Quadrupolar line shapes

As mentioned above, the alkali metal chlorides have cubic symmetry about the chlorine nucleus which requires that the EFG to be essentially zero in a perfect crystal. This results in CT NMR spectra with narrow lines that are free of quad-rupolar effects. As the environment aroimd the chlorine nucleus is transformed to lower symmetry, second-order quadrupolar effects begin to be observed, leading to broadened CT lines with quadrupolar line shapes. The broadening of the signals in chlorine CT NMR spectra as the quadrupolar effects become more significant is the most serious limitation to the types of materials which can be studied with typical SSNMR methods. 

An alternative interpretation of the previous solid-state NMR studies of the backbone motion of collagen has been reported. Based on the analysis of the solid-state NMR chemical shift anisotropy and quadrupolar line shapes for five different isotope labelled collagens, it has been shown that motional averaging of the NMR interactions occurs primarily via small-angle librations about internal bond directions. 

The interactions of synthetic chalcocite surfaces with diethyldithiophos-phate, potassium salt, K[S2P-(OC2H5)2], were studied by P CP MAS and static Cu NMR spectroscopy. The second-order quadrupolar line shape of Cu was simulated, and the NMR interaction parameters, Cq and e(Q), for the copper(I) diethyldithiophosphate cluster were obtained. ... 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

The nucleus is quadrupolar (spin 7/2, natural abundance 99.76%), and thus, the spectra can be affected by both the first- and second-order quadru-pole interaction, though the second-order broadening is generally not the largest source of line broadening in these materials. In general, three major anisotropic interactions influence the line shapes seen in the NMR spectra of solid samples (i) the qua-... 

Hq is the quadmpolar interaction. It is only present for nuclei with I>Vz, which are around 70% of active NMR nuclei. This interaction is also orientahon-dependent however, only the first-order quadrupolar interaction can be averaged out by spinning the sample around the magic angle (MAS)-the second-order quadrupolar interaction cannot be leveled out by this technique, resulting in very broad line shapes or sometimes in invisible sites for SS NMR [109]. In the last... 

Other effects frequently encountered in inorganic systems that can severely affect line shape involve relaxation processes arising from interactions of nuclear quadrupole moments with electric field gradients. For quad-rupolar nuclei (I 1), the quadrupolar contribution to the spin-lattice relaxation time Ti is given approximately by... 

Symmetries of local electrical environments of quadrupolar nuclei (/ 1) profoundly influence relaxation times and resonance line shapes of such nuclei (9, 116). Consider a nucleus for which I = % (Br79, Bn). In the absence of quadrupolar perturbation, the nuclear spin levels are evenly spaced, as shown in I below, and the three possible nuclear resonance transitions have equal energies (Am = 1). If, however, eqQ 0... 

In Fig. 5b, which was obtained at 30°C, the powder pattern displays a severely distorted, intermediate rate line shape. This line shape is characteristic of both fast methyl group rotation and 2 fold molecular re-orientation about the carbonyl bond at a rate comparable to the reciprocal of the quadrupolar coupling constant ( 105 Hz). At room temperature, therefore, the acetone-d6 molecules in the microporous channels of sepiolite are able to undergo restricted re-orientations. 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

Figure 4(B) shows the simulated 17O MAS spectra as a function of i]q from 0 to 1. These calculations were carried out using the same conditions of the 170 stationary NMR spectra. In the similar manner as in the case of the stationary NMR spectra, the MAS spectra exhibit characteristic line shapes from which the information on t]q as well as Cq can be extracted. The anisotropy of CS tensors is removed by MAS and only <5iSO will be obtained if it exists. When the sample spinning frequency is not high enough, the effect of spinning sidebands spaced at the spinning frequency around the central peak needs to be considered in the spectral simulation. The frequency contribution from the second-order quadrupolar interaction under slow/intermediate MAS conditions is given in the literature 46,47 A complicated line shape is expected to appear in the MAS NMR spectrum so that a computer simulation is not trivial (see Figure 15).

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