Phase-adjusted Spinning Sidebands Research Articles

Impacts of agricultural drainage on the quantity and quality of tropical peat soil organic matter in different types of forests

Tropical peatlands serve as huge carbon (C) pools in the global C balance, and degradation due to agricultural use is a major concern. The objective of the present study was to examine the relationship between the C loss rate in agriculturally drained tropical peat soils in relation to groundwater level (GWL) and the chemical structure of the resulting soil organic matter (SOM). Soil mesocosms containing three peat soils collected from three forest types in Malaysia, Mixed peat swamp (MPS), Alan Batu (ABt), and Alan Bunga (ABg) forests, which are classified based on dominant plant species, species diversity, and forest structure, were buried in an oil palm plantation for a period of five years. Soil samples packed in the top layer (0–20 cm depth) and the bottom (60–80 cm depth) layer were then analyzed and the results were compared with the initial values for these soils. Weight loss, total C and N contents, ash content, and pyrophosphate solubility index (PSI; an index of the degree of humification) were measured. Changes in the chemical structure of the SOC were investigated using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with phase-adjusted spinning side bands and thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry (THM-GC/MS). The C loss in the MPS forest soil, 2–12%, was smaller than that in the ABt and ABg forest soils, 20–37%, especially in the bottom layer. The higher C loss in the ABt and ABg soils, without significant difference between the top and bottom layers, can be attributed to the greater accumulation of less decomposed plant components as evidenced by the larger yields of lignin-derived phenols and polysaccharide-related compounds in THM-GC/MS analysis. The C composition as estimated by 13C NMR did not vary during incubation of the ABt and ABg soils while the yields of lignin-derived and other phenols as well as polysaccharide-related compounds in THM-GC/MS were decreased, especially in the top layer. These findings suggest that plant components with a low degree of decomposition are susceptible to drainage regardless of their structure. The progression of the oxidative degradation of lignin in the top layer of all soils was also suggested by the increases in PSI and the vanillic acid/vanillin ratio. The % O-alkyl C and % alkyl C may control the rate of C loss positively and negatively, respectively. PSI also appears to be useful for roughly estimating the decomposability of tropical peat SOM in various GWLs.

Structure of molecule-rich chalcogenide glasses along the join P4Se3-As4Se3: Results from 77Se and 31P NMR and Raman spectroscopy

The atomic structure of unusually molecule-rich glasses along the P4Se3-As4Se3 join with 0 to 70 mol% P4Se3 is studied using Raman, 77Se magic-angle-spinning (MAS) and 2D 31P phase adjusted spinning sideband (PASS) nuclear magnetic resonance (NMR) spectroscopy. When taken together, the spectroscopic results indicate the coexistence of cage-like PxAs4-xSe3 molecular moieties and corner-shared P- and As- containing pyramidal network moieties in the structure of these glasses. Increasing substitution of As with P gives rise to a monotonic increase in the total molecule:network ratio, which is shown to be consistent with the compositional variation in the glass transition temperature.

Correlation of structure and ionic-conductivity in phosphate glass using MAS-NMR and impedance spectroscopy: Influence of sodium salt

