Frequency-switched Lee-Goldburg Research Articles

Radiation-Induced De Novo Defects in Metal-Organic Frameworks Boost CO2 Sorption.

Defects in metal-organic frameworks (MOFs) can significantly change their local microstructures, thus notably leading to an alteration-induced performance in sorption or catalysis. However, achieving de novo defect engineering in MOFs under ambient conditions without the scarification of their crystallinity remains a challenge. Herein, we successfully synthesize defective ZIF-7 through 60Co gamma ray radiation under ambient conditions. The obtained ZIF-7 is defect-rich but also has excellent crystallinity, enhanced BET surface area, and hierarchical pore structure. Moreover, the amount and structure of these defects within ZIF-7 were determined from the two-dimensional (2D) 13C-1H frequency-switched Lee-Goldburg heteronuclear correlation (FSLG-HETCOR) spectra, continuous rotation electron diffraction (cRED), and high-resolution transmission electron microscopy (HRTEM). Interestingly, the defects in ZIF-7 all strongly bind to CO2, leading to a remarkable enhancement of the CO2 sorption capability compared with that synthesized by the solvothermal method.

Atomic-Scale Resolution Insights into Structural and Dynamic Differences between Ofloxacin and Levofloxacin.

This study employs advanced solid-state NMR techniques to investigate the atomic-level structure and dynamics of two enantiomers: ofloxacin and levofloxacin. The investigation focuses on critical attributes, such as the principal components of the chemical shift anisotropy (CSA) tensor, the spatial proximity of 1H and 13C nuclei, and site-specific 13C spin-lattice relaxation time, to reveal the local electronic environment surrounding specific nuclei. Levofloxacin, the levo-isomer of ofloxacin, exhibits higher antibiotic efficacy than its counterpart, and the dissimilarities in the CSA parameters indicate significant differences in the local electronic configuration and nuclear spin dynamics between the two enantiomers. Additionally, the study employs the 1H-13C frequency-switched Lee-Goldburg heteronuclear correlation (FSLGHETCOR) experiment to identify the presence of heteronuclear correlations between specific nuclei (C15 and H7 nuclei and C13 and H12 nuclei) in ofloxacin but not in levofloxacin. These observations offer insights into the link between bioavailability and nuclear spin dynamics, underscoring the significance of NMR crystallography approaches in advanced drug design.

The Floquet–Magnus and Fer Expansions: Application to Control the Spin Dynamics During the Phase Modulated Lee–Goldburg Radiation in Solid-State NMR

Floquet–Magnus and Fer expansion (FE) schemes are used in nuclear magnetic resonance (NMR) for the calculation of effective Hamiltonians and propagators. This work focuses on applying the Floquet–Magnus and FE approaches to control the spin system evolution during the phase modulate Lee–Goldburg radiation experiment. Until now, the Frequency switched Lee–Goldburg and its variant called the Phase module Lee–Goldburg have been treated by only the average Hamiltonian theory and the bimodal Floquet approach. In this paper, we use the two developing expansion schemes in solid-state NMR for the calculation of the effective Hamiltonian and propagator during the spin dynamics. Our work unifies and generalizes existing results of the Floquet–Magnus and FEs and delivers illustrations of novel springs that boost previous applications that are based on the classical information. The generality of this work points to potential applications in problems related to solid-state NMR and theoretical developments of spectroscopy as well as interdisciplinary research areas whenever they include spin dynamics concepts.

Residual proton line width under refocused frequency-switched Lee-Goldburg decoupling in MAS NMR.

Despite many decades of research, homonuclear decoupling in solid-state NMR under magic-angle spinning (MAS) has yet to reach a point where the achievable proton line widths become comparable to the resolution obtained in solution-state NMR. This makes the precise determination of isotropic chemical shifts difficult and thus presents a limiting factor in the application of proton solid-state NMR to biomolecules and small molecules. In this publication we analyze the sources of the residual line width in refocused homonuclear-decoupled spectra in detail by comparing numerical simulations and experimental data. Using a hybrid analytical/numerical approach based on Floquet theory, we find that third-order effective Hamiltonian terms are required to realistically characterize the line shape and line width under frequency-switched Lee-Goldburg (FSLG) decoupling under MAS. Increasing the radio-frequency field amplitude enhances the influence of experimental rf imperfections such as pulse transients and the MAS-modulated radial rf-field inhomogeneity. While second- and third-order terms are, as expected, reduced in size at higher rf-field amplitudes, the line width becomes dominated by first-order terms which severely limits the achievable line width. We expect, therefore, that significant improvements in the line width of FSLG-decoupled spectra can only be achieved by reducing the influence of MAS-modulated rf-field inhomogeneity and pulse transients.

