Solid-state variable-temperature H MAS NMR studies on deuterated polyethylene

Solid-state varible-temperature/magic angle spinning(VT/MAS) ¹H NMR measurements were carried out on deuterated polyethylene. From these experimental results it was found that the ¹H chemical shift induced by conformational and morphological changes of the polyethylene sample is within the linewidth of ≈ 0.5 ppm. Furthermore, from MAS/dipolar decoupling experiments it was found that the resonance frequency of the proton varies linearly with the inverse square of the deuterium decoupling power. This experimental finding is discussed theoretically.     

Solid-state NMR studies of supercapacitors

Electrochemical double-layer capacitors, or ‘supercapacitors’ are attracting increasing attention as high-power energy storage devices for a wide range of technological applications. These devices store charge through electrostatic interactions between liquid electrolyte ions and the surfaces of porous carbon electrodes. However, many aspects of the fundamental mechanism of supercapacitance are still not well understood, and there is a lack of experimental techniques which are capable of studying working devices. Recently, solid-state NMR has emerged as a powerful tool for studying the local environments and behaviour of electrolyte ions in supercapacitor electrodes. In this Trends article, we review these recent developments and applications. We first discuss the basic principles underlying the mechanism of supercapacitance, as well as the key NMR observables that are relevant to the study of supercapacitor electrodes. We then review some practical aspects of the study of working devices using ex situ and in situ methodologies and explain the key advances that these techniques have allowed on the study of supercapacitor charging mechanisms. NMR experiments have revealed that the pores of the carbon electrodes contain a significant number of electrolyte ions in the absence of any charging potential. This has important implications for the molecular mechanisms of supercapacitance, as charge can be stored by different ion adsorption/desorption processes. Crucially, we show how in situ NMR experiments can be used to quantitatively study and characterise the charging mechanism, with the experiments providing the most detailed picture of charge storage to date, offering the opportunity to design enhanced devices. Finally, an outlook for future directions for solid-state NMR in supercapacitor research is offered.     

固体NMR考察有机硅聚合物的热解过程

应用²⁹Si和¹³C固体NMR方法考察了几种有机硅聚合物前驱体热解制备SiC陶瓷的过程,通过²⁹Si和¹³C NMR谱分析了热解产物的结构.结果表明聚碳硅烷及其氧化交联聚合物前驱体在高温下的热解可以获得SiC陶瓷相.     

Recent progress in solid-state NMR studies of drugs confined within drug delivery systems

Recent progress in the application of solid-state NMR (SS NMR) spectroscopy in structural studies of active pharmaceutical ingredients (APIs) embedded in different drug carriers is detailed. This article is divided into sections. The first part reports short characterization of the nanoparticles and microparticles that can be used as drug delivery systems (DDSs). The second part shows the applicability of SS NMR to study non-steroidal anti-inflammatory drugs (NSAIDs). In this section, problems related to API–DDS interactions, morphology, local molecular dynamics, nature of inter- or intramolecular connections, and pore filling are reviewed for different drug carriers (e.g. mesoporous silica nanoparticles (MSNs), cyclodextrins, polymeric matrices and others). The third and fourth sections detail the recent applications of SS NMR for searching for antibiotics and anticancer drugs confined in zeolites, MSNs, amorphous calcium phosphate and other carriers.     

High-resolution solid-state 119Sn and 195Pt NMR studies of MPtSn semiconductors (M = Ti, Zr, Hf, Th)

Solid-state 119Sn and 195Pt magic-angle spinning (MAS) NMR spectra are reported on a series of MPtSn compounds (M = Ti, Zr, Hf, Th). In favorable cases (TiPtSn and ZrPtSn) the spectra reveal expected J-coupling patterns originating from indirect spin coupling between Pt and Sn nuclei. MAS has no effect on the broad and asymmetric spectra of either 119Sn and 195Pt nuclei in HfPtSn.     

Solid-state NMR studies of the adsorption and photooxidation of ethanol on mixed TiO2--SnO2 photocatalysts

In situ solid-state NMR methodologies have been used to investigate the photocatalytic oxidation of ethanol (CH₃CH₂OH) over a series of SnO₂-based photocatalysts. The adsorption of ethanol on commercially available SnO₂ powder was studied using both cross-polarization ¹³C NMR and REDOR experiments, and showed the formation of two surface ethanol species, hydrogen-bonded ethanol at surface hydroxyl groups and ethanol chemisorbed to the SnO₂ surface (Sn–OCH₂CH₃). ¹³C NMR of the adsorbed ethanol was used to characterize the surface of monolayer SnO₂–TiO₂ coupled photocatalysts supported on porous Vycor glass. In situ solid-state NMR studies showed that the photooxidation of ethanol over the monolayer photocatalysts was slower than that over a supported TiO₂ monolayer photocatalyst due to the build-up of reaction intermediates such as acetic acid on the catalyst surface. ¹¹⁹Sn NMR experiments characterized the tin species on the porous Vycor glass support.     

