Monday, April 27, 2009

Science v324, Issue 24

Some Solid-state 27Al MAS NMR can be found in the supporting information.

Science 24 April 2009:Vol. 324. no. 5926, pp. 488 - 492DOI: 10.1126/science.1168162

Greatly Increased Toughness of Infiltrated Spider Silk

Seung-Mo Lee,1,* Eckhard Pippel,1 Ulrich
Gösele,1 Christian Dresbach,2 Yong Qin,1 C. Vinod Chandran,3 Thomas Bräuniger,3 Gerd Hause,4 Mato Knez1,*

In nature, tiny amounts of inorganic impurities, such as metals, are incorporated in the protein structures of some biomaterials and lead to unusual mechanical properties of those materials. A desire to produce these biomimicking new materials has stimulated materials scientists, and diverse approaches have been attempted. In contrast, research to improve the mechanical properties of biomaterials themselves by direct metal incorporation into inner protein structures has rarely been tried because of the difficulty of developing a method that can infiltrate metals into biomaterials, resulting in a metal-incorporated protein matrix. We demonstrated that metals can be intentionally infiltrated into inner protein structures of biomaterials through multiple pulsed vapor-phase infiltration performed with equipment conventionally used for atomic layer deposition (ALD). We infiltrated zinc (Zn), titanium (Ti), or aluminum (Al), combined with water from corresponding ALD precursors, into spider dragline silks and observed greatly improved toughness of the resulting silks. The presence of the infiltrated metals such as Al or Ti was verified by energy-dispersive x-ray (EDX) and nuclear magnetic resonance spectra measured inside the treated silks. This result of enhanced toughness of spider silk could potentially serve as a model for a more general approach to enhance the strength and toughness of other biomaterials.

Wednesday, April 22, 2009

J. Am. Chem. Soc., 2009, 131 (15), pp 5627–5634

Formula Weight Prediction by Internal Reference Diffusion-Ordered NMR Spectroscopy (DOSY)

Deyu Li, Gerald Kagan, Russell Hopson and Paul G. Williard*

Formula weight (FW) information is important to characterize the composition, aggregation number, and solvation state of reactive intermediates and organometallic complexes. We describe an internal reference correlated DOSY method for calculating the FW of unknown species in different solvents with different concentrations. Examples for both the small molecule (DIPA) and the organometallic complex (aggregate 1) yield excellent correlations. We also found the relative diffusion rate is inversely proportional to the viscosity change of the solution, which is consistent with the theoretical Stokes−Einstein equation. The accuracy of the least-squares linear prediction r2 and the percentage difference of FW prediction are directly related to the density change; greater accuracy was observed with decreasing density. We also discuss the guidelines and other factors for successful application of this internal reference correlated DOSY method. This practical method can be conveniently modified and applied to the characterization of other unknown molecules or complexes.

J. Am. Chem. Soc., 2009, 131 (15), pp 5366–5367

Shape Discrimination with Hexapole−Dipole Interactions in Magic Angle Spinning Colloidal Magnetic Resonance

Pietro Tierno*†, Steffen Schreiber‡, Walter Zimmermann‡ and Thomas M. Fischer‡

We have studied the interactions between magnetically driven, DNA-linked anisotropic and isotropic colloidal rotors interacting via induced magnetic dipolar and multipolar forces. We show that a balance between magnetic dipole−dipole and dipole−hexapole interactions near the magic angle allows discrimination between spherical and anisotropic magnetic colloidal rotors.

Monday, April 13, 2009

J. Am. Chem. Soc., Article ASAP

Sensitivity Enhancement in Static Solid-State NMR Experiments via Single- and Multiple-Quantum Dipolar Coherences
T. Gopinath and Gianluigi Veglia*

We present a new method for enhancing the sensitivity in static solid-state NMR experiments for a gain in signal-to-noise ratio of up to 40%. This sensitivity enhancement is different from the corresponding solution NMR sensitivity enhancement schemes and is achieved by combining single- and multiple-quantum dipolar coherences. While this new approach is demonstrated for the polarization inversion spin exchange at magic angle (PISEMA) experiment, it can be generalized to the other separated local field experiments for solid-state NMR spectroscopy. This method will have a direct impact on solid-state NMR spectroscopy of liquid crystals as well as of membrane proteins aligned in lipid membranes.

