Henrik Bildsøe

Satellite transitions in MAS NMR spectra of quadrupolar nuclei

Manifolds of spinning sidebands (ssbs) are observed for the satellite transitions of several half-integer quadrupolar nuclei (spin I12) using one-pulse magic-angle spinning (MAS) NMR experiments of crystalline powders. Numerical simulations of the variety of envelope patterns for the ssbs in such spectra show that the quadrupolar coupling parameters can be determined with high accuracy. The theory of the method is described and evaluated using average Hamiltonian theory to first and second order in the secular approximation. The influence of finite RF pulse excitation and quality factor of the MAS probes on the ssb intensities in the spectra is analyzed. Determination of quadrupole coupling constants and asymmetry parameters for the electric field gradient tensors from 17O, 23Na, and 27Al MAS NMR spectra of various solids illustrates the theory and high performance of the method.

51V MAS NMR spectroscopy: determination of quadrupole and anisotropic shielding tensors, including the relative orientation of their principal-axis systems

Abstract 51 V MAS NMR spectra exhibiting the complete manifold of spinning sidebands (SSBs) from all seven 51 V ( I =7/2) transitions have been observed for NH 4 VO 3 and V 2 O 5 . These spectra are influenced by interactions from both 51 V quadrupole coupling and chemical-shielding anisotropy. A simulation and iterative-fitting program which allows the first determination of quadrupole and anisotropic shielding tensors, including the relative orientation of their principal-axis systems, from MAS NMR spectra is introduced. The 51 V parameters determined from the SSB intensities for NH 4 VO 3 and V 2 O 5 are compared with literature data from static NMR spectra.

Finite rf pulse excitation in MAS NMR of quadrupolar nuclei. Quantitative aspects and multiple-quantum excitation

Abstract The response of quadrupolar nuclei to finite radiofrequency (rf) pulse excitation in magic-angle spinning (MAS) NMR of powders is investigated. A theoretical description and numerical simulations show the impact of rf excitation as function of pulse width, relative strength of the rf field and quadrupole interactions, and spinning speed. The distribution of magnetization over the various multiple-quantum (MQ) transitions is examined for spin I = 3 2 and 5 2 quadrupolar nuclei. A two-pulse MQ-filtered sequence is used to explore one-pulse excitation of MQ coherences. Simulations are compared with experimental 1Q and MQ-filtered 23 Na and 27 Al MAS NMR spectra.

7Li, 23Na, and 27Al quadrupolar interactions in some aluminosilicate sodalites from MAS n.m.r. spectra of satellite transitions

Magic angle spinning n.m.r. spectra of the satellite transitions for the 7 Li, 23 Na, and 27 Al half-integer quadrupolar nuclei are used to probe the quadrupolar interactions of these nuclei in a series of CI-, Br-, and l-aluminosilicate sodalites. Computer simulations of the complete spinning sideband manifolds allow determination of the nuclear quadrupolar coupling constants ( C Q ) and the asymmetry para-meters (η) of the electric-field gradient tensors. These parameters represent a valuable tool for extracting information on the local electronic environments around the exchangeable cage Li + and Na + cations and the 27 Al framework nuclei. Structure refinements of X-ray powder diffraction data were performed to investigate possible correlations between the quadrupolar interactions and structural parameters.

ESCORT editing. An update of the APT experiment

Abstract A new high-accuracy heteronuclear NMR spectral editing technique named ESCORT is presented. The technique does not employ decoupler pulses and can be performed on any FT NMR spectrometer equipped for gated proton decoupling. ESCORT editing is therefore easier to implement than other established editing techniques which require precalibration of the decoupler rf field strength.

