Multiple-quantum NMR Spectroscopy Research Articles

Effect of Degradation Processes Caused by a Small Perturbation on the Growth of the Average Cluster Size of Correlated Spins in Multiple Quantum NMR Spectroscopy of Solids

Effect of Degradation Processes Caused by a Small Perturbation on the Growth of the Average Cluster Size of Correlated Spins in Multiple Quantum NMR Spectroscopy of Solids

High-performance recyclable cross-linked polyurethane with orthogonal dynamic bonds: The molecular design, microstructures, and macroscopic properties

Polyurethane materials (PUs) have been widely used in industry and daily life due to the versatile chemistry. However, despite the rapid advance in synthetic chemistry, it still remains a significant challenge for the facile fabrication of PUs with a single cross-linked network embedded with excellent mechanical properties and recyclability. Herein, in this study, we proposed a simple strategy to fabricate a high-performance recyclable cross-linked PU and revealed the relationship between microscopic structure and macroscopic properties. The UPy (2-ureido-4-[1H]-pyrimidione) motifs were incorporated into the backbone chains of PUs, where the quadruple hydrogen bonding interactions between UPy dimers can significantly enhance the mechanical strength and toughness. Furthermore, a single small molecular Diels-Alder adduct was utilized as the chemical crosslinker, rendering the final cross-linked PUs healable and recyclable. The thermal reversibility of the Diels-Alder reaction was well verified by DSC and solid-state NMR spectroscopy. Notably, it was found that the incorporation of UPy motifs could enhance the strain-induced crystallization (SIC), leading to a large stress at break. The structural and dynamic changes induced by SIC were quantitatively addressed by proton multiple-quantum NMR spectroscopy and SAXS experiments, where SIC further imposed restrictions on the mobility of surrounding polymer chains in the soft domain and led to the change of microphase separated nanostructures. Overall, a simple strategy is proposed here for the facile fabrication of high performance recyclable PUs, and the detailed investigation here on the structure-property relationship may further provide insights into developing high performance polymeric materials.

Probing the 1H spin distribution in hybrid organic-inorganic gels by multiple-quantum NMR spectroscopy

Multiple-quantum (MQ) NMR was used to characterize the organic phase distribution in a series of hybrid organic-inorganic gels namely methyltrimethoxysilane aerogel and QM resins (trimethylsiloxysilicate polymers) with different composition. The Gaussian model of distribution of MQ coherence intensities was used to determine the effective 1H spin cluster size (N). The growth of the effective cluster size with the time of excitation of MQ NMR coherences τ indicates the uniform distribution of organic moieties in the studied gels, but the growth rate is significantly slower than that of adamantane which was used for comparison as an example of the systems with uniform distribution of 1H spins. The growth rate was described by the exponents α in the scaling law of the form N~τα. The obtained values of α were 3.07 for adamantane, 1.27 for aerogel and 1.8 for QM resins. This difference was attributed to the surface distribution of the 1H spins in the studied gels. The growth rate of spin clusters in aerogel is slower in comparison with QM resins with different content of 1H spins which show similar growth rate among each other. This result was explained by the higher local concentration of 1H spins on the surfaces of silica regions in QM resins.

Viscoelasticity and Structures in Chemically and Physically Dual-Cross-Linked Hydrogels: Insights from Rheology and Proton Multiple-Quantum NMR Spectroscopy

Hydrogels have received considerable attention as an innovative material due to their widespread applications in various fields. As a soft and wet material, its mechanical behavior is best understood in terms of the viscoelastic response to the periodic deformation, which is closely related to the microscopic chemically/physically cross-linked structures. Herein, a dual-cross-linked (DC) hydrogel, where a physically cross-linked network by ionic coordination (Fe3+) is imposed on a chemically cross-linked poly(acrylamide-co-acrylic acid) network, was studied in detail by rheology and proton multiple-quantum (MQ) NMR spectroscopy. Rheology experiments revealed the diverse temperature- and strain-frequency-dependent viscoelastic behaviors for DC hydrogels induced by the dynamic Fe3+ coordination interactions, in contrast to the single chemically cross-linked (SC) hydrogels. During the shear experiment, the trivalent Fe3+ complex with moderate/weak binding strength might transform to those with strong binding...