J. Am. Chem. Soc., Article ASAP

Light Penetration and Photoisomerization in Rhodopsin studied by Numerical Simulations and Double-Quantum Solid-State NMR Spectroscopy

Maria Concistrè†, Axel Gansmller†, Neville McLean†, Ole G. Johannessen†, Ildefonso Marn Montesinos†, Petra H. M. Bovee-Geurts‡, Richard C. D. Brown†, Willem J. DeGrip‡ and Malcolm H. Levitt*†

The penetration of light into optically thick samples containing the G-protein-coupled receptor rhodopsin is studied by numerical finite-element simulations and double-quantum solid-state NMR experiments. Illumination with white light leads to the generation of the active bathorhodopsin photostate in the outer layer of the sample but generates a large amount of the side product, isorhodopsin, in the sample interior. The overall yield of bathorhodopsin is improved by using monochromatic 420 nm illumination and by mixing the sample with transparent glass beads. The implications of these findings on the interpretation of previously published rhodopsin NMR data are discussed.

Macromolecules, vol. 42, Issue 7

Local Deformation in Carbon Black-Filled Polyisoprene Rubbers Studied by NMR and X-ray Diffraction

Stphane Dupres, Didier R. Long, Pierre-Antoine Albouy and Paul Sotta*
Laboratoire de Physique des Solides, CNRS/Universit Paris-Sud, UMR 8502, 91405 Orsay Cedex, France
Macromolecules, 2009, 42 (7), pp 2634–2644

Abstract: We study polyisoprene elastomers reinforced with carbon blacks (CBs) of various grades. In addition to mechanical characterization at medium/large elongation ratios, we use deuterium NMR experiments on stretched samples to measure the local strain within the elastomer matrix and X-ray scattering to measure the onset of strain-induced crystallization in the reinforced systems. We show that NMR experiments and measurements of the onset of crystallization are indeed sensitive to the different degrees of reinforcement observed according to the various CB grades and volume fractions. The measurements show a good correlation between macroscopic (mechanical) and microscopic (NMR, crystallization) measurements. This indicates that the techniques used here are valuable techniques for characterizing reinforced systems in which fillers have complex morphologies and dispersion states. The discrepancies between the results of both macroscopic and microscopic measurements are analyzed. The local strain as measured by NMR is lower than the macroscopic strain. This indicates the presence of some degree of local strain (or stress) inhomogeneity within the elastomer matrix in the reinforced systems. This inhomogeneity is more pronounced in the presence of more reinforcing carbon black grades.

Organometallics, vol.28, Issue 7

Chemisorption Pathways and Catalytic Olefin Polymerization Properties of Group 4 Mono- and Binuclear Constrained Geometry Complexes on Highly Acidic Sulfated Alumina

Linda A. Williams and Tobin J. Marks*
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113
Organometallics, 2009, 28 (7), pp 2053–2061

Abstract: Mono- and binuclear “constrained-geometry catalyst” (CGC) group 4 hydrocarbyls Me2Si(Me5C5)(tBuN)ZrMe2 [CGCZrMe2, 1], 1-Me2Si(3-ethylindenyl)(tBuN)ZrMe2 [EICGCZrMe2; Zr1, 2], (μ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si-(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2, Zr2, 3], and (μ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si-(tBuN)](TiMe2)}2 [EBICGC(TiMe2)2, Ti2, 4] undergo rapid chemisorption on highly Brønsted acidic sulfated alumina (AlS) surfaces. 13C CPMAS NMR spectroscopy of the chemisorbed 13CαH3-enriched complexes EICGCZr13Me2/AlS (2*/AlS) and EBICGC(Zr13Me2)2/AlS (3*/AlS) reveals that chemisorption involves two processes, M−C σ-bond protonolysis at the strong surface Brønsted acid sites and heterolytic M−C scission with methide transfer to strong surface Lewis acid sites, forming similar “cation-like” electrophilic organo-group 4 complexes such as EICGCM13Me+. Relative rates of ethylene homopolymerization mediated by the catalysts prepared via chemisorption on AlS are 4/AlS > 2/AlS > 3/AlS > 1/AlS, for ethylene polymerization at 75 psi ethylene and 25 °C. Ethylene/1-hexene copolymerizations mediated by the same set of catalysts display relative polymerization rates of 4/AlS > 3/AlS > 2/AlS > 1/AlS, for copolymerizations at 75 psi ethylene, 0.8 M 1-hexene, and 25 °C.

