Monday, January 05, 2009

Al's Journal Update

Superadiabaticity in magnetic resonance
J. Chem. Phys. 129, 204110 (2008)
Michaël Deschamps, Gwendal Kervern, Dominique Massiot, Guido Pintacuda, Lyndon Emsley, and Philip J. Grandinetti
Adiabaticity plays a central role in modern magnetic resonance experiments, as excitations with adiabatic Hamiltonians allow precise control of the dynamics of the spin states during the course of an experiment. Surprisingly, many commonly used adiabatic processes in magnetic resonance perform well even though the adiabatic approximation does not appear to hold throughout the process. Here we show that this discrepancy can now be explained through the use of Berry's superadiabatic formalism, which provides a framework for including the finite duration of the process in the theoretical and numerical treatments. In this approach, a slow, but finite time-dependent Hamiltonian is iteratively transformed into time-dependent diagonal frames until the most accurate adiabatic approximation is obtained. In the case of magnetic resonance, the magnetization during an adiabatic process of finite duration is not locked to the effective Hamiltonian in the conventional adiabatic frame, but rather to an effective Hamiltonian in a superadiabatic frame. Only in the superadiabatic frame can the true validity of the adiabatic approximation be evaluated, as the inertial forces acting in this frame are the true cause for deviation from adiabaticity and loss of control during the process. Here we present a brief theoretical background of superadiabaticity and illustrate the concept in the context of magnetic resonance with commonly used shaped radio-frequency pulses.

Proton assisted recoupling and protein structure determination
J. Chem. Phys. 129, 245101 (2008)

Gaël De Paëpe, Józef R. Lewandowski, Antoine Loquet, Anja Böckmann, and Robert G. Griffin
We introduce a homonuclear version of third spin assisted recoupling, a second-order mechanism that can be used for polarization transfer between 13C or 15N spins in magic angle spinning (MAS) NMR experiments, particularly at high spinning frequencies employed in contemporary high field MAS experiments. The resulting sequence, which we refer to as proton assisted recoupling (PAR), relies on a cross-term between 1H–13C (or 1H–15N) couplings to mediate zero quantum 13C–13C (or 15N–15N recoupling). In particular, using average Hamiltonian theory we derive an effective Hamiltonian for PAR and show that the transfer is mediated by trilinear terms of the form C1‡C2‡ HZ for 13C–13C recoupling experiments (or N1‡N2‡ HZ for 15N–15N). We use analytical and numerical simulations to explain the structure of the PAR optimization maps and to delineate the PAR matching conditions. We also detail the PAR polarization transfer dependence with respect to the local molecular geometry and explain the observed reduction in dipolar truncation. Finally, we demonstrate the utility of PAR in structural studies of proteins with 13C–13C spectra of uniformly 13C, 15N labeled microcrystalline Crh, a 85 amino acid model protein that forms a domain swapped dimer (MW=2×10.4 kDa). The spectra, which were acquired at high MAS frequencies ( r2 >20 kHz) and magnetic fields (750–900 MHz 1H frequencies) using moderate rf fields, exhibit numerous cross peaks corresponding to long (up to 6–7 Å) 13C–13C distances which are particularly useful in protein structure determination. Using results from PAR spectra we calculate the structure of the Crh protein.

Neutron Powder Diffraction, Multinuclear, and Multidimensional NMR Structural Investigation of Pb5Ga3F19
Inorg. Chem., 2008, 47 (23), pp 10895–10905
Charlotte Martineau, Franck Fayon, Christophe Legein, Jean-Yves Buzaré, François Goutenoire and Emmanuelle Suard
The room temperature structure of Pb5Ga3F19 is investigated by combining neutron diffraction and multinuclear 19F, 71Ga, and 207Pb one-dimensional and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments. Two models built in space group I4cm are reported for the description of the crystalline structure of Pb5Ga3F19. The structure is built from a network of both opposite corner-sharing Ga2F63− octahedra forming infinite chains along the c-axis and isolated Ga1F63− octahedra. The two models present two slightly different views of the strong static disorder of the fluorine ions belonging to the Ga2F63− octahedra. 71Ga NMR results show that the local environment of all Ga2 ions is identical, which indicates a tilt of the Ga2F63− octahedra within the chains. 207Pb NMR experiments confirm that the environment of only one of the two lead sites, Pb1, is strongly affected by the disorder, which gives rise to three broad distinct 207Pb NMR lines for this site. All 19F NMR lines are assigned using the 19F DQ-SQ MAS experiment. 19F−207Pb through-bond and through-space heteronuclear correlation experiments are carried out for the first time, supporting assignment of both the 19F and 207Pb NMR spectra. These correlation experiments also show that both models correctly describe the medium-range order of the structure of Pb5Ga3F19.