Origin of the residual line width under frequency-switched Lee-Goldburg decoupling in MAS solid-state NMR.

Homonuclear decoupling sequences in solid-state nuclear magnetic resonance (NMR) under magic-angle spinning (MAS) show experimentally significantly larger residual line width than expected from Floquet theory to second order. We present an in-depth theoretical and experimental analysis of the origin of the residual line width under decoupling based on frequency-switched Lee-Goldburg (FSLG) sequences. We analyze the effect of experimental pulse-shape errors (e.g.,pulse transients and -field inhomogeneities) and use a Floquet-theory-based description of higher-order error terms that arise from the interference between the MAS rotation and the pulse sequence. It is shown that the magnitude of the third-order auto term of a single homo- or heteronuclear coupled spin pair is important and leads to significant line broadening under FSLG decoupling. Furthermore, we show the dependence of these third-order error terms on the angle of the effective field with the field. An analysis of second-order cross terms is presented that shows that the influence of three-spin terms is small since they are averaged by the pulse sequence. The importance of the inhomogeneity of the radio-frequency (rf) field is discussed and shown to be the main source of residual line broadening while pulse transients do not seem to play an important role. Experimentally, the influence of the combination of these error terms is shown by using restricted samples and pulse-transient compensation. The results show that all terms are additive but the major contribution to the residual line width comes from the rf-field inhomogeneity for the standard implementation of FSLG sequences, which is significant even for samples with a restricted volume.

Micron-scale magnetic resonance imaging of both liquids and solids

We describe and demonstrate a novel apparatus for magnetic resonance imaging (MRI), suitable for imaging of both liquid and solid samples with micron-scale isotropic resolution. The apparatus includes a solenoidal radio-frequency microcoil with 170μm inner diameter and a set of planar gradient coils, all wound by hand and supported on a series of stacked sapphire plates. The design ensures efficient heat dissipation during gradient pulses and also facilitates disassembly, sample changes, and reassembly. To demonstrate liquid state 1H MRI, we present an image of polystyrene beads within CuSO4-doped water, contained within a capillary tube with 100μm inner diameter, with 5.0μm isotropic resolution. To demonstrate solid state 1H MRI, we present an image of NH4Cl particles within the capillary tube, with 8.0μm isotropic resolution. High-resolution solid state MRI is enabled by frequency-switched Lee–Goldburg decoupling, with an effective rotating frame field amplitude of 289kHz. At room temperature, pulsed gradients of 4T/m (i.e., 170Hz/μm for 1H MRI) are achievable in all three directions with currents of 10A or less. The apparatus is contained within a variable-temperature liquid helium cryostat, which will allow future efforts to obtain MRI images at low temperatures with signal enhancement by dynamic nuclear polarization.

High resolution solid state 2D NMR analysis of biomass and biochar.

Solid state NMR methods are required to analyze biomass as a function of its chemical or biological treatment for biofuels, chemicals, or biochar production. The native polymers network in lignocellulosic biomass and other solid materials, such as coal, coke, or biochar, can hardly be analyzed by liquid state NMR due to their poor swelling ability without chemical modification. A (1)H-(13)C two-dimensional heteronuclear correlation (HETCOR) experiment with frequency-switched Lee-Goldburg (FSLG) irradiation is performed on a high field spectrometer (750 MHz). This method leads to previously unattained resolution for biomass and biochar and offers a unique ability to reveal their chemical composition. The formation of aromatic moieties from carbohydrates and lignin thermal conversion is clearly distinguished. This method can be applied to all other carbonaceous materials.

Anomalous Swelling Behavior of Poly(N-vinylimidazole)-l-Poly(tetrahydrofuran) Amphiphilic Conetwork in Water Studied by Solid-State NMR and Positron Annihilation Lifetime Spectroscopy

Poly(N-vinylimidazole) homopolymer (PVIm) and poly(N-vinylimidazole)-l-poly(tetrahydrofuran) (PVIm-l-PTHF), a novel amphiphilic polymer conetwork (APCN), were synthesized to compare their solid state structure and investigate the swelling behavior of this unique conetwork in water. A short-range ordered structure stabilized by second-order interactions between the imidazole pendant groups was found in the PVIm homopolymer and in the PVIm phase of the dry conetwork as revealed by solid-state NMR 13C cross-polarization magic angle spinning (CP MAS) and two-dimensional 1H–13C frequency-switched Lee–Goldburg (FSLG) HETCOR measurements. With increasing swelling ratio, structural and conformational changes were recognized in the hydrophilic PVIm phase of the APCN. In the kinetic swelling study, unexpectedly, four different swelling ranges were identified by gravimetric measurements, solid-state NMR methods, and positron annihilation lifetime (PAL) spectroscopy before the APCN reached its equilibrium swelling st...

