Multiple Quantum Correlation Research Articles

High-resolution indirect detection of spin-3/2 quadrupolar nuclei in solids using multiple-quantum-filtered through-space D-HMQC experiments

Through-space heteronuclear correlation experiments under magic-angle spinning (MAS) conditions can provide unique insights into inter-atomic proximities. In particular, it has been shown that experiments based on two consecutive coherence transfers, 1H → I → 1H, like D-HMQC (dipolar-mediated heteronuclear multiple-quantum correlation), are usually more sensitive for the indirect detection via protons of spin-3/2 quadrupolar nuclei with low gyromagnetic ratio. Nevertheless, the resolution is often decreased by the second-order quadrupolar broadening along the indirect dimension. To circumvent this issue, we incorporate an MQMAS (multiple-quantum MAS) quadrupolar filter into the t1 evolution period of the D-HMQC sequence, which results in a novel pulse sequence called D-HMQC-MQ. The triple-quantum coherences evolving during this filter are excited and reconverted using cosine-modulated long-pulses synchronized with the sample rotation to avoid spinning sidebands in the indirect dimension. The desired coherence transfer pathways during this sequence are selected using two nested cogwheel phase cycles with 56 steps. This high-resolution heteronuclear correlation technique is demonstrated experimentally for the indirect detection via 1H of spin-3/2 isotopes, such as 11B, 23Na and 35Cl, in zinc borate hydrate, NaH2PO4 and l-histidine hydrochloride, respectively. We show that this experiment can be applied at high magnetic fields up to 28.2 T for protons subject to chemical shift anisotropies larger than 20 ppm, provided the MAS frequency is sufficiently stable since the D-HMQC-MQ experiment, like the parent D-HMQC, is sensitive to MAS fluctuations, which can produce t1-noise.

Revealing Brønsted Acidic Bridging SiOHAl Groups on Amorphous Silica-Alumina by Ultrahigh Field Solid-State NMR.

Amorphous silica-aluminas (ASAs) are important acidic catalysts and supports for many industrially essential and sustainable processes. The identification of surface acid sites with their local structures on ASAs is of critical importance for tuning their catalytic properties but still remains a great challenge and is under debate. Here, ultrahigh magnetic field (35.2 T) 27Al-{1H} D-HMQC (dipolar-mediated heteronuclear multiple-quantum correlation) two-dimensional NMR experiments demonstrate two types of Brønsted acid sites in ASA catalysts. In addition to the known pseudobridging silanol acid sites, the use of ultrahigh field NMR provides the first direct experimental evidence for the existence of bridging silanol (BS: SiOHAl) acid sites in ASAs, which has been hotly debated in the past few decades. This discovery provides new opportunities for scientists and engineers to develop and apply ASAs in various reaction processes due to the significance of BS in chemical and fuel productions based on its strong Brønsted acidity.

Through-space 11 B-27 Al correlation: Influence of the recoupling channel.

Through-space heteronuclear correlation (D-HETCOR) experiments based on heteronuclear multiple-quantum correlation (D-HMQC) and refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) sequences have been proven to be useful approaches for the detection of the spatial proximity between half-integer quadrupolar nuclei in solids under magic-angle spinning (MAS) conditions. The corresponding pulse sequences employ coherence transfers mediated by heteronuclear dipolar interactions, which are reintroduced under MAS by radiofrequency irradiation of only one of the two correlated nuclei. We investigate herein using numerical simulations of spin dynamics and solid-state NMR experiments on magnesium aluminoborate glass how the choice of the channel to which the heteronuclear dipolar recoupling is applied affects the transfer efficiency of D-HMQC and D-RINEPT sequences between 11 B and 27 Al nuclei. Experimental results show that maximum transfer efficiency is achieved when the recoupling scheme is applied to the channel, for which the spin magnetization is parallel to the B0 axis in average.

Low-power synchronous helical pulse sequences for large anisotropic interactions in MAS NMR: Double-quantum excitation of 14N.

