5491–5498

Abstract: Understanding the surface reactivity of clay minerals is necessary for accurate prediction of natural weathering rates due to the ubiquity of clays in the environment as weathering products of primary minerals. However, the reactivity of the heterogeneous surfaces of a clay can be difficult to characterize as clay mineral edge sites often react at different rates or via different mechanisms than sites on the basal planes. Ultimately, a method is needed to probe quantitatively the reactive surface sites in order to predict clay mineral dissolution rates. In this study, solid-state NMR spectroscopy has been utilized to investigate surface hydroxyl species and their relation to clay surface reactivity. The surfaces of two kaolinite samples (KGa-1b and KGa-2) and two montmorillonite samples (STx-1b and SWy-2) were reacted with the probe molecule (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS), which binds selectively to reactive non-hydrogen bonded Q3Si hydroxyl sites. Quantification of 19F spins in the TFS-treated samples using 19F magic angle spinning NMR peak intensities provides a sensitive measure of the number of reactive hydroxyl sites on a mass normalized (per gram) basis. The reactive surface site densities of KGa-1b and KGa-2 were found to be proportional to published atomic force microscopy edge site fractions. An example from KGa-1b dissolution after 10 days at pH 2.9 and 21 °C revealed no significant change in Brunauer−Emmett−Teller specific surface area, but a 25% decrease in reactive surface site density. We posit this site density determined by solid-state NMR is proportional to the reactive surface area of each clay mineral and its use in future dissolution studies is warranted to investigate how changes in reactive surface area can be tied to decreases in rates of silicon and aluminum release into solution.

Monday, March 22, 2010

J. Phys. Chem B and C, vol. 114, issues 11

Chemisorbed Thiols on Silica Particles: Characterization of Reactive Sulfur Species

Paula Caregnato*†, Malcolm D. E. Forbes‡, Delia B. Soria§, Daniel O. Mrtire† and Mnica C. Gonzalez†
J. Phys. Chem. C, 2010, 114 (11), pp 5080–5087
DOI: 10.1021/jp911253f

Abstract: Silica particles with surface thiol groups (Sil−SH) were prepared by silylation of silanol groups using 3-mercaptopropyltrimethoxysilane. The particles were characterized by FTIR, Raman, and XPS spectroscopies; thermogravimetry; and solid state 13C and 29Si NMR spectroscopy. Laser flash excitation at 266 nm of Sil−SH water suspensions in the presence of sodium peroxodisulfate generated sulfur-centered radicals that were attached to the silica surface. These radicals were detected at their absorption maxima (330 nm) by transient optical techniques. Absorbance decay signals were fit to first- and second-order decay kinetics and were assigned to thiyl radicals. Formation of disulfide radicals, a major decay channel for free thiyl radicals in solution, was not observed for surface-grafted thiyl radicals.

Reversibility and Improved Hydrogen Release of Magnesium Borohydride

Rebecca J. Newhouse†‡, Vitalie Stavila*‡, Son-Jong Hwang§, Leonard E. Klebanoff‡ and Jin Z. Zhang†
J. Phys. Chem. C, 2010, 114 (11), pp 5224–5232
DOI: 10.1021/jp9116744

Abstract: Desorption and subsequent rehydrogenation of Mg(BH4)2 with and without 5 mol % TiF3 and ScCl3 have been investigated. Temperature programmed desorption (TPD) experiments revealed a significant increase in the rate of desorption as well as the weight percentage of hydrogen released with additives upon heating to 300 °C. Stable Mg(BxHy)n intermediates were formed at 300 °C, whereas MgB2 was the major product when heated to 600 °C. These samples were then rehydrogenated and subsequently characterized with powder X-ray diffraction (pXRD), Raman, and NMR spectroscopy. We confirmed significant conversion of MgB2 to fully hydrogenated Mg(BH4)2 for the sample with and without additives. TPD and NMR studies revealed that the additives have a significant effect on the reaction pathway during both dehydrogenation and rehydrogenation reactions. This work suggests that the use of additives may provide a valid pathway for improving intrinsic hydrogen storage properties of magnesium borohydride.

Comparison of Different Theory Models and Basis Sets in the Calculations of Structures and 13C NMR Spectra of [Pt(en)(CBDCA−O, O′)], an Analogue of the Antitumor Drug Carboplatin

Hongwei Gao*, Xiujuan Wei, Xuting Liu and Tingxia Yan
J. Phys. Chem. B, 2010, 114 (11), pp 4056–4062
DOI: 10.1021/jp912005a

Abstract: Comparisons of various density functional theory (DFT) methods at different basis sets in predicting the molecular structures and 13C NMR spectra for [Pt(en)(CBDCA−O, O′)], an analogue of the antitumor drug carboplatin, are reported. DFT methods including B3LYP, B3PW91, mPW1PW91, PBE1PBE, BPV86, PBEPBE, and LSDA are examined. Different basis sets including LANL2DZ, SDD, LANL2MB, CEP-4G, CEP-31G, and CEP-121G are also considered. It is remarkable that the LSDA/SDD level is clearly superior to all of the remaining density functional methods in predicting the structure of [Pt(en)(CBDCA−O, O′)]. The results also indicate that the B3LYP/SDD level is the best to predict 13C NMR spectra for [Pt(en)(CBDCA−O, O′)] among all DFT methods.

