Multiple-quantum magic-angle spinning NMR with cross-polarization: Spectral editing of high-resolution spectra of quadrupolar nuclei

An experiment is presented that combines the multiple-quantum magic-angle spinning (MQMAS) technique with cross-polarization (CP). As a preliminary test of this new method, we measured and compared the ²⁷Al 3QMAS and ¹⁹F → ²⁷A1 CP 3QMAS spectra of a fluorinated AlPO₄ aluminophosphate. Complete discrimination between the fluorinated and nonfluorinated Al sites was easily achieved, which demonstrates the usefulness of CP MQMAS for spectral editing. Future applications of this experiment will include other spin pairs and heteronuclear correlation NMR spectroscopy.     

Double-quantum homonuclear NMR correlation spectroscopy of quadrupolar nuclei subjected to magic-angle spinning and high magnetic field

We present a new application of the symmetry-based dipolar recoupling scheme, for exciting directly double-quantum (2Q) coherences between the central transition of homonuclear half-integer quadrupolar nuclei. With respect to previously published 2Q-recoupling methods (M. Eden, D. Zhou, J. Yu, Chem. Phys. Lett. 431 (2006) 397), the sequence is used without π/2 bracketing pulses and with an original super-cycling. This leads to an improved efficiency (a factor of two for spin-5/2) and to a much higher robustness to radio-frequency field inhomogeneity and resonance offset. The 2Q-coherence excitation performances are demonstrated experimentally by ²⁷Al NMR experiments on the aluminophosphates berlinite, VPI5, AlPO₄-14, and AlPO₄-CJ3. The two-dimensional 2Q–1Q correlation experiments incorporating these recoupling sequences allow the observation of 2Q cross-peaks between central transitions, even at high magnetic field where the difference in offset between octahedral and tetrahedral ²⁷Al sites exceeds 10 kHz.     

Multiple-Rotor-Cycle QPASS Pulse Sequences: Separation of Quadrupolar Spinning Sidebands with an Application to La NMR

The quadrupolar phase-adjusted spinning sidebands (QPASS) pulse sequence has been recently demonstrated as a useful method for obtaining quadrupolar parameters with magic-angle spinning NMR. The sequence separates spinning sidebands by order in a two-dimensional experiment. A sheared projection of the 2D spectrum effectively yields the infinite spinning rate second-order quadrupolar powder pattern, which can be analyzed to determine quadrupolar coupling constants and asymmetry parameters. The RF power and spinning speed requirements of the original QPASS sequence make it an experimentally demanding technique. A new version of the sequence is demonstrated here and is shown to alleviate many problems associated with the original sequence. New solutions to the determining equations, based on the use of multiple rotor cycles in the QPASS sequence, lead to longer delays between the nine π pulses, provide less chance of pulse overlap, and allow for use of weaker RF field strengths that excite only the central quadrupolar transition. A three-rotor-cycle version of the new experiment is demonstrated on the ¹³⁹La nucleus.     

Double-Resonance Decoupling for Resolution Enhancement of P Solid-State MAS and Al → P MQHETCOR NMR

²⁷Al decoupling has been used to remove residual J-coupling interactions between³¹ P and ²⁷Al in microporous aluminophosphates AlPO₄-14 and AlPO₄-40. In combination with ¹H high-power decoupling, ²⁷Al adapted decoupling yields ³¹P spectra with optimal sensitivity and resolution. The importance of double-resonance decoupling is further demonstrated by incorporating this technique in the MQHETCOR sequence. Unambiguous assignment of all the AlPO₄-14 nuclear magnetic resonances is achieved by combining multiple-quantum evolution in the ²⁷Al dimension and double-resonance decoupling in the ³¹P acquisition domain.     

