Nuclei quadrupolar

10 Quadrupolar Nuclei. - A geometrical description of the quadrupolar interaction has been presented. First- and second-order effects on all transitions were represented by quadrics and quartics. This approach allows a simple and exhaustive description of the Solomon multiple echoes the location of the echoes in the time domain, as well as their nature, is determined without calculation. Experimental evidence of multiple echoes in the case of an Al-O-P cluster is presented. In addition, the selection of double quantum coherences by appropriate phase cycling is presented. DAS and MQ MAS experiments are also described in this particular frame. 

A robust, easy to optimize, and efficient homonuclear correlation NMR experiment for half-integer quadrupolar nuclei in solids has been described and has been experimentally tested on anhydrous Na2HP04 by Na NMR and as-synthesised AIPO4-I4, an aluminophosphate molecular sieve, by Al NMR.  

Two signal enhancement schemes capable of providing large signal-to-noise gains in static and MAS NMR spectra of the central transition of half-integer quadrupolar nuclei have been presented by Schurko et al. Amplitude-modulated double-frequency sweeps (AM-DFS) and rotor-assisted population 

Rotor-assisted population transfer has been developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarisation associated with satellite transitions to the central m = 1/2 -1/2 transi- 

The extension of their previously developed low temperature method to an 18.8 T system has been described by Lipton et al. A new probe was used with a cross coil and variable capacitors that are operational at cryogenic temperatures. The limitations to sensitivity are discussed, including a new diode network, the utilisation of a cryogenic band pass filter, and the consequences of the RF profiles of the coil. Further, details of the spectroscopy of quadrupolar nuclei in a protein are discussed, such as the observation of the outer transitions and how to distinguish them from the desired 1/2 transition. 

Although three-quarters of the magnetically active nuclei are quadrupolar it is still rare for chemists to look beyond a handful of spin-1/2 nuclei in NMR studies. This is principally because of the broad lines usually obtained for these nuclei except in the most symmetrical of molecules. The broad lines originate from rapid relaxation due to coupling of the electric quadrupole of the nucleus 

For a given molecule we only have control over Xc- A.s mentioned above viscosity and hence ic is lower for supercritical solvents than for conventional solvents and so we can expect reduced rates of quadrupolar relaxation in SCFs. This would allow high-resolution NMR spectra of nuclei such as or often present in organic chemistry, or in the ligands of organometallic complexes, and many of the metal nuclei encountered in organometallic chemistry. The extra resolution will hopefully allow detection of couplings not otherwise measurable, and distinction of closely spaced lines. 

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B1.12.4.5 HIGH-RESOLUTION SPECTRA FROM QUADRUPOLAR NUCLEI... 

Werbelow L G 1996 Relaxation theory for quadrupolar nuclei Encyclopedia of Nuclear Magnetic Resonance ed D M Grant and R K Harris (Chichester Wiley) pp 4092-101... 

It is important to note that sodium and chlorine are both quadrupolar nuclei and Eq. (3.4.1) is therefore not true for SPI/SPRITE measurement of these species however, Eq. (3.4.2) is correct when centric scan SPRITE methods are employed. 

The centric scan, one-dimensional, DHK SPRITE measurement was used to study the ingress of lithium. This measurement technique was selected due to the low absolute sensitivity of 7Li (27% of [36]), the small amounts that are present and the short signal lifetimes (bulk Tx of 10 ms and T2 of 120 ps). In addition to the robust, quantitative nature of this technique, lithium is a quadrupolar nucleus and interpretation of the image intensity is more complex than spin % nuclei. Once again Eq. (3.4.2) is quantitatively correct for even quadrupolar nuclei due to the fact the longitudinal steady state does not influence the image intensity. 

The SPI and SPRITE class of measurements for imaging short MR signal lifetimes are quantitative and have signal equations that are readily understood. The centric SPRITE methods are much faster and feature signal equations that are easier to interpret. This feature makes these measurements more readily density weighted and better suited to imaging quadrupolar nuclei. 

NMR-active nuclei with spin > Vi (these include, as we mentioned previously, deuterium) have an electric quadrupole moment and are thus referred to as quadrupolar nuclei. 

Quadrupolar nuclei Those nuclei, which because of their spin quantum number (which is always >1/2), have asymmetric charge distribution and thus posses an electric quadrupole as well as a magnetic dipole. This feature of the nucleus provides an extremely efficient relaxation mechanism for the nuclei themselves and for their close neighbors. This can give rise to broader than expected signals. 

Quadrupolar relaxation Rapid relaxation experienced by quadrupolar nuclei. 

Probing Quadrupolar Nuclei by Solid-State NMR Spectroscopy ... 

We note that optimal control is a universal tool for experiment design and has also, in solid-state NMR spectroscopy, found additional applications in the design of homonu-clear dipolar recoupling [41], broadband rf pulses and quantum gates [71], building blocks of symmetry-based recoupling experiments [129], quadrupolar multiple-quantum MAS experiments [165], and improved pulses for quadrupolar nuclei [166]. Numerous references to further applications with regard to liquid-state NMR can be found in [72]. 

Fernandez C, Pruski M (2011) Probing quadrupolar nuclei by solid-state NMR spectroscopy recent advances. Top Curr Chem. doi 10.1007/128 2011 141... 

Homonuclear Dipolar Recoupling Involving Quadrupolar Nuclei. 179... 

Quadrupolar nuclei, i.e., those with a nuclear spin quantum number 7 greater than or equal to one, outnumber their spin-1/2 counterparts by a factor of roughly 3 1 and are critically important in the study of many solid materials. For example, nB (spin 7 = 3/2, 80% natural abundance), 14N (7=1, 99.6%), 23Na (7 = 3/2, 100%), 170 (7 = 5/2, 0.04%) and 27A1 (7 = 5/2,100%) comprise some of the most important atomic nuclei in chemistry, materials science, and biology. As is well... 

The first term in (24) was already calculated in (9). The second term can be neglected, as the CSA is usually too weak to require the second-order treatment. The last term is the second-order cross-term between the two interactions. A complete review of the second-order cross-terms and their effect on high-resolution solid-state NMR powder spectra of quadrupolar nuclei was recently published by Ashbrook et al. [31]. 

As explained in depth in the next sections, most of the advanced high-resolution experiments on quadrupolar nuclei are based on the selection of specific coherences and on the transfer of these coherences along the selected pathways, which always terminate at the observable SQ coherence p = — 1. Most of the time, the selection is done using nested phase-cycling of the radiofrequency (rf) pulses included in the pulse sequence [34]. Recently, new methods have been proposed to optimize the nested phase-cycling procedure, including the schemes referred to as cogwheel [36-39] and multiplex [40—421. 

Despite the benefits of high resolution, measurements of wideline spectra of quadrupolar nuclei under static or MAS conditions are still commonly used in a variety of applications. For both integer and half-integer spins, simulations of quadrupolar lineshapes can yield full sets of NMR parameters associated with quadrupolar and chemical shift tensors and can be used for studying molecular dynamics. 

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