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
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
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

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
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
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
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
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.
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
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.
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.

J Phys Chem B and C, Vol. 113, Issue 15

Structural Studies of the Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Dichloromethane Using a Combined DFT-NMR Spectroscopic Approach

Sergey A. Katsyuba*, Tatiana P. Griaznova, Ana Vidi and Paul J. Dyson
A.E.Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov Str. 8, 420088 Kazan, Russia, and Institut des Sciences et Ingnierie Chimiques, Ecole Polytechnique Fdrale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
J. Phys. Chem. B, 2009, 113 (15), pp 5046–5051

Abstract: DFT methods in combination with NMR spectroscopy are used to investigate possible variants of the molecular structure of the ion pairs of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], in dichloromethane. According to the computations of the chemical shifts, experimental NMR spectra can be rationalized by an equilibrium between ca. 70−80% of structures with the anion positioned near to the C2 atom of the imidazolium ring and ca. 20−30% of structures with the anion close to the C5 and/or C4 atoms. The content of the latter structures, according to the computed Gibbs free energies, does not exceed 10%. Both the computations and the experimental NMR data suggest that the ratio of the two above-mentioned types of structures of the imidazolium-based ILs is influenced by the concentration/polarity of their dichloromethane solutions.

Connectivity and Proximity between Quadrupolar Nuclides in Oxide Glasses: Insights from through-Bond and through-Space Correlations in Solid-State NMR

Sung Keun Lee*, Michael Deschamps§, Julien Hiet§, Dominique Massiot§ and Sun Young Park
J. Phys. Chem. B, 2009, 113 (15), pp 5162–5167

Abstract:The connectivity and proximity among framework cations and anions in covalent oxide glasses yields unique information whereby their various transport and thermodynamic properties can be predicted. Recent developments and advances in the reconstruction of anisotropic spin interactions among quadrupolar nuclides (spin > 1/2) in solid-state NMR shed light on a new opportunity to explore local connectivity and proximity in amorphous solids. Here, we report the 2D through-bond (J-coupling) and through-space (dipolar coupling) correlation NMR spectra for oxide glasses where previously unknown structural details about the connectivity and proximity among quadrupolar nuclides (27Al, 17O) are determined. Nonbridging oxygen peaks in Ca−aluminosilicate glasses with distinct connectivity, such as Ca−O−Al and Al−O−(Al, Si) are well distinguished in {17O}27Al solid HMQC NMR spectra. Both peaks shift to a lower frequency in direct and indirect dimensions upon the addition of Si to the Ca−aluminate glasses. The 2D 27Al double quantum magic angle spinning NMR spectra for Mg-aluminoborate glasses indicate the preferential proximity between [4]Al and [5]Al leading to the formation of correlations peaks such as [4]Al−[4]Al, [4]Al−[5]Al, and [5]Al−O−[5]Al. A fraction of the [6]Al−[6]Al correlation peak is also noticeable while that of [4,5]Al−[6]Al is missing. These results suggest that [6]Al is likely to be isolated from the [4]Al and [5]Al species, forming [6]Al clusters. The experimental realization of through-bond and through-space correlations among quadrupolar nuclides in amorphous materials suggests a significant deviation from the random distribution among framework cations and a spatial heterogeneity due to possible clustering of framework cations in the model oxide glasses.

Spectroscopic and Computational Characterization of the Base-off Forms of Cob(II)alamin

Matthew D. Liptak, Angela S. Fleischhacker, Rowena G. Matthews§, Joshua Telser and Thomas C. Brunold*
Department of Chemistry, University of Wisconsin-Madison, Madison Wisconsin 53706, Department of Chemistry, and Life Sciences Institute, Department of Biological Chemistry, and Biophysics Research Division, University of Michigan, Ann Arbor, Michigan 48109, and Chemistry Program, Roosevelt University, Chicago, Illinois 60605
J. Phys. Chem. B, 2009, 113 (15), pp 5245–5254