Supercycled symmetry-based double-quantum dipolar recoupling of quadrupolar spins in MAS NMR: I. Theory

Using average Hamiltonian (AH) theory, we analyze recently introduced homonuclear dipolar recoupling pulse sequences for exciting central-transition double-quantum coherences (2QC) between half-integer spin quadrupolar nuclei undergoing magic-angle-spinning. Several previously observed differences among the recoupling schemes concerning their compensation to resonance offsets and radio-frequency (rf) inhomogeneity may qualitatively be rationalized by an AH analysis up to third perturbation order, despite its omission of first-order quadrupolar interactions. General aspects of the engineering of 2Q-recoupling pulse sequences applicable to half-integer spins are discussed, emphasizing the improvements offered from a diversity of supercycles providing enhanced suppression of undesirable AH cross-terms between resonance offsets and rf amplitude errors.     

Temperature calibration for high-temperature MAS NMR to 913 K: 63Cu MAS NMR of CuBr and CuI, and 23Na MAS NMR of NaNbO3

The solid-state phase transitions of CuBr, CuI and NaNbO₃ can be readily observed using ⁶³Cu and ²³Na high-temperature magic-angle spinning nuclear magnetic resonance spectroscopy. Temperature has large, linear effects on the peak maximum of ⁶³Cu in each solid phase of CuBr and CuI, and there is large jump in shift across each phase transition. The ²³Na MAS NMR peak intensities and the line widths in NaNbO₃ also clearly show its high-temperature transition to the cubic phase. These data can be used to calibrate high-temperature MAS NMR probes up to 913 K, which is two hundred degrees higher than the commonly-used temperature calibration based on the chemical shift of ²⁰⁷Pb in Pb(NO₃)₂.     

Influence of sodium ion dynamics on the 23Na quadrupolar interaction in sodalite: a high-temperature 23Na MAS NMR study

High-temperature ²³Na MAS NMR experiments up to 873 K for a number of different sodalites (Na₈[AlSiO₄]₆(NO₃)₂, Na₈[AlSiO₄]₆(NO₂)₂, Na₈[AlSiO₄]₆I₂, Na_7.9[AlSiO₄]₆(SCN)_7.9 · 0.5H₂O, Na₈[AlGeO₄]₆(NO₃)₂, and Na₇[AlSiO₄]₆(H₃O₂) · 4H₂O) were carried out. The spectra of the first five sodalites consist of a quadrupolar MAS pattern with different quadrupolar coupling constants. The quadrupolar interaction for the thiocyanate sodalite, the nitrate aluminosilicate, and germanate sodalite decreases strongly passing a coalescence state on heating, while the quadrupolar interaction of the iodide and nitrite sample shows nearly no change. The basic hydrosodalite shows an asymmetric lineshape at room temperature and, between 350 and 370 K, a second line due to the evaporation of cage-water emerges. The linewidth increases with rising temperature. The temperature dependence of the quadrupolar interaction seems to be a function of the sodalite β-cage expansion. Two conceivable jump mechanisms are proposed for a tetrahedral two-site jump between occupied and unoccupied tetrahedral sites.     

Stimulated-Echo NMR Spectroscopy of Be and Li in Solids: Method and Application to Ion Conductors

The generation of pure quadrupolar stimulated-echo spectra is successfully demonstrated for the spin- probe ⁹Be in a single crystal of triglycine fluoberyllate. This solid exhibits a paraelectric-to-ferroelectric phase transition. From experiments carried out for various mixing times no indications for a slow soft mode could be detected in this crystal. Then ion conducting lithium metal phosphates were studied using ⁷Li, another spin- probe which allows for a non-selective excitation of the entire NMR spectrum. In the indium and the scandium phosphates ultra-slow Li hopping processes could be detected directly via the stimulated-echo technique in a time range of up to four orders of magnitude. Due to the relatively large gyromagnetic ratio and thus strong dipolar interactions of ⁷Li no pure quadrupolar echoes could be generated. However, from a variation of the evolution times the quadrupolar effects could be separated from the dipolar ones. Finally, the differences in the ion hopping times of lithium indium phosphate and of lithium scandium phosphate are briefly discussed.     

