Reintroducing anisotropic interactions in magic-angle-spinning NMR of half-integer quadrupolar nuclei: 3D MQMAS

Selective reintroduction of anisotropic interactions such as the chemical shift anisotropy (CSA) and homonucler dipolar (HMD) coupling were implemented in a high-resolution NMR spectroscopy for half-integer quadrupolar nuclei. Rotary resonance recoupling (R(3)) combined with the multiple-quantum magic-angle spinning (MQMAS) in a three-dimensional (3D) experiment provides not only site-specific high-resolution spectra to yield the quadrupolar interaction parameters but also the CSA or HMD interaction parameters. This 3D experiment provides an avenue for the complete local structural information of half-integer quadrupolar nuclei. Three-dimensional MQMAS experiments incorporating R(3) of HMD and CSA interactions were demonstrated on model compounds containing (11)B, (23)Na, and (87)Rb nuclei.     

Measuring amide nitrogen quadrupolar coupling by high-resolution 14N/13C NMR correlation under magic-angle spinning

The measurement of amide nitrogen 14N quadrupolar coupling by two-dimensional 14N/13C correlation experiment is presented with a natural abundant polypeptide. Directly bonded 14N/13C pairs are correlated through J and residual dipolar coupling under magic-angle spinning using a HMQC-type pulse sequence. The 14N quadrupolar coupling is measured from the isotropic second-order quadrupolar shift obtained by comparing the 14N peak positions with the 15N chemical shifts. The high spectral resolution and sensitivity through 13C detection make this method applicable to many organic, inorganic, and biological molecules for the measurement and the use of 14N quadrupolar coupling as a probe for molecular structure and dynamics.     

Measuring nitrogen quadrupolar coupling with 13C detected wide-line 14N NMR under magic-angle spinning

A (13)C-detected (14)N wide-line NMR experiment is described for measuring nitrogen quadrupolar coupling under magic-angle spinning.     

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.     

Separation of quadrupolar and paramagnetic shift interactions with TOP-STMAS/MQMAS in solid-state lighting phosphors

A new approach for processing satellite-transition magic-angle spinning (STMAS) and multiple-quantum magic-angle spinning (MQMAS) data, based on the two-dimensional one-pulse (TOP) method, which separates the second-rank quadrupolar anisotropy and paramagnetic shift interactions via a double shearing transformation, is described. This method is particularly relevant in paramagnetic systems, where substantial inhomogeneous broadening may broaden the lineshapes. Furthermore, it possesses an advantage over the conventional processing of MQMAS and STMAS spectra because it overcomes the limitation on the spectral width in the indirect dimension imposed by rotor synchronization of the sampling interval. This method was applied experimentally to the Al solid-state nuclear magnetic resonance of a series of yttrium aluminum garnets (YAGs) doped with different lanthanide ions, from which the quadrupolar parameters of paramagnetically shifted and bulk unshifted sites were extracted. These parameters were then compared with density functional theory calculations, which permitted a better understanding of the local structure of Ln substituent ions in the YAG lattice.     

The Microscopic friction tensor for spinning and tumbling motions in liquid pyridine

The dynamic structure of liquid pyridine has been studied by analysing the deuterium and nitrogen ¹⁴N quadrupolar relaxation data of selectively labelled substances. The so-called friction model of molecular reorientation is applied. The coefficients of the microscopic friction tensor for spinning and tumbling motions in liquid pyridine are calculated. The results show motional anisotropy. The friction coefficient for spinning motion is virtually temperature independent.     

Interpolation by fast Wigner transform for rapid calculations of magnetic resonance spectra from powders

We introduce a novel interpolation strategy, based on nonequispaced fast transforms involving spherical harmonics or Wigner functions, for efficient calculations of powder spectra in (nuclear) magnetic resonance spectroscopy. The fast Wigner transform (FWT) interpolation operates by minimizing the time-consuming calculation stages, by sampling over a small number of Gaussian spherical quadrature (GSQ) orientations that are exploited to determine the spectral frequencies and amplitudes from a 10-70 times larger GSQ set. This results in almost the same orientational averaging accuracy as if the expanded grid was utilized explicitly in an order of magnitude slower computation. FWT interpolation is applicable to spectral simulations involving any time-independent or time-dependent and noncommuting spin Hamiltonian. We further show that the merging of FWT interpolation with the well-established ASG procedure of Alderman, Solum and Grant [J. Chem. Phys. 134, 3717 (1986)] speeds up simulations by 2-7 times relative to using ASG alone (besides greatly extending its scope of application), and between 1-2 orders of magnitude compared to direct orientational averaging in the absence of interpolation. Demonstrations of efficient spectral simulations are given for several magic-angle spinning scenarios in NMR, encompassing half-integer quadrupolar spins and homonuclear dipolar-coupled (13)C systems.     