Abstract: The one-electron-reduced form of vitamin B12, cob(II)alamin (Co2+Cbl), is found in several essential human enzymes, including the cobalamin-dependent methionine synthase (MetH). In this work, experimentally validated electronic structure descriptions for two “base-off” Co2+Cbl species have been generated using a combined spectroscopic and computational approach, so as to obtain definitive clues as to how these and related enzymes catalyze the thermodynamically challenging reduction of Co2+Cbl to cob(I)alamin (Co1+Cbl). Specifically, electron paramagnetic resonance (EPR), electronic absorption (Abs), and magnetic circular dichroism (MCD) spectroscopic techniques have been employed as complementary tools to characterize the two distinct forms of base-off Co2+Cbl that can be trapped in the H759G variant of MetH, one containing a five-coordinate and the other containing a four-coordinate, square-planar Co2+ center. Accurate spin Hamiltonian parameters for these low-spin Co2+ centers have been determined by collecting EPR data using both X- and Q-band microwave frequencies, and Abs and MCD spectroscopic techniques have been employed to probe the corrin-centered π → π* and Co-based d → d excitations, respectively. By using these spectroscopic data to evaluate electronic structure calculations, we found that density functional theory provides a reasonable electronic structure description for the five-coordinate form of base-off Co2+Cbl. However, it was necessary to resort to a multireference ab initio treatment to generate a more realistic description of the electronic structure of the four-coordinate form. Consistent with this finding, our computational data indicate that, in the five-coordinate Co2+Cbl species, the unpaired spin density is primarily localized in the Co 3dz2-based molecular orbital, as expected, whereas in the four-coordinate form, extensive Co 3d orbital mixing, configuration interaction, and spin−orbit coupling cause the unpaired electron to delocalize over several Co 3d orbitals. These results provide important clues to the mechanism of enzymatic Co2+Cbl → Co1+Cbl reduction.

Theoretical Study of the Effective Chemical Shielding Anisotropy (CSA) in Peptide Backbone, Rating the Impact of CSAs on the Cross-Correlated Relaxations in l-Alanyl-l-alanine

Ladislav Benda*, Petr Bou, Norbert Mller* and Vladimr Sychrovsk*
Institute of Organic Chemistry and Biochemistry v.v.i., Academy of Sciences of the Czech Republic, Flemingovo nm. 2, 166 10 Praha 6, Czech Republic, and Institute of Organic Chemistry, Johannes Kepler University, Altenbergerstrasse 69, 4040 Linz, Austria
J. Phys. Chem. B, 2009, 113 (15), pp 5273–5281

Abstract:The dependence of the effective chemical shielding anisotropy (effective CSA, Δσeff) on the and ψ peptide backbone torsion angles was calculated in the l-alanyl-l-alanine (LALA) peptide using the DFT method. The effects of backbone conformation, molecular charge including the cation, zwitterion, and anion forms of the LALA peptide, and the scaling taking into account the length of the dipolar vector were calculated for the effective CSAs in order to assess their structural behaviors and to predict their magnitudes which can be probed for the β-sheet and α-helix backbone conformations via measurement of the cross-correlated relaxation rates (CCR rates). Twenty different CSA−DD cross-correlation mechanisms involving the amide nitrogen and carbonyl carbon chemical shielding tensors and the CαHα (α-carbon group), NHN (amide group), CαHN, NHα, C′Hα, and C′HN (α = α1, α2) dipolar vectors were investigated. The X−CαHα (X = N, C′; α = α1, α2) cross-correlations, which had already been studied experimentally, exhibited overall best performance of the calculated effective CSAs in the LALA molecule; they spanned the largest range of values upon variation of the ψ and torsions and depended dominantly on only one of the two backbone torsion angles. The X−NHN (X = N, C′) cross-correlations, which had been also probed experimentally, depended on both backbone torsions, which makes their structural assignment more difficult. The N−NHα2 and N−C′Hα1 cross-correlations were found to be promising for the determination of various backbone conformations due to the large calculated range of the scaled effective CSA values and due to their predominant dependence on the ψ and torsions, respectively. The 20 calculated dependencies of effective CSAs on the two backbone torsion angles can facilitate the structural interpretation of CCR rates.