Aluminum Redistribution during the Preparation of Hierarchical Zeolites by Desilication.

A literature survey reveals a prominent reduction in the concentration of Brønsted acid sites in hierarchically organized zeolites with increasing mesoporous or external surface area independent of the framework type or synthesis route; this suggests a common fundamental explanation. To determine the cause, nature, and impact of the underlying changes in aluminum speciation, this study combines a multitechnique analysis that integrates basic characterization, a detailed synchrotron XRD and multiple-quantum NMR spectroscopy assessment, and catalytic tests to correlate evolution of the properties with performance during successive steps in the preparation of hierarchical MFI-type zeolites by desilication. The findings, subsequently generalized to FAU- and BEA-type materials, identify the crucial impact of calcination on the protonic form, which is an integral step in the synthesis and regeneration of zeolite catalysts; on aluminum coordination; and the associated acidity trends.

Slow Conformational Dynamics in the Cystoviral RNA-Directed RNA Polymerase P2: Influence of Substrate Nucleotides and Template RNA

The RNA-directed RNA polymerase P2 from cystovirus ϕ6 catalyzes the de novo synthesis of positive and negative strands of the viral double-stranded RNA genome. P2 is mobile on the slow, microsecond to millisecond time scale with various motional modes, putatively assisting in RNA translocation and catalysis. Here we investigate the influence of the extreme 3'-end sequence of the single-stranded RNA templates and the nature of the substrate nucleotide triphosphates on these motional modes using multiple-quantum NMR spectroscopy. We find that P2, in the presence of templates bearing the proper genomic 3'-ends or the preferred initiation nucleotide, displays unique dynamic signatures that are different from those in the presence of nonphysiological templates or substrates. This suggests that dynamics may play a role in the fidelity of recognition of the correct substrates and template sequences to initiate RNA polymerization.

MQ NMR and SPME Analysis of Nonlinearity in the Degradation of a Filled Silicone Elastomer

Radiation-induced degradation of polymeric materials occurs through numerous, simultaneous, competing chemical reactions. Although degradation is typically found to be linear in adsorbed dose, some silicone materials exhibit nonlinear dose dependence due to dose-dependent dominant degradation pathways. We have characterized the effects of radiative and thermal degradation on a model filled-PDMS system, Sylgard 184 (commonly used in electronic encapsulation and in biomedical applications), using traditional mechanical testing, NMR spectroscopy, and sample headspace analysis using solid-phase microextraction (SPME) followed by gas chromatography/mass spectrometry (GC/MS). The mechanical data and (1)H spin-echo NMR spectra indicated that radiation exposure leads to predominantly cross-linking over the cumulative dose range studied (0-250 kGy) with a rate roughly linear with dose. (1)H multiple-quantum NMR spectroscopy detected a bimodal distribution in the network structure, as expected from the proposed structure of Sylgard 184. The MQ NMR spectra further indicated that the radiation-induced structural changes were not linear in adsorbed dose and that competing chain scission mechanisms made a greater contribution to the overall degradation process in the range of 50-100 kGy (although cross-linking still dominated). The SPME-GC/MS data were analyzed using principal component analysis (PCA), which identified subtle changes in the distributions of degradation products (the cyclic siloxanes and other components of the material) as a function of age that provide insight into the dominant degradation pathways at low and high adsorbed dose.

