Biomedical subjects

Darren H Brouwer

Publications and source records attributed to Darren H Brouwer.

9 recordsLinked to original sources

PubMed2006

Symmetry-based recoupling of proton chemical shift anisotropies in ultrahigh-field solid-state NMR.

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.

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PubMed2006

Optimization, standardization, and testing of a new NMR method for the determination of zeolite host-organic guest crystal structures.

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.

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PubMed2005

Heteronuclear decoupling interference during symmetry-based homonuclear recoupling in solid-state NMR.

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.

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PubMed2005

A solid-state NMR method for solution of zeolite crystal structures.

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.

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PubMed2005

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.

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.

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PubMed2005

Symmetry-based 29Si dipolar recoupling magic angle spinning NMR spectroscopy: a new method for investigating three-dimensional structures of zeolite frameworks.

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.

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PubMed2004

Effect of molecular oxygen on the variable-temperature 29Si MAS NMR spectra of zeolite-sorbate complexes.

Structure determinations of siliceous zeolite-sorbate host-guest complexes by solid-state NMR require highly resolved 29Si MAS NMR spectra. As the temperature is lowered, the 29Si MAS NMR spectra of many zeolite-sorbate complexes become broadened such that the resolution of the individual 29Si peaks is lost, limiting the application of solid-state NMR for structure determination. It is shown that the 29Si peak widths are related to the 29Si T2 relaxation times and that the source of the 29Si relaxation and the line broadening is paramagnetic molecular oxygen in the channels of the zeolite. Removal of the oxygen by purging the sample with nitrogen gas leads to a dramatic increase in the resolution of the 29Si MAS NMR spectrum of the p-dibromobenzene/ZSM-5 complex. An analysis of the individual 29Si T1 relaxation times reveals that the oxygen molecules are localized mainly in the zigzag channels of ZSM-5, suggesting that the p-dibromobenzene molecules are located in the channel intersections.

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PubMed2003

An efficient peak assignment algorithm for two-dimensional NMR correlation spectra of framework structures.

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 29Si MAS NMR spectra of highly siliceous zeolites based on two-dimensional 29Si 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.

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PubMed2002

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.

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.

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