Dan E. Demco

HIGH-RESOLUTION DOUBLE-QUANTUM NMR SPECTROSCOPY OF HOMONUCLEAR SPIN PAIRS AND PROTON CONNECTIVITIES IN SOLIDS

Abstract The NMR double-quantum response to a three-pulse sequence of dipolar coupled spin-1/2 pairs in powder samples spinning at the magic angle is investigated for arbitrary spinning frequencies. It is shown that double-quantum spinning sideband patterns result from the rotor modulation of the Hamiltonian. The existence of rotor modulated double-quantum spectra is confirmed by experiments with spin pairs of protons in barium chlorate monohydrate. The understanding of these sideband patterns now allow quantitative analysis of the double-quantum spectra to yield structural information in rigid solids. This is demonstrated by application to malonic acid. By combining single and double quantum NMR spectra proton distances within functional groups are determined, as well as connectivities between them. The 1H1H distance in the aliphatic group is 0.18 nm and the shortest distance between two carboxylic protons in the dimer structure is 0.22 nm. The closest aliphatic and carboxylic protons are separated by 0.26 nm, in agreement with neutron diffraction data.

Solid-state nuclear magnetic resonance spectra of dipolar-coupled multi-spin systems under fast magic angle spinning

A general treatment of nuclear magnetic resonance (NMR) spectra under magic-angle spinning (MAS) conditions is provided that is applicable both to homogeneously and inhomogeneously broadened lines. It is based on a combination of Floquet theory and perturbation theory, and allows the factorization of the spin system response into three factors that describe different aspects of the resulting MAS spectrum. The first factor directly reflects the Floquet theorem and describes the appearance of sidebands. The other two terms give the integral intensities of the resulting sidebands and their line shapes and depend on the specific features of the considered interaction. The analytical form of these two factors is derived for multi-spin dipolar interactions under fast MAS. The leading term in the expansion of the integral intensities involves products of only two spin operators whereas the linewidths, which are found to be different for the different sideband orders, are determined predominantly by three-spin te...

Dipolar couplings and internuclear distances by double-quantum nuclear magnetic resonance spectroscopy of solids

The analysis of high-resolution double-quantum nuclear magnetic resonance spinning sidebands for measuring dipolar couplings and internuclear distances in dipolar solids is described. For this purpose, the response of a dipolar-coupled spin system in a rigid solid is investigated with respect to high-resolution multiple-quantum experiments using rotor-frequency synchronized pulse sequences. For isolated, magnetically equivalent spin-1/2 pairs, exact expressions for the double-quantum spinning-sideband patterns are derived. These patterns show spinning sidebands only at odd numbers of the rotor frequency. For longer excitation/reconversion cycles, the double-quantum spinning sidebands are sensitive to changes in the internuclear distances. Using this technique, the dipolar couplings for 13C spin pairs in double-labeled polyethylene were measured in crystalline and amorphous domains, respectively. In the former the dipolar coupling reflects the carbon–carbon distance, in the latter it is reduced due to mole...

Residual dipolar couplings by 1H dipolar-encoded longitudinal magnetization, double- and triple-quantum nuclear magnetic resonance in cross-linked elastomers

The measurements of residual dipolar couplings in elastomer system is desirable, because they reflect the hindrance to molecular motions by the cross-linking, topological constraints and the external factors like mechanical stress. Dipolar-encoded longitudinal magnetization nuclear magnetic resonance (NMR) decay curves, double-quantum and triple-quantum NMR buildup intensities for measuring the residual dipolar couplings, and the associated dynamic order parameters are introduced. It is shown that in the short excitation time regime the effective dipolar network is simplified. In the limit of this model based on localized dipolar couplings, the spin response to two-dimensional pulse sequences used to record multiple-quantum (MQ) NMR coherences was evaluated for longitudinal magnetization, double-, and triple-quantum coherences of methylene, and methyl protons in synthetic 1,4-cis-polyisoprene. The dynamic order parameters can be evaluated from this NMR response using a classical scale-invariant polymer mo...

Nacre-mimetics with synthetic nanoclays up to ultrahigh aspect ratios

Nacre-mimetics hold great promise as mechanical high-performance and functional materials. Here we demonstrate large progress of mechanical and functional properties of self-assembled polymer/nanoclay nacre-mimetics by using synthetic nanoclays with aspect ratios covering three orders in magnitude (25-3,500). We establish comprehensive relationships among structure formation, nanostructuration, deformation mechanisms and mechanical properties as a function of nanoclay aspect ratio, and by tuning the viscoelastic properties of the soft phase via hydration. Highly ordered, large-scale nacre-mimetics are obtained even for low aspect ratio nanoplatelets and show pronounced inelastic deformation with very high toughness, while those formed by ultralarge nanoplatelets exhibit superb stiffness and strength, previously only reachable for highly crosslinked materials. Regarding functionalities, we report formerly impossible glass-like transparency, and excellent gas barrier considerably exceeding earlier nacre-mimetics based on natural nanoclay. Our study enables rational design of future high-performance nacre-mimetic materials and opens avenues for ecofriendly, transparent, self-standing and strong advanced barrier materials.

