Experimental aspects of ^(14)N overtone RESPDOR solid-state NMR spectroscopy under MAS beyond 60 kHz

Nitrogen-14(^(14)N)overtone(OT)spectroscopy under fast magic angle spinning(MAS)conditions(>60 kHz)has emerged as a powerful technique for observing correlations and distances between ^(14)N and ^(1)H,owing to the absence of the first-order quadrupolar broadenings.In addition,^(14)N^(OT) allows selective manipulation of ^(14)N nuclei for each site.Despite extensive theoretical and experimental studies,the spin dynamics of ^(14)N^(OT) remains under debate.In this study,we conducted experimental investigations to assess the spin dynamics of ^(14)N^(OT) using the rotational-echo saturation-pulse double-resonance(RESPDOR)sequence,which monitors population transfer induced by a^(14)N^(OT) pulse.The ^(14)N^(OT) spin dynamics is well represented by a model of a two-energy-level system.Unlike spin-1/2,the maximum excitation efficiency of ^(14)N^(OT) coherences of powdered solids,denoted by p,depends on the radiofrequency field(rf-field)strength due to orientation dependence of effective nutation fields even when pulse lengths are optimized.It is also found that the p factor,contributing to the ^(14)N^(OT) spin dynamics,is nearly independent of the B0 field.Consequently,the filtering efficiency of RESPDOR experiments exhibits negligible dependence on B0 when the ^(14)N^(OT) pulse length is optimized.The study also identifies the optimal experimental conditions for ^(14)N^(OT)/^(1)H RESPDOR correlation experiments.

A review of ^(17)O isotopic labeling techniques for solid-state NMR structural studies of metal oxides in lithium-ion batteries

Recent advances in utilizing ^(17)O isotopic labeling methods for solid-state nuclear magnetic resonance(NMR)investigations of metal oxides for lithium-ion batteries have yielded extensive insights into their structural and dynamic details.Herein,we commence with a brief introduction to recent research on lithium-ion battery oxide materials studied using ^(17)O solid-state NMR spectroscopy.Then we delve into a review of ^(17)O isotopic labeling methods for tagging oxygen sites in both the bulk and surfaces of metal oxides.At last,the unresolved problems and the future research directions for advancing the ^(17)O labeling technique are discussed.

Application of ammonia probe-assisted solid-state NMR technique in zeolites and catalysis

Solid-state NMR(ssNMR)spectroscopy is a powerful technique for characterizing the surface sites of solid acids and organic intermediates formed during the acid catalyzed reaction.As a very useful probe molecule,ammonia is often utilized to determine the density of solidacids’surface sites by ssNMR spectroscopy.The present mini-review summarizes some of the latest research developments on the quantitative characterization of the acid sites and carbenium ions during the zeolite catalytic reaction by ammonia probe-assisted ssNMR spectroscopy.

Deciphering the potassium storage phase conversion mechanism of phosphorus by combined solid-state NMR spectroscopy and density functional theory calculations

Phosphorus is the potential anode material for emerging potassium-ion batteries(PIBs)owing to the highest specific capacity and relatively low operation plateau.However,the reversible delivered capacities of phosphorus-based anodes,in reality,are far from the theoretical capacity corresponding to the formation of K3P alloy.And,their underlying potassium storage mechanisms remain poorly understood.To address this issue,for the first time,we perform high-resolution solid-state31P NMR combined with XRD measurements,and density functional theory calculations to yield a systemic quantitative understanding of(de)potassiation reaction mechanism of phosphorus anode.We explicitly reveal a previously unknown asymmetrical nanocrystalline-to-amorphous transition process via rP←→(K_(3)P_(11),K_(3)P_(7),beta-K_(4)P_(6))←→(alpha-K4P6)←→(K_(1-x)P,KP,K_(4-x)P3,K_(1+x)P)←→(amorphous K4P3,amorphous K3P)that are proceed along with the electrochemical potassiation/depotassiation processes.Additionally,the corresponding KP alloys intermediates,such as the amorphous phases of K_(4)P_(3),K_(3)P,and the nonstoichiometric phases of“K_(1-x)P”,“K_(1+x)P”,“K_(4-x)P_(3)”are experimentally detected,which indicating various complicated K-P alloy species are coexisted and evolved with the sluggish electrochemical reaction kinetics,resulting in lower capacity of phosphorus-based anodes.Our findings offer some insights into the specific multi-phase evolution mechanism of alloying anodes that may be generally involved in conversion-type electrode materials for PIBs.