Solid-state 35/37Cl NMR spectroscopy of hydrochloride salts of amino acids implicated in chloride ion transport channel selectivity: opportunities at 900 MHz

The results of a detailed systematic chlorine solid-state NMR study of several hydrochloride salts of amino acids implicated in chloride ion transport channel selectivity are reported. (35)Cl and (37)Cl NMR spectra have been obtained for stationary and/or magic-angle spinning powdered samples of the following compounds on 500 and/or 900 MHz spectrometers: DL-arginine HCl monohydrate, L-lysine HCl, L-serine HCl, L-glutamic acid HCl, L-proline HCl, L-isoleucine HCl, L-valine HCl, L-phenylalanine HCl, and glycine HCl. Spectral analyses provide information on the anisotropic properties and relative orientations of the chlorine electric field gradient and chemical shift (CS) tensors, which are intimately related to the local molecular and electronic structure. Data obtained at 900 MHz provide unique examples of the effects of CS anisotropy on the NMR spectrum of a quadrupolar nucleus. The range of chlorine quadrupolar coupling constants (C(Q)) measured, -6.42 to 2.03 MHz, demonstrates the sensitivity of this parameter to the chloride ion environment and suggests the applicability of chlorine solid-state NMR as a novel experimental tool for defining chloride binding environments in larger ion channel systems. Salts of hydrophobic amino acids are observed to tend to exhibit larger values of C(Q) than salts of hydrophilic amino acids. A simple model for rationalizing the observed trend in C(Q) is proposed. For salts for which neutron diffraction structures are available, we identify a quantum chemical method which reproduces experimental values of C(Q) with a root-mean-square deviation of 0.1 MHz and a correlation coefficient of 0.9998. On the basis of this, chlorine NMR tensors are predicted for the Cl(-) binding site in ClC channels.     

Dynamic-angle spinning without sidebands

By means of rotor-synchronized π-pulses, it is possible to eliminate the spinning sidebands (while retaining their full intensity in the isotropic centerband) that usually arise in dynamic-angle spinning (DAS) NMR. The theory of this approach, dynamic-angle hopping (DAH-180), is described and illustrated with experimental results on quadrupolar nuclei. A magic-angle hopping (MAH-180) version of magic-angle spinning is also possible and can be used in a two-dimensional NMR experiment to produce sideband-free isotropic—anisotropic correlation spectra for spin- nuclei.     

Solid state separated-local-field NMR spectroscopy on half-integer quadrupolar nuclei: principles and applications to borane analysis

New multidimensional NMR methods correlating the quadrupolar and heteronuclear dipolar interactions affecting a half-integer quadrupolar spin in the solid state are introduced and exemplified. The methods extend separated-local-field magic-angle spinning (SLF MAS) NMR techniques that have been used successfully in spin-(1)/(2) spectroscopy to the study of S >/= (3)/(2) nuclei. In our implementation, these techniques avoid homonuclear proton decoupling requirements by relying on moderately fast MAS rates (6-15 kHz) and use rotor-synchronized constant-time pulse sequences to achieve nearly arbitrary amplifications of the apparent dipolar coupling strengths. The result is a suite of simple 2D NMR experiments, whose line shapes carry valuable information about the structure and dynamics of solids containing quadrupolar and proton nuclei. The potential of these sequences was exploited to gather new insight into the structure and dynamics of a variety of boron-containing samples. These experimental SLF schemes were also extended to 3D NMR experiments that incorporate multiple-quantum MAS, thus enabling the resolution needed to study multiple chemical sites in a solid and providing a useful tool for the assignment of inequivalent sites.     

(39)K NMR of solid potassium salts at 21 T: effect of quadrupolar and chemical shift tensors

39K Solid State NMR spectra (static and magic angle spinning (MAS)) on a set of potassium salts measured at 21.14 T show that the chemical shift range for K(+) ions in diamagnetic salts is well in excess of 100 ppm contrary to previous assumptions that it was quite small. Inequivalent potassium sites in crystals can be resolved through differences in chemical shifts, with chemically similar sites showing differences of over 10 ppm. The quadrupolar coupling constants obtained from MAS and solid echo experiments on powders cover the range from zero for potassium in cubic environments in halides to over 3 MHz for the highly asymmetric sites in K2CO3. Although the quadrupolar effects generally dominate the 39K spectra, in several instances, we have observed subtle but significant contributions of chemical shift anisotropy with values up to 45 ppm, a first such observation. Careful analysis of static and MAS spectra allows the observation of the various chemical shift and quadrupole coupling tensor components as well as their relative orientations, thereby demonstrating that high-field 39K NMR spectroscopy in the solid state has a substantial sensitivity to the local environment with parameters that will be of considerable value in materials characterization and electronic structure studies.     

