Quadrupolar moments

Carbon-13 NMR was utilized to study different aspects of the reactivity of the metal complexes as a function of certain structural features in the selected oxocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) as depicted in Scheme 1 and illustrated in Figs. 1-4. The NMR spectral properties were similar to those obtained from 13C NMR in general, i.e., very sharp lines indicative of fairly long relaxation times in the order of a few seconds. The large quadrupolar moment ofTc-99 (7 = 9/2, 100% abundance) led to a very broad bound 13C signal (Fig. 5), thus excluding the quantitative study of the cyanide exchange by 13C NMR. However, 16N NMR was successfully used instead. 

The linear relationships between the traceless moments 0 and the spherical harmonic moments lmp are obtained by use of the definitions of the functions clmp. For example, for the quadrupolar moment element xx, we obtain the equality (3x2 — l)/2 = a (x2 — y2)/2 + b(3z2 — 1). Solution for a and b for this and corresponding equations for the other moments leads to... 

Proton and fluorine are the most frequently studied nuclei in the solid state NMR of polymers. However, useful information can be obtained from deuterium resonance, (2H I = 1). In the presence of nuclei with spin I > 1/2, which possess a quadrupolar moment, the most dominant interaction occurs between the quadrupolar moment and an electric field gradient tensor V, generated by the C—D bonding electrons. Consequently, the only dominant interaction, besides Hz (Eq. (1)), will be described by the quadrupolar Hamiltonian, which for a single spin has the form ... 

Unfortunately however, the cations most often found in natural and synthetic zeolites are not ideal for NMR studies. The 23Na nucleus has a large quadrupolar moment, while cations of most spin j nuclei (such as Y, Rh, Ag, Cd, Pb, Hg, Os, and Pt) are of relatively little interest to the zeolite chemist. For these reasons much attention has been paid to 7Li, which has a small quadrupolar moment, and to 20ST1, an attractive spin 5 nucleus. On the other hand, the 23Na resonance can be used as a sensitive probe for the electric field gradient in zeolites and the effects brought about by sorbed molecules. 

Solid State 170 NMR Spectral Analysis. The 170 nucleus has a 5/2 spin and so has a quadrupolar moment. This means that static and MAS 170 NMR spectrum contains information about quadrupolar interactions and the chemical shift. In order to obtain the quadrupolar coupling constant and chemical shift separately, we have carried out spectral analysis with theoretical calculation taking into account quadrupolar interactions and chemical shift interactions. 

Although 127I is an NMR active nucleus with 100% abundance and comparative sensitivity to chlorine and bromine, its large quadrupolar moment make for very broad signals and low intensity spectra it is therefore rarely used.1... 

S has a moderate quadrupolar moment, eQ, arising from its non-spherical charge distribution. The interaction between the nuclear quadrupole moment and the electric field gradient, which is generated at the nucleus itself by the surrounding electronic distribution, is modulated by molecular reorientational motion and provides the only effective relaxation mechanism in liquid. The resulting relaxation times are on the order of milliseconds. 

A nucleus with a nuclear spin I> may possess a charge distribution that has a nuclear quadrupolar moment, Q. Such a nucleus may interact with a nonhomogeneous electric field possessing an electric field gradient, which can be expressed in terms of the second derivative of the electrostatic potential, V x... 

Flydrogen atoms inserted in TbFe6Co5Ti crystal lattice orient quadrupolar moment of the electronic 4f-subshell in an electric field created by a neighbouring ions and electrons along c-axis, that caused to orient of magnetic moment of Tb ion along basal plane. It is well known [4,10], that for the rare-earth ions with Stevens coefficient aj > 0 (Sm, Er, Yb) the magnetic moment is parallel, while for... 

The last member of the quadrupolar Group V nuclei, ° Bi, represents another potentially useful nucleus although, surprisingly, the literature seems to be devoid of any chemical applications of its resonance. ° Bi is the only stable bismuth isotope. It has a spin off, a magnetic moment of 4 039 jUn, and is only six times less sensitive than the proton. Its quadrupolar moment of —0-38 barn definitely confines applications to symmetric moieties. 

The second Group Vb transition metal nucleus, Nb, has also been fairly extensively studied. Nb has spin / = (100%), a magnetic moment of 6144/In, and a quadrupolar moment of —0-22 barn. 

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. 

Some of the interested nuclei considered in connection with zeolite science, like 27A1 or possess a quadrupolar moment Q (besides the magnetic moment), that means nuclei with a spin quantum number I larger or equal to one. The quadrupolar moment is a result of the non-spherical distribution of nuclear charge (13). Nuclei with a quadrupolar moment interact with the non-spherical, symmetrical electric field gradient. This quadrupolar... 

However, zero-field experiments suffer from low sensitivity and can only be applied to materials with reasonably long relaxation times T] to allow for the time it makes to shutde the sample in and out of the field. This field cycling technique measures the evolution of the magnetization at zero-field with the sensitivity of high-field NMR. Zero-field NMR is particularly suited to the investigation of nuclei with quadrupolar moments, since it avoids orientation-dependent broadening. 

Although at first glance 2D correlation spectroscopy of C with quadrupolar nuclei seems unfeasible due to relaxation problems, there are two metal nuclei where, owing to the relatively small quadrupolar moment, such a correlation has been performed. Gunther and co-workers were the first... 

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

Be is a spin = 2 nucleus with a relatively small quadrupolar moment (5.3 X 10 ° m ), good receptivity and 100% natural abundance. Despite its suitability for NMR studies, reeent solid-state Be NMR studies are relatively rare, probably due to the extreme toxicity of the element and its compounds, but possibly also because the narrow chemical shift range of Be detracts from its utility as a characterisation tool. Most of the solid state Be NMR studies reported to date have been of minerals which are comparatively non-toxie. Be chemical shifts are commonly reported relative to solid BeO. 

To probe the potential influence of cation size differences on the distribution, Na[ Cs SEDOR experiments have also been conducted on mixed sodium cesium borate glasses containing 30 mol% alkali [57]. Being a nucleus (1=7/2) with a moderately small nuclear electric quadrupolar moment, the Cs nucleus features similar spectroscopic characteristics as Li. Usually sizeable first-order quadrupolar splittings reduce the inversion efficiency of the n-... 

The quadrupolar moments of nuclei with I > 1/2 may interact with electric field gradients in their environments. Depending on the symmetry of the nuclear environment, these interactions can lead to considerable line broadening due to both first- and second-order terms (for a detailed discussion see ref. 17). While both first-and second-order terms each contain angular (relative to the direction of B0) terms only the former contains 3 cos2 0 - 1, like the dipolar interaction term. 

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