Moisture-Induced Non-Equilibrium Phase Segregation in Triple Cation Mixed Halide Perovskite Monitored by In Situ Characterization Techniques and Solid-State NMR

Environmental stability is a major bottleneck of perovskite solar cells.Only a handful of studies are investigating the effect of moisture on the structural degradation of the absorber.They mostly rely on ex situ experiments and on completely degraded samples,which restrict the assessment on initial and final stage.By combining in situ X-ray diffraction under controlled 85%relative humidity,and live observations of the water-induced degradation using liquid-cell transmission electron microscopy,we reveal two competitive degradation paths leading on one hand to the decomposition of state-of-theart mixed cation/anion(Cs_(0.05)(MA_(0.17)FA_(0.83))_(0.95)Pb(Br_(0.17)I_(0.83))_(3)(CsMAFA)into PbI_(2) through a dissolution/recrystallization mechanism and,on the other hand,to a non-equilibrium phase segregation leading to CsPb_(2)Br_(5) and a Cesium-poor/iodide-rich Cs_(0.05)-x(MA_(0.17)FA_(0.83))_(0.95)Pb(Br_(0.17-2y)I_(0.83)+2y)_(3) perovskite.This degradation mechanism is corroborated at atomic-scale resolution through solid-state ^(1)H and ^(133)Cs NMR analysis.Exposure to moisture leads to a film containing important heterogeneities in terms of morphology,photoluminescence intensities,and lifetimes.Our results provide new insights and consensus that complex perovskite compositions,though very performant as champion devices,are comparatively metastable,a trait that limits the chances to achieve long-term stability.

Probing microstructure of solid-state synthesized LiCoO_(2)with MAS NMR spectroscopy

LiCoO_(2)is an important category of active cathode materials in lithium-ion batteries due to its high compacted electrode density,good thermal stability,and stable voltage platform.Recent works on LiCoO_(2)have focused on the realization of higher charging voltages to fully utilize its high theoretical capacity.However,an unambiguous atomic-level local probe is essential for the understanding of structure-function correlation.Here we employ highresolution solid-state nuclear magnetic resonance(NMR)spectroscopy to study the local atomic environments in LiCoO_(2)synthesized with three common sintering methods.While one-dimensional 7Li NMR shows distinct linewidth and subtle dependence on lithium over-stoichiometry,both 7Li and 59Co relaxation times are highly dependent on the sintering method.We prove that the two-step sintering method favors the elimination of unreacted Co3O4,thereby enabling the best discharge capacity in all-solid-state lithium batteries assembled with LiCoO_(2)/LGPS/LiIn,which is in accordance with its narrowest 7Li linewidth and the longest 7Li/59Co T1.

Solid-state NMR spectroscopy at ultrahigh resolution for structural and dynamical studies of MOFs

To characterize the structure and dynamics of metal--organic frameworks(MOFs)indepth at the molecular level,it is necessary to pursue high-resolution solid-state magic angle spinning(MAS)nuclear magnetic resonance(NMR)spectroscopy.Spectral resolution is usually affected by the quality of materials and various experimental conditions,of which magic angle(MA)accuracy is a crucial determinant.The current industrial criteria for MA calibration based on the common standard of KBr were found insufficient in guaranteeing optimal resolution MAS NMR for highly ordered MOFs.To drive towards higher-resolution MAS NMR spectroscopy,we propose_a calibration protocol for more accurate MA with a higher-precision criterion based on 79Br MAS NMR of KBr,where the linewidth ratio of the fifth-order spinning sideband to the central band of KBr should be less than 1.00.As a result,ultrahigh-resolution 13C cross-polarization(CP)MAS NMR of MOF-5 is achieved with minimal linewidths as low as 4 Hz,and therefore MOF-5 can be used as a new standard convenient for verifying MA accuracy and also optimizing 13c CP conditions.Maintaining high-precision MA under variable temperature(VT)was found challenging on certain commercial MAS NMR probes,as was systematically investigated by VT NMR using KBr and MOF-5.Nevertheless,ultrahigh-resolution MAS NMR spectroscopy with stable MA under VT is employed to reveal fine structures and linker dynamics of a series of Zn-based MOFs with highly regulated structures.The ultrahigh-resolution NMR methodcan be generally applied to study a broad range of MOFs and other materials.