In the process of diminishing the safety concerns of sodium-ion batteries, the development of glass-based solid electrolyte materials has received adequate interest. Nevertheless, achieving a high ionic-conductivity at room temperature for glass materials remains a challenging task because of the poor correlation between the conductivity and the glass structure. Here, we attempt to understand the effective influence of NaCl on the structure and ionic-conductivity of the phosphate-based glass network. For this study, $x\mathrm{NaCl}\text{\ensuremath{-}}(100\text{\ensuremath{-}}x$) ($31.725\phantom{\rule{4pt}{0ex}}{\mathrm{Na}}_{2}\mathrm{O}\text{\ensuremath{-}}12.69\phantom{\rule{0.16em}{0ex}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}31.725\phantom{\rule{0.16em}{0ex}}{\mathrm{P}}_{2}{\mathrm{O}}_{5}\text{\ensuremath{-}}8.46\phantom{\rule{0.16em}{0ex}}\mathrm{NaF}\text{\ensuremath{-}}5.40\phantom{\rule{0.16em}{0ex}}{\mathrm{Na}}_{2}{\mathrm{SO}}_{4}\text{\ensuremath{-}}10\phantom{\rule{0.16em}{0ex}}{\mathrm{MoO}}_{3}$) glass systems (mol %) were selected, where $x=0$, 5, 10, 15, and 20 mol %. To investigate structural changes with the addition of different NaCl concentrations, $^{27}\mathrm{Al}, ^{23}\mathrm{Na}, ^{31}\mathrm{P}$ magic angle spinning nuclear magnetic resonance (MAS-NMR), $^{31}\mathrm{P}$ two-dimensional (2D) phase-adjusted spinning sideband (PASS), and $^{31}\mathrm{P}$ 2D J-resolved NMR techniques and Raman spectroscopic techniques were utilized. Impedance spectroscopy and ac conductivity spectra were used to assess ionic-conductivity and sodium-ion dynamics, respectively. Impedance spectral analysis reveals that the ionic-conductivity of the base glass is increased by 2.4 times (from 1.85 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}7}$ to 4.44 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}7}$ S/cm at 373 K) with the addition of 20 mol % of NaCl. Raman spectra confirm the presence of P\ensuremath{-}O\ensuremath{-}Mo and the absence of Mo\ensuremath{-}O\ensuremath{-}Mo bonds in these glass systems, and $^{31}\mathrm{P}$ 2D J-resolved spectra indicate the absence of P\ensuremath{-}O\ensuremath{-}P bonds. Upon increasing the NaCl concentration, significant changes in the shapes of $^{31}\mathrm{P}$ and $^{27}\mathrm{Al}$ MAS-NMR spectra were observed, indicating the effective influence of NaCl on the distribution of alumina and phosphorus structural units. Irrespective of the temperature, sodium-ion dynamic studies show that the mean-square displacement $\ensuremath{\langle}{R}^{2}({t}_{p})\ensuremath{\rangle}$ decreases with increasing NaCl concentration up to 10 mol % and then increases with a further increase in NaCl concentration. This investigation aids in understanding the sodium-ion dynamics and the structural information of a multicomponent glass system to enhance the room-temperature conductivity.

Separation of quadrupolar and paramagnetic shift interactions in high-resolution nuclear magnetic resonance of spinning powders.

Separation and correlation of the shift anisotropy and the first-order quadrupolar interaction of spin I = 1 nuclei under magic-angle spinning (MAS) are achieved by the phase-adjusted spinning sideband (PASS) nuclear magnetic resonance (NMR) experiment. Compared to methods for static samples, this approach has the benefit of higher sensitivity and resolution. Moreover, the PASS experiment has the advantage over previous MAS sequences in the ability to completely separate the shift anisotropy and first-order quadrupolar interactions. However, the main drawback of the pulse sequence is the lower excitation bandwidth. The sequence is comprehensively evaluated using theoretical calculations and numerical simulations and applied experimentally to the 2H NMR of a range of paramagnetic systems: deuterated nickel(II) acetate tetrahydrate, deuterated copper(II) chloride dihydrate, and two forms of deuterated oxyhydride ion conductor BaTiO3-xHy. Our results show that despite the issue with broadband excitation, the extracted shift and quadrupolar interaction tensors and the Euler angles relating the two tensors match well with the NMR parameters obtained with static NMR methods. Therefore, the new application of the PASS experiment is an excellent addition to the arsenal of NMR experiments for 2H and potentially 14N in paramagnetic solids.

Structure and dynamics of sodium alginate as elucidated by chemical shift anisotropy and site-specific spin-lattice relaxation time measurements.

The biocompatible, biodegradable, linear copolymer sodium alginate is fabricated from [Formula: see text] linked [Formula: see text]-D-mannuronic acid (M block) and [Formula: see text]-L-guluronic acid (G-block). It has wide applications in drug delivery, cell encapsulation, and commercial application in the textile, cosmetics, paper, food, biomedical, and pharmaceutical industries. The structure and dynamics of sodium alginate were here investigated by measuring chemical shift anisotropy (CSA) parameters, spin-lattice relaxation time, and molecular correlation time. The principal components of the CSA tensor were determined by two-dimensional phase-adjusted spinning sideband (2DPASS) cross-polarization magic angle spinning (CP-MAS) SSNMR. The alternating M and G blocks of both equatorial and axial links are associated with greater overall flexibility. The molecular correlation time of the carboxyl carbon of both G and M blocks is faster than for the anomeric carbon and pyranose carbon. This is further experimental evidence of the coexistence of two different dynamics within the polysaccharide chains of sodium alginate, which was previously established by 1H-13C dipolar profile analysis. The relaxation time of the para-crystalline region of sodium alginate is comparable with that of chitosan, but it is much shorter than that of cellulose and chitin. The order of the molecular correlation time of sodium alginate and chitosan is also the same. Hence, it can be concluded that sodium alginate exhibits greater flexibility than cellulose and chitin. These types of investigation into the local electronic configuration and nuclear spin dynamics at various carbon nuclei sites of the biopolymer at atomic-scale resolution will help in the design of biomimetic materials.