Improved excitation schemes for multiple-quantum magic-angle spinning for quadrupolar nuclei designed using optimal control theory

We have investigated the use of optimal control theory for the design of improved multiple-quantum excitation schemes for the popular multiple-quantum magic-angle spinning NMR experiment for quadrupolar nuclei with half-integer quadrupolar spin. The advantage of the new low-power experiments, termed OCFASTER, is demonstrated by sensitivity improvements approaching 50% for 87Rb in RbClO4 and RbNO3 as compared to FASTER and standard strong-pulse excitation schemes.     

Investigating the vanadium environments in hydroxylamido V(V) dipicolinate complexes using 51V NMR spectroscopy and density functional theory

Using (51)V magic angle spinning solid-state NMR, SSNMR, spectroscopy and quantum chemical DFT calculations we have characterized the chemical shift and quadrupolar coupling parameters of a series of eight hydroxylamido vanadium(V) dipicolinate complexes of the general formula VO(dipic)(ONR1R2)(H2O) where R1 and R2 can be H, CH3, or CH2CH3. This class of vanadium compounds was chosen for investigation because of their seven-coordinate vanadium atom, a geometry for which there is limited (51)V SSNMR data. Furthermore, a systematic series of compounds with different electronic properties are available and allows for the effects of ligand substitution on the NMR parameters to be studied. The quadrupolar coupling constants, C(Q), are small, 3.0-3.9 MHz, but exhibit variations as a function of the ligand substitution. The chemical shift tensors in the solid state are sensitive to changes in both the hydroxylamide substituent and the dipic ligand, a sensitivity which is not observed for isotropic chemical shifts in solution. The chemical shift tensors span approximately 1000 ppm and are nearly axially symmetric. On the basis of DFT calculations of the chemical shift tensors, one of the largest contributors to the magnetic shielding anisotropy is an occupied molecular orbital with significant vanadium d(z)2 character along the V=O bond.     

Dynamic effects in MAS and MQMAS NMR spectra of half-integer quadrupolar nuclei: calculations and an application to the double perovskite cryolite

Dynamic processes such as chemical exchange or rotations between inequivalent orientations can affect the magic-angle spinning (MAS) and the multiple-quantum (MQ) MAS NMR spectra of half-integer quadrupolar nuclei. The present paper discusses such dynamic multisite MAS and MQMAS effects and applies them to study the dynamic processes that occur in the double perovskite cryolite, Na3AlF6. Dynamic line shape simulations invoking a second-order broadening of the central transition and relying on the semiclassical Bloch-McConnell formalism for chemical exchange were performed for a variety of exchange models possessing different symmetries. Fitting experimental variable-temperature cryolite 23Na NMR data with this formalism revealed that the two inequivalent sodium sites in this mineral undergo an exchange characterized by a broad distribution of rates. To further assess this dynamic process a variety of 27Al and 19F MAS NMR studies were also undertaken; quantitative 27Al-19F dipolar coupling measurements then revealed a dynamic motion of the AlF6 octahedra that were qualitatively consistent with predictions stemming from molecular dynamic simulations on this double perovskite.     

Absolute structural constraints on amyloid fibrils from solid-state NMR spectroscopy of partially oriented samples

We demonstrate that absolute, molecular-level structural information can be obtained from solid-state NMR measurements on partially oriented amyloid fibrils. Specifically, we show that the direction of the fibril axis relative to a carbonyl 13C chemical shift anisotropy (CSA) tensor can be determined from magic-angle spinning (MAS) sideband patterns in 13C NMR spectra of fibrils deposited on planar substrates. Deposition of fibrils on a planar substrate creates a highly anisotropic distribution of fibril orientations (hence, CSA tensor orientations) with most fibrils lying in the substrate plane. The anisotropic orientational distribution gives rise to distorted spinning sideband patterns in MAS spectra from which the fibril axis direction can be inferred. The experimentally determined fibril axis direction relative to the carbonyl CSA tensor of Val12 in fibrils formed by the 40-residue beta-amyloid peptide associated with Alzheimer's disease (Abeta1-40) agrees well with the predictions of a recent structural model (Petkova et al. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 16742-16747) in which Val12 is contained in a parallel beta-sheet in the cross-beta motif characteristic of amyloid fibrils.     