Thursday, April 09, 2009

Chem. Mater., 2009, 21 (7), pp 1187–1197

Solid-State 13C NMR Characterization of Carbon-Modified TiO2

Erin M. Rockafellow, Xiaowen Fang, Brian G. Trewyn, Klaus Schmidt-Rohr* and William S. Jenks*

13C-modified TiO2 was prepared to facilitate study of the dopant atoms and trace their chemical fate throughout the process. In the preannealed material, NMR showed strong evidence of many Ti−O−C bonds. After annealing, surface-bound coke is a major component. NMR also showed that a washing step before annealing led to the generation of orthocarbonate (C(OR)4) centers, observed at 126 ppm, which are located deep inside the TiO2 particles. Both NMR and XPS confirmed the presence of small amounts of regular sp2-hybridized carbonate species in all briefly annealed samples, while annealing for longer times led to a reduction removal of the COn centers. Quantitative NMR also shows the degree of carbon loss that accompanies annealing. Some variation in the chemical degradation of quinoline is noted among the catalysts, but coke-containing TiO2 catalysts are not qualitatively better catalysts for use with visible light with this substrate.

J. Am. Chem. Soc., Article ASAP

Shape Discrimination with Hexapole−Dipole Interactions in Magic Angle Spinning Colloidal Magnetic Resonance

Pietro Tierno*†, Steffen Schreiber‡, Walter Zimmermann‡ and Thomas M. Fischer‡

We have studied the interactions between magnetically driven, DNA-linked anisotropic and isotropic colloidal rotors interacting via induced magnetic dipolar and multipolar forces. We show that a balance between magnetic dipole−dipole and dipole−hexapole interactions near the magic angle allows discrimination between spherical and anisotropic magnetic colloidal rotors.

J. Am. Chem. Soc., Article ASAP

15N−15N Proton Assisted Recoupling in Magic Angle Spinning NMR
Jzef R. Lewandowski†, Gal De Pape‡, Matthew T. Eddy and Robert G. Griffin*

We describe a new magic angle spinning (MAS) NMR experiment for obtaining 15N−15N correlation spectra. The approach yields direct information about the secondary and tertiary structure of proteins, including identification of α-helical stretches and interstrand connectivity in antiparallel β-sheets, which are of major interest for structural studies of membrane proteins and amyloid fibrils. The method, 15N−15N proton assisted recoupling (PAR), relies on a second-order mechanism, third spin assisted recoupling (TSAR), used previously in the context of 15N−13C and 13C−13C polarization transfer schemes. In comparison to 15N−15N proton-driven spin diffusion experiments, the PAR technique accelerates polarization transfer between 15N’s by a factor of 102−103 and is furthermore applicable over the entire range of currently available MAS frequencies (10−70 kHz).

J. Am. Chem. Soc., 2009, 131 (14), pp 5153–5162

Lipophilic Bisphosphonates as Dual Farnesyl/Geranylgeranyl Diphosphate Synthase Inhibitors: An X-ray and NMR Investigation

Yonghui Zhang†, Rong Cao‡, Fenglin Yin‡, Michael P. Hudock‡, Rey-Ting Guo§, Kilannin Krysiak†, Sujoy Mukherjee‡, Yi-Gui Gao†, Howard Robinson, Yongcheng Song†, Joo Hwan No‡, Kyle Bergan†, Annette Leon‡, Lauren Cass†, Amanda Goddard†, Ting-Kai Chang†, Fu-Yang Lin‡, Ermond Van Beek, Socrates Papapoulos, Andrew H.-J. Wang§, Tadahiko Kubo#, Mitsuo Ochi#, Dushyant Mukkamala‡ and Eric Oldfield†‡

Considerable effort has focused on the development of selective protein farnesyl transferase (FTase) and protein geranylgeranyl transferase (GGTase) inhibitors as cancer chemotherapeutics. Here, we report a new strategy for anticancer therapeutic agents involving inhibition of farnesyl diphosphate synthase (FPPS) and geranylgeranyl diphosphate synthase (GGPPS), the two enzymes upstream of FTase and GGTase, by lipophilic bisphosphonates. Due to dual site targeting and decreased polarity, the compounds have activities far greater than do current bisphosphonate drugs in inhibiting tumor cell growth and invasiveness, both in vitro and in vivo. We explore how these compounds inhibit cell growth and how cell activity can be predicted based on enzyme inhibition data, and using X-ray diffraction, solid state NMR, and isothermal titration calorimetry, we show how these compounds bind to FPPS and/or GGPPS.

