Second-order Quadrupolar Shift Research Articles

Local Electronic Structure in AlN Studied by Single-Crystal 27Al and 14N NMR and DFT Calculations.

Both the chemical shift and quadrupole coupling tensors for N and Al in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of N () and Al (), the quadrupolar coupling constants were determined to kHz, and MHz. The chemical shift parameters obtained from the data fit were ppm and ppm for N, and (after correcting for the second-order quadrupolar shift) ppm and ppm for Al. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for Al with MHz, but not for N with kHz.

NMR of quadrupole noble gases in liquid crystals

ABSTRACT NMR spectroscopy of quadrupolar noble gases, 21Ne (spin 3/2), 83Kr (9/2), 131Xe (3/2), dissolved in thermotropic liquid crystals enables the derivation of versatile information about the physical properties of the materials. The spectra display fine structures, triplets and nonets, which reveal quadrupole couplings. These, in turn, can be used in the determination of electric field gradients (EFG), orientational order parameters, and tilt angles. Comparison of 129Xe and 131Xe chemical shifts reveals second-order quadrupole shifts (SOQS) in circumstances where the ratio of quadrupole coupling and magnetic flux density is suitable. The 131Xe SOQS is shown to distinguish between uniaxial and biaxial nematic phases. Both 83Kr and 131Xe NMR spectra may display asymmetry around the central transition because of the SOQS, which can be used to identify biaxiality in nematic phases. As a curiosity, the effect of possible hexadecapole coupling on the 83Kr NMR transitions of krypton in liquid crystals is discussed.

Ternary rare-earth aluminium intermetallics RE10TAl3 (RE = Y, Ho, Tm, Lu; T = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) with an ordered anti-Co2Al5 structure.

Twenty new rare-earth metal rich intermetallic aluminium compounds, RE10TAl3 (RE = Y, Ho, Tm, Lu; T = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt), were synthesized by arc melting the elements. The compounds crystallize, in analogy to e.g. the respective Cd representatives, with a ternary ordered structure as anti-type to the hexagonal Co2Al5 type, with the space group P63/mmc. The three crystallographically independent rare-earth metal sites occupy the aluminium positions of the aristotype, while the transition metal and aluminium atoms are ordered on the two cobalt sites. Like other rare-earth rich compounds the RE10TX3 members also exhibit transition-metal-centred T@RE6 trigonal prisms as striking structural building units. The prepared compounds have been investigated by susceptibility measurements and 27Al solid-state MAS-NMR measurements conducted on the Pauli-paramagnetic Y and Lu compounds. Some compounds show a certain amount of disorder as seen from the single crystal structure analysis and from signal broadening in the NMR investigations. By separating Knight shifts from second-order quadrupolar shifts via field dependent measurements, monotonic trends can be discerned regarding the effect of the T atom valence electron concentration and period number, as well as the effect of the closed 4f shell contributed in the Lu compounds. The results confirm that a comparison of Knight shifts within a series of isotypic compounds can reveal important electronic structure information in intermetallic systems.

ChemInform Abstract: Nitrogen‐Proton Correlation Experiments of Organic Solids at Natural Isotopic Abundance

AbstractReview: 42 refs.

ChemInform Abstract: Xenon in Liquid‐Crystalline Samples

AbstractReview: 18 refs.

14N–1H Heteronuclear Multiple-Quantum Correlation Magic-Angle Spinning NMR Spectroscopy of Organic Solids

Abstract 14N–1H heteronuclear multiple-quantum correlation (HMQC) solid-state magic-angle spinning (MAS) NMR spectra recorded at a 1H Larmor frequency of 850 MHz are presented for the dipeptide β-AspAla. A modified version of the pulse sequence presented by Gan et al. (Chem. Phys. Lett. 435 (2007) 163) that utilises rotary resonance recoupling (R3) at the n = 2 condition (ν 1 = 2ν R ) is employed. Spectra recorded with a short recoupling period (under 200 μs) show two correlation peaks corresponding to the NH and NH3 moieties in the dipeptide. The quadrupolar product, P Q = C Q √ [1 + (η Q 2/3)], is determined experimentally as 3.1 MHz (NH) and 1.0 MHz (NH3) by a comparison of the 14N and 15N isotropic chemical shifts which differ due to the isotropic second-order quadrupolar shift for the spin I = 1 14N nucleus. It is shown that the peak sensitivities increase markedly upon increasing the MAS frequency from 30 to 45 to 60 kHz due to a combination of the reduced residual dipolar broadening of the 1H resonances and a lengthening of the coherence lifetimes under R 3 recoupling. Increasing the recoupling period leads to the observation of additional peaks corresponding to longer range intra- and intermolecular NH proximities. Reasonable agreement is evident upon comparing the experimental build-up of correlation peak intensity to that observed for eight-spin density-matrix simulations.

