SCAM-STMAS: satellite-transition MAS NMR of quadrupolar nuclei with self-compensation for magic-angle misset

Several methods are available for the acquisition of high-resolution solid-state NMR spectra of quadrupolar nuclei with half-integer spin quantum number. Satellite-transition MAS (STMAS) offers an approach that employs only conventional MAS hardware and can yield substantial signal enhancements over the widely used multiple-quantum MAS (MQMAS) experiment. However, the presence of the first-order quadrupolar interaction in the satellite transitions imposes the requirement of a high degree of accuracy in the setting of the magic angle on the NMR probehead. The first-order quadrupolar interaction is only fully removed if the sample spinning angle, chi, equals cos(-1)(1/3) exactly and rotor synchronization is performed. The required level of accuracy is difficult to achieve experimentally, particularly when the quadrupolar interaction is large. If the magic angle is not set correctly, the first-order splitting is reintroduced and the spectral resolution is severely compromised. Recently, we have demonstrated a novel STMAS method (SCAM-STMAS) that is self-compensated for angle missets of up to +/-1 degrees via coherence transfer between the two different satellite transitions ST(+)(m(I)=+3/2<-->+1/2) and ST(-)(m(I)=-1/2<-->-3/2) midway through the t(1) period. In this work we describe in more detail the implementation of SCAM-STMAS and demonstrate its wider utility through 23Na (I=3/2), 87 Rb (I=3/2), 27 Al (I=5/2), and 59 Co (I=7/2) NMR. We discuss linewidths in SCAM-STMAS and the limits over which angle-misset compensation is achieved and we demonstrate that SCAM-STMAS is more tolerant of temporary spinning rate fluctuations than STMAS, resulting in less "t(1) noise" in the two-dimensional spectrum. In addition, alternative correlation experiments, for example involving the use of double-quantum coherences, that similarly display self-compensation for angle misset are investigated. The use of SCAM-STMAS is also considered in systems where other high-order interactions, such as third-order quadrupolar effects or second-order quadrupole-CSA cross-terms, are present. Finally, we show that the sensitivity of the experiment can be improved through the use of amplitude-modulated pulses.     

Satellite-transition MAS NMR of spin I = 3/2, 5/2, 7/2, and 9/2 nuclei: sensitivity,resolution, and practical implementation

The satellite-transition MAS (STMAS) experiment offers an alternative approach to established methods such as dynamic angle spinning (DAS), double rotation (DOR), and multiple-quantum MAS (MQMAS) for obtaining high-resolution NMR spectra of half-integer quadrupolar nuclei. Unlike the multiple-quantum experiment, STMAS involves two-dimensional correlation of purely single-quantum coherences; satellite transitions in t(1) (or F(1)) and the central transition in t(2) (or F(2)). To date, STMAS has primarily been demonstrated for nuclei with spin quantum numbers I = 3/2 and, to a lesser extent, I > 5/2. However, many chemically relevant nuclei possess I > 3/2, such as (17)O and (27)Al (both I = 5/2), (59)Co (I = 7/2), and (93)Nb (I = 9/2). Here, we discuss the application of STMAS to nuclei with spin quantum numbers from I = 3/2 to 9/2. First, we consider the practical implementation of the STMAS experiment using (87)Rb (I = 3/2) NMR as an example. We then extend the discussion to include nuclei with higher spin quantum numbers, demonstrating (27)Al, (45)Sc (I = 7/2), (59)Co, and (93)Nb STMAS experiments on both crystalline and amorphous samples. We also consider the possibility of experiments involving satellite transitions other than m(I) = +/- 1/2 <--> +/- 3/2 and, using (93)Nb NMR, demonstrate the correlation of all single-quantum satellite transitions up to and including m(I) = +/- 7/2 <--> +/- 9/2. The absolute chemical shift scaling factors in these experiments are discussed, as are the implications for isotropic resolution.     

Coherence transfer between spy nuclei and nitrogen-14 in solids

Coherence transfer from 'spy nuclei' such as (1)H or (13)C (S=1/2) was used to excite single- or double-quantum coherences of (14)N nuclei (I=1) while the S spins were aligned along the static field, in the manner of heteronuclear single-quantum correlation (HSQC) spectroscopy. For samples spinning at the magic angle, coherence transfer can be achieved through a combination of scalar couplings J(I,S) and second-order quadrupole-dipole cross-terms, also known as residual dipolar splittings (RDS). The second-order quadrupolar powder patterns in the two-dimensional spectra allow one to determine the quadrupolar parameters of (14)N in amino acids.     