EPR AND HIGH RESOLUTION SOLID-STATE NMR STUDIES ON Fe^(3+) SUBSTITUTED ZSM-5 ZEOLITE

Fe^(3+) substituted ZSM-5 zeolite was studied using EPR and ^(27)A1,^(29)Si MAS-NMR spectroscopy.The results showed that the synthe- sized sample has the typical ZSM-5 structure with Fe^(3+) ionsincorporated in tetrahedric sites in the zeolite framework and there are some para- magnetic centers in the samples.

Recent advances on surface metal hydrides studied by solid-state nuclear magnetic resonance spectroscopy

Metal hydrides (MeH) on solid surfaces, i.e., surface MeH, are ubiquitous but criticalspecies in heterogeneous catalysis, and their intermediate roles have been proposed innumerous reactions such as (de)hydrogenation and alkanes activation, etc., however, thedetailed spectroscopic characterizations remain challenging. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy has become a powerful tool in surface studies, asit provides access to local structural characterizations at atomic level from multipleviews, with comprehensive information on chemical bonding and spatial structures. Inthis review, we summarized and discussed the latest research developments on thesuccessful application of ssNMR to characterize surface MeH species on solid catalystsincluding supported single-site heterogeneous catalysts, bulk metal oxides and metalmodified zeolites. We also discussed the opportunities and challenges in this field, aswell as the potential application/development of state-of-the-art ssNMR technologies toenable further exploration of metal hydrides in heterogeneous catalysis.

Mechanisms of antimicrobial peptides as characterized by solid-state NMR

Antimicrobial peptides(AMP)are small proteins that play critical roles in host defense against microbe invasion.Many AMPs disrupt the cellular membrane of microbe,while the mechanism of action of AMPs can be very sophisticated.Solid-state NMR(SSNMR)technique is powerful in characterizing the mechanism of AMPs in vivo and in vitro.This review summarizes the recent advance of SSNMR technique in AMP mechanisms characterization.We highlight the sample preparation approaches,the SSNMR spectroscopic methods,and a number of outstanding examples of AMP mechanisms elucidated via SSNMR spectroscopy.

Solid-state NMR studies of sulfonated SBA-15 and the synergistic catalysis of fructose into 5-hydroxymethylfurfural with dimethyl sulfoxide

Sulfonic acid functionalized mesoporous SBA-15 was prepared using the grafting method.The structure and acid properties were comprehensively characterized using multi-nuclear and quantitative probe molecule solid-state NMR(SSNMR),together with powder X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscopy(TEM),N2 adsorption-desorption techniques.Its catalytic performance in the conversion of fructose to 5-hydroxymethylfurfural(HMF)in dimethyl sulfoxide(DMSO)was studied.Catalyst dosage,reaction time,reaction temperature and solvent effect have been investigated.A high yield of HMF up to 93%was obtained at a relatively low temperature of 373 K for 180 min.The Brønsted acid of SBA-15_SO3H together with the solvent DMSO was found to synergistically catalyze the reaction.The catalyst preserved most of its activity after five times reuse and the catalytic activity can be recovered by H2O2 process.

Study of the coordinative nature of alkylaluminum modified Phillips CrO_x/SiO_2 catalyst by multinuclear solid-state NMR

Solid-state nuclear magnetic resonance spectroscopy was used to investigate the coordinative states of surface Al species on various alkylaluminum-modified Phillips CrOx/SiO2 catalysts.The alkylaluminum-modified Phillips CrOx/SiO2 catalysts were examined via ethylene homopolymerization.1H and 27Al magic angle spinning（MAS） nuclear magnetic resonance（NMR） spectra clearly demonstrated that the existing states of surface Al species in alkylaluminum-modified catalysts strongly depended on the type of alkylaluminum cocatalyst,concentration of alkylaluminum and the calcination temperature.1H MAS NMR spectra of alkylaluminum-modified Phillips CrOx/SiO2 catalysts,calcined at two different temperatures,exhibited similar trends in peak shift.1H spectra showed that with an increase of Al/Cr ratio and calcination temperature,the main peak shifted to high field,indicating that the dominant surface proton species changed from hydroxyl to ethoxyl and ethyl groups.27Al MAS NMR spectra showed the presence of three different coordination states（6-,5-,and 4-coordinated Al species） in the alkylaluminummodified Phillips catalysts.In comparison of different alkylaluminum cocatalysts,it was found that the reactivity of alkylaluminum modified Phillips catalyst decreased in the order of TEA〉DEAH〉DEAE.The amount of 4-coordinated Al species of Phillips catalysts modified by TEA,DEAE and DEAH also decreased in the order of TEA〉DEAH〉DEAE,indicating that the presence of 4-coordinated Al species is related to the polymerization activity.