Friday, March 12, 2010

J. Phys Chem. B and C. Volume 114, Issues 10

EPR, ENDOR, and HYSCORE Study of the Structure and the Stability of Vanadyl−Porphyrin Complexes Encapsulated in Silica: Potential Paramagnetic Biomarkers for the Origin of Life

Gourier*†, Olivier Delpoux†, Audrey Bonduelle†, Laurent Binet†, Ilaria Ciofini‡ and Herv Vezin§

J. Phys. Chem. B, 2010, 114 (10), pp 3714–3725

Abstract: The possibility of using vanadyl ions as paramagnetic biomarkers for the identification of traces of primitive life fossilized in silica rocks is studied by cw-EPR, ENDOR, HYSCORE, and DFT calculations. It is well-known that porphyrins, which are common to all living organisms, form vanadyl−porphyrin complexes in sediments deposited in oceans. However, the stability of these complexes over a very long time (more than 3 billion years) is not known. By encapsulating vanadyl−porphyrin complexes in silica synthesized by a sol−gel method to mimic SiO2 sediments, we studied the structure and stability of these complexes upon step heating treatments by monitoring the evolution of the g factor and of the hyperfine interactions with 51V, 1H, 14N, 13C, and 29Si nuclei. It is found that vanadyl−porphyrin complexes are progressively transformed into oxygenated vanadyl complexes by transfer of the VO2+ ion from the porphyrin ring to the mineral matrix. The organic component is transformed into carbonaceous matter which contains paramagnetic centers (IOM• centers). To test the validity of this approach, we studied by EPR a 3490 million years old chert (polycrystalline SiO2 rock) containing some of the oldest putative traces of life. This rock contains oxygenated vanadyl complexes and IOM• centers very similar to those found in the synthetic analogues.

Confinement of NaAlH4 in Nanoporous Carbon: Impact on H2 Release, Reversibility, and Thermodynamics

Jinbao Gao†, Philipp Adelhelm†, Margriet H. W. Verkuijlen‡, Carine Rongeat§, Monika Herrich§, P. Jan M. van Bentum‡, Oliver Gutfleisch§, Arno P. M. Kentgens‡, Krijn P. de Jong† and Petra E. de Jongh*†

J. Phys. Chem. C, 2010, 114 (10), pp 4675–4682

Abstract: Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH4 is the only complex metal hydride identified so far that combines favorable thermodynamics with a reasonable hydrogen storage capacity (5.5 wt %) when decomposing in two steps to NaH, Al, and H2. The slow kinetics and poor reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. In an alternative approach we studied the influence of a reduced NaAlH4 particle size and the presence of a carbon support. We focused on NaAlH4/porous carbon nanocomposites prepared by melt infiltration. The NaAlH4 was confined in the mainly 2−3 nm pores of the carbon, resulting in a lack of long-range order in the NaAlH4 structure. The hydrogen release profile was modified by contact with the carbon; even for 10 nm NaAlH4 on a nonporous carbon material the decomposition of NaAlH4 to NaH, Al, and H2 now led to hydrogen release in a single step. This was a kinetic effect, with the temperature at which the hydrogen was released depending on the NaAlH4 feature size. However, confinement in a nanoporous carbon material was essential to not only achieve low H2 release temperatures, but also rehydrogenation at mild conditions (e.g., 24 bar H2 at 150 °C). Not only had the kinetics of hydrogen sorption improved, but the thermodynamics had also changed. When hydrogenating at conditions at which Na3AlH6 would be expected to be the stable phase (e.g., 40 bar H2 at 160 °C), instead nanoconfined NaAlH4 was formed, indicating a shift of the NaAlH4↔Na3AlH6 thermodynamic equilibrium in these nanocomposites compared to bulk materials.