Insertion of indigo molecules in the sepiolite structure as evidenced by 1H–29Si heteronuclear correlation spectroscopy

Despite the numerous studies of the famous indigo-based pigment Maya Blue, there are still many questions regarding the elucidation of its structure. Here, two-dimensional (2D) (1)H-(29)Si heteronuclear correlation (HETCOR) spectroscopy with frequency-switched Lee-Goldburg (FSLG) homonuclear decoupling is applied to sepiolite and sepiolite-indigo complexes. Owing to the high resolution in the (1)H dimension of the 2D (1)H-(29)Si HETCOR spectrum obtained by FSLG homonuclear decoupling, the assignment of the (29)Si cross-polarization magic-angle spinning (CPMAS) spectrum of sepiolite is clearly confirmed. Moreover, 2D (1)H-(29)Si FSLG-HETCOR spectroscopy gives the first direct evidence that some indigo molecules are inserted in the sepiolite structure whereas no interaction between indigo and the external side surface (silanol groups) is observed in the (29)Si CPMAS spectra. These results are consistent with the fact that indigo molecules interact with water coordinated to magnesium and suggest that Maya Blue made from sepiolite is not a surface complex.

Prospects for sub-micron solid state nuclear magnetic resonance imaging with low-temperature dynamic nuclear polarization.

We evaluate the feasibility of (1)H nuclear magnetic resonance (NMR) imaging with sub-micron voxel dimensions using a combination of low temperatures and dynamic nuclear polarization (DNP). Experiments are performed on nitroxide-doped glycerol-water at 9.4 T and temperatures below 40 K, using a 30 mW tunable microwave source for DNP. With DNP at 7 K, a 0.5 microL sample yields a (1)H NMR signal-to-noise ratio of 770 in two scans with pulsed spin-lock detection and after 80 db signal attenuation. With reasonable extrapolations, we infer that (1)H NMR signals from 1 microm(3) voxel volumes should be readily detectable, and voxels as small as 0.03 microm(3) may eventually be detectable. Through homonuclear decoupling with a frequency-switched Lee-Goldburg spin echo technique, we obtain 830 Hz (1)H NMR linewidths at low temperatures, implying that pulsed field gradients equal to 0.4 G/d or less would be required during spatial encoding dimensions of an imaging sequence, where d is the resolution in each dimension.

Understanding of the Plasticizing Effects of Glycerol and PEG 400 on Chitosan Films Using Solid-State NMR Spectroscopy

Differences were found in the plasticizing effects of two commonly used softening materials on amorphous chitosan films. The plasticizing efficiencies for glycerol and PEG 400 were found to be similar in mechanical tests, but the changes in the three-dimensional H-bonded structure monitored by solid-state NMR spectroscopy were different. The analysis of Lee−Goldburg cross-polarization build-up curves demonstrated that, while glycerol decreases the mobility of the acetamide groups, PEG 400 increases it. Further, while glycerol molecules are immobilized in chitosan films, PEG 400 remains mobile in them. The results of two-dimensional (2D) 1H−13C frequency-switched Lee−Goldburg (FSLG) HETCOR experiments supported the mobility difference of the chitosan chains and suggested a specific interaction between the glycerol and the glucosamine units. Our findings were supported by density functional theory calculations. Overall, PEG 400 acts as an external plasticizer, while glycerol acts as an internal plasticizer.

Two-photon Lee-Goldburg nuclear magnetic resonance: Simultaneous homonuclear decoupling and signal acquisition

We present NMR signals from a strongly coupled homonuclear spin system, (1)H nuclei in adamantane, acquired with simultaneous two-photon excitation under conditions of the Lee-Goldburg experiment. Small coils, having inside diameters of 0.36 mm, are used to achieve two-photon nutation frequencies of approximately 20 kHz. The very large rf field strengths required give rise to large Bloch-Siegert shifts that cannot be neglected. These experiments are found to be extremely sensitive to inhomogeneity of the applied rf field, and due to the Bloch-Siegert shift, exhibit a large asymmetry in response between the upper and lower Lee-Goldburg offsets. Two-photon excitation has the potential to enhance both the sensitivity and performance of homonuclear dipolar decoupling, but is made challenging by the high rf power required and the difficulties introduced by the inhomogeneous Bloch-Siegert shift. We briefly discuss a variation of the frequency-switched Lee-Goldburg technique, called four-quadrant Lee-Goldburg (4QLG) that produces net precession in the x-y plane, with a reduced chemical shift scaling factor of 13.

