Distance measurements between carbon and bromine using a split-pulse PM-RESPDOR solid-state NMR experiment

The field dependence of the proton (I) spin–lattice relaxation rate is calculated for a dipole–dipole coupled spin pair, (I = 1/2) − (S = 1), where the quadrupole nucleus (S) is 2H or 14N with asymmetry parameter η = 0. The observed relaxation profile shows a marked enhancement for equal proton Larmor and quadrupole spin frequencies (i.e. ω I = ωQ). This phenomenon is referred to as the quadrupole dip, and has been observed, for instance, in 14N−1H amide groups of immobilized proteins. In this work, an analysis of the observed relaxation enhancement is presented when the dipole–dipole coupling and the quadrupole interaction are modulated by the overall re-orientational motion. A characteristic low field dispersion is observed when (3/2) τRω I ≥ 1, where τR is the rotational correlation time and ω I is the proton Larmor frequency. At higher fields, the relaxation peak exhibits a Lorentzian-like line shape, , which is centred at the quadrupole frequency. The quadrupole spin system shows a spin–lattice T 1Q and a spin–spin relaxation time T 2Q that become equal in the zero field limit. In the slow tumbling limit, the quadrupole spin relaxation times, T 1Q, T 2Q, are equal to (3/2)τR.

Broad developments in experimental NMR techniques have opened new and exciting opportunities for application of solid state nuclear magnetic resonance (SS NMR) in studies of gas hydrates and inclusion compounds in general. Perhaps the most important advance of the last 10 years was the extension into very high magnetic fields beyond 20 T. This progress is especially significant in studies concerned with low-γ, low natural abundance, and quadrupolar nuclei. This work reports our recent exploration of clathrate hydrates and other inclusion compounds (β-quinol, tert-Bu-Calix[4], and dodecasil-3C) with SS NMR of nuclei that were not so long ago completely out of reach for NMR, namely 131Xe, 83Kr, and 33S. Although 129Xe is a widely used NMR probe, applications of the low-γ isotope 131Xe were very scarce. Being a quadrupolar spin 3/2 nucleus, 131Xe provides an additional probe for sampling the electric field gradients in inclusion compounds. Another nucleus that has been seriously under-explored is 83Kr, with its very low γ being the main obstacle, and along with quadrupolar coupling we report the first detection of the chemical shift anisotropy in krypton. The relative values of the Sternheimer antishielding factors for 131Xe and 83Kr, obtained by comparison of the spectra of the two in identical cage environments, are also discussed. Though 33S NMR of solids is notoriously difficult due to its low γ, low natural abundance, and relatively large quadrupolar moment, working at the field of 21.1 T it was possible to acquire, in a reasonable time, natural abundance 33S SS NMR spectra of various H2S and SO2 gas hydrates and inclusion compounds. In most cases the spectra are dominated by the quadrupolar interactions, providing information on the symmetry of the cages encapsulating the guest molecules, and also show the effects of very rapid reorientation of the encaged H2S and SO2. The impact of the introduction of new NMR nuclei on hydrate research is discussed.

Efficient polarization transfer between spin-1/2 and [formula omitted]N nuclei in solid-state MAS NMR spectroscopy

In solid-state NMR, deuteron (2H) spectroscopy can be performed in full analogy to1H spectroscopy, including2H chemical-shift resolution and2H-X dipolar correlation schemes, when the NMR experiments are conducted in a “rotor-synchronized” fashion under fast magic-angle spinning. Here, 2H-X NMR experiments of this type, including2H-15N and2H-1H chemical-shift correlations and distance measurements, are introduced and demonstrated on cytosine monohydrate, whose acidic protons can readily be replaced by deuterons by recrystallization from D2O. In this way,2H NMR spectroscopy provides information complementary to1H NMR data, which is particularly useful for studying hydrogen bonds in supra- or biomolecular systems.

