Al's Literature Update

Calculation of NMR chemical shifts in organic solids: Accounting for motional effects
J. Chem. Phys. 130, 104701 (2009); DOI:10.1063/1.3081630
Jean-Nicolas Dumez and Chris J. Pickard
Abstract:
NMR chemical shifts were calculated from first principles for well defined crystalline organic solids. These density functional theory calculations were carried out within the plane-wave pseudopotential framework, in which truly extended systems are implicitly considered. The influence of motional effects was assessed by averaging over vibrational modes or over snapshots taken from ab initio molecular dynamics simulations. It is observed that the zero-point correction to chemical shifts can be significant, and that thermal effects are particularly noticeable for shielding anisotropies and for a temperature-dependent chemical shift. This study provides insight into the development of highly accurate first principles calculations of chemical shifts in solids, highlighting the role of motional effects on well defined systems.



Dipolar truncation in magic-angle spinning NMR recoupling experiments
J. Chem. Phys. 130, 114506 (2009); DOI:10.1063/1.3089370
Marvin J. Bayro, Matthias Huber, Ramesh Ramachandran, Timothy C. Davenport, Beat H. Meier, Matthias Ernst, and Robert G. Griffin
Abstract:
Quantitative solid-state NMR distance measurements in strongly coupled spin systems are often complicated due to the simultaneous presence of multiple noncommuting spin interactions. In the case of zeroth-order homonuclear dipolar recoupling experiments, the recoupled dipolar interaction between distant spins is attenuated by the presence of stronger couplings to nearby spins, an effect known as dipolar truncation. In this article, we quantitatively investigate the effect of dipolar truncation on the polarization-transfer efficiency of various homonuclear recoupling experiments with analytical theory, numerical simulations, and experiments. In particular, using selectively 13C-labeled tripeptides, we compare the extent of dipolar truncation in model three-spin systems encountered in protein samples produced with uniform and alternating labeling. Our observations indicate that while the extent of dipolar truncation decreases in the absence of directly bonded nuclei, two-bond dipolar couplings can generate significant dipolar truncation of small, long-range couplings. Therefore, while alternating labeling alleviates the effects of dipolar truncation, and thus facilitates the application of recoupling experiments to large spin systems, it does not represent a complete solution to this outstanding problem.


Understanding two-pulse phase-modulated decoupling in solid-state NMR
J. Chem. Phys. 130, 114510 (2009); DOI:10.1063/1.3086936
Ingo Scholz, Paul Hodgkinson, Beat H. Meier, and Matthias Ernst
Abstract:
A theoretical description of the two-pulse phase-modulated (TPPM) decoupling sequence in magic-angle spinning NMR is presented using a triple-mode Floquet approach. The description is formulated in the radio-frequency interaction-frame representation and is valid over the entire range of possible parameters leading to the well-known results of continuous-wave (cw) decoupling and XiX decoupling in the limit of a phase change of 0° and 180°, respectively. The treatment results in analytical expressions for the heteronuclear residual coupling terms and the homonuclear spin-diffusion terms. It also allows the characterization of all resonance conditions that can contribute in a constructive or a destructive way to the residual linewidth. Some of the important resonance conditions are described for the first time since they are not accessible in previous treatments. The combination of the contributions from the residual couplings and the resonance conditions to the effective Hamiltonian, as obtained in a Floquet description, is shown to be required to describe the decoupling behavior over the full range of parameters. It is shown that for typical spin system and experimental parameters a 13C linewidth of approximately 12 Hz can be obtained for TPPM decoupling in an organic solid or a protein. This is a major contribution to the experimentally observed linewidths of around 20 Hz and indicates that decoupling techniques are still one of the limiting factors in the achievable linewidths.


Design of a triple quantum coherence excitation scheme for protons in solid state NMR
J. Chem. Phys. 130, 124506 (2009); DOI:10.1063/1.3098354
Michal Leskes and Shimon Vega
Abstract:
We present a rf scheme designed to excite triple quantum (TQ) coherences for proton solid state NMR. This recoupling scheme is based on the phase modulated Lee Goldburg sequence combined with echo pulses and applied nonsynchronous with the magic angle spinning period. Based on the effective bimodal Floquet Hamiltonian we optimize the conditions for TQ coherence excitation. Numerical simulations are used to further adjust the recoupling conditions as well as define the sequence limitations. Experimental TQ filtered one-dimensional spectra and two-dimensional correlations of TQ to single quantum coherences are presented for standard amino acids. These results are compared with the crystal structures showing that this scheme can aid in resonance assignments and in resolving local spin topologies.