Slow Backbone Dynamics of the C-Terminal Fragment of Human Centrin 2 in Complex with a Target Peptide Probed by Cross-Correlated Relaxation in Multiple-Quantum NMR Spectroscopy

The C-terminal domain of human centrin 2 (C-HsCen2) strongly binds to P1-XPC, a peptide comprising 17 amino acids with a NWKLLAKGLLIRERLKR sequence. This peptide corresponds to residues N847-R863 of XPC, a protein involved in the recognition of damaged DNA during the initial step of the nucleotide excision repair pathway. The slow internal dynamics of the protein backbone in the C-HsCen-P1-XPC complex was studied by measuring the relaxation rates of zero- and double-quantum coherences involving neighboring pairs of carbonyl 13C and amide 15N nuclei. These relaxation rates, which reflect dynamics on time scales in the range of micro- to milliseconds, vary significantly along the protein backbone. Analysis of the relaxation rates at different CaCl2 concentrations and ionic strengths shows that these slow motions are mainly affected by the binding of a Ca2+ ion to the lower-affinity EF-hand III. Moreover, we discuss the possible functional role of residues that undergo differential exchange in the formation of HsCen homodimers.

Swelling Heterogeneities in End-Linked Model Networks: A Combined Proton Multiple-Quantum NMR and Computer Simulation Study

The segmental order of monomodal and bimodal polymer networks is investigated by proton multiple-quantum NMR spectroscopy as applied to poly(dimethylsiloxane) model systems and by Monte Carlo simulations using the bond fluctuation model. The multiple-quantum method is sensitive to chain order parameter distributions and therefore probes heterogeneities in the extent of fast motional averaging of individual network strands subject to topological constraints. These data are in qualitative agreement with the simulations. We find a broadening of the chain order parameter distribution upon swelling accompanied by a nonaffine change of this distribution, indicating a heterogeneous swelling process. Comparing the simulated tensor order parameter with the autocorrelation function of segments, we observe major deviations in the swollen state but also for the long-chain fraction of the dry, bimodal network. These effects are attributed to the fluctuation dependence of the tensor order parameter which comprises information about both orientational order as well as fluctuation properties of fast segment reorientation processes. In general, our results indicate that segmental order is not simply related to the cross-link density. Fluctuations on larger scales such as reorientation of chain clusters, dynamical correlations between segments in the entangled state, and heterogeneities in the segmental fluctuations have an essential influence on the tensorial order. The spatial length scale of the heterogeneities is also investigated by self-diffusion measurements of the solvent molecules.

Measurement of long-range cross-correlation rates using a combination of single- and multiple-quantum NMR spectroscopy in one experiment.

A method is described to determine long-range cross-correlations between the modulations of an anisotropic chemical shift (e.g., of a C' carbonyl carbon in a protein) and the fluctuations of a weak long-range dipolar interaction (e.g., in cross-correlation between the same C' carbonyl and the H(N) proton of the neighboring amide group). Such long-range correlations are difficult to measure because the corresponding long-range scalar couplings are so small that Redfield's secular approximation is often violated. The method, which combines features of single- and double-quantum NMR spectroscopy, allows one to cancel the effects of dominant short-range dipolar interactions (e.g., between the CSA of the amide nitrogen N and the dipolar coupling to its attached proton H(N)) and is designed so that the secular approximation is rescued even if the scalar coupling between the long-range dipolar coupling partners is very small. The cross-correlation rates thus determined in ubiquitin cover a wide range because of local motions and variations of the CSA tensors.

Multiple-quantum dynamics of one-dimensional nuclear spin systems in solids

Multiple-quantum spin dynamics is studied using analytic and numerical methods for one-dimensional finite linear chains and rings of nuclear spins 1/2 coupled by dipole-dipole interactions. An approximation of dipole-dipole interaction between nearest neighbors having the same constants is used to obtain exact expressions for the intensities of the multiple-quantum coherences in the spin systems studied, which are initially in thermal equilibrium and whose evolution is described by a two-spin two-quantum Hamiltonian. An approximation of nearest neighbors with arbitrary dipole-dipole interaction constants is used to establish a simple relationship between the multiple-quantum dynamics and the dynamics of spin systems with an XY Hamiltonian. Numerical methods are developed to calculate the intensities of multiple-quantum coherences in multiple-quantum NMR spectroscopy. The integral of motion is obtained to expand the matrix of the two-spin two-quantum Hamiltonian into two independent blocks. Using the nearest-neighbor approximation the Hamiltonian is factorized according to different values of the projection operator of the total spin momentum on the direction of the external magnetic field. Results of calculations of the multiple-quantum dynamics in linear chains of seven and eight nuclear spins and a six-spin ring are presented. It is shown that the evolution of the intensities of the lowest-order multiple-quantum coherences in linear chains is accurately described allowing for dipole-dipole interaction of nearest and next-nearest neighbors only. Numerical calculations are used to compare the contributions of nearest and remote spins to the intensities of the multiple-quantum coherences.

