Observing quadrupolar nuclei

The following strategy therefore emerges for the study of quadrupolar nuclei observe the central transition of nuclei with noninteger spin, use MAS (to remove dipolar coupling, chemical shift anisotropy, and first-order quadrupolar effects), and work at high fields (to minimize second-order effects). 

There is a redeeming instance that often makes partial spectra of quadrupolar nuclei observable. For odd-half integral nuclei, such as 23Na (7=2), the mz = to m. = -2 transition is not affected by the quadrupolar interaction to first order and remains relatively sharp, while the satellite transitions, such as mz = to mz = 2 and m. = —2 to mz = -2, are broadened by the powder distribution. There is a substantial second-order effect of the quadrupole coupling that broadens even the central transition, but this broadening is often only a few kilohertz to a few tens of kilohertz—large but much smaller than Megahertz. 

There is a redeeming instance that often makes partial spectra of quadrupolar nuclei observable. For odd-half integral nuclei, such as Na (/= ), the j fo = - j transi-... 

The predominant H-bond in proteins is the bridge between the backbone amide proton of one amino acid and the backbone carbonyl oxygen atom of a second amino acid (see insert to Fig. 9.3). Although scalar couplings across H-bonds to the magnetic isotope 170 are conceivable, the fast relaxation of this quadrupolar nucleus would prevent such observations in... 

Vanadium-51 is a spin 7/2 nucleus, and consequently it has a quadrupole moment and is frequently referred to as a quadrupolar nucleus. The nuclear quadrupole moment is moderate in size, having a value of -0.052 x 10 2S m2. Vanadium-51 is about 40% as sensitive as protons toward NMR observation, and therefore spectra are generally easily obtained. The NMR spectroscopy of vanadium is influenced strongly by the quadrupolar properties, which derive from charge separation within the nucleus. The quadrupole moment interacts with its environment by means of electric field gradients within the electron cloud surrounding the nucleus. The electric field gradients arise from a nonspherical distribution of electron density about the... 

We saw in Chapter 7 that the resonance frequency of a quadrupolar nucleus is dependent on the orientation of the molecule in which it resides. Molecular tumbling now causes fluctuating electric fields, which induce transitions among the nuclear quadrupole energy levels. The resulting nuclear relaxation is observed in the NMR just as though the relaxation had occurred by a magnetic mechanism. 

While eq N) is the largest principal component of the electric field gradient (EFG) tensor at the site of the quadrupolar nucleus. All the information about the structure of the molecule and interactions is in the eq N) term. In the gas-phase (at low symmetry positions) the EFG is entirely of intermolecular origin. In the condensed phases (liquids and solids), intermolecular interactions may have a substantial influence on the observed EFG. 

A further factor to be taken into account when assessing the relative ease with which a quadrupolar nucleus may be observed is the width of the line which is determined for a given electric field gradient by the quadrupole moment (also listed in Table 1.2). Often only the central transition is observed, for which the linewidth is determined by its second-order quadrupolar broadening and is proportional to [Q (I(I +... 

Equation (16) is very important because it imcovers two central ideas. First, the second-order frequency splitting depends inversely on the Larmor frequency, thus the importance of this term diminishes with increasing external magnetic field strength. Second, the k=0 term has no orientation dependence (Do 0 = 1) or, in other words, it is an isotropic term. This means that the isotropic shift observed in the NMR spectriun of a quadrupolar nucleus has, in addition to the usual isotropic chemical shift, a contribution from the quadrupole cou-phng, which is given by... 

Recent progress in Ge NMR spectroscopy has been briefly reviewed. The crucial point of heteronuclear NMR spectroscopy is the extent to which it can achieve that which may be carried out by and C NMR spectroscopy. As described in this review, Ge NMR spectroscopy can employ almost all techniques used in H and C NMR spectroscopy. There exists, however, a severe limitation in Ge NMR spectroscopy. Ge is a quadrupolar nucleus and a high symmetry of electric field gradient around germanium is required for proper observation of signals. Acoustic ringing is another problem associated with Ge NMR spectroscopy. 

A further scheme designated specially for observation of satellites in "X spectra relies on the specific shortening of transverse relaxation times T2 for X nuclei adjacent to a quadrupolar nucleus by scalar relaxation of the... 

In addition to mechanistic studies, Co is the transition metal that has found most widespread use in thermal measurements. Since it is a quadrupolar nucleus, a symmetric chemical environment around the central cobalt nucleus is necessary to observe narrow lineshapes for precise temperature measurements. Co... 

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