Multiple-quantum MAS nutation NMR spectroscopy of quadrupolar nuclei☆

Abstract Theory, experiments, and simulations of two-dimensional nutation NMR spectroscopy of half-integer quadrupolar nuclei combined with magic-angle spinning (MAS) are presented. MAS enhances the overall sensitivity and improves resolution in the ω 2 2π dimension of the 2D nutation spectra. Compared to static nutation spectroscopy it is demonstrated that MAS nutation NMR puts less stringent requirements on the magnitude of the radiofrequency field strength, which is beneficial for nuclei with large quadrupole couplings. The finite nutation pulse excites all 2I(I + 1 2 ) multiple-quantum transitions for a spin-I nucleus. Multiple-quantum versions of the 2D nutation experiment are explored. Simulated 2D MAS nutation spectra are compared with experimental 23Na and 27Al spectra of NaNO3 and Ca3A12O6·6H2O (concrete), respectively, and with simulations of corresponding 2D nutation experiments for the static case.

1:1 Correlation between27Al and 29Si chemical shifts and correlations with lattice structures for some aluminosilicate sodalites

Abstract High-speed spinning 27 Al and 29 Si MAS n.m.r. experiments have been performed for five lithium and sodium halide aluminosilicate sodalites. Accurate 27 Al and 29 Si chemical shifts, 27 Al quadrupolar coupling parameters, and 29 Si spin-lattice relaxation times are reported and discussed. A 1:1 linear correlation between the 27 Al and 29 Si chemical shifts ( R L = 0.9998) covering a range of at least 12 ppm is observed. Linear correlations between δ( 27 Al) [or δ ( 29 Si)] and the cubic unit cell a -axis, the Si O Al bond angles (θ), and previously proposed functions describing the local framework geometry and based on θ [e.g. Σd TT = 13.48sin (θ/2)] are observed. The results indicate δ( 27 Al) as alternative to δ( 29 Si) for correlating MAS n.m.r. data with structure parameters at least for aluminosilicates with a ratio Si/Al= 1. The 27 Al second-order quadrupolar effect seems to be influenced by the nature of the anions to a higher degree than by the framework geometry and cations in these systems.

Unusual observation of nitrogen chemical shift anisotropies in tetraalkylammonium halides by 14N MAS NMR spectroscopy.

14N Magic-angle spinning (MAS) NMR spectra for a number of polycrystalline, symmetrical tetraalkylammonium halides with short alkyl chains (C2H(5)- to n-C4H(9)-) have been recorded following a careful setup of the experimental conditions. Analysis of the spectra demonstrates the presence of 14N chemical shift anisotropies (CSAs) on the order of |delta sigma| = 10-30 ppm along with 14N quadrupole coupling constants in the range of 10-70 kHz. The magnitude and sign of the CSAs determined from 14N MAS NMR are confirmed by recording and analysis of the corresponding slow-speed spinning (500-650 Hz) 15N CP/MAS NMR spectra. Most interestingly, it is observed experimentally and demonstrated theoretically and by simulations, that these CSAs are reflected in the spinning sideband (ssb) intensities of the 14N MAS spectra at much higher spinning speeds than can be applied to retrieve the corresponding 15N CSAs from the ssb pattern in the 15N CP/MAS spectra.

Natural-abundance proton-coupled satellite spectra in 13c NMR from double selective population transfer. The flip angle effect

Abstract A new experimental method, useful in the recording of satellite spectra, is presented. The technique, double selective population transfer, has been used to observe protoncoupled 29 Si, 13 C, and 15 N satellites in 13 C NMR and is useful also for determination of the sign of the coupling constant between two rare spins. For homonuclear satellite spectra the so-called “flip angle effect” must be taken into account in order to predict the conditions for optimum intensity enhancements and to determine the relative sign of coupling constants. Calculations of the flip angle effect are presented for a 13 C A − 13 C X − 1 H three-spin system under various conditions of these J -polarization transfer experiments.

Pulse techniques for calibration of the decoupler radiofrequency field strength

Abstract Two new methods for calibration of decoupler radiofrequency field strength are introduced. These two methods, based on the SEMUT and SINEPT techniques, are superior to earlier approaches with respect to sensitivity and accuracy. In addition, the SINEPT technique can be applied for calibration via long-range couplings in complex spin systems. The techniques are analyzed in detail with respect to performance under nonideal conditions such as delay mismatches and off resonance effects.