J. Am. Chem. Soc., 2009, 131 (14), pp 5145–5152

A Solid-State NMR Study of Lead and Vanadium Substitution into Hydroxyapatite

Hlne Pizzala‡, Stefano Caldarelli*†, Jean-Guillaume Eon§, Alexandre Malta Rossi, Danielle Laurencin¶ and Mark E. Smith¶

A systematic study on cationic and anionic substitution in hydroxyapatite structures was carried out, with the aim of understanding the impact of ion exchange on the crystalline structure and properties of these materials. Lead and vanadium were chosen for the exchange, due to their known effects on the redox and catalytic properties of hydroxypatites. Hydroxyapatites with variable Pb and V contents, PbxCa10-x(VO4)y(PO4)6-y(OH)2 (x = 0, 2, 4, 6, 8 and 10 for y = 1; y = 0, 0.5, 1, 2, 3 and 6 for x = 10) were synthesized and characterized by NMR spectroscopy. Solid-state NMR allowed an analysis of the chemical environment of every ion after substitution into the hydroxyapatite network. 43Ca and 207Pb NMR spectra at different lead concentrations provided clear evidence of the preferential substitution of lead into the Ca(II) site, the replacement of the Ca(I) site starting at x = 4 for y = 1. Two NMR distinguishable Pb(I) sites were observed in Pb10(PO4)6(OH)2, which is compatible with the absence of a local mirror plane perpendicular to the c direction. In contrast with 31P NMR, for which only small variations related to the incorporation of Pb are observed, the strong change in the 51V NMR spectrum indicates that lead perturbs the vanadium environment more than the phosphorus one. The existence of a wide variety of environments for OH in substituted apatites is revealed by 1H NMR, and the mobility of the water molecules appears to vary upon introduction of lead into the structure.

J. Am. Chem. Soc., 2009, 131 (14), pp 5014–5015

A Microfluidic High-Resolution NMR Flow Probe

Jacob Bart†, Ard J. Kolkman‡, Anna Jo Oosthoek-de Vries‡, Kaspar Koch‡, Pieter J. Nieuwland‡, Hans (J. W. G.) Janssen‡, Jan (P. J. M.) van Bentum‡, Kirsten A. M. Ampt‡, Floris P. J. T. Rutjes‡, Sybren S. Wijmenga‡, Han (J. G. E.) Gardeniers† and Arno P. M. Kentgens*‡

A microfluidic high-resolution NMR flow probe based on a novel stripline detector chip is demonstrated. This tool is invaluable for the in situ monitoring of reactions performed in microreactors. As an example, the acetylation of benzyl alcohol with acetyl chloride was monitored. Because of the uncompromised (sub-Hz) resolution, this probe holds great promise for metabolomics studies, as shown by an analysis of 600 nL of human cerebrospinal fluid.

Tuesday, April 07, 2009

Angew. Chem. Int. Ed. Engl. up to Apr 7, 2009

Angewandte Chemie International Edition
Volume 48 Issue 17, Pages 3082 - 3086
Published Online: 19 Mar 2009

Gwendal Kervern, Anthony D'Aléo, Loïc Toupet, Olivier Maury, Lyndon Emsley, Guido Pintacuda

lanthanides • NMR crystallography • NMR spectroscopy • paramagnetism • structure elucidation

Shifts for crystals: Solid-state NMR spectroscopy can be used for structure determination of microcrystalline paramagnetic solids at natural isotopic abundance. The protocol makes use of paramagnetic effects, measured on suitably recorded 1H NMR spectra, to define the conformation of a molecule in the lattice and the intermolecular packing in the solid phase. The method is illustrated with a family of lanthanide compounds (see picture).
Received: 29 October 2008; Revised: 7 January 2009
Digital Object Identifier (DOI) 10.1002/anie.200805302

Angewandte Chemie International Edition
Volume 48 Issue 15, Pages 2732 - 2736
Single-Scan 2D Hadamard NMR Spectroscopy

Assaf Tal, Boaz Shapira, Lucio Frydman

Hadamard transform • multidimensional NMR • NMR spectroscopy • spatial/spectral encoding

Scan and deliver: By combining imaging-based spectral/spatial 2D radiofrequency manipulations (see scheme, left) with Hadamard-weighting principles, 2D NMR spectra can be retrieved within a single scan (right). This approach can give homo- or heteronuclear correlations with an enhanced sensitivity over conventional ultrafast 2D NMR spectroscopy.