Investigation of Silver-Containing Layered Materials and Their Interactions with Primary Amines Using Solid-State 109Ag and 15N NMR Spectroscopy and First Principles Calculations
Inorg. Chem., 2008, 47 (23), pp 11245–11256
Hiyam Hamaed, Andy Y. H. Lo, Leslie J. May, Jared M. Taylor, George H. Shimizu and Robert W. Schurko
Silver-containing layered networks of the form [Ag(L)] (L = 4-pyridinesulfonate or p-toluenesulfonate) were treated with primary amines in different ratios. The structures of the parent supramolecular networks are well-known; however, their interactions with primary amines lead to the formation of new layered materials for which single-crystal X-ray structures cannot be obtained. Solid-state 109Ag, 15N, and 13C cross-polarization magic-angle spinning (CP/MAS) NMR experiments, in combination with powder X-ray diffraction experiments and ab initio calculations, are utilized to investigate the interactions between the primary amines and the parent materials, and to propose structural models for the new materials. 109Ag chemical shift (CS) tensor parameters are extremely sensitive to changes in silver environments; hence, 1H-109Ag CP/MAS NMR experiments are used to distinguish and characterize silver sites. The combination of 109Ag and 15N NMR experiments on starting materials and samples prepared with both 15N-labeled and unlabeled amines permits the accurate measurements of indirect 1J(109Ag,15N) and 1J(109Ag,14N) spin−spin coupling constants, providing further information on structure and bonding in these systems. First principles calculations of silver CS tensors and 1J(109Ag,14N) coupling constants in model complexes aid in formulating the proposed structural models for the new materials, which are largely comprised of layers of silver-diamine cations.

The Kagomé Topology of the Gallium and Indium Metal-Organic Framework Types with a MIL-68 Structure: Synthesis, XRD, Solid-State NMR Characterizations, and Hydrogen Adsorption
Inorg. Chem., 2008, 47 (24), pp 11892–11901
Christophe Volkringer, Mohamed Meddouri, Thierry Loiseau, Nathalie Guillou, Jérôme Marrot, Gérard Férey, Mohamed Haouas, Francis Taulelle, Nathalie Audebrand and Michel Latroche
The vanadium-based terephthalate analogs of MIL-68 have been obtained with gallium and indium (network composition: M(OH)(O2C−C6H4−CO2), M = Ga or In) by using a solvothermal synthesis technique using N,N-dimethylformamide as a solvent (10 and 48 h, for Ga and In, respectively, at 100 °C). They have been characterized by X-ray diffraction analysis; vibrational spectroscopy; and solid-state 1H and 1H−1H radio-frequency-driven dipolar recoupling (RFDR), 1H−1H double quantum correlation (DQ), and 13C{1H} cross polarization magic angle spinning (CPMAS) NMR spectroscopy. The three-dimensional network with a Kagomé-like lattice is built up from the connection of infinite trans-connected chains of octahedral units MO4(OH)2 (M = Ga or In), linked to each other through the terephthalate ligands in order to generate triangular and hexagonal one-dimensional channels. The presence of DMF molecules with strong interactions within the channels as well as their departure upon calcination (150 °C under a primary vacuum) of the materials has been confirmed by subjecting MIL-68 (Ga) to solid-state 1H MAS NMR. The 1H−1H RFDR and 1H−1H DQ spectra revealed important information on the spatial arrangement of the guest species with respect to the hybrid organic−inorganic network. 13C{1H} CPMAS NMR of activated samples provided crystallographically independent sites in agreement with X-ray diffraction structure determination. Brunauer−Emmett−Teller surface areas are 1117(24) and 746(31) m2 g−1 for MIL-98 (Ga) and MIL-68 (In), respectively. Hydrogen adsorption isotherms have been measured at 77 K, and the storage capacities are found to be 2.46 and 1.98 wt % under a saturated pressure of 4 MPa for MIL-68 (Ga) and MIL-68 (In), respectively. For comparison, the hydrogen uptake for the aluminum trimesate MIL-110, which has an open framework with 16 Å channels, is 3 wt % under 4 MPa.

NMR evidence of LiF coating rather than fluorine substitution in Li(Ni0.425Mn0.425Co0.15)O2
Journal of Solid State Chemistry 181 (2008) 3303–3307
M. Menetrier, J. Bains, L. Croguennec, A. Flambard, E. Bekaert, C. Jordy, Ph. Biensan, C. Delmas
A series of “Li1+z/2(Ni0.425Mn0.425Co0.15)1−z/2O2−zFz” materials was prepared by a coprecipitation route and their structure was characterized using X-ray diffraction (XRD), as well as 7Li and 19F Magic Angle Spinning (MAS) NMR spectroscopy. Two hypotheses were considered: (i) formation of layered oxyfluoride materials and (ii) formation of a mixture between the layered material and LiF. Structural parameters were refined by the Rietveld method, using XRD diffraction data. The refinement results did not allow us to choose between these two hypotheses: no significant change in crystallinity and structural parameters was observed irrespective of the fluorine ratio. 7Li and 19F MAS NMR analyses showed signals with isotropic positions characteristic of LiF, but envelopes characteristic of very strong dipolar interactions with the electron spins of the material, demonstrating that LiF was not incorporated into the layered oxide structure but was instead present as a coating.

Modulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides
Journal of Solid State Chemistry 182 (2009) 165–171
Denis Foy, GerardDemazeau, PierreFlorian, DominiqueMassiot, Christine Labrugere, GraziellaGoglio

An hydrogenated nitrogen-rich graphitic carbon nitride, structurally related to the theoretical graphitic phase of C3N4, has been synthesized in a bulk well-crystallized form. This new material was prepared by thermal decomposition of thiosemicarbazide up to 600 °C at ambient pressure under nitrogen flow. Its composition was determined by elemental combustion analysis. Powder X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and C13 MAS NMR characterizations were performed. This material can be schematically described with a two-dimensional framework and a composition close to C3N4.17H1.12. In this nitrogen-rich material, C3N3 voids are fully occupied by water molecules which are strongly trapped into the material. A loss of crystallinity associated with a modification of the thermal behavior is observed when the amount of trapped molecules decreases in the graphitic material, order being damaged both between and in the graphitic planes.

No comments: