55 results on '"Darren H. Brouwer"'
Search Results
2. Resolving the discrepancies in reported 13C solid state NMR chemical shifts for native celluloses
- Author
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Darren H. Brouwer and Janelle G. Mikolajewski
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Polymers and Plastics - Abstract
A survey of the literature reporting solid-state 13C NMR spectra of native celluloses reveals inconsistencies in the reported 13C chemical shifts for cellulose Iα and Iβ allomorphs. With reported chemical shifts varying by up to 2 ppm, it is not clear what the correct chemical shifts actually are. Since reliable experimental data are important to future work, such as quantum chemical calculations of NMR parameters or identification of cellulose phases in complex cellulosic materials, it is important that the correct experimental chemical shifts be established with confidence. Through a process of digitization of previously reported spectra and careful consideration of how chemical shifts were referenced in the past, it has been possible to correct previously reported spectra of cellulose Iα and Iβ, putting them on the same chemical shift scale and establishing a definitive set of correctly referenced 13C chemical shifts for cellulose Iα and Iβ allomorphs. In addition, 1D and 2D 13C NMR experiments were carried out on a cellulose Iα-rich bacterial cellulose sample (with 25% 13C enrichment), providing additional evidence for these 13C chemical shifts and a new peak assignment of the 13C signals to the glucose units in cellulose Iα. This work resolves many of inconsistencies in the cellulose solid-state NMR literature and provides a definitive set of 13C chemical shifts that will be important for future work.
- Published
- 2022
3. A combined solid-state NMR and quantum chemical calculation study of hydrogen bonding in two forms of α-d-glucose
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Darren H. Brouwer and Janelle G. Mikolajewski
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Nuclear and High Energy Physics ,Radiation ,General Chemistry ,Instrumentation - Abstract
Hydrogen bonding plays an important role in the structure and function of a wide range of materials. Solid-state
- Published
- 2022
4. A Comprehensive Collection of Solid-State 31p Nmr Spectra of Aluminophosphate Zeolites
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Darren H. Brouwer and Alexa L. White
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History ,Polymers and Plastics ,Mechanics of Materials ,General Materials Science ,General Chemistry ,Business and International Management ,Condensed Matter Physics ,Industrial and Manufacturing Engineering - Published
- 2022
5. NMR crystallography of zeolites: How far can we go without diffraction data?
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Jared Van Huizen and Darren H. Brouwer
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Diffraction ,010405 organic chemistry ,Chemistry ,General Chemistry ,Nuclear magnetic resonance spectroscopy ,010402 general chemistry ,01 natural sciences ,Correlation spectrum ,0104 chemical sciences ,Crystallography ,Solid-state nuclear magnetic resonance ,X-ray crystallography ,General Materials Science ,Density functional theory ,Porous medium ,Zeolite - Abstract
Nuclear magnetic resonance (NMR) crystallography-an approach to structure determination that seeks to integrate solid-state NMR spectroscopy, diffraction, and computation methods-has emerged as an effective strategy to determine structures of difficult-to-characterize materials, including zeolites and related network materials. This paper explores how far it is possible to go in determining the structure of a zeolite framework from a minimal amount of input information derived only from solid-state NMR spectroscopy. It is shown that the framework structure of the fluoride-containing and tetramethylammonium-templated octadecasil clathrasil material can be solved from the 1D 29 Si NMR spectrum and a single 2D 29 Si NMR correlation spectrum alone, without the space group and unit cell parameters normally obtained from diffraction data. The resulting NMR-solved structure is in excellent agreement with the structures determined previously by diffraction methods. It is anticipated that NMR crystallography strategies like this will be useful for structure determination of other materials, which cannot be solved from diffraction methods alone.
- Published
- 2018
6. Quantifying Site-Specific Proton Dynamics in Phosphate Solid Acids by 1H Double Quantum NMR Spectroscopy
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Gillian R. Goward, Darren H. Brouwer, and Gabrielle Foran
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Condensed Matter::Quantum Gases ,Proton ,Chemistry ,Analytical chemistry ,02 engineering and technology ,Crystal structure ,Nuclear magnetic resonance spectroscopy ,Conductivity ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Dipole ,General Energy ,Residual dipolar coupling ,Magic angle spinning ,Physical and Theoretical Chemistry ,0210 nano-technology ,Magnetic dipole–dipole interaction - Abstract
Solid-state magic angle spinning (MAS) NMR was used to investigate changes in proton dynamics in phosphate solid acids that exhibited increased proton conductivity between room temperature and 110 °C. Double quantum dipolar recoupling methods were used to quantify site-specific changes in proton–proton dipolar coupling as a function of temperature. The static dipolar coupling and motionally induced changes to it were compared. This was accomplished by calculating (from crystal structures) and measuring (from the initial parts of the DQ recoupling curves) the root-sum-square of the dipolar coupling, a geometry-independent measure of dipolar coupling strength referred to as the “apparent dipolar coupling”, Dapp. The analysis of KH2PO4 and RbH2PO4 showed that the experimentally determined apparent dipolar couplings were reduced from the calculated values at increased temperatures in dynamic systems. Higher proton conductivity was associated with greater reduction of the apparent dipolar coupling as measured ...
- Published
- 2017
7. Structure determination of a partially ordered layered silicate material with an NMR crystallography approach
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Darren H. Brouwer, Jared Van Huizen, Nicholas Van Huizen, Sylvian Cadars, Kathryn Hotke, Conditions Extrêmes et Matériaux : Haute Température et Irradiation (CEMHTI), Université d'Orléans (UO)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), and Centre National de la Recherche Scientifique (CNRS)-Institut de Chimie du CNRS (INC)-Université d'Orléans (UO)
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Diffraction ,Chemistry ,02 engineering and technology ,Crystal structure ,Nuclear magnetic resonance crystallography ,Nuclear magnetic resonance spectroscopy ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Silicate ,0104 chemical sciences ,Inorganic Chemistry ,chemistry.chemical_compound ,Crystallography ,Crystallinity ,Silanol ,Solid-state nuclear magnetic resonance ,[PHYS.COND.CM-MS]Physics [physics]/Condensed Matter [cond-mat]/Materials Science [cond-mat.mtrl-sci] ,Materials Chemistry ,Physical and Theoretical Chemistry ,0210 nano-technology ,ComputingMilieux_MISCELLANEOUS - Abstract
Structure determination of layered materials can present challenges for conventional diffraction methods due to the fact that such materials often lack full three-dimensional periodicity since adjacent layers may not stack in an orderly and regular fashion. In such cases, NMR crystallography strategies involving a combination of solid-state NMR spectroscopy, powder X-ray diffraction, and computational chemistry methods can often reveal structural details that cannot be acquired from diffraction alone. We present here the structure determination of a surfactant-templated layered silicate material that lacks full three-dimensional crystallinity using such an NMR crystallography approach. Through a combination of powder X-ray diffraction and advanced 29Si solid-state NMR spectroscopy, it is revealed that the structure of the silicate layer of this layered silicate material templated with cetyltrimethylammonium surfactant cations is isostructural with the silicate layer of a previously reported material referred to as ilerite, octosilicate, or RUB-18. High-field 1H NMR spectroscopy reveals differences between the materials in terms of the ordering of silanol groups on the surfaces of the layers, as well as the contents of the inter-layer space.
- Published
- 2017
8. 19F Double Quantum NMR Spectroscopy: A Tool for Probing Dynamics in Proton-Conducting Fluorinated Polymer Materials
- Author
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Gillian R. Goward, Darren H. Brouwer, and Z. Blossom Yan
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chemistry.chemical_classification ,Polymers and Plastics ,Proton ,Carbon-13 NMR satellite ,Chemical shift ,Organic Chemistry ,Analytical chemistry ,02 engineering and technology ,Nuclear magnetic resonance spectroscopy ,Polymer ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Inorganic Chemistry ,chemistry.chemical_compound ,chemistry ,Chemical physics ,Nafion ,Materials Chemistry ,Side chain ,0210 nano-technology ,Ionomer - Abstract
Solid-state NMR spectroscopy is an important technique for probing the structure and local dynamics of materials at the molecular level. For example, 1H double quantum (DQ) NMR is a well-established probe of local dynamics. Here, this concept has been extended to characterize fluorinated ionomer materials for the first time. 19F DQ recoupling NMR experiments are applied to investigate the site-specific local dynamics of the polymer electrolyte material, Nafion 117, under various conditions with respect to temperature and hydration level. The initial rise of the normalized double quantum (nDQ) build-up curves generated from NMR dipolar recoupling experiments is compared as a measure of the motionally averaged 19F–19F dipolar couplings for spectroscopically resolved domains of the polymers. Since the side-chain and backbone fluorines can be distinguished by their chemical shifts, it was possible to demonstrate a difference between the side-chain and backbone local dynamics profiles. The side chain is shown ...
- Published
- 2016
9. High Field Solid-State NMR Spectroscopy Investigation of 15N-Labeled Rosette Nanotubes: Hydrogen Bond Network and Channel-Bound Water
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Takeshi Yamazaki, Souhaila Bouatra, Jae-Young Cho, Zhimin Yan, Darren H. Brouwer, Hicham Fenniri, Mounir El Bakkari, and Grigory Tikhomirov
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Nanotube ,Nanostructure ,Hydrogen bond ,Chemistry ,Analytical chemistry ,02 engineering and technology ,General Chemistry ,Nuclear magnetic resonance spectroscopy ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Biochemistry ,Catalysis ,0104 chemical sciences ,Condensed Matter::Materials Science ,Crystallography ,Colloid and Surface Chemistry ,Solid-state nuclear magnetic resonance ,Molecule ,Bound water ,0210 nano-technology ,Spectroscopy - Abstract
^(15)N-labeled rosette nanotubes were synthesized and investigated using high-field solid-state NMR spectroscopy, X-ray diffraction, atomic force microscopy, and electron microscopy. The results established the H-bond network involved in the self-assembly of the nanostructure as well as bound water molecules in the nanotube’s channel.
- Published
- 2016
10. Minimizing the effects of RF inhomogeneity and phase transients allows resolution of two peaks in the 1H CRAMPS NMR spectrum of adamantane
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Matthew Horvath and Darren H. Brouwer
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Nuclear and High Energy Physics ,Radiation ,Chemistry ,Adamantane ,Analytical chemistry ,Phase (waves) ,General Chemistry ,Nuclear magnetic resonance spectroscopy ,Molecular physics ,Spectral line ,Homonuclear molecule ,chemistry.chemical_compound ,Solid-state nuclear magnetic resonance ,Proton NMR ,Instrumentation ,Decoupling (electronics) - Abstract
One of the limiting factors to achieving highly resolved (1)H NMR spectra with (1)H homonuclear decoupling sequences is imperfections in the applied radiofrequency (RF) pulses, most notably phase transients and RF inhomogeneity. Through a series of simulations and solid-state NMR experiments, it is demonstrated that the combined effects of phase transients and RF inhomogeneity can be minimized by a combination of (i) restricting the sample to small volume of the rotor, (ii) by employing a super-cycled version of the DUMBO decoupling sequence, and (iii) by carefully adjusting the probe tuning such that the asymmetric component of phase transients is minimized. Under these optimal conditions, it was possible to clearly resolve two signals in the (1)H CRAMPS NMR spectrum of adamantane arising from the CH and CH2 protons in the molecule. It is proposed that adamantane could be a very useful setup sample for (1)H CRAMPS NMR as the two peaks are only resolved when the effects of RF inhomogeneity and phase transients are minimized.
- Published
- 2015
11. Solid-state 29Si NMR spectra of pure silica zeolites for the International Zeolite Association Database of Zeolite Structures
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Megan P.Y. Sun, Christian Baerlocher, Sofia Mesa, Claire A. Semelhago, Carraugh C. Brouwer, Erin E. Steckley, Janelle G. Mikolajewski, and Darren H. Brouwer
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Materials science ,Database ,Chemical shift ,Resolution (electron density) ,Solid-state ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,computer.software_genre ,01 natural sciences ,Spectral line ,0104 chemical sciences ,NMR spectra database ,Bond length ,Molecular geometry ,Mechanics of Materials ,General Materials Science ,0210 nano-technology ,Zeolite ,computer - Abstract
With a view to extending the comprehensiveness of the International Zeolite Association Database of Zeolite Structures, this paper reports initial efforts to incorporate solid-state nuclear magnetic resonance (NMR) spectra in the database. A collection of 51 29Si solid-state NMR spectra of pure silica zeolites with 39 different framework types, gleaned from the published scientific literature over the past four decades, is presented here. High resolution 29Si solid-state NMR spectra showing crystallographic resolution of the Si sites were collected, digitized, analyzed, and incorporated into the online Database of Zeolite Structures and can be freely viewed online in an interactive fashion. In addition, with such a large dataset of 29Si solid-state NMR spectra, various correlations between 29Si chemical shifts and geometric parameters describing the local structure around the Si sites were evaluated, showing that the best correlation results when both Si–O–Si bond angles and Si–O bond lengths are considered. It is anticipated that this addition of solid-state NMR spectra to the Database of Zeolite Structures will be of great interest and usefulness to the international community of zeolite scientists.
- Published
- 2020
12. A simulated annealing approach for solving zeolite crystal structures from two-dimensional NMR correlation spectra
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Darren H. Brouwer and Matthew Horvath
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Diffraction ,Nuclear and High Energy Physics ,Radiation ,Chemistry ,General Chemistry ,Nuclear magnetic resonance crystallography ,Crystal structure ,NMR spectra database ,Crystallography ,Ferrierite ,Solid-state nuclear magnetic resonance ,Simulated annealing ,Physical chemistry ,Zeolite ,Instrumentation - Abstract
An improved NMR crystallography strategy is presented for determining the structures of network materials such as zeolites from just a single two-dimensional (2D) NMR correlation spectrum that probes nearest-neighbor interactions, combined with the unit cell parameters and space group information measured in a diffraction experiment. The correlation information contained within a 2D spectrum is converted into a “connectivity matrix” which is incorporated into a cost function whose minimum is searched for using a simulated annealing algorithm. The algorithm was extensively tested on over 150 zeolite frameworks from the International Zeolite Association database of zeolite structures and shown to be very robust and efficient in reconstructing the structures from connectivity information. The structure determination of the pure silica zeolites ITQ-4, Ferrierite, and Sigma-2 from experimental 2D 29 Si double-quantum NMR spectra and powder X-ray diffraction data using this improved approach is also presented.
- Published
- 2015
13. The use of6Li{7Li}-REDOR NMR spectroscopy to compare the ionic conductivities of solid-state lithium ion electrolytes
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Gillian R. Goward, T. L. Spencer, N. W. Plagos, and Darren H. Brouwer
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chemistry ,Phase (matter) ,Analytical chemistry ,General Physics and Astronomy ,Ionic bonding ,Ionic conductivity ,chemistry.chemical_element ,Lithium ,Electrolyte ,Nuclear magnetic resonance spectroscopy ,Physical and Theoretical Chemistry ,Ion transporter ,Ion - Abstract
Garnet-like solid-state electrolyte materials for lithium ion batteries are promising replacements for the currently-used liquid electrolytes. This work compares the temperature dependent Li(+) ion hopping rate in Li6BaLa2M2O12 (M = Ta, Nb) using solid-state (6)Li{(7)Li}-REDOR NMR. The slope of the (6)Li{(7)Li}-REDOR curve is highly temperature dependent in these two phases, and a comparison of the changes of the REDOR slopes as a function of temperature has been used to evaluate the Li(+) ion dynamics. Our results indicate that the Nb phase has a higher overall ionic conductivity in the range of 247 K to 350 K, as well as a higher activation energy for lithium ion hopping than the Ta counterpart. For appropriate relative timescales of the dipolar couplings and ion transport processes, this is shown to be a facile method to compare ion dynamics among similar structures.
- Published
- 2014
14. Structure solution of network materials by solid-state NMR without knowledge of the crystallographic space group
- Author
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Darren H. Brouwer
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Diffraction ,Nuclear and High Energy Physics ,Radiation ,Chemistry ,Structure (category theory) ,Graph theory ,General Chemistry ,Function (mathematics) ,Space (mathematics) ,Symmetry (physics) ,Silicate ,Crystallography ,chemistry.chemical_compound ,Solid-state nuclear magnetic resonance ,Instrumentation - Abstract
An algorithm is presented for solving the structures of silicate network materials such as zeolites or layered silicates from solid-state 29 Si double-quantum NMR data for situations in which the crystallographic space group is not known. The algorithm is explained and illustrated in detail using a hypothetical two-dimensional network structure as a working example. The algorithm involves an atom-by-atom structure building process in which candidate partial structures are evaluated according to their agreement with SiOSi connectivity information, symmetry restraints, and fits to 29 Si double quantum NMR curves followed by minimization of a cost function that incorporates connectivity, symmetry, and quality of fit to the double quantum curves. The two-dimensional network material is successfully reconstructed from hypothetical NMR data that can be reasonably expected to be obtained for real samples. This advance in “NMR crystallography” is expected to be important for structure determination of partially ordered silicate materials for which diffraction provides very limited structural information.
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- 2013
15. From solid-state NMR to crystal structures through combinatorial tiling theory
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Janelle Vanderhout, Chelsey Hurst, Brydon Eastman, and Darren H. Brouwer
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Inorganic Chemistry ,Crystallography ,Materials science ,Solid-state nuclear magnetic resonance ,Structural Biology ,General Materials Science ,Crystal structure ,Physical and Theoretical Chemistry ,Condensed Matter Physics ,Biochemistry - Published
- 2018
16. Probing the Local Structure of Pure Ionic Liquid Salts with Solid- and Liquid-State NMR
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Darren H. Brouwer, Peter G. Gordon, and John A. Ripmeester
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Chemistry ,Vapor pressure ,Chemical shift ,Ab initio ,Analytical chemistry ,Infrared spectroscopy ,Nuclear magnetic resonance spectroscopy ,computational chemistry ,Atomic and Molecular Physics, and Optics ,ionic liquids ,symbols.namesake ,chemistry.chemical_compound ,NMR spectroscopy ,Chemical physics ,solid-state structures ,Ionic liquid ,symbols ,Physical and Theoretical Chemistry ,Raman spectroscopy ,Anisotropy ,chemical shifts - Abstract
Room-temperature ionic liquids (RTILs) are gaining increasing interest and are considered part of the green chemistry paradigm due to their negligible vapour pressure and ease of recycling. Evidence of liquid-state order, observed by IR and Raman spectroscopy, diffraction studies, and simulated by ab initio methods, has been reported in the literature. Here, quadrupolar nuclei are used as NMR probes to extract information about the solid and possible residual order in the liquid state of RTILs. To this end, the anisotropic nature and field dependence of quadrupolar and chemical shift interactions are exploited. Relaxation time measurements and a search for residual second-order quadrupolar coupling were employed to investigate the molecular motions present in the liquid state and infer what kind of order is present. The results obtained indicate that on a timescale of approximately 10(-8) sec or longer, RTILs behave as isotropic liquids without residual order.
- Published
- 2010
17. A structure refinement strategy for NMR crystallography: An improved crystal structure of silica-ZSM-12 zeolite from 29Si chemical shift tensors
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Darren H. Brouwer
- Subjects
Diffraction ,Silicon ,Nuclear and High Energy Physics ,Magnetic Resonance Spectroscopy ,Silicon dioxide ,Molecular Conformation ,Biophysics ,Ab initio ,chemistry.chemical_element ,Crystal structure ,Sensitivity and Specificity ,Biochemistry ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,Isotopes ,Zeolite ,Crystallography ,Chemical shift ,Reproducibility of Results ,Nuclear magnetic resonance spectroscopy ,Silicon Dioxide ,Condensed Matter Physics ,chemistry ,Zeolites ,Algorithms - Abstract
A strategy for performing crystal structure refinements with NMR chemical shift tensors is described in detail and implemented for the zeolite silica-ZSM-12 (framework type code MTW). The 29Si chemical shift tensors were determined from a slow magic-angle spinning spectrum obtained at an ultrahigh magnetic field of 21.1T. The Si and O atomic coordinate parameters were optimized to give the best agreement between experimentally measured and ab initio calculated principal components of the 29Si chemical shift tensors, with the closest Si-O, O-O, and Si-Si distances restrained to correspond with the distributions of the distances found in a set of single-crystal X-ray diffraction (XRD) structures of high-silica zeolites. An improved structure for the silica-ZSM-12 zeolite, compared to a prior structure derived from powder XRD data, is obtained in which the agreement between the experimental and calculated 29Si chemical shift tensors is dramatically improved, the Si-O, O-O, and Si-Si distances correspond to the expected distributions, while the calculated powder XRD pattern remains in good agreement with the experimental powder XRD data. It is anticipated that this "NMR crystallography" structure refinement strategy will be an important tool for the accurate structure determination of materials that are difficult to fully characterize by traditional diffraction methods.
- Published
- 2008
18. Guest Loading and Multiple Phases in Single Crystals of the van der Waals Host p- tert-Butylcalix[4]arene
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Kostantin A. Udachin, Gary D. Enright, John A. Ripmeester, Igor L. Moudrakovski, and Darren H. Brouwer
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Diffraction ,Stereochemistry ,Bilayer ,chemistry.chemical_element ,General Chemistry ,Condensed Matter Physics ,NMR spectra database ,Crystallography ,symbols.namesake ,Dipole ,Xenon ,chemistry ,Calixarene ,symbols ,General Materials Science ,van der Waals force ,Double quantum - Abstract
We have followed the loading of xenon into the low density form of a van der Waals solid host, p- tert-butylcalix[4]arene (tBC), with solid-state NMR and X-ray diffraction (XRD), techniques sensitive to local and long-range order, respectively. Even though there was little change in the unit cell parameters, 13C and 129Xe solid-state NMR spectra indicate that significant structural changes occur in local order even at low levels of loading. In particular, 129Xe double quantum experiments, which probe distance-dependent 129Xe− 129Xe dipolar interactions, reveal that the closest Xe−Xe distances increase with Xe loading into the tBC host, suggesting that the tBC undergoes structural rearrangements as it absorbs Xe. Indeed, in light of the solid-state NMR results, a re-examination of partially loaded single crystals by XRD showed that up to a loading level (Xe/tBC ratio) of 0.25, the structure was closely related to that of the empty form with the typical calixarene bilayer structure; however, at higher loadi...
- Published
- 2008
19. Probing Local Structure in Zeolite Frameworks: Ultrahigh-Field NMR Measurements and Accurate First-Principles Calculations of Zeolite 29Si Magnetic Shielding Tensors
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Gary D. Enright and Darren H. Brouwer
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Silicon ,Magnetic Resonance Spectroscopy ,Chemistry ,Chemical shift ,Isotropy ,Analytical chemistry ,Pulse sequence ,General Chemistry ,Reference Standards ,Biochemistry ,Molecular physics ,Catalysis ,Magnetic field ,Magnetics ,Colloid and Surface Chemistry ,Isotopes ,Electromagnetic shielding ,Zeolites ,Quantum Theory ,Tensor ,Anisotropy ,Spinning - Abstract
The principal components of zeolite 29Si magnetic shielding tensors have been accurately measured and calculated for the first time. The experiments were performed at an ultrahigh magnetic field of 21.1 T in order to observe the small anisotropies of the 29Si shielding interactions that arise for Si atoms in near-tetrahedral geometries. A robust two-dimensional (2D) chemical shift anisotropy (CSA) recoupling pulse sequence was employed that enables quasi-static powder patterns to be resolved according to the isotropic chemical shifts. For the zeolites Sigma-2 and ZSM-12, it is demonstrated that the 29Si chemical shift (CS) tensor components measured by the recoupling experiment are in excellent agreement with those determined from spinning sidebands in slow magic-angle spinning (MAS) experiments. For the zeolite ZSM-5, the principal components of the 29Si CS tensors of 15 of the 24 Si sites were measured using the 2D CSA recoupling experiment, a feat that would not be possible with a slow MAS experiment due to the complexity of the spectrum. A simple empirical relationship between the 29Si CS tensors and local structural parameters could not be established. However, the 29Si magnetic shielding tensors calculated using Hartree-Fock ab initio calculations on clusters derived from the crystal structures are in excellent agreement with the experimental results. The accuracy of the calculations is strongly dependent on the quality of the crystal structure used in the calculation, indicating that the 29Si magnetic shielding interaction is extremely sensitive to the local structure around each Si atom. It is anticipated that the measurement and calculation of 29Si shielding tensors could be incorporated into the "NMR crystallography" of zeolites and other related silicate materials, possibly being used for structure refinements that may lead to crystal structures with very accurate Si and O atomic coordinates.
- Published
- 2008
20. The Structure of Two Anhydrous Polymorphs of Caffeine from Single-Crystal Diffraction and Ultrahigh-Field Solid-State 13C NMR Spectroscopy
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John A. Ripmeester, Victor V. Terskikh, Darren H. Brouwer, and Gary D. Enright
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Diffraction ,Crystallography ,Polymorphism (materials science) ,Chemistry ,Anhydrous ,Space group ,General Materials Science ,General Chemistry ,Nuclear magnetic resonance spectroscopy ,Crystal structure ,Carbon-13 NMR ,Condensed Matter Physics ,Powder diffraction - Abstract
In this work, we examine two anhydrous polymorphs of caffeine, as well as the monohydrate, by high-field (21 T) NMR spectroscopy, and since suitable single crystals of the anhydrous forms could be obtained, the crystal structures were also determined. At high field, the 13C NMR spectra are simplified considerably over those obtained at low field as the effect of the 14N nuclear quadrupoles on the 13C resonances becomes minimal. The spectra of the two anhydrous polymorphs provide information about the number of distinct caffeine sites and indicate structural disorder. The NMR observations are consistent with single-crystal X-ray diffraction, which shows that the structures are indeed complicated by disorder. Furthermore, the space groups obtained previously from powder diffraction were, in fact, incorrect, R3c rather than R3c for the low-temperature polymorph and C2/c rather than Cc for the high-temperature polymorph. As a result, the structures are different from those proposed on the basis of modeling c...
- Published
- 2007
21. Symmetry-based recoupling of proton chemical shift anisotropies in ultrahigh-field solid-state NMR
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Darren H. Brouwer and John A. Ripmeester
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Hydrogen bonding ,Models, Molecular ,Deuterium NMR ,Chemical shift anisotropy ,Nuclear and High Energy Physics ,Magnetic Resonance Spectroscopy ,Proton ,Carbon-13 NMR satellite ,Biophysics ,Analytical chemistry ,Biochemistry ,Molecular physics ,Homonuclear molecule ,Biopolymers ,Magic angle spinning ,Computer Simulation ,Anisotropy ,Chemistry ,Chemical shift ,CRAMPS ,Condensed Matter Physics ,Proton MAS NMR ,Models, Chemical ,Solid-state nuclear magnetic resonance ,Recoupling ,Spin Labels ,Powders ,Protons ,Algorithms - Abstract
A two-dimensional NMR experiment for estimating proton chemical shift anisotropies (CSAs) in solid powders under magic-angle spinning conditions is demonstrated in which 1H CSAs are reintroduced with a symmetry-based recoupling sequence while the individual proton sites are resolved according to their isotropic chemical shifts by magic-angle spinning (MAS) or combined rotation and multiple pulse (CRAMPS) homonuclear decoupling. The experiments where carried out on an ultrahigh-field solid-state NMR instrument (900?MHz 1H frequency) which leads to increased resolution and reliability of the measured 1H CSAs. The experiment is expected to be important for investigating hydrogen bonding in solids.
- Published
- 2007
22. Optimization, Standardization, and Testing of a New NMR Method for the Determination of Zeolite Host−Organic Guest Crystal Structures
- Author
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Darren H. Brouwer and Colin A. Fyfe
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Magic angle ,Relaxation (NMR) ,Analytical chemistry ,Second moment of area ,General Chemistry ,Crystal structure ,Biochemistry ,Catalysis ,Inclusion compound ,chemistry.chemical_compound ,Colloid and Surface Chemistry ,Nuclear magnetic resonance ,chemistry ,X-ray crystallography ,Molecule ,Zeolite - Abstract
An optimized and automated protocol for determining the location of guest sorbate molecules in highly siliceous zeolites from (29)Si INADEQUATE and (1)H/(29)Si cross polarization (CP) magic-angle spinning (MAS) NMR experiments is described. With the peaks in the (29)Si MAS NMR spectrum assigned to the unique Si sites in the zeolite framework by a 2D (29)Si INADEQUATE experiment, the location of the sorbate molecule is found by systematically searching for sorbate locations for which the measured rates of (1)H/(29)Si cross polarization of the different Si sites correlate linearly with (1)H/(29)Si second moments calculated from H-Si distances. Due to the (1)H/(29)Si cross polarization being in the "slow CP regime" for many zeolite-sorbate complexes, it is proposed that the CP rate constants are best measured by (1)H/(29)Si cross polarization drain experiments, if possible, to avoid complications that may arise from fast (1)H and (29)Si T(1)rho relaxations. An algorithm for determining the sorbate molecule location is described in detail. A number of ways to effectively summarize and display the large number of solutions which typically result from a prediction of the structure from the CP MAS NMR data are presented, including estimates of the errors involved in the structure determinations. As a working example throughout this paper, the structure of the low loaded p-dichlorobenzene/ZSM-5 complex is determined under different conditions from solid-state (1)H/(29)Si CP MAS NMR data, and the solutions are shown to be in excellent agreement with the known single-crystal X-ray diffraction structure. This structure determination approach is shown to be quite insensitive to the use of relative rate constants rather than absolute values, to the detailed structure of the zeolite framework, and relatively insensitive to temperature and motions.
- Published
- 2006
23. Determination of the location of naphthalene in the zeolite ZSM-5 host framework by solid-state 1H/29Si CP MAS NMR spectroscopy
- Author
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Darren H. Brouwer and Colin A. Fyfe
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Chemistry ,Organic Chemistry ,Solid-state ,General Chemistry ,Catalysis ,chemistry.chemical_compound ,Reaction rate constant ,Physical chemistry ,Molecule ,ZSM-5 ,Zeolite ,Single crystal ,Magnetic dipole–dipole interaction ,Naphthalene - Abstract
The location of naphthalene in the zeolite ZSM-5 has been determined from solid-state 1H/29Si cross-polarization (CP) magic-angle-spinning (MAS) NMR data alone. With the peaks in the 29Si spectrum assigned to the inequivalent Si sites in the zeolite from a two-dimensional INADEQUATE spectrum, the rates of cross polarization between the 1H nuclei of the guest sorbate molecules and the 29Si nuclei of the zeolite framework were used to determine the location of the naphthalene molecules by exploiting the proportional relationship between cross-polarization rate constants and 1H/29Si dipolar coupling second moments. The NMR structure determination was carried out on three different selectively deuterium-labeled naphthalene molecules (naphthalene-d0, α-naphthalene-d4, and β-naphthalene-d4). The average of the molecule locations in agreement with all three sets of NMR data was found to be in excellent agreement with an existing single crystal XRD structure of the naphthalene–ZSM-5 complex.Key words: solid-state NMR, cross polarization, zeolites, host–guest complexes, structure determination.
- Published
- 2006
24. Solid state NMR investigation of the structure of AlPO4-14A
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J.M. Chezeau, Darren H. Brouwer, and Colin A. Fyfe
- Subjects
Proton ,Chemistry ,Resonance ,Protonation ,General Chemistry ,Condensed Matter Physics ,Crystallography ,Solid-state nuclear magnetic resonance ,Octahedron ,Heteronuclear molecule ,Mechanics of Materials ,Computational chemistry ,Atom ,Molecule ,General Materials Science - Abstract
The as-synthesized aluminophosphate framework material AlPO 4 -14A has a number of unique structural features related to the connectivities in the network of Al and P atoms. One tetrahedral Al atom is present that is linked, via Al–O(H)–Al bonds to two octahedral Al atoms and, consequently to only two P atoms. However, an Al/P ratio of one is maintained as each of these octahedral Al atoms is connected to five P atoms in addition to the Al–O(H)–Al linkage. This connectivity network means that a framework made up of alternating Al and P atoms cannot be obtained by calcination. A variety of 1 H, 27 Al, and 31 P solid state NMR experiments have been employed to characterize the AlPO 4 -14A structure. A fast 1 H MAS experiment showed that the template was in the protonated form and a 1 H{ 27 Al} TRAPDOR experiment identified the resonance of the ‘framework’ proton. The 27 Al and 31 P resonances were completely assigned by the 27 Al → 31 P INEPT heteronuclear correlation experiment and the quadrupolar parameters of the 27 Al signals determined from the 27 Al MQMAS experiment. Lastly, the complete assignment of both 27 Al and 31 P resonances made it possible to qualitatively determine the location and orientation of the template molecule within the framework from two-dimensional dipolar-based 1 H → 31 P cross-polarization and 1 H → 27 Al TEDOR heteronuclear correlation experiments. The data from these experiments confirm in detail the AlPO 4 -14A structure and will be useful benchmarks for the investigation of other structures for which diffraction data are limited.
- Published
- 2006
25. A Solid-State NMR Method for Solution of Zeolite Crystal Structures
- Author
-
Richard J. Darton, Malcolm H. Levitt, Darren H. Brouwer, and Russell E. Morris
- Subjects
Diffraction ,Chemistry ,Pulse sequence ,General Medicine ,Powder xrd ,General Chemistry ,Crystal structure ,Phase problem ,Nuclear magnetic resonance spectroscopy ,Nuclear magnetic resonance crystallography ,Biochemistry ,Catalysis ,Dipole ,Colloid and Surface Chemistry ,Nuclear magnetic resonance ,Solid-state nuclear magnetic resonance ,Physical chemistry ,Zeolite ,Spectroscopy - Abstract
Since zeolites are notoriously difficult to prepare as large single crystals, structure determination usually relies on powder X-ray diffraction (XRD). However, structure solution (i.e., deriving an initial structural model) directly from powder XRD data is often very difficult due to the diffraction phase problem and the high degree of overlap between the individual reflections, particularly for materials with the structural complexity of most zeolites. Here, we report a method for structure determination of zeolite crystal structures that combines powder XRD and nuclear magnetic resonance (NMR) spectroscopy in which the crucial step of structure solution is achieved using solid-state (29)Si double-quantum dipolar recoupling NMR, which probes the distance-dependent dipolar interactions between naturally abundant (29)Si nuclei in the zeolite framework. For two purely siliceous zeolite blind test samples, we demonstrate that the NMR data can be combined with the unit cell parameters and space group to solve structural models that refine successfully against the powder XRD data.
- Published
- 2005
26. Measurement of NMR Cross-Polarization (CP) Rate Constants in the Slow CP Regime: Relevance to Structure Determinations of Zeolite−Sorbate and Other Complexes by CP Magic-Angle Spinning NMR
- Author
-
Colin A. Fyfe, Piotr Tekely, and Darren H. Brouwer
- Subjects
Magnetization ,Reaction rate constant ,Contact time ,Chemistry ,Cross polarization ,Analytical chemistry ,Magic angle spinning ,T1ρ relaxation ,Physical and Theoretical Chemistry ,Atomic physics ,Polarization (waves) ,Zeolite - Abstract
When analyzing I --S variable contact time cross-polarization (CP) curves, the spin dynamics are usually assumed to be describable in the "fast CP regime" in which the growth of the S spin magnetization is governed by the rate of cross polarization while its decay is governed by the rate of I spin T1rho relaxation. However, in the investigation of the structures of zeolite-sorbate and other complexes by polarization transfer this will not necessarily be the case. We discuss the measurement of I --S CP rate constants under the "slow CP regime" in which the rate of T1rho relaxation is fast compared to the rate of cross polarization, leading to a reversal of the usual assumptions such that the rate or growth is governed by the rate of I spin T1rho relaxation while the decay is governed by the rate of cross polarization (and the S spin T1rho relaxation). It is very important to recognize when a system is in the slow CP regime, as an analysis assuming the normal fast CP will lead to erroneous data. However, even when the slow CP regime is recognized, it is difficult to obtain absolute values for the CP rate constants from fits to standard CP curves, since the CP rate constant is correlated to the scaling factor, the contribution from 29Si T1rho relaxation is ignored, and it is difficult to obtain reliable data at very long contact times. The use of a 29Si{1H} CP "drain" or "depolarization" experiment, which measures absolute values of the CP rate constants, is therefore proposed as being most appropriate for theses situations. To illustrate the importance of these observations, measurements of the 1H-29Si CP rate constants in the p-dichlorobenzene/ZSM-5 sorbate-zeolite complex by 29Si{1H} CP and CP drain magic-angle spinning (MAS) NMR experiments are presented and compared and used to determine the location of the guest sorbate molecules in the cavities of the host zeolite framework.
- Published
- 2005
27. Solid-State NMR Studies of the Fluoride-Containing Zeolite SSZ-44
- Author
-
Richard J. Darton, Colin A. Fyfe, Luis A. Villaescusa, Darren H. Brouwer, and Russell E. Morris
- Subjects
Diffraction ,Sideband ,General Chemical Engineering ,Analytical chemistry ,General Chemistry ,Bond length ,chemistry.chemical_compound ,Nuclear magnetic resonance ,chemistry ,Solid-state nuclear magnetic resonance ,Yield (chemistry) ,Materials Chemistry ,Zeolite ,Fluoride ,Spinning - Abstract
Magic-angle spinning NMR has been used to probe the local structure of as-made zeolite SFF containing occluded fluoride and N,N-diethyl-2,6-dimethylpiperidinium. The Si−F bond distance has been measured using two methods: variable contact time cross polarization studies (1.74 A) and spinning sideband intensity fitting at relatively slow spinning speeds (1.79 A). Both measurements yield results that are significantly shorter than the internuclear Si−F distance as measured by single-crystal X-ray diffraction (∼1.9 A), which is based on the average positions of the atoms due to disorder and hence does not reflect the true bond distance.
- Published
- 2004
28. Improvements to a Peak Assignment Algorithm for Two-Dimensional NMR Correlation Spectra of Zeolites Using Graph Theory
- Author
-
E. Keith Lloyd and Darren H. Brouwer
- Subjects
NMR spectra database ,Imagination ,Search engine ,media_common.quotation_subject ,Spectrum (functional analysis) ,Structure (category theory) ,Graph theory ,Spinning ,Algorithm ,Spectral line ,Mathematics ,media_common - Abstract
An improved version of a peak assignment algorithm for two-dimensional NMR correlation spectra of zeolites is described. Both the zeolite framework structure and the two-dimensional NMR correlation spectrum can be expressed as graphs such that the task of peak assignment is to find labelings of the vertices of the graphs which show that the two graphs are homomorphic. In this paper, it is shown that an initial analysis of the structure and spectrum graphs provides information about the possible identities of the peaks which can lead to large gains in the efficiency of the peak assignment algorithm. The improvements in the algorithm are demonstrated for the peak assignment of solid state magic-angle spinning 29Si NMR spectra of the purely siliceous zeolite ZSM-5 from two-dimensional 29Si INADEQUATE NMR spectra.
- Published
- 2004
29. The amblygonite (LiAlPO4F)-montebrasite (LiAlPO4OH) solid solution: A combined powder and single-crystal neutron diffraction and solid-state6Li MAS, CP MAS, and REDOR NMR study
- Author
-
Arthur J. Schultz, Darren H. Brouwer, Bryan C. Chakoumakos, Colin A. Fyfe, Heiko Morell, Lee A. Groat, and Christina Hoffman
- Subjects
Crystallography ,Geophysics ,Geochemistry and Petrology ,Chemistry ,Atom ,Neutron diffraction ,Amblygonite ,Anisotropy ,Absolute zero ,Single crystal ,Solid solution ,Ion - Abstract
The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO{sub 4}(F, OH). These compounds are ideal for studying F {leftrightarrow} OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state {sup 6}Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F {leftrightarrow} OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacementmore » ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the U{sub 3} eigenvalue, which lies between the c and c* directions. U{sub eq} values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder.« less
- Published
- 2003
30. Structural Investigation of Silicalite-I Loaded with n-Hexane by X-ray Diffraction, 29Si MAS NMR, and Molecular Modeling
- Author
-
Hermann Gies, Andrew R. Lewis, Klaus Angermund, Heiko Morell, Darren H. Brouwer, and Colin A. Fyfe
- Subjects
Diffraction ,Crystallography ,Chemistry ,Rietveld refinement ,General Chemical Engineering ,X-ray crystallography ,Materials Chemistry ,Molecule ,General Chemistry ,Crystal structure ,Crystal twinning ,Molecular sieve ,Monoclinic crystal system - Abstract
The room temperature (298 K) structure of zeolite Silicalite-I loaded with approximately eight n-hexane molecules per unit cell was solved from twinned single-crystal X-ray diffraction (XRD) data in the monoclinic space group P121/n1 with a = 19.8247(2) A, b = 20.1292(2) A, c = 13. 4510(2) A, and β = 90.29(8)°. At this temperature, the guest molecules are dynamically disordered and distributed throughout the entire channel system. The structure determined from a Rietveld refinement of room-temperature powder XRD data, which is not affected by the twinning, confirmed this. A twinned crystal refinement was also carried out for data collected at 180 K (P121/n1, a = 19.9310(2) A, b = 20.1730(3) A, c = 13.4191(3) A, β = 90.20(5)°). At 180 K, the sorption sites of the n-hexane molecules are well-defined within the channel system, being located only in the straight and sinusoidal channels, leaving the intersections unoccupied. This ordering is commensurate with the framework structure of Silicalite-I. 29Si HPDEC...
- Published
- 2002
31. NMR crystallography strategies for structure determination of zeolites and layered silicates
- Author
-
Darren H. Brouwer
- Subjects
Inorganic Chemistry ,Crystallography ,Materials science ,Structural Biology ,Structure (category theory) ,General Materials Science ,Physical and Theoretical Chemistry ,Condensed Matter Physics ,Biochemistry - Published
- 2017
32. Location of the Fluoride Ion in Tetrapropylammonium Fluoride Silicalite-1 Determined by 1H/19F/29Si Triple Resonance CP, REDOR, and TEDOR NMR Experiments
- Author
-
Colin A. Fyfe, Darren H. Brouwer, Jean-Michel Chézeau‡, and and Andrew R. Lewis
- Subjects
chemistry.chemical_compound ,Colloid and Surface Chemistry ,Chemistry ,Covalent bond ,Inorganic chemistry ,Resonance ,Physical chemistry ,General Chemistry ,Zeolite ,Biochemistry ,Fluoride ,Catalysis ,Ion - Abstract
The location of the fluoride ion in tetrapropylammonium fluoride silicalite-1 ([TPA]-F-[Si-MFI]), an as-synthesized siliceous zeolite with the MFI topology, has been unambiguously determined using solid-state NMR experiments alone. With the 1H→29Si CP-INADEQUATE experiment, the 12 peaks in the highly resolved 29Si MAS NMR spectrum of [TPA]-F-[Si-MFI] were assigned. Using these peak assignments it was possible to perform 1H/19F/29Si triple resonance CP, REDOR, and TEDOR experiments to measure F−Si distances and thus locate the fluoride ion. It is covalently bonded to Si-9 in the [415262] cage of the zeolite framework and exchanges between two “mirror-related” Si-9 sites, making them equivalent on the NMR time scale. The importance of this result and the general applicability of the approach are discussed.
- Published
- 2001
33. Solid-state NMR and X-ray diffraction structural investigations of the p-nitroaniline/ZSM-5 complex
- Author
-
Darren H. Brouwer and Colin A. Fyfe
- Subjects
Diffraction ,Chemistry ,General Chemistry ,Condensed Matter Physics ,Resonance (chemistry) ,Crystal ,Dipole ,Crystallography ,Nuclear magnetic resonance ,Solid-state nuclear magnetic resonance ,Mechanics of Materials ,X-ray crystallography ,General Materials Science ,Orthorhombic crystal system ,Single crystal - Abstract
The complex of zeolite ZSM-5 loaded with p-nitroaniline to four molecules per unit cell is of current interest as a potential non-linear optical material. A previous single crystal X-ray diffraction (XRD) investigation has claimed that the p-nitroaniline molecules line up as oriented dipole chains inside the channels of ZSM-5, and it was concluded that this ordering explains how second harmonic generation arises. In another XRD study, it was not possible to distinguish between order and disorder of the p-nitroaniline molecules. The present results from solid-state NMR and single crystal XRD experiments indicate that the p-nitroaniline molecules are disordered in the ZSM-5 channels, i.e., they do not form ordered dipole chains throughout the entire crystal. The quantitative 29Si-MAS spectrum at room temperature shows 48 peaks of equal intensity indicating disorder of the guest molecules, and the 15N-MAS spectrum at 180 K of the 15N-labeled p-nitroaniline in ZSM-5 shows a splitting of the NO2 resonance, which also implies disorder. The single crystal XRD results show that the p-nitroaniline molecules are disordered over four-sites at the channel intersection even when refined in the less symmetric Pn21a space group. The final structure was refined in the orthorhombic Pnma space group to R=0.0465. The nature of this disorder is discussed. In particular, the possibilities of disorder arising from the average of polar domains or local disorder are examined in the light of the solid-state NMR results. That the p-nitroaniline molecules are disordered in ZSM-5 implies that the mechanism by which second harmonic generation arises may be more complex than is presently assumed.
- Published
- 2000
34. Nucleation and Growth of Silver at Zeolite A-Modified Electrodes
- Author
-
Darren H. Brouwer and Mark D. Baker
- Subjects
Horizontal scan rate ,Supersaturation ,Materials science ,Chemical engineering ,Electrode ,Materials Chemistry ,Nucleation ,Physical and Theoretical Chemistry ,Chronoamperometry ,Cyclic voltammetry ,Zeolite ,Tin oxide ,Surfaces, Coatings and Films - Abstract
In this paper the nucleation and growth of silver deposits at tin oxide and tin oxide modified with silver ion-exchanged zeolite A (AgxNa12-xA) is studied. We show via chronoamperometry that, in the case of the zeolite-modified electrodes, the lateral motion of silver cations to active sites is impeded by the zeolite layer, resulting in distinct nucleation and growth at defect sites and/or at the bulk tin oxide surface. Solution-phase silver deposition at tin oxide fits the Scharifker−Mostany model extremely well, showing that on average only one active site for silver nucleation and growth is available for each 10 zeolite particles. Furthermore, the scan rate and loading dependence of the cyclic voltammetry are explained in terms of ion-exchange kinetics and supersaturation of silver atoms around the active nucleation site. A general model pertaining to the nucleation and growth of silver at zeolite-modified electrodes is presented. Cyclic voltammetry recorded in aqueous electrolytes containing cations o...
- Published
- 1997
35. Some New Halogen-containing Hydrate-formers for Structure I and II Clathrate Hydrates1
- Author
-
Mark Lee, Eric B. Brouwer, Darren H. Brouwer, John A. Ripmeester, Dan Parks, and Graham Maclaurin
- Subjects
chemistry.chemical_compound ,Chemistry ,Polymer chemistry ,Halogen ,Clathrate hydrate ,Physical chemistry ,Molecule ,General Chemistry ,Fluorocarbon ,Hydrofluorocarbon ,Hydrate - Abstract
The clathrate hydrate-forming capabilities of 15 hydrofluorocarbon (HFC), fluorocarbon (FC), hydrochlorofluorocarbon (HCFC) and chlorofluorocarbon (CFC) guests are reported. These molecules form st...
- Published
- 1997
36. A general protocol for determining the structures of molecularly ordered but noncrystalline silicate frameworks
- Author
-
Osamu Terasaki, Zheng Liu, Darren H. Brouwer, Juergen Eckert, Bradley F. Chmelka, and Sylvian Cadars
- Subjects
Quantum chemical ,Diffraction ,Structure (category theory) ,General Chemistry ,Space (mathematics) ,Biochemistry ,Catalysis ,Silicate ,chemistry.chemical_compound ,Crystallography ,Colloid and Surface Chemistry ,chemistry ,Group (periodic table) ,Order (group theory) ,Spectroscopy - Abstract
A general protocol is demonstrated for determining the structures of molecularly ordered but noncrystalline solids, which combines constraints provided by X-ray diffraction (XRD), one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles quantum chemical calculations. The approach is used to determine the structure(s) of a surfactant-directed layered silicate with short-range order in two dimensions but without long-range periodicity in three-dimensions (3D). The absence of long-range 3D molecular order and corresponding indexable XRD reflections precludes determination of a space group for this layered silicate. Nevertheless, by combining structural constraints obtained from solid-state (29)Si NMR analyses, including the types and relative populations of distinct (29)Si sites, their respective (29)Si-O-(29)Si connectivities and separation distances, with unit cell parameters (though not space group symmetry) provided by XRD, a comprehensive search of candidate framework structures leads to the identification of a small number of candidate structures that are each compatible with all of the experimental data. Subsequent refinement of the candidate structures using density functional theory calculations allows their evaluation and identification of "best" framework representations, based on their respective lattice energies and quantitative comparisons between experimental and calculated (29)Si isotropic chemical shifts and (2)J((29)Si-O-(29)Si) scalar couplings. The comprehensive analysis identifies three closely related and topologically equivalent framework configurations that are in close agreement with all experimental and theoretical structural constraints. The subtle differences among such similar structural models embody the complexity of the actual framework(s), which likely contain coexisting or subtle distributions of structural order that are intrinsic to the material.
- Published
- 2013
37. Comparing quantum-chemical calculation methods for structural investigation of zeolite crystal structures by solid-state NMR spectroscopy
- Author
-
Richard J. Darton, Igor L. Moudrakovski, Darren H. Brouwer, and Russell E. Morris
- Subjects
Diffraction ,Models, Molecular ,Magnetic Resonance Spectroscopy ,Gaussian ,NMR crystallography ,Nuclear magnetic resonance crystallography ,Crystal structure ,Energy minimization ,Crystallography, X-Ray ,symbols.namesake ,General Materials Science ,Spectroscopy ,chemical shielding tensors ,Molecular Structure ,Chemistry ,quantum-chemical calculations ,General Chemistry ,Reference Standards ,NMR ,29Si ,Crystallography ,Solid-state nuclear magnetic resonance ,CASTEP ,symbols ,Zeolites ,solid-state NMR ,Physical chemistry ,Quantum Theory - Abstract
Combining quantum-chemical calculations and ultrahigh-field NMR measurements of 29Si chemical shielding (CS) tensors has provided a powerful approach for probing the fine details of zeolite crystal structures. In previous work, the quantum-chemical calculations have been performed on ‘molecular fragments’ extracted from the zeolite crystal structure using Hartree–Fock methods (as implemented in Gaussian). Using recently acquired ultrahigh-field 29Si NMR data for the pure silica zeolite ITQ-4, we report the results of calculations using recently developed quantum-chemical calculation methods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) and compare these calculations to those calculated with Gaussian. Furthermore, in the context of NMR crystallography of zeolites, we report the completion of the NMR crystallography of the zeolite ITQ-4, which was previously solved from NMR data. We compare three options for the ‘refinement’ of zeolite crystal structures from ‘NMR-solved’ structures: (i) a simple target-distance based geometry optimization, (ii) refinement of atomic coordinates in which the differences between experimental and calculated 29Si CS tensors are minimized, and (iii) refinement of atomic coordinates to minimize the total energy of the lattice using CASTEP quantum-chemical calculations. All three refinement approaches give structures that are in remarkably good agreement with the single-crystal X-ray diffraction structure of ITQ-4. Copyright © 2010 John Wiley & Sons, Ltd.
- Published
- 2010
38. Interplay between Solid-State NMR and Single-Crystal X-Ray Diffraction
- Author
-
Darren H. Brouwer
- Subjects
Diffraction ,Crystallography ,Solid-state nuclear magnetic resonance ,Chemistry ,X-ray crystallography ,Crystal structure ,Nuclear magnetic resonance crystallography ,Spectroscopy ,Single crystal ,Characterization (materials science) - Abstract
The complementary relationship between solid-state nuclear magnetic resonance (NMR) spectroscopy and diffraction techniques, specifically single-crystal X-ray diffraction (XRD), is explained and illustrated. It is demonstrated how solid-state NMR and single-crystal XRD can be combined advantageously, allowing structural chemists to investigate the detailed structures of materials in a way that could not be achieved by using a single technique alone. By using few examples involving zeolites and organic host-guest complexes, the importance of a combined approach with solid-state NMR and single-crystal XRD for detailed structural characterization is illustrated. Keywords: solid-state NMR; X-ray diffraction; crystallography; zeolites; host-guest complexes; crystal structure; disorder; dynamics
- Published
- 2008
39. (35)Cl solid-state NMR of halide ionic liquids at ultrahigh fields
- Author
-
John A. Ripmeester, Peter G. Gordon, and Darren H. Brouwer
- Subjects
NMR spectra database ,chemistry.chemical_compound ,Solid-state nuclear magnetic resonance ,Field (physics) ,chemistry ,Chemical physics ,Ionic liquid ,Analytical chemistry ,Halide ,Fluorine-19 NMR ,Physical and Theoretical Chemistry ,Electric field gradient ,Spectral line - Abstract
This Letter describes recent work investigating the solid-state NMR spectra of (35)Cl nuclei in an assortment of ionic liquids under static and MAS conditions at field strengths of 9.4 and 21.1 T. At high field it was possible to resolve and extract information from multiple unique crystallographic sites and to resolve otherwise complex spectra that were analyzed to extract information regarding the electric field gradient (EFG) and chemical shift tensors, including their relative orientation. The NMR parameters were found to be typical of organic salts in general.
- Published
- 2008
40. NMR crystallography of p-tert-butylcalix[4]arene host-guest complexes using 1H complexation-induced chemical shifts
- Author
-
Saman Alavi, John A. Ripmeester, and Darren H. Brouwer
- Subjects
Magic angle ,Crystallography ,Magnetic Resonance Spectroscopy ,Molecular Structure ,Chemistry ,Chemical shift ,General Physics and Astronomy ,Aromaticity ,Nuclear magnetic resonance spectroscopy ,Inclusion compound ,NMR spectra database ,chemistry.chemical_compound ,Calixarene ,Molecule ,Computer Simulation ,Physical and Theoretical Chemistry ,Calixarenes - Abstract
(1)H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host-guest complexes, providing important spatial information about the location of the guest molecules in the host cavities.
- Published
- 2008
41. NMR crystallography of zeolites: refinement of an NMR-solved crystal structure using ab initio calculations of 29Si chemical shift tensors
- Author
-
Darren H. Brouwer
- Subjects
Quantum chemical ,Diffraction ,Chemistry ,Ab initio ,General Chemistry ,Nuclear magnetic resonance crystallography ,Crystal structure ,Biochemistry ,Nmr data ,Catalysis ,Condensed Matter::Materials Science ,Crystallography ,Colloid and Surface Chemistry ,Ab initio quantum chemistry methods ,Zeolite - Abstract
An NMR structure refinement method for the NMR crystallography of zeolites is presented and demonstrated to give an NMR-determined crystal structure for the zeolite Sigma-2 that is in very good agreement with the single-crystal X-ray diffraction structure. The Si coordinates of the zeolite framework were solved from 29Si double-quantum NMR data obtained at a low magnetic field strength (7.0 T) and the Si and O coordinates were subsequently refined using the principal components of 29Si chemical shift tensors experimentally measured at an ultrahigh-field (21.1 T) and calculated using ab initio quantum chemical methods.
- Published
- 2008
42. A double quantum (129)Xe NMR experiment for probing xenon in multiply-occupied cavities of solid-state inclusion compounds
- Author
-
John A. Ripmeester, Saman Alavi, and Darren H. Brouwer
- Subjects
Coupling constant ,Models, Molecular ,Magic angle ,Magnetic Resonance Spectroscopy ,Time Factors ,Chemistry ,Analytical chemistry ,General Physics and Astronomy ,chemistry.chemical_element ,Nuclear magnetic resonance spectroscopy ,Reference Standards ,Molecular physics ,Inclusion compound ,NMR spectra database ,Molecular dynamics ,chemistry.chemical_compound ,Xenon ,Physics::Atomic and Molecular Clusters ,Quantum Theory ,Xenon Isotopes ,Physical and Theoretical Chemistry ,Chromans ,Spin (physics) - Abstract
A method is presented for detecting multiple xenon atoms in cavities of solid-state inclusion compounds using (129)Xe double quantum NMR spectroscopy. Double quantum filtered (129)Xe NMR spectra, performed on the xenon clathrate of Dianin's compound were obtained under high-resolution Magic-Angle Spinning (MAS) conditions, by recoupling the weak (129)Xe-(129)Xe dipole-dipole couplings that exist between xenon atoms in close spatial proximity. Because the (129)Xe-(129)Xe dipole-dipole couplings are generally weak due to dynamics of the atoms and to large internuclear separations, and since the (129)Xe Chemical Shift Anisotropy (CSA) tends to be relatively large, a very robust dipolar recoupling sequence was necessary, with the symmetry-based SR26 dipolar recoupling sequence proving appropriate. We have also attempted to measure the (129)Xe-(129)Xe dipole-dipole coupling constant between xenon atoms in the cavities of the xenon-Dianin's compound clathrate and have found that the dynamics of the xenon atoms (as investigated with molecular dynamics simulations) as well as (129)Xe multiple spin effects complicate the analysis. The double quantum NMR method is useful for peak assignment in (129)Xe NMR spectra because peaks arising from different types of absorption/inclusion sites or from different levels of occupancy of single sites can be distinguished. The method can also help resolve ambiguities in diffraction experiments concerning the order/disorder in a material.
- Published
- 2007
43. Heteronuclear decoupling interference during symmetry-based homonuclear recoupling in solid-state NMR
- Author
-
Andreas Brinkmann, Giancarlo Antonioli, Ildefonso Marin-Montesinos, Malcolm H. Levitt, Wai Cheu Lai, and Darren H. Brouwer
- Subjects
Condensed Matter::Quantum Gases ,Nuclear and High Energy Physics ,Carbon Isotopes ,Alanine ,Condensed matter physics ,Molecular Structure ,Nitrogen Isotopes ,Chemistry ,Biophysics ,Glycine ,Decoupling (cosmology) ,Condensed Matter Physics ,Biochemistry ,Molecular physics ,Homonuclear molecule ,Biological materials ,Dipole ,Heteronuclear molecule ,Solid-state nuclear magnetic resonance ,Fumarates ,Condensed Matter::Superconductivity ,Physics::Atomic and Molecular Clusters ,Magic angle spinning ,Physics::Atomic Physics ,Nuclear Magnetic Resonance, Biomolecular - Abstract
We examine the influence of continuous-wave heteronuclear decoupling on symmetry-based double-quantum homonuclear dipolar recoupling, using experimental measurements, numerical simulations, and average Hamiltonian theory. There are two distinct regimes in which the heteronuclear interference effects are minimized. The first regime utilizes a moderate homonuclear recoupling field and a strong heteronuclear decoupling field; the second regime utilizes a strong homonuclear recoupling field and a weak or absent heteronuclear decoupling field. The second regime is experimentally accessible at moderate or high magic-angle-spinning frequencies and is particularly relevant for many realistic applications of solid-state NMR recoupling experiments to organic or biological materials.
- Published
- 2005
44. Measuring the silion fluoride bond distance in zeolites
- Author
-
Richard J. Darton, Colin A. Fyfe, Luis A. Villaescusa, Russell E. Morris, and Darren H. Brouwer
- Subjects
Diffraction ,Bond length ,Crystallography ,Magic angle ,Chemistry ,Yield (chemistry) ,Magic angle spinning ,Measure (mathematics) ,Spinning ,Molecular physics ,Intensity (heat transfer) - Abstract
A common misconception is that X-ray diffraction is the best way to measure bond distances. However, in some cases where disorder is present it can yield incorrect answers. The silicon-fluoride bond distance in fluoride-containing zeolite SFF has been measured using two magic angle spinning NMR techniques. Both techniques, variable contact time cross polarization and spinning side band intensity fitting give shorter Si-F bond distances than X-ray diffraction.
- Published
- 2004
45. An efficient peak assignment algorithm for two-dimensional NMR correlation spectra of framework structures
- Author
-
Darren H. Brouwer
- Subjects
Physics ,Quality Control ,Nuclear and High Energy Physics ,Silicon ,Crystallography ,Magnetic Resonance Spectroscopy ,Chemical shift ,Relaxation (NMR) ,Statistics as Topic ,Biophysics ,Structure (category theory) ,Graph theory ,Condensed Matter Physics ,Biochemistry ,Spectral line ,NMR spectra database ,Correlation ,Isotopes ,Models, Chemical ,Zeolites ,Ideal (ring theory) ,Algorithm ,Algorithms - Abstract
An algorithm is described for efficiently assigning the resonances in NMR spectra to the inequivalent atoms in the structure under study based on the information in two-dimensional NMR correlation experiments and the ‘connectivities’ known from the structure. The algorithm, which is based on basic graph theory concepts, finds all possible assignments sets which are consistent with the experimentally observed correlations and known connectivities in a very efficient manner. It is designed to deal with less than ideal experimental data in which there may be overlapping peaks and uncertainty about the presence or absence of correlation peaks. The algorithm was primarily developed for assigning the peaks in the high-resolution solid-state 29 Si MAS NMR spectra of highly siliceous zeolites based on two-dimensional 29 Si INADEQUATE spectra and is described using the zeolites ZSM-12 and ZSM-5 as working examples. Peak assignment for zeolite frameworks is particularly challenging since there is often little or no information to distinguish peaks from one another such as characteristic chemical shifts, relative intensities, or different relaxation times. The algorithm may be a useful tool for easily, reliably, and efficiently working out peak assignments from other types of correlation experiments on other types of systems and further examples are provided in the Supplementary material.