优化初始脉冲增强多量子跃迁及卫星跃迁魔角旋转谱灵敏度

该文提出一个能使多量子跃迁、卫星跃迁魔角旋转及其变种方法的灵敏度明显增强的方法．在通常多量子激发或卫星跃迁激发脉冲之前施加一个预备脉冲,对初始态优化从而使循环延迟时间显著减小．利用几个代表性核种(23Na,11B和87Rb)在两个不同磁场下演示了这一方法．该方法可在低至4.7 T的磁场及6 kHz的转速下在任何常规固体核磁共振实验中实现,无需附加硬件或软件．此外,尽管完整的理论解说将另行发表,文中给出具体的实验步骤,使该方法可由用户灵活订制实验,针对每个样品优化实验参数．     

Quadrupolar transfer pathways

A set of graphical conventions called quadrupolar transfer pathways is proposed to describe a wide range of experiments designed for the study of quadrupolar nuclei with spin quantum numbers I=1, 3/2, 2, 5/2, etc. These pathways, which inter alea allow one to appreciate the distinction between quadrupolar and Zeeman echoes, represent a generalization of the well-known coherence transfer pathways. Quadrupolar transfer pathways not merely distinguish coherences with different orders -2I < or = p< or = +2I, but allow one to follow the fate of coherences associated with single transitions that have the same coherence order p=m(I)(r)-m(I)(s) but can be distinguished by a satellite order q=(m(I)(r))(2)-(m(I)(s))(2).     

Rotor-synchronized acquisition of quadrupolar satellite-transition NMR spectra: practical aspects and double-quantum filtration

The very broad resonances of quadrupolar (spin I > 1/2) nuclei are resolved by magic angle spinning (MAS) into a large number of spinning sidebands, each of which often remains anisotropically broadened. The quadrupolar interaction can be removed to a first-order approximation if the MAS NMR spectrum is acquired in a rotor-synchronized fashion, aliasing the spinning sidebands onto a centreband and thereby increasing the signal-to-noise ratio in the resulting, possibly second-order broadened, spectrum. We discuss the practical aspects of this rotor-synchronization in the direct (t(2)) time domain, demonstrating that the audiofrequency filters in the receiver section of the spectrometer have a significant impact on the precise timings needed in the experiment. We also introduce a novel double-quantum filtered rotor-synchronized experiment for half-integer spin quadrupolar (spin I = 3/2, 5/2, etc.) nuclei that makes use of central-transition-selective inversion pulses to both excite and reconvert double-quantum coherences and yields a simplified spectrum containing only the ST(1) (m(I) = +/-1/2 <--> +/-3/2) satellite-transition lineshapes. For spin I = 5/2 nuclei, such as (17)O and (27)Al, this spectrum may exhibit a significant resolution increase over the conventional central-transition spectrum.     

Probing through bond connectivities with MQMAS NMR

Polarization transfer from quadrupolar (27Al) to spin-1/2 (31P) nuclei via J-coupling is employed to measure two-dimensional 27Al-31P heteronuclear correlation spectra with isotropic resolution. The proposed experiment, MQ-J-HETCOR, uses multiple quantum magic angle spinning (MQMAS) NMR for elimination of the second-order quadrupolar broadening and INEPT, INEPTR, INEPT+ and DEPT sequences for the polarization transfer. The experimental conditions leading to best sensitivity and resolution are detailed using AlPO4-14 as a test sample.     

Improved quantitation in 3QMAS of spin 5/2 nuclei by RF power modulation of FAM-II

High-resolution NMR of quadrupolar I = 5/2 nuclei using triple-quantum magic angle spinning (3QMAS) techniques can provide more accurate quantitative information on sites with small quadrupolar coupling constants by changing the pulse strength in addition to the pulse length in the FAM-II multiple-quantum conversion sequence. These effects are illustrated using (27)Al NMR of yttrium aluminium garnet and andalusite.     

Efficient triple-quantum excitation in modified RIACT MQMAS NMR for I=3/2 nuclei

A new approach involving the creation of triple-quantum (TQ) coherences from both TQ and central transitions (CT) is investigated, in order to enhance the efficiency of triple-quantum excitation for I=3/2 nuclei. The RIACT excitation scheme, a soft pi/2 and hard spin-locking pulse, is shown to induce both adiabatic coherence transfer between CT and TQ coherences and TQ nutation. By combining the RIACT scheme with the presaturation of the satellite transitions, a significant improvement in the TQ excitation can be achieved mainly through enhanced CT polarization via the RIACT mechanism, in particular for nuclei with moderate to large quadrupole coupling constants (> or = 2.0 MHz). There also exists a nontrivial contribution from the TQ transition, which depends on the size of the quadrupole interaction.     