HCN polymers characterized by solid state NMR: Chains and sheets formed in the neat liquid
J. Chem. Phys. 130, 134503 (2009); DOI:10.1063/1.3092908
Irena Mamajanov and Judith Herzfeld
Abstract:
Hydrogen cyanide polymerizes readily under a variety of conditions and significant prebiotic roles have been suggested for these polymers due to the abundance of HCN in universe. However, the structures of HCN polymers have been more speculative than grounded in experimental data. Here we show that 13C and 15N solid state NMR spectra of polymers formed in neat HCN are inconsistent with the previously proposed structures and suggest instead that the polymers are formed by simple monomer addition, first in head-to-tail fashion to form linear, conjugated chains, and then laterally to form saturated two-dimensional networks. This interpretation of the NMR spectra finds support in other information about the polymerization of neat HCN, including the presence of free radicals. As expected from the literature, formation of the HCN tetramer, diaminomaleonitrile, is also observed, but only when the reaction is catalyzed exclusively by base and then in crystalline form.


Crystal Structure Solid-State Cross Polarization Magic Angle Spinning 13C NMR Correlation in Luminescent d10 Metal-Organic Frameworks Constructed with the 1,2-Bis(1,2,4-triazol-4-yl)ethane Ligand
Inorg. Chem., 2009, 48 (5), pp 2166–2180 DOI: 10.1021/ic802069k
Hesham A. Habib, Anke Hoffmann, Henning A. Hppe, Gunther Steinfeld and Christoph Janiak
Abstract:
Hydrothermal reactions of 1,2-bis(1,2,4-triazol-4-yl)ethane (btre) with copper(II), zinc(II), and cadmium(II) salts have yielded the dinuclear complexes [Zn2Cl4(μ2-btre)2] (1) and [Zn2Br4(μ2-btre)2] (2), the one-dimensional coordination polymer ∞1[Zn(NCS)2(μ2-btre)] (3), the two-dimensional networks ∞2[Cu2(μ2-Cl)2(μ4-btre)] (4), ∞2[Cu2(μ2-Br)2(μ4-btre)] (5), and ∞2{[Cd6(μ3-OH)2(μ3-SO4)4(μ4-btre)3(H2O)6](SO4)·6H2O} (6), and the three-dimensional frameworks ∞3{[Cu(μ4-btre)]ClO4·0.25H2O} (7), ∞3{[Zn(μ4-btre)(μ2-btre)](ClO4)2} (8), ∞3{[Cd(μ4-btre)(μ2-btre)](ClO4)2} (9), and ∞3[Cu2(μ2-CN)2(μ4-btre)] (10, 2-fold 3D interpenetrated framework). The copper-containing products 4, 5, 7, and 10 contain the metal in the +1 oxidation state, from a simultaneous redox and self-assembly reaction of the Cu(II) starting materials. The cyanide-containing framework 10 has captured the CN− ions from the oxidative btre decomposition. The perchlorate frameworks 7, 8, or 9 react in an aqueous NH4+PF6− solution with formation of the related PF6−-containing frameworks. The differences in the metal-btre bridging mode (μ2-κN1:N1′, μ2-κN1:N2 or μ4-κN1:N2:N1′:N2′) and the btre ligand symmetry can be correlated with different signal patterns in the 13C cross polarization magic angle spinning (CPMAS) NMR spectra. Compounds 2, 4, 5 and 7 to 10 exhibit fluorescence at 403−481 nm upon excitation at 270−373 nm which is not seen in the free btre ligand.