- Published
- 2003
46. Combined solid state NMR and X-ray diffraction investigation of the local structure of the five-coordinate silicon in fluoride-containing as-synthesized STF zeolite
- Author
-
Colin A. Fyfe, Darren H. Brouwer, Andrew R. Lewis, Russell E. Morris, and Luis A. Villaescusa
- Subjects
Silicon ,chemistry.chemical_element ,General Chemistry ,Nuclear magnetic resonance spectroscopy ,Crystal structure ,Biochemistry ,Catalysis ,chemistry.chemical_compound ,Crystallography ,Colloid and Surface Chemistry ,Nuclear magnetic resonance ,chemistry ,Solid-state nuclear magnetic resonance ,Aluminosilicate ,X-ray crystallography ,Zeolite ,Fluoride - Abstract
The local structure of the [SiO(4/2)F]- unit in fluoride-containing as-synthesized STF zeolite has been experimentally determined by a combination of solid-state NMR and microcrystal X-ray diffraction to be very close to trigonal bipyramidal. Because the fluoride ions are disordered over two sites, the resulting local structure of the [SiO(4/2)F]- unit from a conventional XRD refinement is an average between tetrahedral SiO(4/2) and five-coordinate [[SiO(4/2)F]-, giving an apparent F-Si distance longer than expected. The correct F-Si distance was determined by slow spinning MAS and fast spinning (19)F/(29)Si CP and REDOR solid-state NMR experiments and found to be between 1.72 and 1.79 A. In light of this, the X-ray structure was re-refined, including the disorder at Si3. The resulting local structure of the [SiO(4/2)F]- unit was very close to trigonal bipyramidal with a F-Si distance of 1.744 (6) A, in agreement with the NMR results and the prediction of Density Functional Theory calculations. In addition, further evidence for the existence of a covalent F-Si bond is provided by a (19)F--(29)Si refocused INEPT experiment. The resonance for the five-coordinate species at -147.5 ppm in the (29)Si spectrum is a doublet due to the (19)F/(29)Si J-coupling of 165 Hz. The peaks in this doublet have remarkably different effective chemical shift anisotropies due to the interplay of the CSA, dipolar coupling, and J-coupling tensors. The distortions from tetrahedral geometry of the neighboring silicon atoms to the five-coordinate Si3 atom are manifested in increased delta(aniso) values. This information, along with F-Si distances measured by (19)F--(29)Si CP experiments, makes it possible to assign half of the (29)Si resonances to unique tetrahedral sites. As well as determining the local geometry of the [SiO(4/2)F]- unit, the work presented here demonstrates the complementarity of the solid-state NMR and X-ray diffraction techniques and the advantages of using them together.
- Published
- 2002
47. A graph theory approach to structure solution of network materials from two-dimensional solid-state NMR data
- Author
-
Kevin P. Langendoen and Darren H. Brouwer
- Subjects
Diffraction ,Series (mathematics) ,Chemistry ,Structure (category theory) ,Graph theory ,General Chemistry ,Condensed Matter Physics ,Spectral line ,Crystallography ,Solid-state nuclear magnetic resonance ,General Materials Science ,Biological system ,Two-dimensional nuclear magnetic resonance spectroscopy ,Spin-½ - Abstract
An NMR crystallography strategy is presented for solving the structures of materials such as zeolites and related network materials from a combination of the unit cell and space group information derived from a diffraction experiment and a single two-dimensional NMR correlation spectrum that probes nearest-neighbour interactions. By requiring only a single 2D NMR spectrum, this strategy overcomes two limitations of previous approaches. First, the structures of materials having poor signal-to-noise in solid-state NMR experiments can be investigated using this approach since a series of 2D spectra is not required. Secondly, the structures of aluminophosphate materials can potentially be determined from 27Al/31P solid-state NMR experiments since this approach does not require the isolated spin pairs which have been important for determining structures of silicate materials by 29Si solid-state NMR. Using concepts from graph theory, the structure solution strategy is described in detail using a hypothetical two-dimensional network structure. A collection of two-dimensional network structures generated by the algorithm under various initial conditions is presented. The algorithm was tested on a series of 27 zeolite framework types found in the International Zeolite Association’s zeolite structure database. Finally, the structure of the zeolite ITQ-4 was solved from powder X-ray diffraction data and a single 29Si double quantum NMR correlation spectrum. The limitations of the strategy are discussed and new directions for this approach are outlined.
- Published
- 2013
48. Symmetry-Based 29Si Dipolar Recoupling Magic Angle Spinning NMR Spectroscopy: A New Method for Investigating Three-Dimensional Structures of Zeolite Frameworks
- Author
-
Colin A. Fyfe, Darren H. Brouwer, Malcolm H. Levitt, and Per Eugen Kristiansen
- Subjects
Chemistry ,Pulse sequence ,General Chemistry ,Nuclear magnetic resonance spectroscopy ,Biochemistry ,Catalysis ,Spectral line ,Homonuclear molecule ,Dipole ,Colloid and Surface Chemistry ,Nuclear magnetic resonance ,Chemical physics ,Magic angle spinning ,Molecule ,Zeolite - Abstract
A new 29Si solid-state MAS NMR experiment is described for investigating the framework structures of pure silica zeolites. The symmetry-based homonuclear dipolar recoupling sequence SR26411 has been incorporated into a two-dimensional NMR experiment to probe the Si-O-Si bonding connectivities and long-range Si-Si distances in zeolite frameworks. This dipolar recoupling sequence is shown to have a number of advantages over the J-coupling-based INADEQUATE experiment. For the clathrasil Sigma-2, it is demonstrated that there is excellent agreement between experimental double-quantum build-up curves obtained from a series of two-dimensional double-quantum correlation spectra and simulated curves which consider all Si-Si distances out to 8 A. This result suggests that this experiment could be used to solve zeolite frameworks with unknown structures.
- Published
- 2004
49. Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of 29Si–O–29Si scalar couplings
- Author
-
Bradley F. Chmelka, Darren H. Brouwer, and Sylvian Cadars
- Subjects
Diffraction ,Coupling constant ,Nuclear magnetic resonance ,Solid-state nuclear magnetic resonance ,Chemistry ,Chemical physics ,X-ray crystallography ,Cluster (physics) ,Scalar (physics) ,General Physics and Astronomy ,Physical and Theoretical Chemistry ,Spin (physics) ,Characterization (materials science) - Abstract
Subtle structural details of siliceous zeolites are probed by using two-bond scalar (J) coupling constants to characterize covalently bonded 29Si-O-29Si site pairs and local framework order. Solid-state two-dimensional (2D) 29Si{29Si} NMR measurements and first-principles calculations of 2J(29Si-O-29Si) couplings shed insights on both the local structures of siliceous zeolites Sigma-2 and ZSM-12, as well as the sensitivity of J couplings for detailed characterization analyses. DFT calculations on a model linear silicate dimer show that 2J(Si-O-Si) couplings have complicated multiple angular dependencies that make semi-empirical treatments impractical, but which are amenable to cluster approaches for accurate J-coupling calculations in zeolites. DFT calculations of 2J(29Si-O-29Si) couplings of the siliceous zeolite Sigma-2, whose framework structure is known to high accuracy from single-crystal X-ray diffraction studies, yield excellent agreement between calculated and experimentally measured 2J(Si-O-Si) couplings. For the siliceous zeolite ZSM-12, calculated 2J(29Si-O-29Si) couplings based on less-certain powder X-ray diffraction analyses deviate significantly from experimental values, while a refined structure based on 29Si chemical-shift-tensor analyses shows substantially improved agreement. 29Si J-coupling interactions can be used as sensitive probes of local structures of zeolitic frameworks and offer new opportunities for refining and solving complicated structures, in combination with complementary scattering, modeling, and other nuclear spin interactions.
- Published
- 2009
50. NMR crystallography of p-tert-butylcalix[4]arene host–guest complexes using 1H complexation-induced chemical shifts.
- Author
-
Darren H. Brouwer, Saman Alavi, and John A. Ripmeester
- Abstract
1H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host–guest complexes, providing important spatial information about the location of the guest molecules in the host cavities. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
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