Multi-dimensional 1H– 13C HETCOR and FSLG-HETCOR NMR study of sphingomyelin bilayers containing cholesterol in the gel and liquid crystalline states

13C cross polarization magic angle spinning (CP-MAS) and 1H MAS NMR spectra were collected on egg sphingomyelin (SM) bilayers containing cholesterol above and below the liquid crystalline phase transition temperature ( T m). Two-dimensional (2D) dipolar heteronuclear correlation (HETCOR) spectra were obtained on SM bilayers in the liquid crystalline ( L α ) state for the first time and display improved resolution and chemical shift dispersion compared to the individual 1H and 13C spectra and significantly aid in spectral assignment. In the gel ( L β ) state, the 1H dimension suffers from line broadening due to the 1H– 1H homonuclear dipolar coupling that is not completely averaged by the combination of lipid mobility and MAS. This line broadening is significantly suppressed by implementing frequency switched Lee–Goldburg (FSLG) homonuclear 1H decoupling during the evolution period. In the liquid crystalline ( L α ) phase, no improvement in line width is observed when FSLG is employed. All of the observed resonances are assignable to cholesterol and SM environments. This study demonstrates the ability to obtain 2D heteronuclear correlation experiments in the gel state for biomembranes, expands on previous SM assignments, and presents a comprehensive 1H/ 13C NMR assignment of SM bilayers containing cholesterol. Comparisons are made to a previous report on cholesterol chemical shifts in dimyristoylphosphatidylcholine (DMPC) bilayers. A number of similarities and some differences are observed and discussed.

Characterization of microporous aluminophosphate IST-1 using 1H Lee–Goldburg techniques

The presence of two independent methylamine species in microporous aluminophosphate IST-1 (|(CH 3NH 2) 4(CH 3NH + 3) 4(OH −) 4|[Al 12P 12O 48]) has been shown previously by synchrotron powder X-ray diffraction. One of these species, [N(1)–C(1)], links to a six-coordinated framework Al-atom [Al (1)], while the other methylamine [N(2)–C(2)] is protonated and hydrogen-bonded to three O-atoms [O (1), O (2) and O (12)]. We revisit the structure of IST-1 and report the complete assignment of the 1H NMR spectra by combining X-ray data and high-resolution heteronuclear/homonuclear solid-state NMR techniques based on frequency-switched Lee–Goldburg homonuclear decoupling and 31P– 31P homonuclear recoupling. Careful analysis of the 2D 1H–X homonuclear correlation (X = 1H) and 2D heteronuclear correlation (X = 13C, 31P and 27Al) spectra allowed the distinction of both methylamine species and the assignment of all 31P and 13C resonances. For the first time at a relatively high (9.4 T) magnetic field, symmetric doublet patterns have been observed in the 13C spectra, caused by the influence of the 14N second-order quadrupolar interaction.

Two-dimensional 13C– 1H heteronuclear correlation NMR spectroscopic studies for the inclusion complex of cyclomaltoheptaose (β-cyclodextrin) with a new Helicobacter pylori eradicating agent (TG44) in the amorphous state

The interaction of a newly developed Helicobacter pylori eradicating agent (TG44, 4-methylbenzyl-4′-[ trans-4-(guanidinomethyl)cyclohexylcarbonyloxy]-biphenyl-4-carboxlylate monohydrochloride) with cyclomaltoheptaose (β-cyclodextrin, β-CyD) in the solid state was studied by high-speed frequency-switched Lee-Goldburg (FSLG) 13C– 1H heteronuclear correlation (HETCOR) NMR experiments. The TG44/β-CyD solid complex in a 1:1 stoichiometry was prepared by the grinding method. Powder X-ray diffractometry confirmed that the complex is in an amorphous state. The solid-state 13C signals of TG44 and β-CyD were significantly broadened by the complexation. As the temperature increased, the 13C signals of the aromatic moieties of TG44 were insignificantly influenced, whereas those of the cyclohexyl moiety became sharper. The T 1 ρ H values of the aromatic moieties of TG44 were almost the same as those of the β-CyD carbons, whereas those of other TG44 carbons gave much smaller values. The 13C– 1H HETCOR spectra gave the intermolecular correlation peaks between the aromatic carbons of TG44 and the β-CyD protons or between the biphenyl protons of TG44 and the β-CyD carbons, when measured using longer contact times (500 and 1500 μs). On the basis of these solid NMR spectroscopic data together with aqueous NMR data, we assume that β-CyD includes predominantly the biphenyl moiety of TG44 in the solid state. 13C– 1H HETCOR spectroscopy is particularly useful for the determination of inclusion modes of the complexes that occurring in an amorphous form.

