Quadrupolar Nucleus Experiments

General Principles, NMR as a Highly Sensitive Analytical Tool (pg to ng Amounts) 

The identification and structural characterization of biological materials, obtained for example from plants, was traditionally carried out via the classical sequence involving extraction, separation, isolation and characterization, a sequence which requires large amounts of substance and a great deal of time. Industrial problems, for example the search for small amounts of contaminants in industrial products or in waste water, also require intensive analytical studies. 

A direct combination of separation and analysis techniques is thus invaluable. GLC-MS and HPLC-MS coupling are now routinely used. Because of the high sensitivity of modern NMR instruments the coupling of HPLC and NMR is now used in many NMR laboratories, and we shall discuss the principles 

The coupling of HPLC in tandem with NMR requires two separate systems  

A long capillary with a computer-controlled switching valve (the instruments must be separated by 2-3 metres because of the strong magnetic field) connects the exit from the HPLC with the probehead. The latter is completely different in its construction from conventional probeheads instead of the NMR tube there is a small flow cell, the volume of which is 40-100 pi. The transmitter and receiver coils are attached directly to the cell in order to maximize the sensitivity. 

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Some appreciation for the nitrogen nuclear characteristics may be obtained from Table 1. Because the relative sensitivities are comparable, the approximately 300-fold higher natural abundance of would seem to make it the nucleus of choice. It is even more sensitive than at natural abundance. However, like all nuclei with spin quantum number I > 1/2, possesses an electric quadrupole moment that arises from a nonspherical electric charge distribution in the nucleus itself. When placed in an electric field gradient, such as that characteristic of most molecular electron distributions, a quadrupolar nucleus experiences random fluctuating electric fields. The characteristic frequencies of these motions have components at the resonance frequency and hence afford an efficient relaxation mechanism. As a result, spin-lattice relaxation times (Tj ) are very short, 0.1-10 ms. Because Tj = To for in most molecules Lie in solution, linewidths are corres-... 

Although you would expect deuterium (D) NMR to be every bit as useful as NMR, it is not carried out routinely as deuterium has a natural abundance of 0.015% and very low sensitivity. Furthermore, as the spin is greater than %, it is a quadrupolar nucleus and the signals are usually broad. Deuterium shows signals in the same chemical shift range as H (5 0-12), and NMR is most often used in experiments using D-labelled compounds to investigate reaction mechanisms. 

It is this absorption of electromagnetic energy that is measured in NQR spectroscopy. Radiation in the radio-frequency region is actually employed to effect transitions among the various orientations of a quadru-polar nucleus in a nonspherical field. In the experiment, which is generally carried out on a powder sample, one set of resonances is exposed for each chemically or crystallographically inequivalent quadrupolar nucleus... 

Residual coupling effects due to quadrupolar nuclei in mas spectra. It was seen that for both dipolar and spin-spin coupling between nuclei the first-order anisotropic effects are proportional to P2(cos0) so that under MAS if a nucleus experiences both these effects, only the isotropic effects of J remain. However, the... 

It should be noted that in both the TRAPDOR and REAPDOR experiments, the rf field on the dephasing quadrupolar nucleus remains on for a considerable time, which can result in a Bloch-Siegert shift (Bloch and Siegert 1940) appearing as a misset in the zero-order phasing of the reduced echo compared to the full echo spectrum. 

Trebosc et al. introduced a frequency-selective (FS) REDOR approach to a multi-spin system Sl y where S is a quadrupolar nucleus. FS-REDOR may be used for accurate trough-space distance measurements in spin pairs that involve quadrupolar nuclei [106]. The experiment reveals heteronuclear dipolar and scalar couplings, which can be reintroduced selectively, site after site. Importantly, FS-REDOR may also be used under high-resolution provided by MQMAS, STMAS or I-STMAS. 

Another very important experiment for spectral editing is a heterocorrelation experiment combining MQMAS (in the indirect dimension) with CP. Introducing CP at the MQMAS echo position following a split-fj MQMAS scheme creates a spin-j detected spectra (in the dimension), which is modulated by the isotropic frequency of the quadrupolar nucleus in F. HETCOR has been demonstrated on Na- P pairs, on 2 A1- P pairs and lately on a- H pairs,where proton resolution during F was obtained with the wPMLGS multiple pulse decoupling technique. ... 

A new approach to combining CP with MQ MAS in a 2D NMR experiment has been demonstrated, involving CP from H to the single-quantum coherences of a quadrupolar nucleus.In two separate methods, pure-absorption line shapes have been obtained using a z-filter and a reversed split-ti method. 

CP from H to the MQ coherences of a quadrupolar nucleus has been used in combination with the 2D MQ MAS NMR experiment in order to extract high-resolution CPMAS NMR spectra. The technique has been demonstrated on Na (S = 3/2), Al (both S = 5/2), and " Sc (S = 7/2) nuclei, showing the applicability of MQ CP to systems with differing spin quantum number, gyromagnetic ratio, and relative nuclide abundance. The utility of this 2D MAS NMR experiment for spectral editing and site-specific measurement of CP intensities has been demonstrated. The possibility of direct CP to higher order MQ coherences has also been considered and three-, five-, and seven-quantum cross-polarized " Sc MAS NMR spectra have been presented. 

Since the short relaxation times associated with a quadrupolar nucleus drastically reduce the time delay to be applied in an NMR experiment between two pulses, measuring times are short or, in other words, distinct NMR signals can often be detected with a limited time spent down to micromolar concentrations. Along with this apparent advantage, quadrupolar nuclei provide information in addition to the classical parameters chemical shift (or shielding) and nuclear spin-spin coupling constants. Variations in linewidths for quadrupolar nuclei are another sensitive quantity allowing for the evaluation of the electronic and the steric situation in the first coordination sphere of a vanadium compound, its periphery, its (local) symmetry and its interaction with the matrix, i.e. counter-ions, solvent molecules and other constituents present in solution. 

To understand the varied chemistry of the aluminas, techniques need to be developed for studying the surface independent of the bulk. Structural and dynamical aspects of the surface do have their origins in the bulk, but the specific details delineating the surface will be different. Clearly it would be advantageous to apply the same surface selective CP methodology developed for the silicas [12-16] to the surface of the aluminas. Before addressing this particular point, however, we need to consider the feasibility of the experiment. Are the aluminum atoms at the surface indeed observable by NMR methods If surface aluminum atoms are observable, we must then recognize that the spin of interest, Al, is not a spin-1/2 nuclide (/ for Al is 5/2) hence Al has a nonzero nuclear electric quadrupole moment. Cross-polarization from protons to a quadrupolar nucleus presents the experimenter with another layer of complication in compari-... 

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