Solid-State NMR Studies of the Local Structure of NaAlH4/C Nanocomposites at Different Stages of Hydrogen Desorption and Rehydrogenation

Margriet H. W. Verkuijlen†, Jinbao Gao‡, Philipp Adelhelm‡, P. Jan M. van Bentum*†, Petra E. de Jongh‡ and Arno P. M. Kentgens*†

J. Phys. Chem. C, 2010, 114 (10), pp 4683–4692

Abstract: Structural properties of NaAlH4/C nanocomposites were studied using 23Na and 27Al solid-state NMR. The samples were synthesized by melt infiltration of a highly porous carbon support, with typical pore sizes of 2−3 nm. Physical mixtures of high surface carbon with alanates in different stages of hydrogen desorption show somewhat broadened resonances and a small negative chemical shift compared to pure alanates. This is most likely caused by a susceptibility effect of the carbon support material, which shields and distorts the applied magnetic field. After melt infiltration, 23Na and 27Al spectra are broadened with a small downfield average shift, which is mainly caused by a chemical shift distribution and is explained by a larger disorder in the nanoconfined materials and a possible charge transfer to the carbon. Our measurements show that the local structure of the nanoconfined alanate is the similar to bulk alanate because a comparable chemical shift and average quadrupolar coupling constant is found. In contrast to bulk alanates, in partly desorbed nanocomposite samples no Na3AlH6 is detected. Together with a single release peak observed by dehydrogenation experiments, this points toward a desorption in one single step. 23Na spectra of completely desorbed NaAlH4/C and NaH/C nanocomposites confirm the formation of metallic sodium at lower temperatures than those observed for bulk alanates. The structural properties observed with solid-state NMR of the nanoconfined alanate are restored after a rehydrogenation cycle. This demonstrates that the dehydrogenation of the NaAlH4/C nanocomposite is reversible, even without a Ti-based catalyst.

Transferred Hyperfine Interaction between a Tetrahedral Transition Metal and Tetrahedral Lithium: Li6CoO4

Dany Carlier*, Michel Mntrier and Claude Delmas

J. Phys. Chem. C, 2010, 114 (10), pp 4749–4755

Abstract: Li6CoO4 presents an antifluorite-type structure, with both the Co and Li ions in tetrahedral oxygen coordination. 7Li MAS NMR shows remarkably different shifts (+885 and −232 ppm) for the two different crystallographic types of Li. In order to assign the signals and to understand the mechanisms whereby the electron spins on the e orbitals of Co2+ ions (e4 t23 electronic configuration) are transferred toward the two different types of Li with opposite polarization, we have carried out GGA and GGA+U calculations of the electronic structure using the VASP code. Spin density maps in selected planes of the structure reveal (as expected) that lobes of the t2 orbitals point toward the faces of the CoO4 tetrahedra and can thus overlap with the neighboring Li(2) through empty square pyramidal sites. As concerns Li(1), a mechanism is evidenced where the (filled) e orbitals of Co2+ are polarized by the electron spins in the t2 ones. These polarized e orbitals overlap with Li(1) through the common edge of the tetrahedra. The relative magnitude of the experimental shifts for the two types of Li are however not fully reproduced by the calculations, and the influence of the U parameter as well as of the pseudopotential method used is discussed.

A Combined Hydrogen Storage System of Mg(BH4)2−LiNH2 with Favorable Dehydrogenation

X. B. Yu*†‡, Y. H. Guo‡, D. L. Sun‡, Z. X. Yang§, A. Ranjbar†, Z. P. Guo†, H. K. Liu† and S. X. Dou†

J. Phys. Chem. C, 2010, 114 (10), pp 4733–4737

Abstract: The decomposition properties of Mg(BH4)2−LiNH2 mixtures were investigated. Apparent NH3 release appeared from 50 to 300 °C for the Mg(BH4)2−LiNH2 mixtures with mole ratios of 1:1.5, 1:2, and 1:3, while only hydrogen release was detected for the mixture with a mole ratio of 1:1. In the case of the Mg(BH4)2−LiNH2 (1:1) sample, the onset of the first-step dehydrogenation starts at 160 °C, with a weight loss of 7.2 wt % at 300 °C, which is improved significantly compared to the pure Mg(BH4)2 alone. From Kissinger’s method, the activation energy, Ea, for the first and second step dehydrogenation in Mg(BH4)2−LiNH2 (1:1) was estimated to be about 121.7 and 236.6 kJ mol−1, respectively. The improved dehydrogenation in the combined system may be ascribed to a combination reaction between [BH4] and [NH2], resulting in the formation of Li−Mg alloy and amorphous B−N compound.