Selective residual dipolar couplings in cross-linked elastomers by 1H double-quantum NMR spectroscopy

1H double-quantum (DQ) solid-state NMR spectroscopy under fast magic-angle spinning (MAS) is introduced as a new spectroscopic tool for the investigation of the structure and local chain dynamics of elastomers. Dipolar connectivities between the protons of the various functional groups can be directly established from the highly resolved DQ solid-state NMR spectra as is shown for a series of cross-linked poly(styrene-co-butadiene). More quantitatively, residual dipolar couplings within and between the functional groups are evaluated selectively from the build-up curves of the double-quantum signals in the limit of the spin-pair approximation. In particular, the CH-CH and the CH2-CH couplings of butadiene, which both act predominantly along the chain-segment direction, have been measured relative to the CH2 coupling. The total build-up intensity is correlated with the cross-link density.

SPINNING-SIDEBAND PATTERNS IN MULTIPLE-QUANTUM MAGIC-ANGLE SPINNING NMR SPECTROSCOPY

Recent interest has focused on solid-state NMR experiments which excite multiple-quantum (MQ) coherences in the presence of magic-angle spinning (MAS). Such experiments have been applied to both dipolar-coupled spin I = 1/2 and half-integer quadrupolar systems. A feature common to both cases is the observation of interesting spinning sideband patterns in the indirect (MQ) dimension. In this paper, the origin of these patterns is reviewed in terms of two distinct mechanisms: first, rotor encoding of the dipolar or quadrupolar interaction caused by the change in the Hamiltonian active during the MQ reconversion period relative to the excitation period (reconversion rotor encoding, RRE); and, second, rotor modulation of the interaction during the evolution of the MQ coherences in the t 1 dimension (evolution rotor modulation, ERM). Only the first mechanism is present for total spin coherences, while for lower-order MQ coherences both mechanisms contribute to the pattern. For dipolar and quadrupolar model sys...

Study of order and dynamic processes in tendon by NMR and MRI.

Tendons are composed of a parallel arrangement of densely packed collagen fibrils that results in unique biomechanical properties of strength and flexibility. In the present review we discuss several advanced magnetic resonance spectroscopy (MRS) and imaging (MRI) techniques that have allowed us to better understand the biophysical properties of tendons and ligaments. The methods include multiple quantum and T2 filtering combined with NMR and MRI techniques. It is shown in detail how these techniques can be used to extract a number of useful parameters: 1) the 1H‐1H and 1H‐2H dipolar interactions; 2) the proton exchange rates between water and collagen, and between water molecules; 3) the distribution of fibril orientations; and 4) the anisotropy of diffusion. It is shown that relaxation data as a function of angular dependence can be obtained in vivo using mobile NMR sensors. Finally, this article describes how double quantum filtered (DQF) MRI can be used to image and monitor the healing process in injured tendons. J. Magn. Reson. Imaging 2007.

Deactivation and regeneration of a naphtha reforming catalyst

Abstract A series of naphtha reforming catalysts from different stages of the deactivation (coking) and the regeneration (decoking) processes were investigated by NMR and chemical engineering methods. The dependence of the tortuosity on the coke content was determined for both processes by NMR measurements of the intraparticle self-diffusion coefficients of adsorbed liquid n -heptane. The shrinkage of the accessible pore volume as a function of increasing coke content due to the deactivation process is compared to nitrogen adsorption (BET) measurements which show an equivalent behavior. A crude model was adapted to predict qualitatively the relationship between the tortuosity and the average pore diameter. Longitudinal ( T 1 ) and transverse ( T 2 ) NMR relaxation times measured for protons of adsorbed liquid n -heptane, provide information on the pore morphology changes which can be corroborated by the tortuosity measurements. The chemical composition of the coke layer, which was investigated by 1 H magic angle spinning (MAS) and 13 C cross polarization (CP)/MAS NMR spectroscopy, is shown to change during both deactivation and decoking processes. Moreover, the micro-structure of the fresh catalyst and the fully regenerated catalyst was investigated by scanning electron microscopy (SEM). The experimental results indicate that a full recovery of the activity of the clean catalyst is not achieved by the regeneration process, and that the quality of regeneration depends on the coke content reached during the deactivation/regeneration cycle.

Anisotropy of collagen fiber orientation in sheep tendon by 1H double-quantum-filtered NMR signals

The anisotropy of the angular distribution of collagen fibrils in a sheep tendon was investigated by 1H double-quantum (DQ) filtered NMR signals. Double-quantum build-up curves generated by the five-pulse sequence were measured for different angles between the direction of the static magnetic field and the axis of the tendon plug. Proton residual dipolar couplings determined from the DQ build-up curves in the initial excitation/reconversion time regime which mainly represent the bound water are interpreted in terms of a model of spin-1/2 pairs with their internuclear axes oriented on average along the fibril direction in the presence of proton exchange. The angular distribution of collagen fibrils around the symmetry axis of the tendon measured by the anisotropy of the residual dipolar couplings was described by a Gaussian function with a standard deviation of 12 degrees +/-1 degrees and with the center of the distribution at 4 degrees +/-1 degrees. The existence of this distribution is directly reflected in the finite value of the residual dipolar couplings at the magic angle, the value of the angular contrast, and the oscillatory behavior of the DQ build-up curves. The 1H residual dipolar couplings were also measured from the doublets recorded by the DQ-filtered signals. From the angular dependence of the normalized splitting the angular distribution of the collagen fibrils was evaluated using a Gaussian function with a standard deviation of 19 degrees +/-1 degrees and with the center of distribution at 2 degrees +/-1 degrees. The advantages and disadvantages of these approaches are discussed.