Homonuclear correlation experiments for quadrupolar nuclei, spinning away from the magic angle

We present a set of homonuclear correlation experiments for half-integer quadrupolar spins in solids. In all these exchange-type experiments, the dipolar interaction is retained during the mixing time by spinning the sample at angles other than the “regular magic angle” (54.7°). The second-order quadrupolar interaction is averaged by different strategies for the different experiments. The multiple-quantum off magic angle spinning (MQOMAS) exchange experiment is essentially a regular MQMAS experiment where the quadrupolar interaction is averaged by combining magic angle spinning with a multiple- to single-quantum correlation scheme. The sample is spun at the magic angle at all times except during the mixing time which is added to establish homonuclear correlation. In the multiple-quantum P₄ magic angle spinning (MQP₄MAS) exchange experiment, the sample is spun at one of the angles at which the fourth-order Legendre polynomial vanishes (P₄ magic angle), the remaining second-order quadrupolar interaction now governed by a second-rank tensor is refocussed by the multiple to single-quantum correlation scheme. In the dynamic angle spinning (DAS) exchange experiment, the second-order quadrupolar interaction is averaged by correlating the evolution from two complementary angles. These experiments are demonstrated and compared, in view of their specific advantages and disadvantages, for ²³Na in the model compound Na₂SO₃.     

High-resolution NMR of quadrupolar nuclei using mixed multiple-quantum coherences

A multiple-quantum magic angle spinning (MQMAS) NMR experiment of quadrupolar nuclei is demonstrated, which uses two different multiple quantum coherences in t(1) to refocus the quadrupolar broadening. This experiment has the potential of achieving improved resolution over current techniques.     

I-STMAS,a new high-resolution solid-state NMR method for half-integer quadrupolar nuclei

In complement to the previously proposed multiple-quantum magic-angle-spinning (MQMAS) and satellite transition MAS (STMAS) sequences, we describe a new two-dimensional high-resolution method, inverse-STMAS (I-STMAS) that allows second-order quadrupolar averaging. Like STMAS, I-STMAS correlates second-order quadrupole dephasing occurring on coherences related to the central transition (CT) and satellite transitions (STs), but does it in a reverse manner: CT evolves during the t₁ period while STs are detected during t₂. Although STMAS and I-STMAS are symmetric, there are some interesting and useful differences between the two methods. For example, we show that during the acquisition time t₂, it is possible to over-sample the data and then to process them to suppress the CT–CT correlation resonance.     

Indirect detection of nitrogen-14 in solids via protons by nuclear magnetic resonance spectroscopy

This Communication describes the indirect detection of 14N nuclei (spin I=1) in solids by nuclear magnetic resonance (NMR) spectroscopy. The two-dimensional correlation method used here is closely related to the heteronuclear multiple quantum correlation (HMQC) experiment introduced in 1979 to study molecules in liquids, which has recently been used to study solids spinning at the magic angle. The difference is that the coherence transfer from neighboring 1H nuclei to 14N is achieved via a combination of J couplings and residual dipolar splittings (RDS). Projections of the two-dimensional correlation spectra onto the 14N dimension yield powder patterns which reflect the 14N quadrupolar interaction. In contrast to the indirect detection of 14N via 13C nuclei that was recently demonstrated [Gan, J. Am. Chem. Soc. 128 (2006) 6040; Cavadini et. al., J. Am. Chem. Soc., 128 (2006) 7706], this approach may benefit from enhanced sensitivity, and does not require isotopic enrichment in 13C, although the 1H line-widths may have to be reduced upon selective deuteration.     

Enhancing MQMAS sensitivity using signals from multiple coherence transfer pathways

Multiple-quantum magic-angle spinning experiment removes second-order quadrupolar broadening from the central-transition of half-integer quadrupolar nuclei. This paper presents a novel scheme to enhance the sensitivity of MQMAS using signals from multiple coherence transfer pathways. The enhancement can be obtained in two ways. The first method uses the multiplex phase cycling to acquire MQMAS spectra from various coherence transfer pathways simultaneously. An addition of spectra collected with no extra time enhances the efficiency of the experiment. The second method, soft-pulse-added-mixing, is designed based on a complete alias of coherence transfer pathways. By properly fixing the soft-pulse phase, signals from various coherence transfer pathways can add constructively resulting higher signal intensities. The two methods are demonstrated for sensitivity enhancement with samples of spin-3/2 and 5/2.     

Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers

Rotor-assisted population transfer (RAPT) was developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarization associated with satellite transitions to the central m=12-->-12 transition. After a single RAPT transfer, there still remains polarization in the satellite transitions that can be transferred to the central transition. This polarization is available without having to wait for the spin system to return to thermal equilibrium. We describe a new RAPT scheme that uses the remaining polarization of the satellites to obtain a further enhancement of the central transition by performing RAPT-enhanced experiments multiple times before waiting for re-equilibration of the spin system. For 27Al (I=5/2) in albite we obtain a multiple RAPT enhancement of 3.02, a 48% increase over single RAPT. For 93Nb (I=9/2) in NaNbO(3) we obtain a multiple RAPT enhancement of 5.76, an 89% increase over single RAPT. We also describe a data processing procedure for obtaining the maximum possible signal-to-noise ratio.     