Effects of T2-relaxation in MAS NMR spectra of the satellite transitions for quadrupolar nuclei: a 27Al MAS and single-crystal NMR study of alum KAl(SO4)2.12H2O

Asymmetries in the manifold of spinning sidebands (ssbs) from the satellite transitions have been observed in variable-temperature 27Al MAS NMR spectra of alum (KAl(SO4)2.12H2O), recorded in the temperature range from -76 to 92 degrees C. The asymmetries decrease with increasing temperature and reflect the fact that the ssbs exhibit systematically different linewidths for different spectral regions of the manifold. From spin-echo 27Al NMR experiments on a single-crystal of alum, it is demonstrated that these variations in linewidth originate from differences in transverse (T2) relaxation times for the two inner (m=1/2<-->m=3/2 and m=-1/2<-->m=-3/2) and correspondingly for the two outer (m=3/2<-->m=5/2 and m=-3/2<-->m=-5/2) satellite transitions. T2 relaxation times in the range 0.5-3.5 ms are observed for the individual satellite transitions at -50 degrees C and 7.05 T, whereas the corresponding T1 relaxation times, determined from similar saturation-recovery 27Al NMR experiments, are almost constant (T1=0.07-0.10 s) for the individual satellite transitions. The variation in T2 values for the individual 27Al satellite transitions for alum is justified by a simple theoretical approach which considers the cross-correlation of the local fluctuating fields from the quadrupolar coupling and the heteronuclear (27Al-1H) dipolar interaction on the T2 relaxation times for the individual transitions. This approach and the observed differences in T2 values indicate that a single random motional process modulates both the quadrupolar and heteronuclear dipolar interactions for 27Al in alum at low temperatures.     

A method for detection of spectral spin diffusion from minor peaks, and its application to F MAS NMR of antimony(III)-doped fluorapatite

A new technique for detecting spectral spin diffusion in solids under MAS NMR conditions that is particularly well suited for accurately measuring cross-relaxation from minor spectral components is presented. The pulse sequence, SINK (Saturation Inter-Nuclear Kinetics), selectively saturates the magnetization of a minor spectral component with a series of rotor-synchronized DANTE pulse trains and monitors spin diffusion to other peaks with a non-selective 90° pulse. We have used SINK with ¹⁹F MAS NMR on samples of calcium fluorapatite doped with Sb³⁺ to measure spin diffusion between a weak peak at 68.6 ppm due to fluoride ions associated with Sb³⁺ and other peaks in the spectrum. The SINK experiment clearly demonstrates that spin diffusion from the former peak to the main resonance of fluorapatite at 64.0 ppm is faster than spin diffusion to a second antimony-related peak at 65.6 ppm. These results strengthen our previous conclusion that antimony(III) occupies a phosphate site in the apatite lattice, with an SbO₃³⁻ group replacing a PO₄³⁻ group. The SINK experiment also enables the detection of a “hidden” peak at approximately 62.9 ppm that is otherwise obscured by the intense main peak at 64.0 ppm.     

Dipolar Coupling Information in Multispin Systems: Application of a Compensated REDOR NMR Approach to Inorganic Phosphates

Anexperimental strategy has been developed for measuring multiple dipole–dipole interactions in inorganic compounds using the technique of rotational echo double resonance (REDOR) NMR. Geometry-independent information about the dipole couplings between the observe nuclear species S (arbitrary quantum number) and the heteronuclear species I (spin- ) can be conveniently obtained from the experimental curve of ΔS/S₀ versus dipolar evolution time by limiting the analysis to the initial data range 0 < ΔS/S₀ < 0.30. Numerical simulations have been carried out on a three-spin system of type SI₂ in order to assess the effect of the I–I homonuclear dipole–dipole coupling and the influence of experimental imperfections such as finite pulse length and misadjustments of the 180° pulses applied to the I-spin species. The simulations show further that within the initial data range the effects of such misadjustments can be internally compensated by a modified sequence having an additional 180° pulse on the I channel in the middle of the dipolar evolution periods. Experimental ²⁷Al{³¹P} REDOR results on the multispin systems Al(PO₃)₃, AlPO₄, [AlPO₄]₁₂(C₃H₇)₄NF, and Na₃PO₄ confirm the general utility of this approach. Thus, for applications to unknown systems the compensation strategy obviates calibration procedures with model compounds.     