Chemical order in binary Se-Te glasses: Results from high-resolution 2D 77Se and 125Te MATPASS NMR spectroscopy

The structure of binary TexSe100-x (0 ≤ x ≤ 35) glasses is studied using high-resolution two-dimensional magic-angle-turning phase-adjusted spinning sidebands (2D MATPASS) 77Se and 125Te nuclear magnetic resonance (NMR) spectroscopy. The 77Se (125Te) isotropic NMR spectra are consistent with the presence of three different kinds of Se (Te) sites, which correspond to Se–Se (Te)–Se, Te–Se (Te)–Se, and Te–Se (Te)–Te environments. The compositional variation of the relative fractions of the three Se sites indicates that the Se and Te atoms are randomly distributed to form [Se, Te]n copolymer chains. The 77Se and 125Te NMR isotropic chemical shifts for Se and Te atoms in isostructural environments are found to display a linear correlation. The systematic compositional variation of the CSA of Se–Se–Se sites can be related to the appearance of Te atoms as next-nearest neighbors of Se atoms in the [Se, Te]n chains.

Chemical compositional analysis of soil fulvic acids using Fourier transform ion cyclotron resonance mass spectrometry.

Soil fulvic acids (FAs) are considered to be a highly reactive pool of soil organic matter. The functions of FAs are related to their chemical structures, the details of which are largely unidentified. To better understand them, Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) must be a useful but generally unused tool. The structural properties of the components of five FA samples from a variety of soils were determined using FTICR-MS with negative-mode electrospray ionization. The peaks were assigned to molecular formulae, which were categorized into seven compound groups based on the H/C-O/C van Krevelen diagram. Ramp 13 C cross polarization/magic angle spinning nuclear magnetic resonance (NMR) spectra with phase-adjusted spinning side bands were also recorded to estimate the C composition. From FTICR-MS, molecular formulae were assigned to 1746-2605 peaks across the m/z range of 200-700. Those aligned in the lignin-like, tannin-like, and condensed aromatic regions of the van Krevelen diagram accounted for 49-58%, 4-20%, and 18-39% of the total peak magnitude, respectively. The proportion of the summed peak magnitudes that were detected in the lignin-like and condensed aromatic regions correlated positively to the aromatic C% as estimated by 13 C NMR. From Kendrick mass defect analysis using a carboxyl group, 94 molecular formulae were assigned to condensed aromatic acids, of which the maximum ring number was 4-7, as potential structures. A high proportion of lignin-like formulae and condensed aromatics, including those probably condensed aromatic acids with small ring numbers, as well as the existence of tannin-like formulae, which were generally lacking in soil humic acids, was suggested as a common feature of soil FAs.

Elucidating the internal structure and dynamics of [formula omitted]chitin by 2DPASS-MAS-NMR and spin-lattice relaxation measurements

The structure and dynamics of the second most abundant biopolymer α−chitin were studied by high resolution solid state 13C cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS-NMR) spectral analysis, 13C relaxation measurements at eight chemically different carbon sites and chemical shift anisotropy measurement by two-dimensional phase-adjusted spinning sidebands (2DPASS) magic angle spinning (MAS) solid state NMR method.13C spin-lattice relaxation time was measured by high resolution Torchia CP method. Spin-lattice relaxation rate (1/T1) of side chain carbon nuclei were remarkably high, because those nuclei possess higher degree of motional freedom. Chemical shift anisotropy parameters of eight chemically different carbon nuclei were determined by 2DPASS-MAS-NMR experiment. Large value of chemical shift anisotropy was observed for carbonyl group carbon (C7) nuclei, because of electrostatic effect, hydrogen bonding and molecular magnetic susceptibility. 13C relaxation mechanism is mainly governed by chemical shift anisotropy interaction, especially at high value of external magnetic field (11.74 T). Thus, the correlation time at different carbon sites were also calculated by using the spin-lattice relaxation times and chemical shift anisotropy values. The correlation time of side chain carbon (C8) was two orders of magnitude less than the carbonyl group carbon. These types of investigations would enlighten the correlation between the structure and dynamics of long polysaccharide chain compound.

