Probing through bond connectivities with MQMAS NMR

Practical considerations on the use of low RF-fields and cosine modulation in high-resolution NMR of I = 3/2 spin quadrupolar nuclei in solids

Despite its ease in experimental set up, the low sensitivity of MQMAS experiments is often a limiting factor in many practical applications. This is mainly due to the large radiofrequency (RF) field requirement of the two short hard-pulses often used for the optimum MQ excitation and conversion steps. Very recently, two novel MQMAS experiments have been proposed for I = 3/2 nuclei, namely lp-MQMAS and coslp-MQMAS, enabling an efficient MQ excitation/conversion with a reduced RF requirement, by utilizing two long pulses lasting one rotor period each, with or without cosine modulation. In this study, we focus on the practical considerations of these new methods and discuss their pros and cons to elucidate their appropriate use under both moderate and fast spinning conditions. Using four I = 3/2 (⁸⁷Rb, ⁷¹Ga, ³⁵Cl and ²³Na) nuclei at a moderate magnetic field (B₀ = 14.1 T), we show the superior use of these experiments, especially for samples with large C_Q values and/or low-gamma nuclei. Compared to all other existing sequences, the coslp-MQMAS method with initial WURST signal enhancement is the most robust, efficient and resolved high-resolution 2D method for spin 3/2 nuclei. Furthermore, using {²³Na}-¹H spin systems, we demonstrate the sensitivity advantage of the WURST coslp-MQ-HETCOR acquisition upon ¹H detection and fast MAS conditions.

Accelerating the acquisition of high-resolution quadrupolar MQ/ST-HETCOR 2D spectra under fast MAS via 1H detection and through-space population transfers

Recently, we established an experimental setup protocol to perform the population transfer from half-integer quadrupolar spin to ¹H nuclei under fast MAS in the context of MQ-HETCOR experiments. In this article, we further develop the high-resolution 2D HETCOR methods by ST-based approaches, making use of the sensitivity advantage of STMAS over its MQMAS counterpart. In a similar manner to the previous work, which utilized CP and RINEPT for the population transfer, we also demonstrate the experimental setup protocol for PRESTO. Using {²³Na}-¹H and {²⁷Al}-¹H spin systems of powder samples, we compare a series of MQ/ST-HETCOR 2D spectra to discuss the pros and cons of the distinct MQ/ST-based approaches for spin 3/2 and 5/2 nuclei, respectively. We also incorporate two experimental tricks to reduce the experimental time of such long 2D experiments, the Optimized Rotor-Synchronization (ORS) and the Non-Uniform Sampling (NUS), in the context of high-resolution spectra of half-integer quadrupolar spin nuclei.

A comparison of through-space population transfers from half-integer spin quadrupolar nuclei to 1H using MQ-HETCOR and MQ-SPAM-HETCOR under fast MAS

In this article, we compare the various schemes of magnetization transfer from half-integer quadrupolar spins to ¹H nuclei and we establish an efficient protocol to perform these transfers under MQMAS high-resolution with the MQ-HETCOR and MQ-SPAM-HETCOR experiments under fast MAS. The MQMAS efficiencies are analyzed with SIMPSON simulations, and the CPMAS and RINEPT magnetization transfers are compared at 62.5 kHz MAS using {²³Na}-¹H and {²⁷Al}-¹H MQ-HETCOR and MQ-SPAM-HETCOR experiments performed on NaH₂PO4, Na₂HPO₄, Na citrate dihydrate and ipa-AlPO-14 powder samples. We discuss the pros and cons of these approaches, aiming to record 2D spectra of the best possible quality under fast MAS. We also incorporate some experimental approaches to reduce the total experiment time of such long 2D experiments.

