Friday, June 05, 2009

J. Phys. Chem. C, vol. 113, Issues 20 to 23

MgCl2·4(CH3)2CHOH: A New Molecular Adduct and Super Active Polymerization Catalyst Support

K. S. Thushara
, Renny Mathew, T. G. Ajithkumar, P. R. Rajamohanan, Sumit Bhaduri*§ and Chinnakonda S. Gopinath*

J. Phys. Chem. C, 2009, 113 (20), pp 8556–8559
DOI: 10.1021/jp9026546
Abstract: A new molecular adduct, MgCl2·4(CH3)2CHOH, has been synthesized and characterized for structural aspects and demonstrated for super active ethylene polymerization activity with TiCl4 to ultrahigh molecular weight polyethylene in high yield.

Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS 13C NMR Investigations

Niki Baccile
*, Guillaume Laurent, Florence Babonneau, Franck Fayon§, Maria-Magdalena Titirici and Markus Antonietti

J. Phys. Chem. C, 2009, 113 (22), pp 9644–9654
DOI: 10.1021/jp901582x
Abstract: The local structure of carbon spheres obtained via the hydrothermal carbonization process is characterized by using a combination of advanced solid-state 13C NMR techniques. Glucose was chosen as the starting product because it offers the possibility of 13C isotopic enrichment and is regarded as a model compound for more complex polysaccharides and biomass, as reported in recent studies. A number of 13C solid-state MAS NMR techniques (single-pulse, cross-polarization, inversion recovery cross-polarization, INEPT, 13C−13C proton-driven magnetization exchange, and 13C−13C double-quantum−single-quantum correlation experiments) were combined to retrieve information about binding motifs and C−C closest neighbor relations. We found that the core of the carbonaceous scaffold is composed of furan rings cross-linked by domains containing short keto-aliphatic chains instead of otherwise expected graphene-type sheets, as mainly reported either for hydrothermal carbon spheres or for biomass-related carbons obtained by low-temperature (<350>

Optimized Synthesis and Structural Characterization of the Borosilicate MCM-70

Dan Xie
, Lynne B. McCusker*, Christian Baerlocher, Lisa Gibson#, Allen W. Burton and Son-Jong Hwang§
J. Phys. Chem. C, 2009, 113 (22), pp 9845–9850
DOI: 10.1021/jp903500q

Abstract:A structure analysis of the borosilicate zeolite MCM-70, whose synthesis had been patented in 2003, was reported in 2005. Unfortunately, that structure analysis was somewhat ambiguous. Anisotropic line broadening made it difficult to model the peak shape, some peaks in the electron density map could not be interpreted satisfactorily, the framework geometry was distorted, and MAS NMR results were partially contradictory. In an attempt to resolve some of these points, an optimization of the synthesis was undertaken, and the structure was reinvestigated. The structure was solved from synchrotron powder diffraction data collected on an as-synthesized sample (Pmn21, a = 13.3167(1) Å, b = 4.6604(1) Å, c = 8.7000(1) Å) using a powder charge-flipping algorithm. The framework topology, with a 1-dimensional, 10-ring channel system, is identical to the one previously reported. However, the B in this new sample was found to be ordered in the framework, fully occupying one of the four tetrahedral sites. Two extra-framework K+ ion positions, each coordinated to five framework O atoms and one water molecule, were also found. The solid state 29Si, 11B and 1H NMR results are fully consistent with this ordered structure.

LiSc(BH4)4 as a Hydrogen Storage Material: Multinuclear High-Resolution Solid-State NMR and First-Principles Density Functional Theory Studies

Chul Kim
, Son-Jong Hwang*, Robert C. Bowman, Jr., Joseph W. Reiter, Jason A. Zan, James G. Kulleck, Houria Kabbour§#, E. H. Majzoub and V. Ozolins

J. Phys. Chem. C, 2009, 113 (22), pp 9956–9968
DOI: 10.1021/jp9011685
Abstract: A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH4)4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH4 and ScCl3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH4)4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH4)3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I (#82) is predicted to be the lowest energy state for LiSc(BH4)4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB2 and [B12H12]−2 anion containing (e.g., Li2B12H12) along with residual LiBH4. Reaction of a desorbed LiSc(BH4)4 + 4LiCl mixture (from 4LiBH4/ScCl3 sample) with hydrogen gas at 70 bar resulted only in an increase in the contents of Li2B12H12 and LiBH4. Full reversibility to reform the LiSc(BH4)4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications.

A 47/49Ti Solid-State NMR Study of Layered Titanium Phosphates at Ultrahigh Magnetic Field

Jianfeng Zhu
, Nick Trefiak, Tom K. Woo and Yining Huang*

J. Phys. Chem. C, 2009, 113 (23), pp 10029–10037
DOI: 10.1021/jp901235w
Abstract: Layered titanium phosphates (TiPs) have many potentially important applications in ion exchange, catalysis, intercalation, and sorption. Characterization of metal local environments by solid-state 47/49Ti NMR has been difficult due to many unfavorable 47/49Ti NMR properties. In this work, we have directly characterized the local structures around Ti in several representative layered TiPs, including α-, β-, and γ-TiP, by examining the 47/49Ti static NMR spectra of these materials at an ultrahigh magnetic field of 21.1 T. The 47/49Ti chemical shielding and electric field gradient (EFG) tensors have been extracted from spectral analysis. The observed 47/49Ti spectra are mainly determined by the second-order quadrupolar interactions. The quadrupole coupling constants (CQ) are sensitive to the distortion of the TiO6 octahedron in this series of layered TiPs. Quantum mechanical calculations have been performed on several model clusters as well as periodic systems. The results indicate that, in addition to the oxygens in the first coordination sphere of Ti, the atoms in the second and third coordination spheres and beyond also have significant effects on the EFG at the metal center, and this long-range effect contributes substantially to the CQ. A relationship between observed CQ and the Ti−O bond length distortion parameter appears to exist, and this empirical correlation is also confirmed by theoretical calculations. Using sodium-exchanged α-TiP (α-Na-TiP) with an unknown structure as an example, we show that the 47/49Ti NMR spectra can provide partial information on the local environment of the metal center. For this material, the ion exchange does not affect the Ti local environment significantly. It appears that the layer in α-TiP is more robust compared to that of the zirconium analogue.

Electron−Nuclear Spin Dynamics in a Bacterial Photosynthetic Reaction Center

Eugenio Daviso
, A. Alia, Shipra Prakash, Anna Diller, Peter Gast, Johan Lugtenburg, Jrg Matysik* and Gunnar Jeschke§

J. Phys. Chem. C, 2009, 113 (23), pp 10269–10278
DOI: 10.1021/jp900286q
Abstract: The solid-state photo-CIDNP effect is known to occur in natural photosynthetic reaction centers (RCs) where it can be observed by magic-angle spinning (MAS) NMR as strong modification of signal intensities under illumination compared to experiments performed in the dark. The origin of the effect has been debated. In this paper, we report time-resolved photo-CIDNP MAS NMR data of reaction centers of quinone depleted Rhodobacter sphaeroides. It is demonstrated that the build-up of nuclear polarization on the primary donor and the bacteriopheophytin acceptor depends on the presence and lifetimes of the molecular triplet states of the donor and carotenoid. Analysis of the data proves that up to three electron−nuclear spin-coupling mechanisms and two transient effects are working concomitantly in the spin-chemical machinery of the reaction center

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