Oxygen sites in the zeolite stilbite: a comparison of static, MAS, VAS, DAS and triple quantum MAS NMR techniques

Static, magic angle spinning (MAS), variable angle spinning (VAS), dynamic angle spinning (DAS) and triple quantum magic angle spinning (3QMAS) NMR techniques were applied to separate and quantify oxygen signals from Al–O–Si and Si–O–Si sites of ¹⁷O-enriched samples of the mineral stilbite, a natural zeolite. DAS experiments showed that there was a distribution of quadrupolar coupling constants, asymmetry parameters and isotropic chemical shifts. Two methods were used to study the quantification problem of DAS and 3QMAS. Our results showed that DAS was quantitative. In 3QMAS, signal intensity from sites with larger quadrupolar coupling constants was reduced because of less efficient excitation. All techniques have shown a clear difference in rates of exchange between the two types of sites with interchannel H₂O molecules.     

Anisotropy-correlated spectroscopy of quadrupolar nuclei

The two-dimensional anisotropy-correlated NMR (2DAC) spectra of half-integer quadrupolar nuclei may be recorded by using an exchange sequence in conjunction with magic angle spinning (MAS) during evolution and detection, and off-MAS during mixing. Application of this experiment to boron oxides is described, in addition to an analysis of the spin diffusion rates in such materials.     

Zur Bestimmung chemischer Verschiebungen der NMR-Frequenzen bei Quadrupolkernen aus den MAS-NMR-Spektren

Determination of Chemical Shifts of NMR-Frequencies of Quadrupolar Nuclei from the MAS-NMR Spectra The general expressions for the NMR central transition of rotating samples with quadrupolar nuclei of half-integer spins, derived by BEHRENS [1, 2] for arbitrary angles of inclination of the spinning axis considering second-order quadrupolar effects, are presented for the practically interesting case of magic angle spinning (MAS) in a form analogous to the expressions for the resting sample. The theory is tested and used for the exact determination of the chemical shift values from the MAS-²⁷Al-NMR spectra of two representative aluminates.     

Sensitivity enhancement of the central-transition signal of half-integer spin quadrupolar nuclei in solid-state NMR: features of multiple fast amplitude-modulated pulse transfer

Sensitivity enhancement of solid-state NMR spectrum of half-integer spin quadrupolar nuclei under both magic-angle spinning (MAS) and static cases has been demonstrated by transferring polarisation associated with satellite transitions to the central m=-1/2-->1/2 transition with suitably modulated radio-frequency pulse schemes. It has been shown that after the application of such enhancement schemes, there still remains polarisation in the satellite transitions that can be transferred to the central transition. This polarisation is available without having to wait for the spin system to return to thermal equilibrium. We demonstrate here the additional sensitivity enhancement obtained by making use of this remaining polarisation with fast amplitude-modulated (FAM) pulse schemes under both MAS and static conditions on a spin-3/2 and a spin-5/2 system. Considerable signal enhancement is obtained with the application of the multiple FAM sequence, denoted as m-FAM. We also report here some of the salient features of these multiple FAM sequences with respect to the nutation frequency of the pulses and the spinning frequency.     

Double-quantum homonuclear correlations of spin I=5/2 nuclei

The challenges associated with acquiring double-quantum homonuclear Nuclear Magnetic Resonance correlation spectra of half-integer quadrupolar nuclei are described. In these experiments the radio-frequency irradiation amplitude is necessarily weak in order to selectively excite the central transition. In this limit only one out of the 25 double-quantum coherences possible for two coupled spin I=5/2 nuclei is excited. An investigation of all the 25 two spins double quantum transitions reveals interesting effects such as a compensation of the first-order quadrupolar interaction between the two single quantum transitions involved in the double quantum coherence. In this paper a full numerical study of a hypothetical two spin I=5/2 system is used to show what happens when the RF amplitude during recoupling is increased. In principle this is advantageous, since the required double quantum coherence should build up faster, but in practice it also induces adiabatic passage transfer of population and coherence which impedes any build up. Finally an optimized rotary resonance recoupling (oR(3)) sequence is introduced in order to decrease these transfers. This sequence consists of a spin locking irradiation whose amplitude is reduced four times during one rotor period, and allows higher RF powers to be used during recoupling. The sequence is used to measure (27)Al DQ dipolar correlation spectra of Y(3)Al(5)O(12) (YAG) and gamma alumina (γAl(2)O(3)). The results prove that aluminium vacancies in gamma alumina mainly occur in the tetrahedral sites.