We develop a theoretical framework for a class of pulse sequences in the nuclear magnetic resonance (NMR) of rotating solids, which are applicable to nuclear spins with anisotropic interactions substantially larger than the spinning frequency, under conditions where the radiofrequency amplitude is smaller than or comparable to the spinning frequency. The treatment is based on average Hamiltonian theory and allows us to derive pulse sequences with well-defined relationships between the pulse parameters and spinning frequency for exciting specific coherences without the need for any detailed calculations. This framework is applied to the excitation of double-quantum spectra of 14N and is used both to evaluate the existing low-power pulse schemes and to predict the new ones, which we present here. It is shown that these sequences can be designed to be γ-encoded and therefore allow the acquisition of sideband-free spectra. It is also shown how these new double-quantum excitation sequences are incorporated into heteronuclear correlation NMR, such as 1H-14N dipolar double-quantum heteronuclear multiple-quantum correlation spectroscopy. The new experiments are evaluated both with numerical simulations and experiments on glycine and N-acetylvaline, which represent cases with "moderate" and "large" quadrupolar interactions, respectively. The analyzed pulse sequences perform well for the case of a "moderate" quadrupolar interaction, however poorly with a "large" quadrupolar interaction, for which future work on pulse sequence development is necessary.

Two-dimensional single- and multiple-quantum correlation spectroscopy in zero-field nuclear magnetic resonance

We present single- and multiple-quantum correlation J-spectroscopy detected in zero (<1μG) magnetic field using a 87Rb vapor-cell magnetometer. At zero field the spectrum of ethanol appears as a mixture of 13C isotopomers, and correlation spectroscopy is useful in separating the two composite spectra. We also identify and observe the zero-field equivalent of a double-quantum transition in 13C2-acetic acid, and show that such transitions are of use in spectral assignment. Two-dimensional spectroscopy further improves the high resolution attained in zero-field NMR since selection rules on the coherence-transfer pathways allow for the separation of otherwise overlapping resonances into distinct cross-peaks.

Diketopeprazin and Methyl-5-docosenoate from endophytic fungi Aureobasidium pollulan BSS6 with α-Glucosidase inhibition and its validation through molecular docking

Endophytic fungi have been a focus of attention for producing diverse classes of bioactive metabolites but little is known about endophytic fungi Aureobasidium pullulan, isolated from the bark of frankincense producing tree (Boswellia sacra). Extensive chromatographic methods resulted in the isolation of two known diketopeprazin (1 and 2) along with three fatty acids derivatives (3–5) from the ethyl acetate fraction of Aureobasidium pollulan BSS6. The structures of mentioned compounds were characterized by advanced 1D and 2D spectroscopic techniques such as, ultraviolet–visible (UV), infrared (IR), electrospray ionization mass spectrometry (ESI-MS) 1H NMR, 13C NMR, heteronuclear multiple bond correlation (HMBC), heteronuclear multiple-quantum coherence (HMQC) and correlation spectroscopy (COSY). The resulted compounds were evaluated for α-glucosidase inhibition potential to know their hyperglycemic effect in type 2 diabetes mellitus. Compound 3 showed significant inhibition having an IC50 value of 23.3 µM. To know the binding energy and pharmacophore of compound 3 with in active site of enzyme, the molecular docking was performed. Compound 3 has shown π-alkyl type of interactions with Asp214 and Glu276, key amino acid residues in the catalytic triad of α-glucosidase. In conclusion, the current study reports novel sources of anti-diabetic constituent's from Aureobasidium sp. for the first time.