Temperature calibration for high-temperature MAS NMR to 913 K: 63Cu MAS NMR of CuBr and CuI, and 23Na MAS NMR of NaNbO3

The solid-state phase transitions of CuBr, CuI and NaNbO₃ can be readily observed using ⁶³Cu and ²³Na high-temperature magic-angle spinning nuclear magnetic resonance spectroscopy. Temperature has large, linear effects on the peak maximum of ⁶³Cu in each solid phase of CuBr and CuI, and there is large jump in shift across each phase transition. The ²³Na MAS NMR peak intensities and the line widths in NaNbO₃ also clearly show its high-temperature transition to the cubic phase. These data can be used to calibrate high-temperature MAS NMR probes up to 913 K, which is two hundred degrees higher than the commonly-used temperature calibration based on the chemical shift of ²⁰⁷Pb in Pb(NO₃)₂.     

Solid-state NMR studies of some tin(II) compounds

High-resolution NMR spectra of [Formula: see text] nuclei, particularly (119)Sn and (31)P, in solid tin(II) phosphite, SnHPO(3), and tin(II) phosphate, SnHPO(4), are presented. The results are discussed in relation to the crystal structures. Spinning sideband analysis has been carried out for both nuclei, giving information on the shielding tensors. Satellite peaks allow the indirect Sn,Sn coupling constants to be determined. Surprisingly large values of 2600+/-200Hz and 4150+/-200Hz are reported for SnHPO(3) and SnHPO(4) respectively. The satellite peaks were investigated by using a single Hahn echo for each refocusing time, which showed that the observed splittings result from (119)Sn, (117)Sn coupling. For SnHPO(3), the calculated relative intensities of the satellites for six intra-layer coupling interactions are in agreement with the experiment values, but for SnHPO(4) the coupling appears to be inter-layer in nature. Tin-119 (and in one case phosphorus-31) shielding tensor data derived from MAS NMR are also reported for four other crystalline tin(II) compounds, namely tin diphosphate, tin oxalate, tin sulphate and calcium tin ethylenediamine tetraacetate.     

An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2

The substitution of the divalent cations Mg²⁺ and Zn²⁺ into the aluminophosphate (AlPO) framework of STA-2 has been studied using an “NMR crystallographic” approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and first-principles calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH⁻ or F⁻, and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined.For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg²⁺ for Al³⁺ and, where required, by hydroxide anions coordinated to the framework [27] Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations. The calculations also show that ³¹P chemical shifts depend not only on the topologically-distinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)₂OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the ³¹P isotropic chemical shift and the average 〈P–O–M〉 bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured.     

Atomic refinement with correlated solid-state NMR restraints

The orientation data provided by solid-state NMR can provide a great deal of structural information about membrane proteins. The quality of the information provided is, however, somewhat degraded by sign degeneracies in measurements of the dipolar coupling tensor. This is reflected in the dipolar coupling penalty function used in atomic refinement, which is less capable of properly restraining atoms when dipolar sign degeneracies are present. In this report we generate simulated solid-state NMR data using a variety of procedures, including back-calculation from crystal structures of alpha-helical and beta-sheet membrane proteins. We demonstrate that a large fraction of the dipolar sign degeneracies are resolved if anisotropic dipolar coupling measurements are correlated with anisotropic chemical shift measurements, and that all sign degeneracies can be resolved if three data types are correlated. The advantages of correlating data are demonstrated with atomic refinement of two test membrane proteins. When refinement is performed using correlated dipolar couplings and chemical shifts, perturbed structures converge to conformations with a larger fraction of correct dipolar signs than when data are uncorrelated. In addition, the final structures are closer to the original unperturbed structures when correlated data are used in the refinement. Thus, refinement with correlated data leads to improved atomic structures. The software used to correlate dipolar coupling and chemical shift data and to set up energy functions and their derivatives for refinement, CNS-SS02, is available at our web site.     

Double-quantum NMR spectroscopy of P species submitted to very large CSAs

We introduce an original pulse sequence, , which is a block super-cycled sequence employing as basic element a π pulse sandwiched by ‘window’ intervals. This homonuclear dipolar recoupling method allows the efficient excitation of double-quantum coherences between spin-1/2 nuclei submitted to very large chemical shift anisotropy. We demonstrate that this technique can be employed in double-quantum ↔ single-quantum ³¹P homonuclear correlation experiment at high magnetic field (B₀  14 T) and high MAS frequencies (ν_R  30 kHz). The performances of are compared to those of the double-quantum recoupling methods, such as BABA and bracketed fp-RFDR, which were already employed at fast MAS rates. The sequence displays a higher robustness to CSA and offset than the other existing techniques.     