High-Resolution NMR of S = 3/2 Quadrupole Nuclei by Detection of Double-Quantum Satellite Transitions via Protons

Indirect NMR detection via protons under fast magic-angle spinning can help overcome the low sensitivity and resolution of low-γ quadrupole nuclei such as ³⁵Cl. A robust and efficient method is presented for indirectly acquiring the double-quantum satellite-transition (DQ-ST) spectra of quadrupole nuclei. For a spin S = 3/2, the DQ-STs have a much smaller second-order quadrupolar broadening, one-ninth compared to that of the central transition. Thus, they can provide a factor of up to 18 in resolution enhancement. The indirect detection of DQ-STs via protons is carried out using the heteronuclear multiple-quantum coherence (HMQC) experiment with the transfer of populations in double-resonance (TRAPDOR) recoupling mechanism. The resolution enhancement by detecting DQ-STs and the high efficiency of the TRAPDOR-HMQC experiment are demonstrated by ³⁵Cl NMR of several active pharmaceutical ingredients (APIs).

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 ¹J_71Ga-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.

A simulated annealing approach for solving zeolite crystal structures from two-dimensional NMR correlation spectra

An improved NMR crystallography strategy is presented for determining the structures of network materials such as zeolites from just a single two-dimensional (2D) NMR correlation spectrum that probes nearest-neighbor interactions, combined with the unit cell parameters and space group information measured in a diffraction experiment. The correlation information contained within a 2D spectrum is converted into a "connectivity matrix" which is incorporated into a cost function whose minimum is searched for using a simulated annealing algorithm. The algorithm was extensively tested on over 150 zeolite frameworks from the International Zeolite Association database of zeolite structures and shown to be very robust and efficient in reconstructing the structures from connectivity information. The structure determination of the pure silica zeolites ITQ-4, Ferrierite, and Sigma-2 from experimental 2D (29)Si double-quantum NMR spectra and powder X-ray diffraction data using this improved approach is also presented.

New methods and applications in solid-state NMR spectroscopy of quadrupolar nuclei

Solid-state nuclear magnetic resonance (NMR) spectroscopy has long been established as offering unique atomic-scale and element-specific insight into the structure, disorder, and dynamics of materials. NMR spectra of quadrupolar nuclei (I > (1)/2) are often perceived as being challenging to acquire and to interpret because of the presence of anisotropic broadening arising from the interaction of the electric field gradient and the nuclear electric quadrupole moment, which broadens the spectral lines, often over several megahertz. Despite the vast amount of information contained in the spectral line shapes, the problems with sensitivity and resolution have, until very recently, limited the application of NMR spectroscopy of quadrupolar nuclei in the solid state. In this Perspective, we provide a brief overview of the quadrupolar interaction, describe some of the basic experimental approaches used for acquiring high-resolution NMR spectra, and discuss the information that these spectra can provide. We then describe some interesting recent examples to showcase some of the more exciting and challenging new applications of NMR spectra of quadrupolar nuclei in the fields of energy materials, microporous materials, Earth sciences, and biomaterials. Finally, we consider the possible directions that this highly informative technique may take in the future.

Theoretical study of homonuclear J coupling between quadrupolar spins: single-crystal, DOR, and J-resolved NMR

The theory describing homonuclear indirect nuclear spin-spin coupling (J) interactions between pairs of quadrupolar nuclei is outlined and supported by numerical calculations. The expected first-order multiplets for pairs of magnetically equivalent (A2), chemically equivalent (AA'), and non-equivalent (AX) quadrupolar nuclei are given. The various spectral changeovers from one first-order multiplet to another are investigated with numerical simulations using the SIMPSON program and the various thresholds defining each situation are given. The effects of chemical equivalence, as well as quadrupolar coupling, chemical shift differences, and dipolar coupling on double-rotation (DOR) and J-resolved NMR experiments for measuring homonuclear J coupling constants are investigated. The simulated J coupling multiplets under DOR conditions largely resemble the ideal multiplets predicted for single crystals, and a characteristic multiplet is expected for each of the A2, AA', and AX cases. The simulations demonstrate that it should be straightforward to distinguish between magnetic inequivalence and equivalence using J-resolved NMR, as was speculated previously. Additionally, it is shown that the second-order quadrupolar-dipolar cross-term does not affect the splittings in J-resolved experiments. Overall, the homonuclear J-resolved experiment for half-integer quadrupolar nuclei is demonstrated to be robust with respect to the effects of first- and second-order quadrupolar coupling, dipolar coupling, and chemical shift differences.