Phase transition and relaxation processes by 87Rb and 39K nuclear magnetic measurements of RbKSO4 single crystals

Abstract 87 Rb and 39 K nuclear magnetic resonance (NMR) spectra of RbKSO 4 single crystals were measured at room temperature. 87 Rb central line has the angular dependences of second-order quadrupolar shifts. From these results, the quadrupole coupling constant and the asymmetry parameter were determined at room temperature. In addition, the spin–lattice relaxation rate, 1/ T 1 , and the spin–spin relaxation rate, 1/ T 2 , were measured as a function of temperature. The values of 1/ T 1 for the 87 Rb and 39 K nuclei were found to increase with increasing temperature, and 1/ T 1 was determined to be proportional to T n . Therefore, for the 87 Rb and 39 K nuclei, Raman processes with n =2 are more significantly in nuclear quadrupole relaxation than direct processes.

Water molecular reorientation in ice and tetrahydrofuran clathrate hydrate from lineshape analysis of 17O spin-echo NMR spectra

Lineshape analysis of 17O spin-echo NMR spectra has been used to study water molecular reorientations in ice-Ih and THF Structure II clathrate hydrate. The kinetics was determined by the changes of the lineshapes of the 17O central transitions at different temperatures. A model involving 12 orientations and 4-step jumps of water molecular orientations was proposed. Semi-classical exchange formalism was employed to simulate the lineshape of the central transitions of the 17O nuclei. Lineshape analysis gave the quadrupolar coupling constant CQ = 6.43 MHz and the asymmetry parameter η = 0.935, for both ice-Ih and THF gas hydrate. The theoretical lineshape simulations resulted in activation energies of water molecular reorientations Ea = 55.2 ± 2.1 kJ mol–1 and Ea = 30.5 ± 0.8 kJ mol–1 for ice-Ih and THF hydrate, respectively. The range of dynamic rates in THF clathrate hydrate is such that before melting, a pseudo-isotropic lineshape is observed that retains a second-order quadrupolar shift. The water reorientation process is discussed in light of recent results on Bjerrum defect injection obtained from molecular dynamics simulation and structural studies.

Detection of Phase Biaxiality in Liquid Crystals by Use of the Quadrupole Shift inXe131NMR Spectra

An experimental method to unambiguously distinguish between uniaxial and biaxial liquid crystal phases is introduced. The method is based on the second order quadrupole shift (SOQS) observable in 131Xe NMR spectra of xenon dissolved in liquid crystals. It is shown that besides revealing the biaxiality, the 131Xe SOQS offers a novel method to determine the tilt angle in smectic C phases. As an example, the 131Xe SOQS in a ferroelectric liquid crystal is reported. It yields up a biaxial phase in between isotropic and smectic C phases.

10.1007/s11451-008-3017-5

The orientation dependences of the second-order quadrupole shifts of the central component in the 23Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na1/2Bi1/2TiO3 structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111] p direction and a small displacement statistically or dynamically disordered over six equally probable [100] p -type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111] p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.

High-field QCPMG NMR of large quadrupolar patterns using resistive magnets

Spectroscopy in a high magnetic field reduces second-order quadrupolar shift while increasing chemical shift. It changes the scale between quadrupolar and chemical shift of half-integer quadrupolar spins. The application of QCPMG multiple echo for acquiring large quadrupolar pattern under the high magnetic field of a 25 T resistive magnet is presented for acquiring large quadrupolar patterns. It shows that temporal field fluctuations and spatial homogeneity of the Keck magnet at the NHMFL contribute about ±20 ppm in line broadening. NMR patterns which have breadths of hundreds to thousands of kilohertz can be efficiently recorded using a combination of QCPMG and magnetic field stepping with negligible hindrance from the inhomogeneity and field fluctuations of powered magnets.

Local and Average Structure of Relaxor Na1/2Bi1/2TiO3 from the Point of View of NMR

23 Na NMR spectra have been measured in a very low magnetic field at temperatures between 203 K and 780 K. The orientation dependences of the second-order quadrupole shifts of the central component were studied. The achieved spectral resolution is sufficient to test various models of short –range order in the Na 1/2 Bi 1/2 TiO 3 (NBT) structure using computer simulations of the spectral line shape. The polar monoclinic areas were found to coexist with the tetragonal phase insertions till the lowest investigated temperature 203 K. In the temperature range 203 K – 640 K the NMR spectra also display the presence of areas with very small deviation from the cubic structure (matrix).

Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR

Using a two-dimensional multiple-quantum (MQ) double rotation (DOR) experiment the contributions of the chemical shift and quadrupolar interaction to isotropic resonance shifts can be completely separated. Spectra were acquired using a three-pulse triple-quantum z-filtered pulse sequence and subsequently sheared along both the ν 1 and ν 2 dimensions. The application of this method is demonstrated for both crystalline (RbNO 3) and amorphous samples (vitreous B 2O 3). The existence of the two rubidium isotopes ( 85Rb and 87Rb) allows comparison of results for two nuclei with different spins ( I = 3/2 and 5/2), as well as different dipole and quadrupole moments in a single chemical compound. Being only limited by homogeneous line broadening and sample crystallinity, linewidths of approximately 0.1 and 0.2 ppm can be measured for 87Rb in the quadrupolar and chemical shift dimensions, enabling highly accurate determination of the isotropic chemical shift and the quadrupolar product, P Q . For vitreous B 2O 3, the use of MQDOR allows the chemical shift and electric field gradient distributions to be directly determined—information that is difficult to obtain otherwise due to the presence of second-order quadrupolar broadening.