1H, 29Si and 119Sn double and triple resonance NMR spectroscopy of the small-pore framework sodium stannosilicate Na2SnSi3O9·2H2O

The small-pore framework sodium stannosilicate AV-10, chemical composition Na2SnSi3O9·2H2O and known crystallographic structure, was synthesized by hydrothermal crystallization. This stannosilicate is built up of a three-dimensional network of corner-shared SiO4 tetrahedra and SnO6 octahedra. The SnO6 sites are linked to six SiO4 tetrahedra (Sn(6Si)) while each of the two crystallographically different SiO4 units are connected to two SnO6 and SiO4 units (Si(2Si,2Sn)). This material was used as model compound for developing a solid-state MAS NMR strategy aimed on the challenges and possibilities for structural studies, particularly considering the short and medium range order to verify the connectivity of SiO4 and SnO6 of such compounds despite the low natural abundances of 4.68% for 29Si and 8.59% for 119Sn nuclei as a real challenge. 29Si{119Sn} and 119Sn{29Si} REDOR (Rotational-Echo Double-Resonance) NMR measurements after 1H cross-polarization (CP) were carried out. The REDOR curves show a significant change after the “normal” quadratic short time evolution from which both (i) the shortest internuclear 29Si – 119Sn distances (and vice versa) and (ii) the number of corner-sharing SiO4 tetrahedra around the SnO6 octahedra (and vice versa) can be obtained. Based on these data, optimized 29Si{119Sn} and 119Sn{29Si} REPT-HMQC (Recoupled Polarization Transfer-Heteronuclear Multiple-Quantum Correlation, again after 1H CP) experiments were implemented, which directly show those heterogroup connectivity as correlation peaks in a 2D spectrum. This information was also obtained using 2D29Si{119Sn}-J-Coupling NMR experiments. Furthermore, 2D29Si INADEQUATE NMR experiments are also feasible, showing the connectivity of SiO4 tetrahedra. The combination of REDOR, REPT-HMQC, J-Coupling and INADEQUATE experiments yielded a complete analysis of the short and medium range structure of this microporous stannosilicate, in agreement with the previously published structure obtained Ab Initio from powder X-Ray diffraction data (XRD).

Two-dimensional NMR lineshape analysis of single, multiple, zero and double quantum correlation experiments

NMR spectroscopy provides a powerful approach for the characterisation of chemical exchange and molecular interactions by analysis of series of experiments acquired over the course of a titration measurement. The appearance of NMR resonances undergoing chemical exchange depends on the frequency difference relative to the rate of exchange, and in the case of one-dimensional experiments chemical exchange regimes are well established and well known. However, two-dimensional experiments present additional complexity, as at least one additional frequency difference must be considered. Here we provide a systematic classification of chemical exchange regimes in two-dimensional NMR spectra. We highlight important differences between exchange in HSQC and HMQC experiments, that on a practical level result in more severe exchange broadening in HMQC spectra, but show that complementary alternatives to the HMQC are available in the form of HZQC and HDQC experiments. We present the longitudinal relaxation optimised SOFAST-H(Z/D)QC experiment for the simultaneous acquisition of sensitivity-enhanced HZQC and HDQC spectra, and the longitudinal and transverse relaxation optimised BEST-ZQ-TROSY for analysis of large molecular weight systems. We describe the application of these experiments to the characterisation of the interaction between the Hsp90 N-terminal domain and a small molecule ligand, and show that the independent analysis of HSQC, HMQC, HZQC and HDQC experiments provides improved confidence in the fitted dissociation constant and dissociation rate. Joint analysis of such data may provide improved sensitivity to detect and analyse more complex multi-state interaction mechanisms such as induced fit or conformational selection.

DNA/RNA of gum cancer cells-anti–cancer nano drugs ligands structure determination with the two–dimensional NMR molecular line shape analysis of single, multiple, zero and double quantum correlation experiments

DNA/RNA of gum cancer cells-anti–cancer nano drugs ligands structure determination with the two–dimensional NMR molecular line shape analysis of single, multiple, zero and double quantum correlation experiments