Multiple resonance heteronuclear decoupling under MAS: dramatic increase of spectral resolution at moderate magnetic field and MAS frequencies

The effects of multiple-resonance heteronuclear decoupling under magic angle spinning (MAS) on the resolution of one-dimensional (19)F and (31)P and various two-dimensional MAS NMR spectra and on the residual non-refocusable coherence lifetimes in fluorinated aluminophosphate AlPO(4)-CJ2, i.e. a compound that contains numerous highly abundant nuclei but no homonuclear spin bath, has been investigated. The design of the four-channel ((1)H, (19)F, (27)Al, (31)P) MAS probe used for this study is first described. (1)H and (1)H-(27)Al double-resonance decouplings allows lengthening the optimized transverse relaxation T(2)(opt) and increasing the resolution in the (19)F and (31)P dimensions. Under the application of multi-nuclear decoupling, a two-dimensional (19)F-(31)P CP-HETCOR correlation spectrum for AlPO(4)-CJ2 is recorded with unprecedented high-resolution in the two dimensions. Moreover, because (1)H-decoupling increases the (19)F T(2)(opt), it has been applied during the entire duration of the 2D NMR experiments, allowing the direct use of residual small interactions to generate (19)F-(19)F and (19)F-(27)Al 2D NMR correlation spectra in AlPO(4)-CJ2.

19F-decoupling of half-integer spin quadrupolar nuclei in solid-state NMR: application of frequency-swept decoupling methods

In solid-state NMR studies of minerals and ion conductors, quadrupolar nuclei like (7)Li, (23)Na or (133)Cs are frequently situated in close proximity to fluorine, so that application of (19)F decoupling is beneficial for spectral resolution. Here, we compare the decoupling efficiency of various multi-pulse decoupling sequences by acquiring (19)F-decoupled (23)Na-NMR spectra of cryolite (Na(3)AlF(6)). Whereas the MAS spectrum is only marginally affected by application of (19)F decoupling, the 3Q-filtered (23)Na signal is very sensitive to it, as the de-phasing caused by the dipolar interaction between sodium and fluorine is three-fold magnified. Experimentally, we find that at moderate MAS speeds, the decoupling efficiencies of the frequency-swept decoupling schemes SW(f)-TPPM and SW(f)-SPINAL are significantly better than the conventional TPPM and SPINAL sequences. The frequency-swept sequences are therefore the methods of choice for efficient decoupling of quadrupolar nuclei with half-integer spin from fluorine.

Detection and characterization of intermolecular multiple-quantum coherence NMR signals of IS (I=1/2; S=3/2) spin systems

Intermolecular multiple-quantum coherences (iMQCs) have some intrinsic properties different from conventional single-quantum coherences in solution NMR. In this paper, we extended our study to heteronuclear iMQCs in IS (I=1/2, S=3/2) spin systems. A sample of sodium chloride (NaCl) water solution was taken as an example. Heteronuclear COSY revamped by asymmetric Z-gradient echo detection (CRAZED) experiments were performed. One- and two-dimensional heteronuclear iMQC spectra were obtained. The quantum-mechanical treatment was used to deduce the signal expressions. Magic angle experiments validate that the signals are indeed from intermolecular dipolar interaction and insensitive to the imperfection of radio-frequency (RF) flip angles. Both experimental results and theoretical analysis indicate that heteronuclear CRAZED experiment allows coherence transfer from spin-3/2 nuclei to spin-1/2, and vice verse. Furthermore, the dependences of iMQC signal intensities on RF pulse flip angles follow the same rules as those for heteronuclear IS (I=1/2, S=1/2 or 1) spin systems.

