Nuclear quadrupolar effects

Chemists pay much less attention to the NMR relaxation rates than to the coupling constants and chemical shifts. From the point of view of the NMR spectroscopist, however, the relaxation characteristics are far more basic, and may mean the difference between the observation or not of a signal. For the quadrupolar nucleides such as 14N the relaxation characteristics are dominated by the quadrupole relaxation. This is shown by the absence of any nuclear Overhauser effect for the 14N ammonium ion despite its high symmetry, which ensures that the quadrupole relaxation is minimized. Relaxation properties are governed by motional characteristics normally represented by a correlation time, or several translational, overall rotational and internal rotational, and thus are very different for solids, liquids and solutions. 

The usefulness of quadrupolar effects on the nuclear magnetic resonance c I 7 yi nuclei in the defect solid state arises from the fact that point defects, dislocations, etc., give rise to electric field gradients, which in cubic ciystals produce a large effect on the nuclear resonance line. In noncubic crystals defects of course produce an effect, but it may be masked by the already present quadrupole interaction. Considerable experimental data have been obtained by Reif (96,97) on the NMR of nuclei in doped, cubic, polycrystalline solids. The effect of defect-producing impurities is quite... 

Further studies reported by Johnson, Geanangel and Shore 17> have led to the isolation of KB5H8 as a microcrystalline white powder of limited thermal stability. The temperature dependence of the LiBsHg UB nmr spectrum (Fig. 3) was ascribed to boron nuclear quadrupolar relaxation effects caused by the increased viscosity of the solution at lower temperatures 17>. The spectmm of the potassium salt showed the same temperature dependence as LiBsHs but at temperatures 60 to 70 degrees lower 17>. The magnetic equivalence of the basal boron atoms... 

Except for some quadrupolar effects, all the interactions mentioned are small compared with the Zeeman interaction between the nuclear spin and the applied magnetic field, which was discussed in detail in Chapter 2. Under these circumstances, the interaction may be treated as a perturbation, and the first-order modifications to energy levels then arise only from terms in the Hamiltonian that commute with the Zeeman Hamiltonian. This portion of the interaction Hamiltonian is often called the secular part of the Hamiltonian, and the Hamiltonian is said to be truncated when nonsecular terms are dropped. This secular approximation often simplifies calculations and is an excellent approximation except for large quadrupolar interactions, where second-order terms become important. 

Quadrupolar nuclei constitute most of the magnetic nuclei within the Periodic Table of the elements. However, the lack of suitable instrumentation as well as the misconception of the deleterious nature of these nuclei have impeded a more widespread utilization of their resonances. Quadrupolar relaxation resulting from the interaction of the nuclear quadrupolar moment with finite electric field gradients is the principal source of nuclear relaxation in nearly all compounds. However, albeit generally eonsidered a nuisance, the phenomenon may as well be exploited to the experimenter s advantage. In contrast to spin-j nuclei whose relaxation behaviour is principally dictated by the dynamics of the molecules in solution, structural and electronic effects play the key role in the relaxation process of quadrupolar nuclei. 

An important result of the relaxation studies on Li carried out by Wehrli [9,40] was the finding that an appreciable H, Li nuclear Overhauser effect (NOE) exists which amounts to ti = 2.61 and 1.19 for Li in aqueous LiCl and n-butyllithium in n-hexane, respectively. The theoretical value is Ti=y( H)/ 2y( Li) = 3.40 (Table 1). Due to the inefficient quadrupolar relaxation, the Li nucleus shows considerable dipolar interactions with neighbouring protons which can yield valuable structural information. Similar effects for Li are uncommon because of the stronger dominance of quadrupolar relaxation for this nucleus, but appreciable H, Li NOEs have been found for systems like 7 [37,38]. 

Hq Quadrupolar eq I lO -lO Interaction of nuclear quadrupolar moment with the eleetric field gradient (q), effectively an l interaction... 

Quadrupolar nuclei, those with 1 > 1/2, have a nonspherical distribution of nuclear charge and therefore interact with the electric field gradient in the solid. Since 74% of all nuclei with spin are quadrupolar, their study is of considerable interest. The quadrupole interaction broadens and shifts the NMR signals and may also affect their relative intensities. Since many chemists working in catalysis are unfamiliar with quadrupolar effects, we give a brief summary of the most important considerations to be borne in mind. In practice, two distinct cases must be considered nuclei with noninteger spin (of which Al is perhaps the most important to catalysis) and nuclei with integer spin, such as H. 

The NMR signals of insensitive nuclear spins can be enhanced by transferring polarization from a more sensitive species to which they are coupled. The well-known pulse sequences as the polarization transfer techniques are insensitive nuclei enhanced by polarization transfer (INEPT), distortionless enhancement by polarization transfer (DEPT), and reverse insensitive nuclei enhanced by polarization transfer (RINEPT) The INEPT sequence is an alternative to the nuclear Overhauser effect. The INEPT experiment does not require any particular relaxation mechanism and therefore a better enhancement factor can be obtained. Furthermore it is demonstrated that INEPT sequence can be used to determine the multiplicity of each signal in a NMR spectrum. More recently, the INEPT and DEPT experiments were used for the coherence transfer via heteronuclear J-coupling between spin-1/2 and quadrupolar nuclei in the solids. " Fyfe et showed that coherence transfer via the scalar coupling between spin-1/2 and quadrupolar nuclei can be obtained in the solid state by using INEPT experiment. 

The reason here is that the l N nucleus has a small magnetic moment, which is hard to drive by r.f. fields, the difficulty being compounded in the case of frozen solution samples by line broadening due to the l N nuclear quadrupolar coupling. Echo envelope modulation effects are, on the other hand, quite easy to see for nuclei with small moments. For the simple case of an I = 1/2 nucleus weakly coupled to an S = 1/2 electron, it can be shown that the modulation depth is independent of the nuclear moment and depends only on the ratio between the Zeeman field and the local field at the nucleus due to the electron.15 Breadth of the shfs line is, moreover, not a serious obstacle to detection, provided that at least one modulation cycle can be seen in the echo envelope. 

The effectiveness of these types of measurements is limited since idealized isolated two-level systems are seldom encountered in practice. The pulsed nature of the laser sources which are used for excitation also places a limitation on the frequency width of the sources. The latter more often than not is broad enough to obscure features in the spectra, which are of interest, in which small energy splittings are involved. Examples of this are nuclear quadrupolar and magnetic... 

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