PM, Vector Composing, Armstrong PM, QAM-based PM, Equivalent Analysis Abstract. Both the Armstrong phase modulation (PM) and QAM-based PM are typical vector composing circuits in the course “Radio Frequency Circuits”. Due to having innate defects of nonlinear phase distortion and parasitical AM, the characteristics of Armstrong PM are not good unless the modulation exponent is small enough. Therefore, the Armstrong PM is an approximate circuit. The QAM- based PM is a quite different circuit from the Armstrong PM, and its defects are not given in general teaching material. The equivalence of two circuits is proved in this paper by mathematical method. So, the QAM-based PM has same pros and cons with Armstrong PM and can be replaced with the latter completely.

Unveiling the impact of second-order 1H-17O Quadrupolar-Dipolar interaction on solid-state NMR spectroscopy

Previous studies have shown that racemic mixtures of 40- and 42-residue amyloid-β peptides (d,l-Aβ40 and d,l-Aβ42) form amyloid fibrils with accelerated kinetics and enhanced stability relative to their homochiral counterparts (l-Aβ40 and l-Aβ42), suggesting a "chiral inactivation" approach to abrogating the neurotoxicity of Aβ oligomers (Aβ-CI). Here we report a structural study of d,l-Aβ40 fibrils, using electron microscopy, solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations. Two- and three-dimensional solid-state NMR spectra indicate molecular conformations in d,l-Aβ40 fibrils that resemble those in known l-Aβ40 fibril structures. However, quantitative measurements of 13C-13C and 15N-13C distances in selectively labeled d,l-Aβ40 fibril samples indicate a qualitatively different supramolecular structure. While cross-β structures in mature l-Aβ40 fibrils are comprised of in-register, parallel β-sheets, our data indicate antiparallel β-sheets in d,l-Aβ40 fibrils, with alternation of d and l molecules along the fibril growth direction, i.e., antiparallel "rippled sheet" structures. The solid-state NMR data suggest the coexistence of d,l-Aβ40 fibril polymorphs with three different registries of intermolecular hydrogen bonds within the antiparallel rippled sheets. DFT calculations support an energetic preference for antiparallel alignments of the β-strand segments identified by solid-state NMR. These results provide insight into the structural basis for Aβ-CI and establish the importance of rippled sheets in self-assembly of full-length, naturally occurring amyloidogenic peptides.

Tracking the evolution and differences between guest-induced phases of Ga-MIL-53 via ultra-wideline 69/71Ga solid-state NMR spectroscopy

Spinning-rate encoded chemical shift correlations from rotational resonance solid-state NMR experiments

Solid-state NMR spectroscopy is one of the most widely used methods for investigating crystal structures, along with the X-ray and neutron diffraction methods. Solid-state NMR can provide structural information including isotropic chemical shift, dipolar and quadrupolar couplings, spin diffusion, and chemical shift tensor. Among these, the chemical shift tensor is of particular significance because the electronic environment around a nucleus is directly reflected on the chemical shift tensor. However, full information of the chemical shift tensor, including principal values and axes, is difficult to obtain experimentally because a large single crystal is required for the measurement. On the other hand, we have proposed the use of a magnetically oriented microcrystal array (MOMA) as an alternative to a single crystal.[1,2] A MOMA is a composite in which microcrystals are aligned three-dimensionally, prepared by using a time-dependent magnetic field. We recently demonstrated that the13C chemical shift tensors of L-alanine crystal can be completely determined by application of the standard procedure in the single-crystal rotation method to a MOMA of L-alanine microcrystals,[3] as shown in Figure 1. The L-alanine MOMA produces sharp resonance peaks without resolution enhancement by magic angle spinning (MAS). In addition, we observed that the positions of the13C resonance peaks vary systematically as a function of the angle ψ that is the sample-rotation angle about the axis inclined by the magic angle with respect to the NMR magnetic field. From the ψ-dependence of the chemical shifts,13C chemical shift tensor was completely determined. We confirmed that the combination of MOMA with the single-crystal rotation method can be applied to other nuclei such as31P and15N. These results clearly show that the MOMA method is a powerful tool for obtaining full information of the chemical shift tensor from a microcrystalline powder without MAS.