Effective combined water and sideband suppression for low-speed tissue and in vivo MAS NMR.

High-resolution magic angle spinning (HR-MAS) NMR is a powerful technique that can provide metabolic profiles and structural constraints on intact biological and environmental samples such as cells, tissues and living organisms. However, centripetal force from fast spinning can lead to a loss of sample integrity. In analyses focusing on structural organization, metabolite compartmentalization or in vivo studies, it is critical to keep the sample intact. As such, there is growing interest in slow spinning studies that preserve sample longevity. In this study, for example, reducing the spinning rate from 2500 to 500Hz during the analysis of a living freshwater shrimp increased the 100% survivability threshold from ~14 to 40h. Unfortunately, reducing spinning rate decreases the intensity of the isotropic signals and increases both the intensity and number of spinning sidebands, which mask spectral information. Interestingly, water suppression approaches such as excitation sculpting and W5 WATERGATE, which are effective at higher spinning rates, fail at lower spinning rates (<2500Hz) while simpler approaches such as presaturation are not able to effectively suppress water when the ratio of water to biomass is very high, as is the case in vivo. As such there is a considerable gap in NMR approaches which can be used to suppress water signals and sidebands in biological samples at lower spinning rates. This research presents simple but practically important sequences that combine PURGE water suppression with both phase-adjusted spinning sidebands and an analogue of TOSS termed TOSS.243. The result is simple and effective water and sideband suppression even in extremely dilute samples in pure water down to ~100Hz spinning rate. The approach is introduced, described and applied to a range of samples including, ex vivo worm tissue, Daphnia magna (water fleas), and in vivo Hyalella azteca (shrimp).

Size distribution of carbon layer planes in biochar from different plant type of feedstock with different heating temperatures

Biochar application to soil is a strategy to decelerate the increase in the atmospheric carbon concentration. The composition of condensed aromatic clusters appears to be an important determinant of the degradation rate of char in soil. The objective of the present study was to determine the size distribution of carbon layer planes in biochars produced from different types of feedstock (a broadleaf and a coniferous tree and two herbs) using different heating treatment temperatures (HTT; 400 °C–800 °C) using X-ray diffraction 11 band profile analysis. 13C nuclear magnetic resonance with the phase-adjusted spinning side bands of the chars indicated different spectral features depending on the HTT and similar carbon composition among the plant types at each HTT. Both the content and composition of carbon layer planes in biochar produced using the same HTT were also similar among the plant types. The carbon layer plane size in the 400 °C and 600 °C chars was distributed from 0.24 to 1.68 or 1.92 nm (corresponding to 37 or 52 rings) with the mean size of 0.79–0.92 and 0.80–1.14 nm, respectively. The carbon layer planes in the 800 °C chars ranged from 0.72–0.96 nm (7–14 rings) to 2.64–3.60 nm (91–169 rings) and the mean values were 1.47–1.89 nm. The relative carbon layer plane content in the 600 °C and 800 °C chars was typically 2 and 3 times that in the 400 °C chars. These results indicate the progression of the formation and/or the size development of graphite-like structures, suggesting that a char produced at a higher HTT would have better carbon sequestrating characteristics.

Application of (13)C ramp CPMAS NMR with phase-adjusted spinning sidebands (PASS) for the quantitative estimation of carbon functional groups in natural organic matter.

The composition of carbon (C) functional groups in natural organic matter (NOM), such as dissolved organic matter, soil organic matter, and humic substances, is frequently estimated using solid-state (13)C NMR techniques. A problem associated with quantitative analysis using general cross polarization/magic angle spinning (CPMAS) spectra is the appearance of spinning side bands (SSBs) split from the original center peaks of sp (2) hybridized C species (i.e., aromatic and carbonyl C). Ramp CP/phase-adjusted side band suppressing (PASS) is a pulse sequence that integrates SSBs separately and quantitatively recovers them into their inherent center peaks. In the present study, the applicability of ramp CP/PASS to NOM analysis was compared with direct polarization (DPMAS), another quantitative method but one that requires a long operation time, and/or a ramp CP/total suppression side band (ramp CP/TOSS) technique, a popular but non-quantitative method for deleting SSBs. The test materials were six soil humic acid samples with various known degrees of aromaticity and two fulvic acids. There were no significant differences in the relative abundance of alkyl C, O-alkyl C, and aromatic C between the ramp CP/PASS and DPMAS methods, while the signal intensities corresponding to aromatic C in the ramp CP/TOSS spectra were consistently less than the values obtained in the ramp CP/PASS spectra. These results indicate that ramp CP/PASS can be used to accurately estimate the C composition of NOM samples.