A 13C and 15N Solid-State NMR Study of Structural Disorder and Aurophilic Bonding in Au(I) and Au(III) Cyanide Complexes
Inorg. Chem., 2009, 48 (5), pp 2316–2332 DOI: 10.1021/ic8022198
Kristopher J. Harris and Roderick E. Wasylishen
Abstract:
Solid-state nuclear magnetic resonance has been used to study several cyanoaurates. Carbon-13 and nitrogen-15 NMR spectra of samples enriched with isotopically labeled 13C,15N cyanide ligands were recorded for stationary samples and samples spinning at the magic angle. Several salts of the dicyanoaurate(I) anion, M[Au(CN)2], where M = n-butylammonium, potassium, and thallium, were studied via solid-state NMR. A gold(III) cyanide, K[Au(CN)4], was also investigated. Carbon-13 and nitrogen-15 chemical shift tensors are reported for each salt, as are the measured 13C,15N direct dipolar coupling constants together with the related derived cyanide bond lengths, r(C,N). The value for r(C,N) in [(n-C4H9)4N][Au(CN)2], 1.17(5) Å, was determined to be more realistic than a previously reported X-ray diffraction value of 1.03(4) Å. Large 13C NMR line widths from Tl[Au(CN)2], 250−315 Hz, are attributed to coupling with 197Au (I = 3/2) and/or 203/205Tl (I = 1/2), as confirmed by measurements of the transverse relaxation constant, T2. Investigation of the carbon-13 chemical shifts for cyanide ligands bound to gold involved in a variety of metallophilic bonding environments demonstrates that the chemical shift is sensitive to metallophilic bonding. Differences in Au−Tl metallophilic bonding are shown to cause a difference in the isotropic carbon-13 chemical shift of up to 15.7 ppm, while differences in Au−Au aurophilic bonding are found to be responsible for a change of up to 5.9 ppm. The disordered polymeric material gold(I) monocyanide, AuCN, was also investigated using 13C and 15N SSNMR. Two-dimensional 13C,13C double-quantum dipolar-recoupling spectroscopy was used to probe connectivity in this material. The 13C NMR site multiplicity in AuCN is explained on the basis of sensitivity of the carbon-13 chemical shift to aurophilic bonding of the directly bonded gold atom. This assignment allows estimation of the position of the linear [−M−CN−]∞ chain’s position with respect to the neighboring polymer chain. For the samples studied, a range of 7 ± 2% to 25 ± 5% of the AuCN chains are found to be “slipped” instead of aligned with the neighboring chains at the metal position.


High-Temperature Chlorination-Reduction Sequence for the Preparation of Silicon Hydride Modified Silica Surfaces
Chem Eur. J. Volume 15 Issue 4, Pages 936 - 946
Nicolas Plumeré, Bernd Speiser, Prof. Dr., Hermann A. Mayer, Prof. Dr., Dominik Joosten, Dr., Lars Wesemann, Prof. Dr.
Abstract:
A general method for the functionalization of silica surfaces with silicon hydride (Si-H) groups is described for four different preparations of silica. The silica surface is reduced in a two-step chlorination-reduction procedure within a simple gas-flow system at high temperatures. After initial dehydroxylation of the silica surface, silicon chloride groups are formed by the reaction with thionyl chloride. The chlorination activates otherwise inaccessible surface siloxane moieties. A high silicon-hydride surface concentration results from the subsequent reduction of the chlorinated surface with hydrogen. The physical properties of the resulting silica are analyzed using scanning electron microscopy, as well as dynamic light scattering and Brunauer-Emmet-Teller measurements. The chlorination-reduction sequence has no significant impact on the structure, surface area and mesopore size of the silica materials used. The surface of the materials is characterized by diffuse reflectance infrared Fourier transform (DRIFT) and 29Si CP/MAS NMR spectroscopy. The silicon-hydride groups are mostly of the -type. The use of high temperatures (>800 °C) results in the condensation of internal and surface silanol groups. Therefore, materials with both a fully condensed silica matrix as well as a surface free of silanol groups are obtained. The materials are ideal precursors for further molecular silica surface modification, as demonstrated with a ferrocene derivative.


