Lineshape quadrupole static

As a consequence of the small quadrupole moment of Li, the quadrupolar interaction in solid state NMR spectra is much smaller for Li than for Li. This has been used to advantage for the determination of the Li chemical shift anisotropy from the Li static solid state powder spectrum of 2,4,6-tris(isopropyl)phenyllithium (see below) . Applying MAS up to 10 kHz, the CSA contributions to the lineshape can be completely ehminated in most Li spectra of organolithium compounds. If the measurement of the quadrupolar... 

Some typical CT-only spectra simulated for static samples considering solely the quadrupole interaction are shown in Figure lA, for sites with different values of the asymmetry parameter r/q. The widths of these lineshapes depend on the rjq value and are proportional to the parameter A, defined for a nucleus with 1=5/2 as ° ... 

Under anisotropic conditions, NMR lineshapes for a quadrupolar nucleus are dominated by chemical shielding and (first and second order) quadrupolar interactions. Dipolar interaction is usually a minor contribution only. First-order quadrupole interaction lifts the degeneracy of the allowed 21 (i.e. seven in the case of V / = V2) Zeeman transitions as shown in Figure 3.7, giving rise to seven equidistant lines, viz. a central line (mj = + V2 -V2. unaffected by quadrupole interaction) and six satellite lines. The overall breadth of the spectrum is determined by the size of the nuclear quadrupole coupling constant Cq the deviations from axial symmetry and hence the shape of the spectral envelope are governed by the asymmetry parameter. Static solid-state NMR thus provides additional parameters, in particular the quadrupole coupling constant, which correlates with the electronic situation in a vanadium compound. [ 1 The central component reflects the anisotropy of the chemical shift. 

Fig. 25. Random walk simulations for static 2H NMR powder lineshapes arising from a quadrupole echo 90°x-t-90°v-t-FID pulse sequence for the model of an isotropic 3° jump.36 (a) Jump correlation time, tj = 411 gs correlation time for the motion, xc = 100 ms, echo delays x as given in the figure. Dotted line is the spectrum for an isotropic random jump with xj = xc = 100 ms and an echo delay x — 200 gs. (b) Jump correlation times xj and motional correlation times xc as given in the figure, echo delay x = 100 gs.

One-dimensional quadrupole echo NMR lineshape analysis of powder samples is particularly informative when fast, discrete jumps occur between sites of well-defined geometry as, for example, in a phenyl group undergoing two-site exchange. In this case, the characteristic Pake-pattern is transformed into an axially asymmetric lineshape with an apparent asymmetry parameter r] 9 0 (see Equation (6.2.3)) [1-8]. The asymmetric lineshapes, shown on the left in Fig. 6.2.2, can be derived by considering how the individual components of the principal EFG tensor become averaged by the discrete jumps. Within the molecular frame, and in units of as defined by Equation (6.2.2), the static axially symmetric tensor consists of the components = 1, = — 1/2, and V y = — 112. This traceless tensor satisfies the... 

These could consist of a whole range of Mo(VI) and Mo(IV) mixed oxygen/ sulfur compounds. There is no evidence for the presence of MoOi, except in one case. Simulation of the static powder lineshapes allowed the deconvolution of the various components, thus yielding values of quadrupole parameters as well as relative intensity data. From the spikelet experiments it was ascertained that all species are present as both static adsorbed and dynamically active phases. The Mo spectrum of the used catalyst shows the presence of the tetrahedral molybdenum-oxo species, along with a much reduced MoSt resonance (relative to the fresh system), perhaps suggesting that MoSt is the active site in the... 

The use of NMR spectroscopy for probing molecules adsorbed on alumina dispersions is briefly cited. Soderlind and Blum [66] used deuterium NMR to probe the dynamics and structure of a surfactant (an alkylbenzenesulfonate, SHBS) adsorbed onto alumina. The phenyl ring was deuterium-labeled, and from the analysis of the lineshapes of H spectra, valuable information on the degree of motional freedom in the adsorbed layer was obtained. As deuterium possesses a quadrupole moment, it is sensitive to the orientation of the molecule with respect to the external magnetic field. This sensitivity generates a quadrupolar splitting, which is defined as the separation between the two allowed transitions for the deuterium nucleus. In liquid systems, the quadrupole interaction is averaged out, but for static systems powder... 

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