High-field QCPMG-MAS NMR of half-integer quadrupolar nuclei with large quadrupole couplings

Journal of Magnetic Resonance, 1998

By combination of fast magic-angle spinning (MAS) and detecfor the central transition on the order of hundreds of kilohertz tion of the free-induction decay during a rotor-synchronized quadgenerally reduces the spectral sensitivity. rupolar Carr-Purcell-Meiboom-Gill (QCPMG) train of refocus- We recently introduced the QCPMG experiment, i.e., the ing pulses, the sensitivity of quadrupolar-echo MAS NMR spectra quadrupolar version (9, 10) of the Carr-Purcell-Meiboomfor the central transition of half-integer quadrupolar nuclei exhib-Gill (CPMG) 12) experiment, as a means to increase iting large quadrupolar couplings may be significantly enhanced. the sensitivity of quadrupolar-echo (QE) spectra (13)

The Journal of Physical Chemistry A, 1997

A novel approach to quadrupolar-echo (QE) NMR of half-integer quadrupolar nuclei in static powders is analyzed. By acquisition of the QE spectrum during a Carr-Purcell-Meiboom-Gill (CPMG) train of selective π pulses, the second-order quadrupolar line shape for the central transition is split into a comb of sidebands leading to a considerable increase in the sensitivity compared to a conventional QE spectrum. The applicability of the method for determination of magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors is examined. Through numerical simulation and iterative fitting of experimental 87 Rb (RbClO 4 and RbVO 3 ) and 59 Co spectra (Co(NH 3 ) 5 Cl 3 ), it is demonstrated that the quadrupolar CPMG experiment represents a useful method for studying half-integer quadrupolar nuclei exhibiting large quadrupolar coupling combined with anisotropic chemical shielding interactions. Sensitivity enhancements by a factor of up to about 30 are observed for the samples studied.

Journal of Magnetic Resonance, 1997

Journal of the American Chemical Society, 2002

Solid State Nuclear Magnetic Resonance, 1997

Several aspects of the Multiple-Quantum Magic-Angle Spinning (MQMAS) technique (L. Frydman and J.S. Harwood, J. Am. Chem. Soc., 117 (1995) 5367) are compared with Dynamic-Angle Spinning (DAS). Examples of MQMAS spectra are shown for nuclei with CQ up to 3.6 MHz, and for 27Al () with CQ up to 10 MHz. The MQMAS linewidth is largely independent of the magnitude of the homonuclear dipolar interaction, while the spinning sideband manifold is similar to that observed in DAS experiments. MQMAS is technically simple and routinely useful for studying nuclei with short spin-lattice relaxation times, but care must be taken in its use for quantitative studies as the excitation of the triple-quantum coherence is not uniform. In this regard, MQMAS is most useful for samples with small quadrupolar coupling constants. In the specific case of 17O, DAS would give spectra with excellent resolution in comparison to MQMAS. The different advantages of DAS and MQMAS make them useful complementary techniques in many cases.Two additional methods are also presented for extracting the chemical shift anisotropy (CSA) directly for quadrupolar nuclei using the multiple-quantum scheme.

Chemical Physics Letters, 1996

It has been recently shown that second-order anisotropies can be removed from solid phase NMR spectra of quadrupolar nuclei via the combined use of magic-angle spinning (MAS) and bidimensional multiple-quantum (MQ) spectroscopy. The present study investigates the conditions under which the acquisition of such high-resolution MQMAS NMR spectra are optimized. The excitation and conversion pulse lengths that maximize 0 -~ _ 3(t 1) ~ -l(t 2) coherence transfer pathway signals for arbitrary spin numbers were calculated, as were the pulse lengths that provide NMR spectra free from dispersive line shape distortions. For the case of spin-5/2 nuclei, the conditions which optimize experiments involving quintuplequantum coherences were also determined.

Solid State Nuclear Magnetic Resonance, 2000

We derive a complete analytical solution for the powder magic angle spinning MAS nuclear magnetic resonance Ž . Ž . NMR lineshape in the presence of second-order quadrupole interaction, considering a radiofrequency rf pulse of finite ² : width, a finite MAS frequency, and a non-zero asymmetry parameter. I is calculated using two approaches. The first x Ž . applies time-dependent perturbation theory in the presence of the rf pulse and stationary perturbation theory SPT in its absence. The second is based on the Magnus expansion of the density matrix in the interaction representation during the pulse and SPT in its absence. We solve the problem in the laboratory frame using the properties of the Fourier transform and spin operators. Diagonalisation is not required. Both approaches agree well with each other under all conditions and also with the transition probability approach for the central transition. The Magnus expansion exists at all times and the effect of ² : the non-secular terms is negligible. We describe an analytical method of averaging I over the Euler angles and simulate x the 11 B MAS NMR lineshapes for crystalline and vitreous B O . A critical analysis is given of all earlier calculations of the 2 3 MAS NMR lineshape.

Applications, Theory and Instrumentation, 2006

Topics in Current Chemistry, 2011

Solid-state nuclear magnetic resonance (NMR) of quadrupolar nuclei has recently undergone remarkable development of capabilities for obtaining structural and dynamic information at the molecular level. This review summarizes the key achievements attained during the last couple of decades in solid-state NMR of both integer spin and half-integer spin quadrupolar nuclei. We provide a concise description of the first-and second-order quadrupolar interactions, and their effect on the static and magic angle spinning (MAS) spectra. Methods are explained for efficient excitation of single-and multiple-quantum coherences, and acquisition of spectra under low-and high-resolution conditions. Most of all, we present a coherent, comparative description of the high-resolution methods for half-integer quadrupolar nuclei, including double rotation (DOR), dynamic angle spinning (DAS), multiple-quantum magic angle spinning (MQMAS), and satellite transition magic angle spinning (STMAS). Also highlighted are methods for processing and analysis of the spectra. Finally, we review methods for probing the heteronuclear and homonuclear correlations between the quadrupolar nuclei and their quadrupolar or spin-1/2 neighbors.

Applications, Theory and Instrumentation, 2006