New bioactive C15 acetogenins from the red alga Laurencia obtusa

Background and Objective: With regard to the uniqueness of the red algae of the genus Laurencia as the source of C15-acetogenins, along with the diversity of biological applications; the acetogenin content of the Red Sea Laurencia obtusa was investigated. Materials and Methods: Fractionation and purification of the CH2Cl2/MeOH extract were carried out by applying several chromatographic techniques, including column and preparative thin-layer chromatography; followed by a series of 1H nuclear magnetic resonance measurements to give rise of some interesting notes. Toxicity to Artemia salina was evaluated. The apoptosis induced by these two compounds was demonstrated by DNA fragmentation assay and microscopic observation. Results: A new rare chloroallene-based C15 acetogenin, laurentusenin (1) along with a new furan ring containing C15 acetogenin, laurenfuresenin (2), were isolated from the red alga L. obtusa. Comparing 1D and 2D NMR, MS, ultraviolet and infrared radiation spectral data for the newly isolated compounds with the reported bromoallene containing acetogenins spectral data was played the crucial role for characterization of their chemical structures. 1 and 2 exhibited bare toxicity (LD50 >12 mM) in test organism, A. salina and induced apoptotic death confirmed by DNA fragmentation and microscopic investigations. Conclusion: The isolated metabolite 1 showed unusual substituted allene side chain, while 2 inserted furan ring as a new acetogenin nucleus. Both compounds may play a role in apoptosis induction and initiation and propagation of inflammatory responses. Abbreviations used: NMR: Nuclear magnetic resonance; MS: Mass spectrometry; UV: Ultraviolet spectroscopy; IR: infrared radiation; EIMS: Electron ionization mass spectra; TLC: Thin-layer chromatography; PPP: Platelet poor plasma; PRPDS: platelet-rich plasma derived serum. DEPT: Destortionless Enhancement by Polarization Transfer; NOESY: Nuclear Overhauser Effect Spectroscopy; HSQC: Heteronuclear Single Quantum Coherence; HMBC: Heteronuclear Multiple-quantum Correlation; 1H-1H COSY: Correlation Spectroscopy.

Reducing t1 noise through rapid scanning.

The so-called t1 noise, which arises due to random instabilities in the spectrometer hardware, remains the primary source of noise that limits the sensitivity of most 2D NMR experiments, particularly in the expanding group of solid-state NMR methods that utilize dipolar-recoupling. In this communication we revisit the relationship between the signal intensity and the t1 noise produced. It is shown that since the latter scales linearly with the signal strength, the use of a conventional relaxation delay of 1.3T1 may prove far from optimal. In cases where the fluctuations occur on a shorter timescale than the recycle delay, a considerably faster repetition rate should be used to maximize the time sensitivity in a 2D experiment than what is used to maximize the sensitivity in 1D. This is demonstrated with the acquisition of 1H{13C} Dipolar-mediated Heteronuclear Multiple-Quantum Correlation (D-HMQC) type spectra in which the sensitivity could be nearly doubled by choosing a very short relaxation delay corresponding to 0.2T1.

Observation of proximities between spin-1/2 and quadrupolar nuclei in solids: Improved robustness to chemical shielding using adiabatic symmetry-based recoupling

We introduce a novel heteronuclear dipolar recoupling based on the R21−1 symmetry, which uses the tanh/tan (tt) shaped pulse as a basic inversion element and is denoted R21−1(tt). Using first-order average Hamiltonian theory, we show that this sequence is non-γ-encoded and that it reintroduces the |m| = 1 spatial component of the Chemical Shift Anisotropy (CSA) of the irradiated isotope and its heteronuclear dipolar interactions. Using numerical simulations and one-dimensional (1D) 27Al-{31P} through-space D-HMQC (Dipolar Heteronuclear Multiple-Quantum Correlation) experiments on VPI-5, we compare the performances of this recoupling to those of other non-γ-encoded |m| = 1 heteronuclear recoupling schemes: REDOR (Rotational-Echo DOuble Resonance), SFAM (Simultaneous Frequency and Amplitude Modulation) and R42−1(tt). Such comparison indicates that the R21−1(tt) scheme is more robust to CSA, offset and radiofrequency field inhomogeneities than the other schemes. We take advantage of the high robustness of R21−1(tt) to CSA and offset to demonstrate the possibility to correlate the signals of 207Pb isotope with those of neighboring half-integer spin quadrupolar nuclei. Such approach is demonstrated experimentally by acquiring 11B-{207Pb} D-HMQC 2D spectra of Pb4O(BO3)2 crystalline powder.

