Double Resonance NMR Spectroscopy Research Articles

Precise Structural and Dynamical Details in Zeolites Revealed by Coupling-Edited 1H-17O Double Resonance NMR Spectroscopy.

Despite the extensive industrial and research interests in zeolites, their intrinsic catalytic nature is not fully understood due to the complexity of the hydroxyl-aluminum moieties. 17O NMR would provide irreplaceable opportunities for much-needed fine structural determination given the ubiquitous presence of oxygen atoms in nearly all species; however, the low sensitivity and quadrupolar nature of oxygen-17 make its NMR spectroscopic elucidation challenging. Here, we show that state-of-the-art double resonance solid-state NMR techniques have been combined with spectral editing methods based on scalar (through-bond) and dipolar (through-space) couplings, which allowed us to address the subtle protonic structures in zeolites. Notably, the often-neglected and undesired second-order quadrupolar-dipolar cross-term interaction ("2nd-QD interaction") can actually be exploited and can help gain invaluable information. Eventually, a comprehensive set of 1H-17O/1H-27Al double resonance NMR with J-/D-coupling spectral editing techniques have been designed in this work and enabled us to reveal atomic-scale precise structural and dynamical details in zeolites including: 1) The jump rate of the bridging acid site (BAS) proton is relatively low, i.e., far less than 100 s-1 at room temperature. 2) The Al-OH groups with 1H chemical shift at 2.6-2.8 ppm, at least for nonseverely dealuminated H-ZSM-5 catalysts, exhibit a rigid bridging environment similar to that of BAS. 3) The Si-OH groups at 2.0 ppm are not hydrogen bonded and undergo fast cone-rotational motion. The results in this study predict the 2nd-QD interaction to be universal for any rigid -17O-H environment, such as those in metal oxide surfaces or biomaterials.

Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by Dynamic Nuclear Polarization-Enhanced Double-Resonance NMR Spectroscopy.

The characterization of nanometer-scale interactions between carbon-containing substrates and alumina surfaces is of paramount importance to industrial and academic catalysis applications, but it is also very challenging. Here, we demonstrate that dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) allows the unambiguous description of the coordination geometries and conformations of the substrates at the alumina surface through high-resolution measurements of 13C-27Al distances. We apply this new technique to elucidate the molecular-level geometry of 13C-enriched methionine and natural abundance poly(vinyl alcohol) adsorbed on γ-Al2O3-supported Pd catalysts, and we support these results with element-specific X-ray absorption near-edge measurements. This work clearly demonstrates a surprising bimodal coordination of methionine at the Pd-Al2O3 interface.

Toward a biorelevant structure of protein kinase C bound modulators: design, synthesis, and evaluation of labeled bryostatin analogues for analysis with rotational echo double resonance NMR spectroscopy.

Protein kinase C (PKC) modulators are currently of great importance in preclinical and clinical studies directed at cancer, immunotherapy, HIV eradication, and Alzheimer's disease. However, the bound conformation of PKC modulators in a membrane environment is not known. Rotational echo double resonance (REDOR) NMR spectroscopy could uniquely address this challenge. However, REDOR NMR requires strategically labeled, high affinity ligands to determine interlabel distances from which the conformation of the bound ligand in the PKC-ligand complex could be identified. Here we report the first computer-guided design and syntheses of three bryostatin analogues strategically labeled for REDOR NMR analysis. Extensive computer analyses of energetically accessible analogue conformations suggested preferred labeling sites for the identification of the PKC-bound conformers. Significantly, three labeled analogues were synthesized, and, as required for REDOR analysis, all proved highly potent with PKC affinities (∼1 nM) on par with bryostatin. These potent and strategically labeled bryostatin analogues are new structural leads and provide the necessary starting point for projected efforts to determine the PKC-bound conformation of such analogues in a membrane environment, as needed to design new PKC modulators and understand PKC-ligand-membrane structure and dynamics.

Solid-State MAS NMR Studies of Brønsted Acid Sites in Zeolite H-Mordenite

(17)O-(1)H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross-polarization magic angle spinning (CPMAS) NMR, CP rotational-echo double resonance (CP-REDOR) NMR, and heteronuclear correlation (HETCOR) NMR spectroscopy to distinguish between Brønsted acid sites with different O-H distances. The accessibility of the various Brønsted acid sites was quantified by adsorbing the basic probe molecule trimethylphosphine in known amounts. On the basis of these experiments, locations of different Brønsted acid sites in H-Mordenite (H-MOR) were proposed. The use of (17)O chemical shift correlations to help assign sites is discussed.