Friday, March 05, 2010

J. Phys. Chem. B. and J. Phys. Chem. C, Volumes 114, Issues 9

LiBH4 in Carbon Aerogel Nanoscaffolds: An NMR Study of Atomic Motions

David T. Shane†, Robert L. Corey†‡, Charlie McIntosh†, Laura H. Rayhel†, Robert C. Bowman, Jr.§, John J. Vajo, Adam F. Gross and Mark S. Conradi*†

J. Phys. Chem. C, 2010, 114 (9), pp 4008–4014
DOI: 10.1021/jp9107365

Hydrogen NMR of LiBH4 in the pores of carbon aerogel nanoscaffolds shows the coexistence of motionally narrowed and broad components. The fraction of mobile, diffusing hydrogen, already evident at room temperature, increases continuously with temperature. Thus, a broad distribution of environments is present, as in some ball-milled hydrides. With decreasing pore size from 25 to 13 nm, the narrowed fraction increases, suggesting that the narrow resonance is from the most defective regions, the grain boundaries. The broad component eventually exhibits narrowing in the same temperature window as for bulk material, confirming the bulk-like structure of those regions. Hole-burning measurements reveal magnetization exchange between the broad and narrow resonance lines, confirming the close spatial proximity of the atoms in each line. The solid−solid transition is clearly evident in 7Li line shapes, with a 10−15 °C depression from the bulk. More rapid decay of the quadrupolar satellite signals in spin echoes, compared to the central transition, is due to lithium atoms diffusing between differently oriented nanocrystallites. Our results suggest that crystallites in neighboring pores have similar orientations but are incoherent for diffraction. Remarkably, the T1 data of hydrogen and 7Li are continuous in the vicinity of the transition, in contrast with the bulk T1 data, suggesting that some rapid lithium motion remains below the transition.

Metal Carbonation of Forsterite in Supercritical CO2 and H2O Using Solid State 29Si, 13C NMR Spectroscopy

Ja Hun Kwak, Jian Zhi Hu*, David W. Hoyt, Jesse A. Sears, Chongming Wang, Kevin M. Rosso and Andrew R. Felmy

J. Phys. Chem. C, 2010, 114 (9), pp 4126–4134
DOI: 10.1021/jp1001308

Ex situ natural abundance magic angle spinning (MAS) NMR was used for the first time to study fundamental mineral carbonation processes and reaction extent relevant to geologic carbon sequestration (GCS) using a model silicate mineral forsterite (Mg2SiO4)+supercritical CO2 with and without H2O. Run conditions were 80 °C and 96 atm. With H2O but without CO2, 29Si MAS NMR reveals that the reaction products contain only two peaks of similar intensities located at about −84.8 and −91.8 ppm, which can be assigned to surface Q1 and Q2 species, i.e., SiO4 tetrahedra sharing one and two corners with other tetrahedra, respectively. Using scCO2 without H2O, no reaction is observed within 7 days. Using both scCO2 and H2O, the surface reaction products for silica are mainly Q4 species (−111.6 ppm) accompanied by a lesser amount of Q3 (−102 ppm) and Q2 (−91.8 ppm) species. No surface Q1 species were detected, indicating the carbonic acid formation and magnesite (MgCO3) precipitation reactions are faster than the forsterite hydrolysis process. Thus, it can be concluded that the Mg2SiO4 hydrolysis process is the rate limiting step of the overall mineral carbonation process. 29Si NMR combined with XRD, TEM, SAED, and EDX further reveals that the reaction is a surface reaction with the Mg2SiO4 crystallite in the core and with condensed Q2, Q3, and Q4 species forming highly porous amorphous surface layers. 13C MAS NMR unambiguously identified a reaction intermediate as Mg5(CO3)4(OH)2·5H2O, i.e., the dypingite.

Solid State 2H NMR Analysis of Furanose Ring Dynamics in DNA Containing Uracil

Monica N. Kinde-Carson†, Crystal Ferguson§, Nathan A. Oyler‡, Gerard S. Harbison† and Gary A. Meints*§

J. Phys. Chem. B, 2010, 114 (9), pp 3285–3293
DOI: 10.1021/jp9091656

Abstract: DNA damage has been implicated in numerous human diseases, particularly cancer, and the aging process. Single-base lesions, such as uracil, in DNA can be cytotoxic or mutagenic and are recognized by a DNA glycosylase during the process of base excision repair. Increased dynamic properties in lesion-containing DNAs have been suggested to assist recognition and specificity. Deuterium solid-state nuclear magnetic resonance (SSNMR) has been used to directly observe local dynamics of the furanose ring within a uracil:adenine (U:A) base pair and compared to a normal thymine:adenine (T:A) base pair. Quadrupole echo lineshapes, T1Z, and T2e relaxation data were collected, and computer modeling was performed. The results indicate that the relaxation times are identical within the experimental error, the solid lineshapes are essentially indistinguishable above the noise level, and our lineshapes are best fit with a model that does not have significant local motions. Therefore, U:A base pair furanose rings appear to have essentially identical dynamic properties as a normal T:A base pair, and the local dynamics of the furanose ring are unlikely to be the sole arbiter for uracil recognition and specificity in U:A base pairs.