Improving reliability of chemometric models for authentication of species origin of heparin by switching from 1D to 2D NMR experiments

Nuclear magnetic resonance (NMR) spectroscopy is regarded as one of the most powerful and versatile analytical approaches to assure the quality of heparin preparations. In particular, it was recently demonstrated that by using 1H NMR coupled with chemometrics heparin and low molecular weight heparin (LMWH) samples derived from three major animal species (porcine, ovine and bovine) can be differentiated [Y.B. Monakhova et al. J. Pharm. Anal. 149 (2018) 114–119].In this study, significant improvement of existing chemometric models was achieved by switching to 2D NMR experiments (heteronuclear multiple-quantum correlation (HMQC) and diffusion-ordered spectroscopy (DOSY)). Two representative data sets (sixty-nine heparin and twenty-two LMWH) belonged to different batches and distributed by different commercial companies were investigated. A trend for animal species differentiation was observed in the principal component analysis (PCA) score plot built based on the DOSY data. A superior model was constructed using HMQC experiments, where individual heparin (LMWH) clusters as well as their blends were clearly differentiated. The predictive power of different classification methods as well as unsupervised techniques (independent components analysis, ICA) clearly proved applicability of the model for routine heparin and LMWH analysis.The switch from 1D to 2D NMR techniques provides a wealth of additional information, which is beneficial for multivariate modeling of NMR spectroscopic data for heparin preparations.

Heteronuclear Correlation Experiments Such as Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC), Heteronuclear Multiple-Quantum Correlation Spectroscopy (HMQC) and Heteronuclear Multiple-Bond Correlation Spectroscopy (HMBC) Comparative Study On Malignant and Benign Human Endocrinology and Thyroid Cancer Cells and Tissues Under Synchrotron Radiation

Heteronuclear Correlation Experiments Such as Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC), Heteronuclear Multiple-Quantum Correlation Spectroscopy (HMQC) and Heteronuclear Multiple-Bond Correlation Spectroscopy (HMBC) Comparative Study On Malignant and Benign Human Endocrinology and Thyroid Cancer Cells and Tissues Under Synchrotron Radiation

Isolation, elucidation, and molecular docking studies of active compounds from Phyllanthus niruri with angiotensin-converting enzyme inhibition

Background: Phyllanthus niruri, in Indonesia, is known as “Meniran” has a long history of use in ethnic or traditional medicine worldwide, mainly as an antihypertensive agent. Objective: The present study was designed to isolate and identify active compounds with angiotensin-converting enzyme (ACE) inhibition activity from P. niruri herb and confirm the mechanism of action, affinity, and domain specificity interactions of the isolated compounds. Materials and Methods: Some fractions of P. niruri methanolic extract were subjected to column chromatography and preparative thin-layer chromatography to get active compounds. Structural elucidation was determined via spectroscopic methods. ACE inhibition activity was measured using hippuryl-L-histidyl-L-leucine as a substrate in vitro assay. Furthermore, confirmation of the mechanism of action, affinity, and domain specificity interaction of the isolated compounds on ACE complex macromolecule (protein database id: 1O86) was performed by in silico molecular docking studies. Results: In this work, four active compounds were isolated from aerial part of P. niruri, including hypophyllantin (50% inhibition concentration [IC50] = 0.180 μg/mL), phyllantin (IC50 = 0.140 μg/mL), methyl gallate (IC50 = 0.015 μg/mL), and quercetin 3-O-β-D-glucopyranosyl-(1'''-6'')-α-rhamnoside (IC50 = 0.086 μg/mL). In silico molecular docking method emphasizes ligand-residue interactions, thereby predicting the inhibitory activity of these compounds. After docking to an ACE complex macromolecule, quercetin 3-O-β-D-glucopyranosyl-(1'''-6'')-α-rhamnoside obtained more interactions than lisinopril. Conclusion: The results were obtained from in silico and in vitro experiments and confirm the potential active compound is an ACE inhibitor and a new antihypertensive agent. Abbreviations Used: P. niruri: Phyllanthus niruri; ACE: Angiotensin-converting enzyme; HHL: Hippuryl-L-histidyl-L-leucine; HA: Hippuric acid; PDB: Protein database; IC50: 50% inhibition concentration; FH: N-hexane fraction; FE: Ethyl acetate fraction; TLC: Thin layer chromatography; UV-VIS: Ultraviolet-visible; NMR: nuclear magnetic resonance; FTIR: Fourier–Transform infrared; MS: Mass spectrometry; HMQC: Heteronuclear Multiple-Quantum Correlation; HMBC: Heteronuclear multiple bond correlation; TADOK: Tugas Akhir Mahasiswa Doktor.