Electronic Structure, Chemical Bonding, and Solid-State NMR Spectroscopy of the Digallides of Ca, Sr, and Ba
Chem Eur. J. Volume 15 Issue 7, Pages 1673 - 1684
Frank Haarmann, Dr., Katrin Koch, Dipl.-Phys., Daniel Grüner, Dr., Walter Schnelle, Dr., Oliver Pecher, Dipl.-Chem., Raul Cardoso-Gil, Dr., Horst Borrmann, Dr., Helge Rosner, Dr., Yuri Grin, Prof. Dr
Abstract:
Delving into digallides: The characteristics of the chemical bonding of the digallides of the alkaline-earth metals (see figure) have been studied by application of experimental methods, such as single-crystal X-ray diffraction and solid-state NMR spectroscopy, in combination with quantum mechanical calculations. Combined application of 69,71Ga NMR spectroscopy and quantum mechanical calculations reveals the chemical bonding in the digallides of Ca, Sr, and Ba. An analysis of the electron localization function (ELF) shows honeycomb-like 63 nets of the Ga atoms as the most prominent structural features in SrGa2 and BaGa2. For CaGa2 a description of a 3+1-coordinated Ga atom is revealed by the ELF and by an analysis of interatomic distances. The NMR spectroscopic signal shift is mainly due to the Knight shift and is almost equal for the investigated digallides, whereas the anisotropy of the signal shift decreases with the radius of the alkaline-earth metals. Calculated and observed values of the electric field gradient (EFG) are in good agreement for CaGa2 and BaGa2 but differ by about 21 % for SrGa2 indicating structural instability. Better agreement is achieved by considering a puckering of the Ga layers. For BaGa2 an instability of the structure is indicated by a peak in the density of states at the Fermi level, which is shifted to lower energies when taking puckering of the Ga layers into account. Both structural modifications are confirmed by crystallographic information. The Fermi velocity of the electrons is strongly anisotropic and is largest in the (001) plane of the crystal structure. This results in an alignment of the crystallites with the [001] axis perpendicular to the magnetic field as observed in 69,71Ga NMR spectroscopy and magnetic susceptibility experiments. The electron transport is predominantly mediated by the Ga-Ga px- and py-like electrons in the (001) plane. The specific heat capacity of BaGa2 was determined and indicated the absence of phase transitions between 1.8 and 320 K.

The Extra-Framework Sub-Lattice of the Metal-Organic Framework MIL-110: A Solid-State NMR Investigation
Chem. Eur. J. Volume 15 Issue 13, Pages 3139 - 3146
Mohamed Haouas, Dr., Christophe Volkringer, Dr., Thierry Loiseau, Dr., Gérard Férey, Prof. Dr., Francis Taulelle, Dr.
Abstract:
A changeable character: Differences in the dynamics of occluded moieties within the large pores (see graphic) of aluminium 1,3,5-benzene tricarboxylate framework solid MIL-110 are a function of the synthesis pH. Host-guest proton-transfer processes lead to a reversible change in the character of the framework from cationic to neutral, depending on the nature of the extra-framework moieties. The aluminium 1,3,5-benzene tricarboxylate framework solid MIL-110, which crystallises either at pH0 or at pH4, has been investigated by solid-state NMR spectroscopy. At pH0 the solid (MIL-110{pH0}) is the unique stable thermodynamic product, whereas at pH4 MIL-110{pH4} is observed as a kinetic product in competition with the MIL-96 phase. Diffraction studies and 27Al NMR spectroscopy prove that the framework is identical in both cases. The nature and dynamics of occluded moieties within the large pores look quite different for the two compounds. MIL-110{pH4} and MIL-110{pH0} both show the presence of occluded 1,3,5-benzene tricarboxylate (btc), with additional nitrates and water molecules. However, the proportions of btc, nitrates and water are functions of the pH, leading to an identical framework and a quite different extra-framework. With the extra-framework moieties, the framework undergoes a proton transfer which is a function of the synthesis pH. Washing the MIL-110{pH0} phase with water produces a different extra-framework structure, richer in water and poorer in btc and nitrates. The hydroxyl groups of the inorganic aluminium cluster of the framework are involved in a proton transfer, which leads for all cases to a cationic framework and an anionic extra-framework. 1H-1H DQ 2D NMR spectra (DQ=double quantum) give evidence for the interaction of extra-framework btc with the terminal water carried by Al(2,3) and shows their proximity to the closest hydroxyl groups. A structure for the non-diffracting extra-framework is proposed as the most plausible topology. It provides an efficient picture for the creation of many substituted MIL-110 compounds that would have a large number of applications.