X‐ray Diffraction and Solid‐State NMR Studies of a Germanium Binuclear Complex

A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry).

1H?15N correlation spectroscopy of nanocrystalline proteins

The limits of resolution that can be obtained in 1H-15N 2D NMR spectroscopy of isotopically enriched nanocrystalline proteins are explored. Combinations of frequency switched Lee-Goldburg (FSLG) decoupling, fast magic angle sample spinning (MAS), and isotopic dilution via deuteration are investigated as methods for narrowing the amide 1H resonances. Heteronuclear decoupling of 15N from the 1H resonances is also studied. Using human ubiquitin as a model system, the best resolution is most easily obtained with uniformly 2H and 15N enriched protein where the amides have been exchanged in normal water, MAS at approximately 20 kHz, and WALTZ-16 decoupling of the 15N nuclei. The combination of these techniques results in average 1H lines of only approximately 0.26 ppm full width at half maximum. Techniques for optimizing instrument stability and 15N decoupling are described for achieving the best possible performance in these experiments.

Heteronuclear dipolar recoupling in solid-state nuclear magnetic resonance by amplitude-, phase-, and frequency-modulated Lee–Goldburg cross-polarization

This paper presents a theoretical, numerical, and experimental study of phase- and frequency-switched Lee-Goldburg cross-polarization (FSLG-CP) under magic-angle spinning conditions. It is shown that a well-defined amplitude modulation of one of the two radio-frequency (rf) fields in the FSLG-CP sequence results in highly efficient heteronuclear dipolar recoupling. The recoupled dipolar interaction is gamma-encoded and, under ideal conditions, the effective spin Hamiltonian is equivalent to that in continuous-wave Lee-Goldburg CP. In practice, however, FSLG-CP is less susceptible to rf field mismatch and inhomogeneity, and provides better suppression of (1)H spin diffusion. The performance of FSLG-CP is experimentally demonstrated on liquid-crystalline samples exhibiting motionally averaged dipolar couplings.

Controlling the effects of pulse transients and RF inhomogeneity in phase-modulated multiple-pulse sequences for homonuclear decoupling in solid-state proton NMR

The effects of pulse imperfections and RF inhomogeneity on NMR spectra obtained with phase-modulated multiple-pulse NMR sequences are analyzed. The emphasis is on the combined effects of frequency offset, RF inhomogeneity, and pulse phase transients. To enable a theoretical description of the transients associated with phase changes under continuous RF irradiation, the nature of the transients is investigated in depth. As monitored in our 300 MHz spectrometer, they are found to be caused by linear elements of the RF circuitry. The validity of their representation as δ-function pulses and the significance of their decomposition into antisymmetric and symmetric components are discussed. A practical method for quantitative control of the antisymmetric phase transients is proposed. The linearity property allows the development of a theoretical description of the spin dynamics caused by the transients. This leads to a vector-Hamiltonian model for phase-modulated Lee–Goldburg experiments. It quantitatively predicts both the frequency shift and the line broadening caused by antisymmetric phase transients and their coupling with RF inhomogeneity. The model is shown to be equally applicable to frequency-switched Lee–Goldburg experiments. A noteworthy discovery is that for a given magnitude of the antisymmetric phase transients a frequency offset exists at which the inhomogeneity broadening is essentially canceled. This explains the common observation that for best resolution one side of resonance is preferred over the other. It also suggests a strategy for enhancing resolution without having to resort to severe sample volume restriction. Numerical calculations verified the theoretical predictions and allowed extension of the model to BLEW-12 and DUMBO-1. Experimental verification is presented. The deviations from theoretical predictions are discussed.

Reduction of rf power of observed nuclei for 1H-homonuclear decoupled cross-polarization in solid-state NMR spectroscopy

A new approach to markedly reduce rf power for observed nuclei during 1H-homonuclear decoupled cross-polarization is described. The reduction of rf power to satisfy the Hartmann–Hahn matching condition for observed nuclei and homonuclear dipolar decoupling at 1H nuclei were achieved simultaneously based on ‘time averaging’ of nutation frequency of 1H nuclei by frequency switched Lee–Goldburg decoupling with unequal duration times for ±LG irradiations. The rf nutation frequency and rf field for observed nuclei were shown experimentally to be reduced by factor 3 and 10, respectively, at 13C-NMR signals of MBBA in the liquid crystalline state.