Second-order quadrupolar shifts as an NMR probe of fast molecular-scale dynamics in solids

Molecular-scale dynamics on the nanosecond timescale or faster can have a measurable influence on isotropic NMR frequencies of quadrupolar nuclei. Although previously studied in solution, where it is usually referred to as the ‘dynamic shift’, this effect is less well known in solids. Here we demonstrate that multiple-quantum NMR measurements of isotropic quadrupolar shifts are a simple way to probe nanosecond timescale motions in solids. We measure the 11B (spin I = 3/2) shifts of the resolved boron sites in ortho-carborane as a function of temperature and interpret the results in terms of the known rapid tumbling dynamics.

23Na NMR study of the local order in the Na1/2Bi1/2TiO3 structure in a weak magnetic field

The orientation dependences of the second-order quadrupole shifts of the central component in the 23Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na1/2Bi1/2TiO3 structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111]p direction and a small displacement statistically or dynamically disordered over six equally probable [100]p-type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111]p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.

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.

Possible spin triplet superconductivity inNaxCoO2·yH2O —59Co NMR studies

We report 59Co NMR studies on magnetically oriented powder samples of Co oxide superconductors,NaxCoO2·yH2O,with Tc∼4.7 K. From the two-dimensional powder pattern in the NMR spectrum, theab-plane Knight shift in the normal state was estimated from the magnetic field dependence ofthe second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift showsa Curie–Weiss-like temperature dependence, similarly to the bulk magnetic susceptibilityχ. From the analysisof the so-called K–χ plot, the spin and the orbital components ofK and the positive hyperfine coupling constant were estimated. The onset temperature of thesuperconducting transition in the Knight shift does not change much in an appliedmagnetic field up to 7 T, which is consistent with the reported high upper critical fieldHc2. The Knight shift at 7 T shows an invariant behaviour belowTc. No coherencepeak just below Tc was observed in the temperature dependence of the nuclear spin–lattice relaxation rate1/T1 in either case (NMR, nuclear quadrupole resonance). Weconclude that the invariant behaviour of the Knight shift belowTc and unconventionalbehaviours of 1/T1 may indicate spin triplet superconductivity with p- or f-wave symmetry.

Symmetry-based recoupling of 17O– 1H spin pairs in magic-angle spinning NMR

We have performed magic-angle-spinning solid-state NMR experiments in which protons are recoupled to oxygen-17 nuclei by applying a symmetry-based recoupling sequence at the proton Larmor frequency. Two-dimensional quadrupole-dipole correlation spectra are produced, in which the second-order quadrupolar shift of the oxygen-17 central transition is correlated with the recoupled heteronuclear dipole–dipole interaction. These spectra are sensitive to the relative orientation of the electric field gradient at the site of the oxygen-17 nucleus and the O–H internuclear vector. We also demonstrate experiments in which polarization is transferred from protons to oxygen-17, and show that oxygen-17 signals may be selected according to the protonation state of the oxygen site. We discuss the small observed value of the heteronuclear dipolar splitting in the central-transition oxygen-17 spectra.

Amorphization of Al 32Ge 68 in a slow heating run studied by NMR

Amorphization of Al 32Ge 68 at normal pressure, starting from the thermobarically quenched high-pressure crystalline metallic γ-phase, was studied in a slow heating run by performing NMR field-sweep experiments. In the temperature interval from 77 to 300 K the 27Al central line exhibits a continuous broadening and a positive frequency shift. Close to 300 K rather abrupt changes were observed, where the shift changed its sign, and the line width jumped to its highest value. The discontinuous course of the phase changes indicates that the amorphization process proceeds via a sequence of intermediate metastable states, which can be ‘overheated’ in a slow heating run and consequently transform in an explosion-like manner to the final amorphous state at a well-defined temperature. The frequency shift could be decomposed into the negative second-order quadrupolar shift and the positive Knight shift. The change of the shift sign from positive to negative at 300 K reflects the vanishing tendency of the Knight shift upon heating and is compatible with the conduction-electron localization upon structure amorphization.

Spin-locking mechanism of spin I=3/2 quadrupolar nuclei undergo magic angle spinning

The spin-locking mechanism of the spin I=3/2 quadrupolar nuclei under magic angle spinning (MAS) has been theoretically and experimentally investigated, and the criterion of adiabatic passage around zero-crossings of the quadrupole splitting was inferred from the time-dependent Shrödinger equation in this article. The theory, numerical simulations, and experiments conducted in this work all indicated that second-order quadrupole interaction and off-resonance play important roles in the spin-locking of the quadrupolar nuclei, and they were responsible for the great loss of the spin-locking signals. The spin-locking for a spin I=3/2 nucleus might be achieved by minimizing the effect of the second-order quadrupole interaction by using a radio frequency (RF) offset. This offset was realized by setting the RF to the opposite position of the isotropic second-order quadrupolar shift of single quantum coherences.