Angewandte Chemie International Edition
Volume 48 Issue 9, Pages 1673 - 1676
Revisiting Prussian Blue Analogues with Solid-State MAS NMR Spectroscopy: Spin Density and Local Structure in [Cd3{Fe(CN)6}2][sdot]15 H2O

by Alexandrine Flambard, Frank H. Köhler, Rodrigue Lescouëzec

NMR spectroscopy • Prussian blue analogues • solid-state structures • spin density

No legendary Prussian order! The distribution of vacancies in Prussian blue analogues is not random, and the spin density on the Cd2+ ion varies depending on the number of paramagnetic ions in its surroundings. This conclusion follows from 113Cd solid-state magic-angle spinning NMR studies of [Cd3{Fe/Co(CN)6}2]15 H2O, where the presence of small but significant spin density on the observed 113Cd nucleus leads to improved spectral resolution.
Received: 5 November 2008

Digital Object Identifier (DOI) 10.1002/anie.200805415 About DOI

Monday, April 06, 2009

MRC; Magn. Reson. Chem. up to April 6, 2009

MRC; Magn. Reson. Chem. up to April 6, 2009

95Mo NMR: a useful tool for structural studies in solution
from Magnetic Resonance in Chemistry by José Angel Brito, Helena Teruel, Stéphane Massou, Montserrat Gómez

Oxomolybdenum(VI) complexes containing diverse ligands from an electronic and topological point of view have been analysed by means of 95Mo NMR in solution with the purpose of using this technique as a tool to study their coordination chemistry and reactivity. The relationship between the electronic density on the metal tuned by the electron-donor ability of the coordinated ligands and the 95Mo chemical shift has been proved for mono- and bimetallic complexes showing a hexa- or hepta-coordination around the metal centre. The different origins of the signal broadening (associated either to the symmetry of the metallic polyhedron or to the presence of isomers or to the ligand de-coordination) have been also considered to rationalise the obtained data. Copyright © 2009 John Wiley & Sons, Ltd.

NMR • 95Mo • chemical shift • transverse relaxation • oxomolybdenum(VI) complexes • electron-donor ligands • electron-withdrawing ligands • polydentated ligands

Digital Object Identifier (DOI)

Thursday, April 02, 2009

J Phys Chem C and B, vol. 113, Issue 13 and 14

Designing Amphotropic Smectic Liquid Crystals Based on Phosphonium Salts for Partial Ordering of Solutes as Monitored by NMR Spectroscopy

Astghik A. Shahkhatuni, Kefeng Ma and Richard G. Weiss*
Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227
J. Phys. Chem. B, 2009, 113 (13), pp 4209–4217

Abstract: The ordering parameters of selected solutes from NMR spectroscopic measurements have been assessed in the thermotropic and amphotropic smectic liquid-crystalline phases of a wide structural range of phosphonium salts with three equivalent long n-alkyl chains, one shorter chain, and various anions. The nature of the added liquids that convert the salts to amphotropic phasesalcohols and other small organic moleculesand their concentrations have been determine. These factors are correlated with the NMR-derived parameters in order to understand how the phases can be optimized to maximize information about the solutes. The various salts cover a range of liquid crystallinity from −40 to 100 °C. The phosphonium salts are easily aligned in the strong magnetic fields of the spectrometers. In several of the systems, a coexistence of isotropic and anisotropic phases is observed over a wide range of temperatures. The order parameters of the amphotropic liquid-crystalline phases vary from high to very low values, and some of the systems provide good spectral resolution for the solute molecules. Also, structural and orientational parameters of a model molecule, 13C-enriched acetonitrile, have been calculated in various systems in order to evaluate more precisely the applicability of the host systems for determining solute structures by NMR. The results, in toto, indicate that several of the phosphonium salts are very promising as hosts to determine solute structures.