Probing quadrupolar nuclei by solid-state NMR spectroscopy: recent advances

Solid-state nuclear magnetic resonance (NMR) of quadrupolar nuclei has recently undergone remarkable development of capabilities for obtaining structural and dynamic information at the molecular level. This review summarizes the key achievements attained during the last couple of decades in solid-state NMR of both integer spin and half-integer spin quadrupolar nuclei. We provide a concise description of the first- and second-order quadrupolar interactions, and their effect on the static and magic angle spinning (MAS) spectra. Methods are explained for efficient excitation of single- and multiple-quantum coherences, and acquisition of spectra under low- and high-resolution conditions. Most of all, we present a coherent, comparative description of the high-resolution methods for half-integer quadrupolar nuclei, including double rotation (DOR), dynamic angle spinning (DAS), multiple-quantum magic angle spinning (MQMAS), and satellite transition magic angle spinning (STMAS). Also highlighted are methods for processing and analysis of the spectra. Finally, we review methods for probing the heteronuclear and homonuclear correlations between the quadrupolar nuclei and their quadrupolar or spin-1/2 neighbors.

Determination of J coupling constants between spin-1/2 and quadrupolar nuclei in inorganic solids from spin echo and refocused INEPT experiments: a case study on AlPO₄ berlinite

A systematic study utilizing rotor-synchronized homonuclear ((31)P, (27)Al) and heteronuclear ({(31)P}(27)Al and {(27)Al}(31)P) spin echo, and {(27)Al}(31)P refocused INEPT experiments (employing soft pulses for selective excitation of the central transition for the quadrupolar (27)Al (I=5/2)) have been performed on AlPO(4) berlinite at 30 kHz MAS to better understand the J modulation behavior involving half-integer quadrupolar nuclei in solid materials with framework structure. Analyses of the J modulation on either the (27)Al or (31)P coherence in both the {(31)P}(27)Al and {(27)Al}(31)P spin echo experiments, and both periods of the refocused INEPT experiment yield consistent results for the (2)J(AlP) (Al-O-P) coupling constant (ca. 25 Hz). It is noted that the coupling of each (27)Al to four (31)P spins during the first ((27)Al) evolution period of the refocused INEPT, and the populations of (31)P coupled to different numbers (0-4) of (27)Al in the ± 1/2 Zeeman states during (31)P coherence evolution, which have been neglected in previous studies, must be taken into account for proper treatment. Analysis of J modulation on the spin ((27)Al) coupled to spin-1/2 nuclei in general gives more accurate results. Weak long-range homonuclear (4)J(PP) (P-O-Al-O-P) coupling was also observed from the (31)P spin echo and INADEQUATE experiments.

1D to 3D NMR study of microporous alumino-phosphate AlPO(4)-40

From one- to two- and three-dimensional MAS NMR solid-state experiments involving (31)P and (27)Al, we show that the structure of microporous alumino-phosphate AlPO(4)-40 contains at least four times more sites than expected, and we attribute two types of Al(IV) sites. The newly described (27)Al-(31)P MQ-HMQC opens new possibilities of describing details of three-dimensional bounded networks.

Correlation NMR spectroscopy involving quadrupolar nuclei

We review the recent developments proposed for integer or half-integer quadrupolar nuclei, focussing on the methods to observe them under high-resolution and to analyze their through-space and through-bond connectivities.

Indirectly detected through-bond chemical shift correlation NMR spectroscopy in solids under fast MAS: studies of organic-inorganic hybrid materials

Indirectly detected, through-bond NMR correlation spectra between (13)C and (1)H nuclei are reported for the first time in solid state. The capabilities of the new method are demonstrated using naturally abundant organic-inorganic mesoporous hybrid materials. The time performance is significantly better, almost by a factor of 10, than in the corresponding (13)C detected experiment. The proposed scheme represents a new analytical tool for studying other solid-state systems and the basis for the development of more advanced 2D and 3D correlation methods.