Unveiling chain–chain interactions in CO2-based crystalline stereocomplexed polycarbonates by solid-state NMR spectroscopy and DFT calculations

CO2-based stereocomplexed polycarbonates derived from the intermolecularly interlocked interaction between the enantiopure polymers with the opposite configuration exhibit high crystallinity, excellent thermal and mechanical stabilities. Deep insights into the mechanism of stereocomplexation are of particular importance to the design and manufacture of new promising and sustainable polycarbonates with enhanced physicochemical properties. Our solid-state NMR experiments linking with DFT computations clearly reveal the specific chain-chain interactions in a typical stereocomplexed poly（4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0] octane carbonate）（PCXC）.13C CP/MAS NMR,1H DUMBO MAS NMR and 13C/1H relaxation-time measurements indicate that the formation of stereocomplex reduces the local mobilities of carbonyl, methine and methylene groups in each chain of PCXC significantly. Through a combination of two-dimensional 1H-13C HETCOR NMR and DFT calculation analysis, the cis-/trans-conformations and packing models of PCXC chains in the amorphous, enantionpure isotactic and stereocomplexed polycarbonates are identified. The splitting of 13C and 1H NMR chemical shifts of methine groups in the backbone carbon region demonstrates the ordered interlock interactions between the R-and S-chain in the stereocomplexed PCXC.

Solid-State NMR Spectroscopic Approaches to Investigate Dynamics, Secondary Structure and Topology of Membrane Proteins

Solid-state NMR spectroscopy is routinely used to determine the structural and dynamic properties of both membrane proteins and peptides in phospholipid bilayers [1-26]. From the perspective of the perpetuated lipids, 2H solid-state NMR spectroscopy can be used to probe the effect of embedded proteins on the order and dynamics of the acyl chains of phospholipid bilayers [8-13]. Moreover, 31P solid-state NMR spectroscopy can be used to investigate the interaction of peptides, proteins and drugs with phospholipid head groups [11-14]. The secondary structure of 13C = O site-specific isotopically labeled peptides or proteins inserted into lipid bilayers can be probed utilizing 13C CPMAS solid-state NMR spectroscopy [15-18]. Also, solid-state NMR spectroscopic studies can be utilized to ascertain pertinent informa- tion on the backbone and side-chain dynamics of 2H- and 15N-labeled proteins, respectively, in phospholipid bilayers [19-26]. Finally, specific 15N-labeled amide sites on a protein embedded inside oriented bilayers can be used to probe the alignment of the helices with respect to the bilayer normal [2]. A brief summary of all these solid-state NMR ap- proaches are provided in this minireview.

Detecting water-protein chemical exchange in membrane- bound proteins/peptides by solid-state NMR spectroscopy--Dedicated to Professor Xiuwen Han on the occasion of her 80th birthday

Water plays an important role in many essential biological processes of membrane proteins in hydrated lipid environments.In general,the 1H polarization transfers berween water molecules and site--specific protons in proteins can be classified as coherent(via dipolar spin diffusion)and incoherent(via chemical exchange and nuclear Overhauser effect)transfers.Solid-state NMR is the technique of choice for studying such water-protein interactions in membrane-bound proteins/peptides through the detection of'H polarization transfers from water to the proteins.These polarization transfer mechanisms often exist simultaneously and are difficult to quantify individually.Here,we review water-protein polarization transfer techniques in solid state NMR with a focus on the recent progress for the direct detection of site-specific kinetic water-protein chemical exchange processes on the sub-millisecond time scale in membrane-bound proteins.The measurements of the pure chemical exchange ki-netics provide a unique opportunity to understand the role that water plays in the structure-function relationships of membrane bound species at the water-bilayer interface.In addi-tion,the perspective of chemical exchange saturation transfer(CEST)experiments in membrane-bound proteins/peptides is further discussed.

SOLID-STATE HIGH RESOLUTION NMR STUDY ON POLY (2, 6-DIMETHYL-1,4-PHENYLENE OXIDE)

Experiments including C-13 spin-lattice relaxation, C-13 heteronuclear dipolar dephasing and H-1 spin diffusion are performed on poly (2,6-dimethyl-1,4-phenylene oxide) (PPO). The results show that the rotation of the methyl groups in solid PPO is partially restricted, which results in a surprisingly efficient spin diffusion between the aromatic proton and methyl proton characterized by a diffusion time of 150 mu s. The results also show that the aromatic ring in solid PPO is rigid and twisted, which causes all aromatic carbons to be chemically unequivalent.