The acquisition of ideal powder line shapes remains a recurring challenge in solid-state wideline nuclear magnetic resonance (NMR). Certain species, particularly quadrupolar spins in sites associated with large electric field gradients, are difficult to excite uniformly and with good efficiencies. This paper discusses some of the opportunities that arise upon departing from standard spin-echo excitation approaches and switching to echo sequences that use low-power, frequency-swept radio frequency (rf) pulses instead. The reduced powers demanded by such swept rf fields allow one to excite spins in different crystallites efficiently and with orientation-independent pulse angles, while the large bandwidths of interest that are needed by the measurement can be covered, thanks to the use of broadband frequency sweeps. The fact that the spins' evolution and ensuing dephasing starts at the beginning of such rf manipulation calls for the use of spin-echo sequences; a number of alternatives capable of providing the desired line shapes both in the frequency and in the time domains are introduced and experimentally demonstrated. Sensitivity- and lineshape-wise these experiments are competitive vis-a-vis current implementations of wideline quadrupolar NMR based on hard rf pulses; additional opportunities that may derive from these ideas are also briefly discussed.

Today, nuclear magnetic resonance (NMR) is among the most efficient tools in structural studies. Measurement of interatomic distances is the most common way of determining high‐resolution structures of molecules using NMR techniques. In this article, we describe NMR techniques for static powder samples, based on a two‐dimensional single‐echo scheme, enhanced with adiabatic cross‐polarization. They can significantly increase the accuracy of measuring internuclear distances and turn NMR into a high‐precision crystallographic technique, complementing the X‐ray, and neutron‐scattering methods. Experimental examples are presented for intramolecular CN and CC distances in α‐crystalline form of glycine. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A: 56–67, 2008.

Probing 29Si-17O connectivities and proximities by solid-state NMR

The solid-state structure of (CH3)3SnF was reinvestigated by X-ray diffraction techniques as well as by multi-nuclear solid-state NMR spectroscopy. Trimethyltin fluoride crystallizes from hot ethanol in the orthorhombic space group Pnma at room temperature and changes to a low-temperature orthorhombic phase (space group: Cmcm) below -70 °C. In both modifications, trimethyltin fluoride adopts a linear chain structure with symmetric fluorine bridges, in contrast to previous reports. During its synthesis, (CH3)3SnF precipitates in another, poorly crystalline modification, as shown by powder X-ray diffraction. Solid-state MAS NMR experiments of both room-temperature phases of (CH3)3SnF (non-recrystallized and recrystallized) were carried out for the (1)H, (13)C, (19)F, and (119)Sn nuclei. The (119)Sn{(19)F, (1)H} and (19)F{(1)H} NMR spectra offer unambiguous determination for the (19)F and (119)Sn shielding tensors. The (119)Sn{(1)H} solid-state NMR spectra are in agreement with pentacoordination of Sn in this compound for the non-recrystallized and the recrystallized modifications. Based on the solid-state NMR results, the non-recrystallized modification of (CH3)3SnF also consists of linear, symmetrically fluorine-bridged chains, and differs from the recrystallized orthorhombic phase only in packing of the chains.

A general strategy of structural analysis of alumina silicate by combining various solid-state NMR measurements such as single pulse, multi-quantum magic angle spinning, double-quantum homo-nuclear correlation under magic angle spinning (DQ-MAS), and cross-polarization hetero-nuclear correlation (CP-HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for (1) H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of (27) Al in single pulse, DQ-MAS, and even in triple-quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ-MAS methods for homo-nuclear correlations between both (27) Al-(27) Al and (29) Si-(29) Si and by CP-HETCOR for hetero-nuclear correlations between (27) Al-(29) Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina-silicate minerals.