An Efficient NMR Experiment for Analyzing Sugar-Puckering in Unlabeled DNA: Application to the 26-kDa Dead Ringer–DNA Complex

We present a new NMR experiment for estimating the type and degree of sugar-puckering in high-molecular-weight unlabeled DNA molecules. The experiment consists of a NOESY sequence preceded by a constant-time scalar coupling period. Two subexperiments are compared, each differing in the amount of time the (3)J(H3'H2') and (3)J(H3'H2") couplings are active on the H3' magnetization. The resultant data are easy to analyze, since a comparison of the signal intensities of any resolved NOE cross peak originating from H3' atoms of the duplex can be used to estimate the sum of the (3)J(H3'H2') and (3)J(H3'H2") couplings and thus the puckering type of the deoxyribose ring. Isotope filters to eliminate signals of the (13)C-labeled component in the F1-dimension are implemented, facilitating analyses of high-molecular-weight protein-DNA complexes containing (13)C-labeled protein and unlabeled DNA. The utility of the experiment is demonstrated on the 26-kDa Dead Ringer protein-DNA complex and reveals that the DNA uniformly adopts the S-type configuration when bound to protein.     

A theoretical study of magnetic phase transitions in ultra-thin films: Application to NiO

Phase diagrams and critical temperatures of projected onto fcc (1 0 0) layers, which is believed to be applicable to first-row transition metal oxides such as VO, MnO and NiO, are obtained from mean field theory and Monte Carlo simulations. Within the regime, J_se > 0, which includes MnO and NiO, both approaches predict bicritical behaviour of the AF₂ and AF₃ antiferromagnetic spin alignments for odd numbers of layers greater than one and monocritical behaviour for even numbers of layers, even when films are described by single values of J_d and J_se. The ferromagnetic alignment, on the other hand, exhibits monocritical behaviour for all thicknesses from the monolayer through to the bulk. For values of (x = J_d/J_se) which are close to those obtained from first principles calculations for NiO and also those derived from measured magnon spectra, estimates of the thickness dependence of the critical temperature from Monte Carlo simulations are similar to that derived from linear polarised X-ray absorption spectra of NiO(1 0 0) ultra-thin films grown epitaxially on MgO(1 0 0) [D. Alders, L.H. Tjeng, F.C. Voogt, T. Hibma, G.A. Sawatzky, J. Vogel, M. Sacchi, S. Iacobucci, Phys. Rev. B 57 (1998) 11623].     

A non-perturbative open-shell theory for atomic and molecular systems: Application to transbutadiene

A non-perturbative theory is proposed in this article in which an energy independent effective Hamiltonian is obtained for open-shell systems. We have given a diagrammatic version of theory to facilitate the analysis of the problem. The theory has been applied to a model 4-π electron problem, for calculating the lowestπ-π* singlet and triplet energy levels of transbutadiene. Comparison with full Cl calculation indicates the excellent workability of the theory.     

3D NMR Experiments for MeasuringN Relaxation Data of Large Proteins: Application to the 44 kDa Ectodomain of SIV gp41

A suite of 3D NMR experiments for measuring¹⁵N–{¹H} NOE,¹⁵NT₁, and¹⁵NT_1ρvalues in large proteins, uniformly labeled with¹⁵N and¹³C, is presented. These experiments are designed for proteins that exhibit extensive spectral overlap in the 2D¹H–¹⁵N HSQC spectrum. The pulse sequences are readily applicable to perdeuterated samples, which increases the spectral resolution and signal-to-noise ratio, thereby permitting the characterization of protein dynamics to be extended to larger protein systems. Application of the pulse sequences is demonstrated on a perdeuterated¹³C/¹⁵N-labeled sample of the 44 kDa ectodomain of SIV gp41.