Quadrupolar and Chemical Shift Tensors Characterized by 2D Multiple-Quantum NMR Spectroscopy

The present work discusses a new 2D NMR method for characterizing the principal values and relative orientations of the electric field gradient and the chemical shift tensors of half-integer quadrupolar sites. The technique exploits the different contributions that quadrupolar and shielding interactions impart on the evolution of multiple-quantum and of single-quantum coherences, in order to obtain 2D powder lineshapes that are highly sensitive to these nuclear spin coupling parameters. Different spinning variants of this experiment were assayed, but it was concluded that a static version can yield the highest sensitivity to the values of the principal components and to the relative geometries of the local coupling tensors. It was found that correlating the central transition evolution with the highest available order of the spin coherence was also helpful for maximizing this spectral information. Good agreement between data obtained on 87Rb (S = 32) and 59Co (S = 72) samples and ideal theoretical lineshape predictions of this experiment was obtained, provided that heterogeneities in the multiple-quantum excitation and conversion processes were suitably accounted by procedures similar to those described in the spin-12 multiple-quantum NMR literature.

Enhanced effect of magnetic field gradients using multiple quantum NMR spectroscopy applied to self-diffusion coefficient measurement

NMR spectroscopy can be used to measure molecular self-diffusion coefficients using a modified spin-echo pulse sequence known as the longitudinal eddy current delay (LED) method which incorporates pulsed magnetic field gradients. The effect of such gradients on multiple quantum coherences is investigated and shown to be of benefit for the measurement of diffusion coefficients using modest gradient strengths. The multiple quantum coherences involved in the spin operator 8I(x)I(y)I(y)S(y) of a weakly coupled SI(3) spin system ((13)CH(3) at natural abundance, in alanine in this case) were studied. The multiple quantum coherences included the quadruple-quantum (QQ) order of (3I + S), double-quantum (DQ) order of (31 - S) and (31 + S), and zero-quantum (ZQ) order of (I - S) with effective magnetogyric ratios of (3 gamma(H) + gamma(C)), (3 gamma(H) - gamma(C)), (gamma(H) + gamma(C)) and (gamma(H) - gamma(C)) respectively. The results show that the use of higher level quantum coherences for diffusion coefficient measurement can have the practical effect of significantly enhancing the gradient strength and this is important for slowly diffusing species.

Enhanced effect of magnetic field gradients using multiple quantum NMR spectroscopy applied to self-diffusion coefficient measurement

NMR spectroscopy can be used to measure molecular self-diffusion coefficients using a modified spin-echo pulse sequence known as the longitudinal eddy current delay (LED) method which incorporates pulsed magnetic field gradients. The effect of such gradients on multiple quantum coherences is investigated and shown to be of benefit for the measurement of diffusion coefficients using modest gradient strengths. The multiple quantum coherences involved in the spin operator 8I x I y I y S y of a weakly coupled SI3 spin system (13CH3 at natural abundance, in alanine in this case) were studied. The multiple quantum coherences included the quadruple-quantum (QQ) order of (3I + S), double-quantum (DQ) order of (3I - S) and (3I + S), and zero-quantum (ZQ) order of (I - S) with effective magnetogyric ratios of (3γH + γC), (3γH - γC), (γH + γC) and (γH - γC) respectively. The results show that the use of higher level quantum coherences for diffusion coefficient measurement can have the practical effect of significantly enhancing the gradient strength and this is important for slowly diffusing species.