“Grafting From” Polymerization of Vinylidene Fluoride (VDF) from Silica to Achieve Original Silica–PVDF Core–Shells

A new method of surface modification based on the “grafting from” polymerization process enabled to prepare original silica nanoparticles covered with PDVF “hair”. This strategy involved two steps: (i) the radical addition of 1,4-diiodoperfluorobutane initiated by tert-butylperoxypivalate onto the double bonds of silica nanoparticles (specific area of 150 m2 g–1) led to silica that bore C4F8I end-groups on its surface, and (ii) its use as original macrochain transfer agent in the “grafting from” polymerization of vinylidene fluoride (VDF) initiated by bis(4-tert-butylcyclohexyl) peroxydicarbonate. The characterizations of both modified silica were achieved by elemental analysis, 1H and 19F magic angle spinning solid state rotational-echo double resonance NMR spectroscopy, thermogravimetry, and water contact angle (WCA) assessments. The grafting efficiency was confirmed by three features: (i) the increase of weight percentage of fluorine atom (that ranged from 0 for the starting unsaturated silica to 2.50% for silica-bearing −C4F8I end-group and to 3.32% for original silica–PVDF core–shells), (ii) a high thermostability (that reached more than 400 °C under air), and (iii) a good hydrophobicity: the WCA values were ranging from 84° to 109° and to 126° for vinyl silica, for silica coated with −C4F8I, and for PVDF-covered silica nanoparticles, respectively.

Characterization of Reactive Sites in Supported Catalysts by 51V/15N Rotational Echo Double Resonance NMR Spectroscopy: Formation of Phenylimido Groups at Surface-Bound Oxovanadium Sites

Silica-supported oxovanadium groups were reacted in a gas-solid reaction with aniline at 175 degrees C. The reaction was clean as monitored in situ by UV-vis spectroscopy and resulted in the elimination of water as the principal product of the reaction and the disappearance of the terminal V=O stretch in the Raman spectrum. 15N MAS solid-state NMR spectroscopy showed only a single nitrogen-containing species on the surface. Proton-dephased 15N NMR showed only weak attenuation of its intensity, indicating that there are no protons directly bonded to the nitrogen. The formation of a vanadium-imido covalent bond was characterized by 51V/15N rotational echo double resonance NMR spectroscopy where the quadrupolar 51V nucleus was monitored and the spin-1/2 15N nucleus was dephased.

Measurement of 15N– 1H bond lengths by rotational-echo double-resonance NMR spectroscopy

A method based on rotational-echo double-resonance (REDOR) NMR is proposed to measure 1H– 15N bond lengths under magic-angle-spinning. Protons are selectively polarized and evolve under the frequency-switched Lee–Goldburg (FSLG) homonuclear decoupling, while two 15N π pulses are applied to restore 1H– 15N dipolar interactions. Such a method relies on the 1H– 15N dipolar couplings and requires no knowledge of their chemical shift tensors allowing one to precisely extract their couplings. This method could be applicable for measuring relatively long distances between amide protons and other nucleus such as 13C if applying the π pulses to 13C.

Determination of intermolecular distance for a model peptide of Bombyx mori silk fibroin, GAGAG, with rotational echo double resonance.

Rotational echo double resonance NMR spectroscopy is applied for the determination of the distance of intermolecular chains of pentapeptide, GAGAG (G: Gly, A: Ala), a model typical of the crystalline domain in Bombyx mori silk fibroin. 1:4 mixture of G[1-(13)C]AGAG and GAG[(15)N]AG with antiparallel beta-sheet structure was used to determine the distance of intermolecular hydrogen bonding between adjacent molecules within pleated sheet and the (13)C-(15)N interatomic distance was determined to be 4.3 A. On the other hand, 1:4 mixture of GAG[1-(13)C]AG and GAG[(15)N]AG gave information on the interpleated sheet arrangement. When we assumed the same distances between two interpleated sheets, the distance was calculated to be 5.3 A and the angle (15)N-(13)C-(15)N was 180 degrees.

Modern solid state double resonance NMR strategies for the structural characterization of adsorbate complexes involved in the MTG process

The reaction pathways in the methanol to gasoline conversion process are still not well understood. Although recent years have witnessed remarkable progress in recent years in the in situ characterization of these reactions, the nature of the bonding of the intermediates to the Bronsted acid sites of the zeolite catalyst is still the subject of intense debate. Our approach is to study the geometry of the binding of the methanol molecule to the zeolite surface using modern double resonance NMR spectroscopy. As model systems we chose the host/guest complexes formed in the systems zeolite NaX loaded with 13CH3I and zeolite HZSM-5 loaded with 13CH3OH, treated at various temperatures. By exploring the dipolar coupling strength between 13C and 27Al or 1H and 27Al using 1H–{27Al} and 13C–{27Al} rotational echo adiabatic passage double resonance (REAPDOR) NMR and simulation of the resulting dipolar evolution curves we have been able to determine carbon–aluminium and hydrogen–aluminium internuclear distances and thus to identify the nature of the adsorbate–zeolite complexes formed as a function of the processing conditions.