51V NMR Chemical Shifts Calculated from QM/MM Models of Peroxo Forms of Vanadium Haloperoxidases

K. R. Geethalakshmi, Mark P. Waller, Walter Thiel and Michael Bhl*
Max-Planck-Institut fr Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mlheim an der Ruhr, Germany, and School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, U.K.
J. Phys. Chem. B, 2009, 113 (13), pp 4456–4465

Abstract: QM/MM models of the peroxo forms of vanadium-containing haloperoxidases (VHPOs) are critically assessed in terms of active site geometries, hydrogen bonds within the active site, isotropic and anisotropic 51V NMR chemical shifts, and TD-DFT excitation energies. The geometric stability within the active site of the protein is comparable to the respective native forms, as indicated by low standard deviations in bond lengths across a number of local minima sampled along MD trajectories. There is a significant calculated upfield shift in δ(51V) upon formation of the peroxo from the respective native forms for both the vanadium-containing chloroperoxidase (VCPO) and vanadium-containing bromoperoxidase (VBPO) models, which is in qualitative agreement with 51V NMR experiments of VBPO in solution. The models show appreciable differences between the anisotropic chemical shifts of the different protonation states of the peroxo form of VHPO. The most likely candidates for the peroxo forms of the VHPO enzymes appear to be unprotonated or have a single proton on either of the equatorial oxygen ligands, based on QM/MM modeling in combination with X-ray, 51V NMR, and UV−vis data.

Calcium-Induced Formation of Subdomains in Phosphatidylethanolamine−Phosphatidylglycerol Bilayers: A Combined DSC, 31P NMR, and AFM Study

Laura Picas, M. Teresa Montero, Antoni Morros§, Miquel E. Cabaas, Bastien Seantier#, Pierre-Emmanuel Milhiet# and Jordi Hernndez-Borrell*
J. Phys. Chem. B, 2009, 113 (14), pp 4648–4655

Abstract: We study the effect of Ca2+ on the lateral segregation of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) (3:1, mol/mol). Supported lipid bilayers (SLBs) were observed by atomic force microscopy (AFM). Since SLBs are formed from liposomes of POPE:POPG, we examined the effect of calcium on these suspensions by differential scanning calorimetry (DSC) and 31P nuclear magnetic resonance spectroscopy (31P NMR). AFM images revealed the existence of two separated phases, the higher showing a region with protruding subdomains. Force spectroscopy (FS) was applied to clarify the nature of each phase. The values of breakthrough force (Fy), adhesion force (Fadh), and height extracted from the force curves were assigned to the corresponding gel (Lβ) and fluid (Lα) phase. The endotherms obtained by DSC suggest that, in the presence of Ca2+, phase separation already exists in the suspensions of POPE:POPG used to form SLBs. Due to the temperature changes applied during preparation of SLBs a 31P NMR study was performed to assess the lamellar nature of the samples before spreading them onto mica. With in situ AFM experiments we showed that the binding of Ca2+ to POPG-enriched domains only induces the formation of subdomains in the Lβ phase.

Alumina-Promoted Sulfated Mesoporous Zirconia Catalysts

Chi-Chau Hwang and Chung-Yuan Mou*
J. Phys. Chem. C, 2009, 113 (13), pp 5212–5221
Abstract: Mesoporous zirconia, hydrothermally synthesized from surfactant templating, was directly impregnated with aluminum sulfate to give the acidic Al-promoted sulfated mesoporous zirconia (AS/MP-ZrO2). A series of AS/MP-ZrO2 catalysts were characterized by Brunauer−Emmett−Teller and X-ray diffraction for their texture properties and crystalline phases. The catalytic behavior for n-butane isomerization was found to be strongly promoted at relatively low temperature by the addition of a proper amount of alumina as a promoter. 27Al S.S. magic-angle spinning nuclear magnetic resonance results indicated that Zr atoms were partially substituted by Al, giving a considerable increased concentration of Brønsted acids. X-ray photoelectron spectroscopy and diffuse-reflectance infrared Fourier-transformed spectra (DRIFT) analysis were then employed to identify and relatively quantify properties of acid sites on catalyst surface. A balanced distribution of acid sites strength was proven to prevent a catalyst from deactivating rapidly due to coke formation on the catalyst surface. A small concentration of olefins formed by oxidation of n-butane and proven to be key intermediates during n-butane isomerization on sulfated zirconia was found by the Baeyer test. Electron paramagnetic resonance and in situ DRIFT results show that this occurs via oxidative dehydrogenation of butane by the sulfate groups to form butene which leads to butyl carbenium species for skeleton isomerization. A modified biomolecular mechanism for the isomerization of butane is examined to explain the catalysis results.