Determination of 27Al-29Si connectivities in zeolites with 2D 27Al-->29Si RAPT-CPMG-HETCOR NMR

The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr-Purcell-Meiboom-Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of approximately 4, which enabled acquisition of a high quality two-dimensional 27Al-29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.

Indirect covariance NMR spectroscopy of through-bond homo-nuclear correlations for quadrupolar nuclei in solids under high-resolution

Indirect covariance NMR spectroscopy is demonstrated in solids, and we show that it can be used to obtain through-bond 2D homo-nuclear correlation spectra for quadrupolar nuclei under high-resolution. These spectra, generated with indirect covariance from a hetero-nuclear correlation spectrum, are equivalent to those recorded with the through-bond homo-nuclear hetero-nuclear single-quantum correlation (H-HSQC) method very recently proposed. However, the indirect covariance method can save a lot of experiment time, compared to the H-HSQC experiments, which allows introducing a high-resolution quadrupolar filter, thus providing a much better resolution, even on medium-field spectrometers. The covariance concept can be used to generate many different "indirectly-detected" high-resolution homo-nuclear correlation spectra with through-space or through-bond correlations for spin 1/2 or quadrupolar nuclei. We also propose a simple method that decreases the noise in all (direct or indirect) covariance methods.

Efficiency of the Refocused31P−29Si MAS-J-INEPT NMR Experiment for the Characterization of Silicophosphate Crystalline Phases and Amorphous Gels

One- and two-dimensional refocused MAS-J-INEPT NMR experiments in the solid state (through-bond polarization transfer) involving the highly abundant 31P spin and the rare 29Si spin are described for the crystalline silicophosphate phase Si5O(PO4)6 and complex mixtures of SiP2O7 polymorphs. The evaluation of the 2JP-O-Si coupling constants for all 29Si sites is obtained by the careful analysis of the INEPT build-up curves under fast MAS. The results are in agreement with the crystallographic data, taking into account the various J coupling paths. The efficiency of the experiment is demonstrated by its application to more complex systems such as silicophosphate amorphous gels (obtained by the sol-gel process).

Three-dimensional through-bond homonuclear-heteronuclear correlation experiments for quadrupolar nuclei in solid-state NMR applied to (27)Al-O-(31)P-O-(27)Al networks

We present here the first 3D homonuclear/heteronuclear correlation experiment applied to quadrupolar nuclei and making use of the sole scalar J-coupling. This experiment, based on the 2D-Homonuclear-Heteronuclear Single Quantum Correlation (H-HSQC) experiment, uses a relayed transfer from the (27)Al central transition to neighbouring (31)P spins and to the central transition of a second (27)Al. It confirms the correlation map characterizing the two (27)Al and the (31)P NMR signatures of (27)Al-O-(31)P-O-(27)Al chemically bonded molecular motifs.

HMQC and refocused-INEPT experiments involving half-integer quadrupolar nuclei in solids

Hetero-nuclear coherence transfers in HMQC and refocused-INEPT experiments involving half-integer quadrupolar nuclei in solids are analyzed. 1D and 2D schemes are considered under MAS for the general case of multi-spin systems SI(n) (n4), where S is an observed nucleus. These results are also discussed in the context of high-resolution schemes featuring MQMAS or STMAS. The theoretical predictions are verified experimentally in a series of 1D and 2D experiments performed at 9.4 and 18.8T.

Toward a better description of gallo-phosphate materials in solid-state NMR: 1D and 2D correlation studies

We show that weak 2J(71Ga-O-31P), typically approximately 12 Hz in GaPO4, can be used to efficiently establish heteronuclear 31P-71Ga correlation using a MAS HMQC experiment in gallo-phosphate materials. The experiment demonstrated for cristobalite GaPO4 is then applied to Ga(PO3)3, where it allows the differentiation of the signature of three different Ga sites overlapping in the 1D spectrum.