On the use of cross polarization in solid-state NMR:1 H spinlock versus adiabatic demagnetization in the rotating frame

Cross polarization(CP)is a widely used solid-state nuclear magnetic resonance(NMR)technique for enhancing the polarization of dilute S spins from much larger polarization of abundant I spins such as 1 H.To achieve such a polarization transfer,the I spin should either be spin-locked or be converted to the dipolar ordered state through adiabatic demagnetization in the rotating frame.In this work,we analyze the spin dynamics of the Hartmann-Hahn CP(HHCP)utilizing the 1 H spin-locking,and the dipolar-order CP(DOCP)having the 1 H adiabatic demagnetization.We further propose an adiabatic demagnetization CP(ADCP)where a constant radio-frequency pulse is applied on the S spin while 1 H is adiabatically demagnetized.Our analyses indicate that ADCP utilizes the adiabatic passage to effectively achieve the polarization transfer from the 1 H to S spins.In addition,the dipolar ordered state generated during the 1 H demagnetization process could also be converted into the observable S polarization through DOCP,further enhancing the polarized signals.It is shown by both static and magic-angle-spinning(MAS)NMR experiments that ADCP has dramatically broadened the CP matching condition over the other CP schemes.Various samples have been used to demonstrate the polarization transfer efficiency of this newly proposed ADCP scheme.

Solid-state NMR in the field of drug delivery:State of the art and new perspectives

In the last decades,a variety of nuclear magnetic resonance(NMR)techniques have been applied with success in the field of advanced functional materials,including the important area of drug delivery.In such field,solid-state NMR(SSNMR)is an irreplaceable tool in the arsenal of characterization techniques,offering unique and comprehensive perspectives for the description of chemical structure,spatial connectivity and interfacial phenomena of solid dosage forms.This review focuses on the widespread applications of SSNMR in drug delivery field,an overview of selected case studies is provided,together with possible developments.

Investigation of Li-ion transport in Li7P3S11 and solid-state lithium batteries

The high Li-ion conductivity of the Li7P3S11 sulfide-based solid electrolyte makes it a promising candidate for all-solid-state lithium batteries. The Li-ion transport over electrode-electrolyte and electrolyteelectrolyte interfaces, vital for the performance of solid-state batteries, is investigated by impedance spectroscopy and solid-state NMR experiments. An all-solid-state Li-ion battery is assembled with the Li7P3S11 electrolyte, nano-Li2S cathode and Li-In foil anode, showing a relatively large initial discharge capacity of 1139.5 m Ah/g at a current density of 0.064 m A/cm^ 2 retaining 850.0 m Ah/g after 30 cycles. Electrochemical impedance spectroscopy suggests that the decrease in capacity over cycling is due to the increased interfacial resistance between the electrode and the electrolyte. 1D exchange ^7Li NMR quantifies the interfacial Li-ion transport between the uncycled electrode and the electrolyte, resulting in a diffusion coefficient of 1.70(3) ×10^-14cm^2/s at 333 K and an energy barrier of 0.132 e V for the Li-ion transport between Li2S cathode and Li7P3S11 electrolyte. This indicates that the barrier for Li-ion transport over the electrode-electrolyte interface is small. However, the small diffusion coefficient for Li-ion diffusion between the Li2S and the Li7P3S11 suggests that these contact interfaces between electrode and electrolyte are relatively scarce, challenging the performance of these solid-state batteries.

Efficient transesterification over two-dimensional zeolites for sustainable biodiesel production

Basic zeolites have shown great potential as efficient catalysts for biodiesel production in the transesterification reactions.However,conventional three-dimensional(3D)basic zeolites generally suffer from limited base sites and severe mass-transfer restriction,thereby suppressing their catalytic activity.Herein,2D basic zeolites with large external surface areas,hierarchical characteristics,and abundant accessible and stable base sites are prepared by expansion,delamination and subsequent solid-state ion-exchange(SSIE)approach.The facile SSIE method provides more advantages in stabilizing and dispersing high concentration of strong basic sites than the conventional liquid-phase ion-exchange(LIE)approach.Due to the excellent mass transportation and stable basic sites,the 2D Na/ITQ-2 prepared by the SSIE approach shows remarkably enhanced activity and recyclability in the transesterification of triglycerides to produce biodiesel,compared to 3D zeolites and other reported basic zeolites.This work will open the boulevard to the rational design of 2D basic catalysts and expand the potential application of 2D zeolites to biodiesel production and other industrial reactions involving bulky molecules.