Structure of (NH4)3GaF6 investigated by multinuclear magic-angle spinning NMR spectroscopy in comparison with rietveld refinement

The structure of ammonium gallium cryolite (NH(4))(3)GaF(6) was investigated by (19)F and (69,71)Ga magic-angle spinning (MAS) NMR in comparison with X-ray powder diffraction followed by Rietveld refinement. In agreement with previous thermodynamic measurements, NMR experiments on (NH(4))(3)GaF(6) support the model of rigid GaF(6) octahedra. At high spinning speeds (30 kHz), the scalar coupling between the six equivalent (19)F nuclei and (69,71)Ga can be directly observed in the powder spectra. The coupling constants are J(19)F(69)Ga = 197 Hz and J(19)F(71)Ga = 264 Hz. To explain the (71)Ga spectra recorded at 3 kHz a small distribution of quadrupolar frequencies has to be included. The spread of the spinning sidebands hints to a largest nu(Q) value of 28 kHz for (71)Ga. This can be explained by the occurrence of highly symmetric GaF(6) octahedra, which are tilted against the surrounding atoms. In addition, the incomplete motional excitation does not average out the quadrupolar effects. NMR findings are in discrepancy to those of Rietveld refinement. As result it appears that X-ray diffraction is not sensitive enough to deliver proper results.     

2H NMR calculations on polynuclear transition metal complexes: on the influence of local symmetry and other factors

It is now well-known that (2)H solid-state NMR techniques can bring a better understanding of the interaction of deuterium with metal atoms in organometallic mononuclear complexes, clusters or nanoparticles. In that context, we have recently obtained experimental quadrupolar coupling constants and asymmetry parameters characteristic of deuterium atoms involved in various bonding situations in ruthenium clusters, namely D(4)Ru(4)(CO)(12), D(2)Ru(6)(CO)(18) and other related compounds [Gutmann et al., J. Am. Chem. Soc., 2010, 132, 11759], which are model compounds for edge-bridging (μ-H) and face-capping (μ(3)-H) coordination types on ruthenium surfaces. The present work is in line with density functional theory (DFT) calculations of the electric field gradient (EFG) tensors in deuterated organometallic ruthenium complexes. The comparison of quadrupolar coupling constants shows an excellent agreement between calculated and observed values. This confirms that DFT is a method of choice for the analysis of deuterium NMR spectra. Such calculations are achieved on a large number of ruthenium clusters in order to obtain quadrupolar coupling constants characteristic of a given coordination type: terminal-D, η(2)-D(2), μ-D, μ(3)-D as well as μ(4)-D and μ(6)-D (i.e. interstitial deuterides). Given the dependence of such NMR parameters mainly on local symmetry, these results are expected to remain valid for large assemblies of ruthenium atoms, such as organometallic ruthenium nanoparticles.     

Experimental and theoretical studies of 45Sc NMR interactions in solids

Solid-state 45Sc NMR spectroscopy, ab initio calculations, and X-ray crystallography are applied to examine the relationships between 45Sc NMR interactions and molecular structure and symmetry. Solid-state 45Sc (I = 7/2) magic-angle spinning (MAS) and static NMR spectra of powdered samples of Sc(acac)3, Sc(TMHD)3, Sc(NO3)3.5H2O, Sc(OAc)3, ScCl3.6H2O, ScCl3.3THF, and ScCp3 have been acquired. These systems provide a variety of scandium coordination environments yielding an array of distinct 45Sc chemical shielding (CS) and electric field gradient (EFG) tensor parameters. Acquisition of spectra at two distinct magnetic fields allows for the first observations of scandium chemical shielding anisotropy (CSA). 45Sc quadrupolar coupling constants (CQ) range from 3.9 to 13.1 MHz and correlate directly with the symmetry of the scandium coordination environment. Single-crystal X-ray structures were determined for Sc(TMHD)3, ScCl3.6H2O, and Sc(NO3)3.5H2O to establish the hitherto unknown scandium coordination environments. A comprehensive series of ab initio calculations of EFG and CS tensor parameters are in excellent agreement with the observed parameters. Theoretically determined orientations of the NMR interaction tensors allow for correlations between NMR tensor characteristics and scandium environments. Solid-state 45Sc, 13C, and 19F NMR experiments are also applied to characterize the structures of the microcrystalline Lewis acid catalyst Sc(OTf)3 (for which the crystal structure is unknown) and a noncrystalline, microencapsulated, polystyrene-supported form of the compound.     

Third-order effect in solid-state NMR of quadrupolar nuclei

The theory and experimental observation of the third-order effect in solid-state NMR of quadrupolar nuclei are presented. The third-order effect consists of spherical harmonic terms up to rank l=6 and shifts NMR frequencies between two spin states that are not symmetric such as satellite transitions. Two-dimensional satellite transition magic-angle spinning experiment averages both the first and the second-order quadrupolar interactions making the quantitative measurement of the third-order effect possible. The third-order quadrupolar effect in andalusite has been measured at 11.7 T and its powder patterns are fitted with numerical simulations.