Advances in solid-state NMR of membrane proteins

Solid-state nuclear magnetic resonance (SSNMR) is an NMR spectroscopy applied to condensed-phase systems, including membrane proteins. Membrane protein fold and function are dependent upon interactions with surrounding bilayer components. Structural and functional analyses are thus challenging, and new approaches are needed to better characterise these systems. SSNMR is uniquely suited to the examination of membrane proteins in native environments, and has the capabilities to elucidate complex protein mechanisms and structures. Notable research implementing SSNMR is aimed at developing new strategies and technology to efficiently target membrane proteins within synthetic and biological membranes. Significant advances have been made: observation of protein function in native environments, emergence of in situ methods to examine integral proteins within natural membranes, sensitivity enhancement techniques and cutting-edge structure determination methods. We present how these advances are applied to answer outstanding questions in structural biology. Experiments have shown consistent results for protein investigations in biological membranes and synthetic lipid compositions, indicating that SSNMR is an innovative and direct approach for the study of these systems.     

结晶PMMA/SAN共混体系的相容性和PMMA结晶行为的NMR研究

我们用固体NMR方法研究了SAN与结晶PMMA的相容性,并提出了可能的相容性机理;同时我们也研究了PMMA的结晶行为,发现它不同于SMA/PMMA中只存在少量结晶区的PMMA的行为.SAN/结晶PMMA是部分相容的体系,由于PMMA中的羰基与SAN中的苯基的相互吸引才导致共混物相容,且SAN只与无定形PMMA区相容,结晶区可用NMR方法检测到,这说明SAN/PMMA的相容性比SMA/PMMA差,主要原因是SAN与PMMA的相互作用弱于SMA与PMMA的相互作用.交替共聚利于相容.     

Use of SPAM and FAM pulses in high-resolution MAS NMR spectroscopy of quadrupolar nuclei

The merits of SPAM and FAM pulses for enhancing the conversion of triple- to single-quantum coherences in the two-dimensional MQMAS experiment are compared using (87)Rb (spin I=3/2) and (27)Al (I=5/2) NMR of crystalline and amorphous materials. Although SPAM pulses are more easily optimized, our experiments and simulations suggest that FAM pulses yield greater signal intensity in all cases. In conclusion, we argue that, as originally suggested, SPAM and FAM pulses are best implemented in phase-modulated whole-echo MQMAS experiments and that the use of SPAM pulses to record separate echo and antiecho data sets, which are then combined, generally yields lower signal-to-noise ratios.     

Solid-state NMR investigation on the interactions between a synthetic montmorillonite and two homopolypeptides

Interactions of two homopolypeptides (polylysine and polyglutamic acid) with a synthetic montmorillonite were studied by ¹H MAS, ¹H–²⁷Al HETCOR and ¹H–¹³C CP-MAS NMR experiments. ¹H–²⁷Al HETCOR with ¹H spin-diffusion NMR appears to be a powerful probe for the identification of the polypeptide fragments, which interact with the montmorillonite interlayer surfaces. In particular, selective interactions were observed between the polypeptide side-chains and the montmorillonite octahedral aluminum atoms. ¹H–¹³C CP-MAS NMR experiments were used to assess the dynamics of the two polypeptides through the measurement of the t_1/2 characteristic time of selected carbons. Results indicate that the local mobility of the side chains and their interaction with the montmorillonite layers depend on the nature of the adsorbed polypeptides.     

Stationary interphases with extended alkyl chains: A comparative study on chain order by solid-state NMR spectroscopy

Stationary interphases with long n-alkyl chains (n = 18, 22, 30, 34) have been examined by solid-state NMR spectroscopy. The determination of the silane functionality and the degree of cross-linking of silane ligands on the silica surface was performed by ²⁹Si CP/MAS NMR spectroscopy. High-speed ¹H MAS and ¹³C CP/MAS NMR spectroscopy were utilized to assess alkyl chain order and mobility of the different bonded phases. For this purpose, ¹H NMR line widths and ¹³C chemical shifts have been evaluated. It is shown that stationary phase order and rigidity increase with alkyl chain length. In addition, the temperature-dependent trans/gauche conformational change occurs at higher temperatures for a polymeric C₃₄ phase compared with a C₃₀ sorbent. This behaviour is discussed in the context of previously reported Chromatographic (HPLC) shape selectivity differences.