Dynamics on the Microsecond Timescale in Microporous Aluminophosphate AlPO-14 as Evidenced by 27Al MQMAS and STMAS NMR Spectroscopy

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.

Through-bond homonuclear correlation experiments in solid-state NMR applied to quadrupolar nuclei in Al–O–P–O–Al chains

Through-bond homonuclear correlation experiments can be realised in solids between spins of type X, separated by four chemical bonds, in X-O-Y-O-X motifs, provided a J coupling between X and Y exists: central transitions of quadrupolar 27Al spins can be correlated via the J2 scalar coupling between 27Al (X) and 31P (Y) in materials featuring Al-O-P-O-Al motifs.

Quadrupolar coupling selective cross-polarization in solid state NMR

A new strategy is presented for achieving selective heteronuclear polarization transfers from half-integer quadrupolar spins in magic-angle spinning (MAS) NMR. By combining cross-polarization with a recently introduced RAPT pulse sequence that selectively excites the signal of a half-integer quadrupolar nucleus based on its quadrupolar coupling constant magnitude, we demonstrate that hetero-nuclei in its close proximity may be selectively excited. Selective 23Na --> 1H polarization transfers are demonstrated in Na2MoO4 x 2 H2O, Na2HPO4 x 2 H2O and a mixture of NaHCO3 and Na2HPO4 x 2 H2O.

SPAM-MQ-HETCOR: an improved method for heteronuclear correlation spectroscopy between quadrupolar and spin-1/2 nuclei in solid-state NMR

The recently introduced concept of soft pulse added mixing (SPAM) is used in two-dimensional heteronuclear correlation (HETCOR) NMR experiments between half-integer quadrupolar and spin-1/2 nuclei. The experiments employ multiple quantum magic angle spinning (MQMAS) to remove the second order quadrupolar broadening and cross polarization (CP) or refocused INEPT for magnetization transfer. By using previously unexploited coherence pathways, the efficiency of SPAM-MQ-HETCOR NMR is increased by a factor of almost two without additional optimization. The sensitivity gain is demonstrated on a test sample, AlPO(4)-14, using CP and INEPT to correlate (27)Al and (31)P nuclei. SPAM-3Q-HETCOR is then applied to generate (27)Al-(31)P spectra of the devitrified 41Na(2)O-20.5Al(2)O(3)-38.5P(2)O(5) glass and the silicoaluminophosphate ECR-40. Finally, the method allowed the acquisition of the first high resolution solid-state correlation spectra between (27)Al and (29)Si.

Measurement of J-couplings between spin-(1/2) and quadrupolar nuclei by frequency selective solid-state NMR

We report a REDOR-based scheme for the measurement of heteronuclear J-couplings in solid samples with well defined structure, containing spin-12 and quadrupolar nuclei, which can be used with selective RF irradiation to target a specific spin pair, and which provides direct information about the number of coupled spins.

Increasing the sensitivity of 2D high-resolution NMR methods applied to quadrupolar nuclei

Gan and Kwak recently proposed a soft-pulse added mixing (SPAM) idea in the classical two-pulse multiple-quantum magic-angle spinning scheme. In the SPAM method, a soft pi/2 pulse is added after the second hard-pulse (conversion pulse) and all coherence orders in between them are constructively used to obtain the signal. We, here, further extend this idea to distributed samples where the signal mainly results from echo pathways and that from anti-echo pathways dies out after a few t1 increments. We show that, with a combination of SPAM and collection of fewer anti-echoes, an enhancement of the signal to noise ratio by a factor of ca. 3 may be obtained over the z-filtered version. This may prove to be useful even for samples with long T2' relaxation times.