Characterization of Silicon Nanocrystal Surfaces by Multidimensional Solid-State NMR Spectroscopy

The chemical and photophysical properties of silicon nanocrystals (Si NCs) are strongly dependent on the chemical composition and structure of their surfaces. Here we use fast magic angle spinning (MAS) and proton detection to enable the rapid acquisition of dipolar and scalar 2D 1H–29Si heteronuclear correlation (HETCOR) solid-state NMR spectra and reveal a molecular picture of hydride-terminated and alkyl-functionalized surfaces of Si NCs produced in a nonthermal plasma. 2D 1H–29Si HETCOR and dipolar 2D 1H–1H multiple-quantum correlation spectra illustrate that resonances from surface mono-, di-, and trihydride groups cannot be resolved, contrary to previous literature assignments. Instead the 2D NMR spectra illustrate that there is large distribution of 1H and 29Si chemical shifts for the surface hydride species in both the as-synthesized and functionalized Si NCs. However, proton-detected 1H–29Si refocused INEPT experiments can be used to unambiguously differentiate NMR signals from the different surf...

Synthesis, modeling and biological evaluation of hybrids from pyrazolo[1,5c]pyrimidine as antileishmanial agents.

A new series of pyrazolo[1,5-c]pyrimidines were synthesized by different hybridization strategies. All structures were confirmed by IR, 1H, 13C, 1H-13C heteronuclear multiple-quantum correlation (HMQC) spectra and microanalysis. They were evaluated for their in vitro antileishmanial activity against miltefosine and amphotericin B deoxycholate as reference drugs. The most active compounds 2a and 9a demonstrated superior potencies to miltefosine by ten- and six-fold, respectively, for the promastigote form, and by 5.5-fold for the amastigote form. Their binding scenario to Leishmania major pteridine reductase was rationalized by docking experiments. In addition, all compounds were safe for the experimental animals orally up to 150mg/kg and parenterally up to 75mg/kg. This study provides novel chemotype class for antileishmanial activity. [Formula: see text].

71Ga-77Se connectivities and proximities in gallium selenide crystal and glass probed by solid-state NMR