Advanced solid‐state NMR spectroscopy of strongly dipolar coupled spins under fast magic angle spinning

Continuing recent progress in the design of magic angle spinning (MAS) equipment now allows spinning frequencies of up to 35 kHz in commercial MAS probes. Such high frequencies make MAS an important tool for the manipulation of strong dipolar couplings, in particular when supplemented by suitable pulse techniques. This article describes convenient and simple strategies for designing techniques that are suited to application under such conditions. The strategies are then applied to widely used pulse techniques for line narrowing, recoupling of spin interactions, and multiple-quantum NMR spectroscopy. Representative results illustrate the usefulness of the techniques. ©1998 John Wiley & Sons, Inc. Concepts Magn Reson 10: 99–128, 1998

Multidimensional solid state NMR: A unique tool for the characterisation of complex materials

AbstractA survey is given on new developments in multidimensional solid state NMR which evolves as an indispensable tool of materials characterisation. After a short introduction NMR is compared with other techniques, in particular X‐ray and neutron scattering. The unique information available by solid state NMR is demonstrated by specific examples of structural studies of chain conformation, advanced aspects of chain dynamics, phase separation and interfacial effects as well as non‐linear plastic deformation in polymers. Emphasis is placed on the new development of high resolution multiple quantum NMR spectroscopy of abundant nuclei in solids with first experimental examples on the location of hydrogen positions in organic solids, structure of inorganic glasses and chain order in polymer melts and elastomers.

Further Developments in MQMAS NMR Spectroscopy for Spin-[formula omitted]Nuclei

It is shown that multiple-quantum NMR spectroscopy for quadrupolar spins in powder solids gives enhanced resolution over single-quantum spectra for the case of triple-quantum (TQ) NMR for spin-32. The problem of generating TQ coherence evenly over all molecular orientations is considered and solutions are found. The effects of sample spinning on the excitation of the TQ coherence for spin-32is considered in detail. It is shown that selection of a suitable spinning speed is essential if sensible spectra are to be recorded. Finally, it is predicted that the effects of the quadrupole interaction can be removed completely from triple-quantum spectra of spin-32by spinning the sample at an angle of 70.1° or 30.6° with respect to the applied field, and experimental results are given. Conclusions derived in this work are applicable to the newly termedmultiple-quantummagic-angle-spinning experiment.

Solid-state proton multiple-quantum NMR spectroscopy with fast magic angle spinning

The feasibility of multiple-quantum NMR spectroscopy with high resolution for protons in solids is explored. A new multiple-quantum excitation sequence suitable for use with fast magic angle spinning is described, and its performance is compared to that of both static and slow-spinning multiple-quantum methods. Modified sequences with scale the rate of development of the multiple-quantum coherences are also demonstrated, and two-dimensional double-quantum spectra of adamantane and polycarbonate are presented.

Distribution and motion of organic guest molecules in zeolites

Hexamethyltienzene (HMB), adamantane, and naphthalene are adsorbed into zeolite Na-Y by a vapor phase impregnation method. Results from thermogravimetric analyses, Raman, and 13 C NMR measurements indicate that the organic species reside in the supercages of Na-Y. Proton multiple-quantum NMR spectroscopy is used to investigate the distribution of the organic species adsorbed within the cavities of Na-Y and the organic species occluded during the synthesis process into the intracrystalline voids of thecubic and hexagonal polytypes of faujasite and ZSM-18

Multiple quantum NMR spectroscopy methods for measuring the apparent self‐diffusion coefficient of In Vivo lactic acid

Nuclear magnetic resonance spectroscopy methods are presented for measuring the apparent self-diffusion coefficient of lactic acid in vivo. These techniques incorporate diffusion-sensitive gradients into multiple quantum NMR pulse sequences that are employed for spectral editing. Both diffusion-sensitive zero quantum (ZQ) and double quantum (DQ) spectral editing techniques have been developed and the DQ method has been used to measure the apparent self-diffusion coefficient of in vivo lactic acid in H-MESO-1 human tumor xenografts subcutaneously implanted in athymic mice.