Isotropic High Field NMR Spectra of Li-Ion Battery Materials with Anisotropy &gt;1 MHz

The use of a magic-angle turning and phase-adjusted spinning sideband NMR experiment to resolve and quantify the individual local environments in the high field (7)Li and (31)P NMR spectra of paramagnetic lithium-ion battery materials is demonstrated. The use of short radio frequency pulses provides an excitation bandwidth that is sufficient to cover shift anisotropy of >1 MHz in breadth, allowing isotropic and anisotropic components to be resolved.

TOP-PASS: A processing algorithm to reduce 2D PASS acquisition time

A slow speed MAS spectrum contains a pattern of spinning sideband resonances separated by integer multiples of the rotor frequency and centered about an isotropic frequency. The 2D signal acquired in a two-dimensional Phase Adjusted Spinning Sideband (PASS) experiment correlates this slow speed MAS spectrum, obtained in the direct dimension, to an indirect dimension spectrum containing the same pattern of spinning sideband resonances centered about a frequency of zero. An affine transformation is used to convert the acquired 2D PASS signal into a 2D signal that correlates a spectrum of pure isotropic frequencies to a spectrum of spinning sideband resonances with no isotropic frequency contributions. The conventional affine transform applied to 2D PASS consists of an active shear of the signal parallel to the indirect time domain coordinate followed by a passive scaling of the indirect time domain coordinate. Here we show that an alternative affine transform, previously employed in the Two-dimensional One Pulse (TOP) experiment, can be employed to create the same 2D signal correlation with an enhanced spectral width in the anisotropic (spinning sideband) dimension. This enhancement can provide a significant reduction in the minimum experiment time required for a 2D PASS experiment, particularly for spectra where the individual spinning sideband patterns are dispersed over a wider spectral range than the isotropic resonance frequencies. The TOP processing consists of an active shear of the signal parallel to the direct time domain, followed by an active shear of the signal parallel to the new indirect time domain coordinate followed by a passive scaling of the new direct time domain coordinate. A theoretical description of the affine transformation in the context of 2D PASS is given along with illustrative examples of 29Si in Clinoenstatite and 13C in l-Histidine.

On the magic-angle turning and phase-adjusted spinning sidebands experiments

The underlying relation between the magic-angle turning (MAT) and phase-adjusted spinning sidebands (PASS) experiments is examined. The MAT experiment satisfies the PASS conditions for separating spinning sidebands with a non-constant total evolution time and only requires linear t 1 increments of up to one rotor period. The time-domain data of the two experiments are related by a shearing transformation. A combination of the linear evolution-time increments of MAT and simple data processing of PASS are particularly attractive for the implementation of MAT for measuring chemical shift anisotropy.

Analytical solutions to several magic-angle spinning NMR experiments

Using the Anderson–Weiss (AW) formalism, analytical expressions of the NMR signal are obtained for the following magic-angle spinning (MAS) experiments: total suppression of sidebands (TOSS); phase adjusted spinning sidebands (PASS); rotational-echo double-resonance (REDOR); rotor-encoded REDOR (REREDOR); cross-polarization magic-angle spinning (CPMAS); exchange induced sidebands (EIS); one-dimensional exchange spectroscopy by sideband alternation (ODESSA); time-reverse ODESSA (trODESSA); centerband-only detection of exchange (CODEX). In order to test the validity of the AW approach, the Gaussian powder approximation is compared with exact powder calculations. A quantitative study of the effect of molecular dynamics on the efficiency of the TOSS and REDOR pulse sequences is then presented.

Solid‐state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside

Three flavonoids of pharmaceutical importance-baicalein, baicalin, and wogonoside-were isolated from a Chinese medicinal plant Scutellaria baicalensis Georgi and studied by 13C NMR in solution and solid state. Two-dimensional (2D) NMR spectroscopy in the liquid phase and dipolar dephasing (DD) experiments in magic-angle spinning (MAS) spectra enabled the assignment of 13C resonances. The cross-polarization (CP) time constants T(CH) and relaxation times T(H) (1rho) were obtained from the variable-contact time experiments. The principal elements of the 13C chemical shift tensor were determined in the spectra recorded under slow sample spinning (2 kHz) using phase-adjusted spinning sideband (PASS)-2D NMR technique, and were verified by density functional theory gauge-independent atomic orbital (DFT GIAO) calculations of shielding constants. Analysis of the 13C delta(ii) and comparison with shielding parameters calculated for different conformers of compounds 1-3 enabled the selection of the most reliable geometry in the solid phase. In all three compounds, an intramolecular hydrogen bond C5--OH...=C4 is formed; the existence of baicalein and baicalin with 'anticlockwise' orientation of OH groups is more probable.