Short and Medium Range Order in Sodium Aluminoborate Glasses. 2. Site Connectivities and Cation Distributions Studied by Rotational Echo Double Resonance NMR Spectroscopy

A quantitative NMR strategy is developed for the study of the network connectivity (intermediate range structure) and the cation distribution in the sodium aluminoborate glass system. The strategy is based on the analysis of rotational echo double resonance (REDOR) spectroscopy applied to the glasses and the crystalline model compounds Li6Al2(BO3)4, NaBO2, and Na2B4O7. The heterodipolar multispin interaction between the quadrupolar spin systems 27Al ↔ 11B and 23Na ↔ 11B is analyzed in terms of approximate second moments, which are extracted from REDOR data measured at short dipolar evolution times. On the basis of 27Al{11B} and 11B{27Al}-REDOR results, the framework connectivity distribution is extracted and compared to statistical and preferential bonding distribution scenarios, respectively. The sodium aluminoborate system is characterized by a large degree of bonding regularity in the framework. Four-coordinated aluminum is preferentially linked to BO3/2 and BO2/2O- units and vice versa, while linking ...

Structural models for covalent non-oxidic glasses: Atomic distribution and local order in CdGeAs2-xPx glasses studied by use of 31P and 113Cd spin-echo and 31P-113Cd spin-echo double-resonance NMR spectroscopy.

The structure of glasses in the system ${\mathrm{CdGeAs}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ is discussed on the basis of complementary solid-state NMR experiments, including $^{31}\mathrm{P}$ and $^{113}\mathrm{Cd}$ magic-angle spinning (MAS) and spin-echo techniques, as well as $^{31}\mathrm{\ensuremath{-}}^{113}$Cd spin-echo double resonance (SEDOR) NMR. Computer simulations of atomic distribution models and experimental studies on crystalline model systems are used to quantify the results. The analysis reveals striking differences in the short-range order between the glassy and the stoichiometrically analogous crystalline materials. The structure of glasses in the system ${\mathrm{CdGeAs}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ is characterized by the presence of a substantial fraction of homopolar pnictogen-pnictogen bonds and by a distribution of cadmium relative to phosphorus that is close to random. These results lend credence to the bond-switching model invoked for the structural description of amorphous tetrahedral semiconductors.

Structural models for covalent non-oxidic glasses. Atomic distribution and local order in glassy CdGeP2 studied by phosphorus-31 and cadmium-113 MAS, spin-echo, and phosphorus-31-cadmium-113 spin-echo double resonance NMR spectroscopy

Structural models for covalent non-oxidic glasses. Atomic distribution and local order in glassy CdGeP2 studied by phosphorus-31 and cadmium-113 MAS, spin-echo, and phosphorus-31-cadmium-113 spin-echo double resonance NMR spectroscopy

Preparation, crystal and molecular structure of, and NMR parameters for, the exopolyhedral heterocyclic platinaundecaborane [μ-2,7-(SCSNEt 2)-7-(PMe 2Ph)- NIDO-7-PtB 10H 11

The compound [μ-2,7-(SCSNEt 2)-7-(PMe 2Ph)- nido-7-PtB 10H 11] has been obtained in a yield of 52% from the reaction of [7,7-(PMe 2Ph)- nido-7-PtB 10H 12] and [AuBr 2(S 2CNEt 2)], and identified by single crystal X-ray diffraction analysis and multi-element single and double resonance NMR spectroscopy. The yellow-orange compound crystallizes in the monoclinic space group P2 1/ n with a 1179.2(2), b = 1244.9(5), c = 1641.4(2) pm, β = 95.45(1)°, Z = 4, and the structure ( R 0.0209, R w = 0.0211 for 3719 observed reflections) is that of a nido-7-platinaundecaborane with an exopolyhedral N,N-diethyldithiocarbamate ligand bridging the Pt(7) and B(2) positions to give a -Pt-B-C-S- five-membered ring. The tetrahapto platinum-to-borane bonding has a considerable twist distortion relative to other nido-7-platinaundecaboranes which do not possess this cyclic feature. The NMR parameters exhibit no anomalies and are consistent with the crystal molecular structure. A plot of δ( 11B) vs δ( 1H) for directly bound exo-terminal hydrogen atoms shows good correlation with the slope 16 : 1.

Azabicyclo chemistry—V : Synthesis and stereochemistry of 7-substituted cis-octahydroindoles prepared via a nitrenium intermediate

As a potential route to 9-functionalized-2-azabicyclo[3.3.1]nonanes we have investigated the silver nitrate catalyzed solvolysis of N-chloro-N-methyl-2-cyclohexene-1-ethanamine ( 4a) in methanol. By this route we obtained 1-methyl-7(a)-methoxy- cis-octahydroindole ( 5a), 1-methyl-7-chloro- cis-octahydroindole ( 6a), 2-methy ( 8a) in the approximate ratio of 75:1:3:23, respectively. The structure proofs of 5a and 8a were accomplished by chemical interrelating the two and by double resonance NMR spectroscopy, while the structures of 6a and 7a were assigned by mass spectroscopy. Appropriate deuterium studies substantiated the mass spectral fragmentation pathways. Mechanistically we favor the formation of the products by invoking the intermediacy of a nitrenium ion.