We introduce two-dimensional (2D) 71Ga-77Se through-bond and through-space correlation experiments. Such correlations are achieved using (i) the J-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer (J-RINEPT) method with 71Ga excitation and 77Se Carr-Purcell-Meiboon-Gill (CPMG) detection, as well as (ii) the J- or dipolar-mediated Hetero-nuclear Multiple-Quantum Correlation (J- or D-HMQC) schemes with 71Ga excitation and quadrupolar CPMG (QCPMG) detection. These methods are applied to the crystalline β-Ga2Se3 and the 0.2Ga2Se3-0.8GeSe2 glass. Such glass leads to a homogeneous and reproducible glass–ceramic, which is a good alternative to single-crystalline Ge and polycrystalline ZnSe materials for making lenses transparent in the IR range for thermal imaging applications. We show that 2D 71Ga-77Se correlation experiments allow resolving the 77Se signals of molecular units, which are not resolved in the 1D 77Se CPMG spectrum. Additionally, the build-up curves of the J-RINEPT and the J-HMQC experiments allow the estimate of the 71Ga-77Se J-couplings via one and three-bonds in the three-dimensional network of β-Ga2Se3. Furthermore, these build-up curves show that the one-bond 1J71Ga-77Se couplings in the 0.2Ga2Se3-0.8GeSe2 glass are similar to those measured for β-Ga2Se3. We also report 2D 71Ga Satellite Transition Magic-Angle Spinning (STMAS) spectrum of β-Ga2Se3 using QCPMG detection at high magnetic field and high Magic-Angle Spinning frequency using large radio frequency field. Such spectrum allows separating the signal of β-Ga2Se3 and that of an impurity.

γ-Independent through-space hetero-nuclear correlation between spin-1/2 and quadrupolar nuclei in solids

We introduce novel sequences using indirect detection to correlate quadrupolar nuclei and spin-1/2 isotopes, other than 1H and 19F. These sequences use γ-encoded symmetry-based RNnν schemes that reintroduce the space component |m| = 1 of the heteronuclear dipolar coupling. These schemes can be applied to the indirectly detected spin in Dipolar-mediated Heteronuclear Multiple-Quantum Correlation (D-HMQC) sequence or to the detected isotope in a novel sequence, named Dipolar-mediated Heteronuclear Universal-Quantum Correlation (D-HUQC). We show that the signal of these sequences using γ-encoded recoupling does not depend on the γ Euler angle relating the inter-nuclear vector between the coupled spins to the MAS rotor-fixed frame. Therefore, the transfer efficiency of these sequences is in principle higher than that of D-HMQC methods using non-γ-encoded recoupling. Furthermore, numerical simulations show that the heteronuclear correlation experiments employing γ-encoded recoupling are more robust to Chemical Shift Anisotropy (CSA) of the irradiated spin and MAS frequency fluctuations. These results are confirmed by 13C-{15N} heteronuclear correlation on glycine and 31P-27Al ones on VPI-5 and Na7(AlP2O7)4PO4. These experiments indicate that R1635 recoupling produces the highest signal-to-noise ratio in heteronuclear correlation 2D experiments when the detected spin-1/2 nuclei are subject to large CSA.

Low-power broadband solid-state MAS NMR of 14N.

We propose two broadband pulse schemes for 14N solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) that achieves (i) complete population inversion and (ii) efficient excitation of the double-quantum spectrum using low-power single-sideband-selective pulses. We give a comprehensive theoretical description of both schemes using a common framework that is based on the jolting-frame formalism of Caravatti et al. [J. Magn. Reson. 55, 88 (1983)]. This formalism is used to determine for the first time that we can obtain complete population inversion of 14N under low-power conditions, which we do here using single-sideband-selective adiabatic pulses. It is then used to predict that double-quantum coherences can be excited using low-power single-sideband-selective pulses. We then proceed to design a new experimental scheme for double-quantum excitation. The final double-quantum excitation pulse scheme is easily incorporated into other NMR experiments, as demonstrated here for double quantum-single quantum 14N correlation spectroscopy, and 1H-14N dipolar heteronuclear multiple-quantum correlation experiments. These pulses and irradiation schemes are evaluated numerically using simulations on single crystals and full powders, as well as experimentally on ammonium oxalate ((NH4)2C2O4) at moderate MAS and glycine at ultra-fast MAS. The performance of these new NMR methods is found to be very high, with population inversion efficiencies of 100% and double-quantum excitation efficiencies of 30%-50%, which are hitherto unprecedented for the low radiofrequency field amplitudes, up to the spinning frequency, that are used here.