1H relaxation times of metabolites in biological samples obtained with nondestructive ex‐vivo slow‐MAS NMR

Methods suitable for measuring (1)H relaxation times such as T(1), T(2) and T(1rho) of metabolites in small, intact biological objects including live cells, excised organs and tissues, oil seeds etc. are developed in this work. This was achieved by combining inversion-recovery, spin-echo, or a spin-lock segment with the phase-adjusted spinning sideband (PASS) technique, which was applied at low sample-spinning rates. Here, PASS was used to produce high-resolution (1)H spectra in a nondestructive way so that the relaxation parameters of individual metabolite could be determined. The methodologies were demonstrated by measuring (1)H T(1), T(2), and T(1rho) of metabolites in excised rat liver at a spinning rate of 40 Hz.

Determination of the 13C chemical shift anisotropies of cellulose I and cellulose II

The chemical shift anisotropies (CSAs) of cellulose Iα and Iβ, the two crystalline constituents of bacterial cellulose produced by Acetobacter xylinum (DSM 14666), and regenerated cellulose II are reported for each of the spectroscopically resolved carbon resonances using the phase adjusted spinning sideband (PASS) experiment. The data are compared with experimental results using the recoupling of anisotropy information (RAI) technique and with theoretical calculations of the structure of cellulose, including the hydrogen bonding systems.

Formula omitted] and [formula omitted] NMR and XPS characterization of PbO–PbCl 2–CdCl 2 glasses

A structural characterization of PbO–PbCl 2–CdCl 2 oxychloride glasses was achieved using 207 Pb and 113 Cd nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS). 207 Pb static NMR spectra exhibit two broad components, whose intensity change with oxygen–chlorine substitution in the glass composition. 207 Pb phase-adjusted spinning sidebands (PASS) NMR shows that the lead first co-ordination sphere in PbO–PbCl 2–CdCl 2 glasses contains both oxygen and chlorine atoms and that the Pb bonding environment varies from an ionic environment to a more covalent bonding state. From the study of crystalline Pb 3O 2Cl 2, the correlation between 207 Pb isotropic and anisotropic chemical shift is extended to oxychloride compounds. Oxygen 1s photoelectron spectra contain two components that are consistent with a glass formation through Pb–O–Pb bonds. 113 Cd NMR spectra show that Cd is mainly ionically bonded to chlorine and acts as network modifier for all glass compositions.

Multiple-Rotor-Cycle 2D PASS Experiments with Applications to 207Pb NMR Spectroscopy

Thetwo-dimensional phase-adjusted spinning sidebands (2D PASS) experiment is a useful technique for simplifying magic-angle spinning (MAS) NMR spectra that contain overlapping or complicated spinning sideband manifolds. The pulse sequence separates spinning sidebands by their order in a two-dimensional experiment. The result is an isotropic/anisotropic correlation experiment, in which a sheared projection of the 2D spectrum effectively yields an isotropic spectrum with no sidebands. The original 2D PASS experiment works best at lower MAS speeds (1–5 kHz). At higher spinning speeds (8–12 kHz) the experiment requires higher RF power levels so that the pulses do not overlap. In the case of nuclei such as 207Pb, a large chemical shift anisotropy often yields too many spinning sidebands to be handled by a reasonable 2D PASS experiment unless higher spinning speeds are used. Performing the experiment at these speeds requires fewer 2D rows and a correspondingly shorter experimental time. Therefore, we have implemented PASS pulse sequences that occupy multiple MAS rotor cycles, thereby avoiding pulse overlap. These multiple-rotor-cycle 2D PASS sequences are intended for use in high-speed MAS situations such as those required by 207Pb. A version of the multiple-rotor-cycle 2D PASS sequence that uses composite pulses to suppress spectral artifacts is also presented. These sequences are demonstrated on 207Pb test samples, including lead zirconate, a perovskite-phase compound that is representative of a large class of interesting materials.