Quadrupolar lineshape

Despite the benefits of high resolution, measurements of wideline spectra of quadrupolar nuclei under static or MAS conditions are still commonly used in a variety of applications. For both integer and half-integer spins, simulations of quadrupolar lineshapes can yield full sets of NMR parameters associated with quadrupolar and chemical shift tensors and can be used for studying molecular dynamics. 

Vosegaard and Massiot [144] showed that it is possible to create a high-resolution 2D spectrum correlating the chemical shifts with the second-order quadrupolar lineshapes from several spectra recorded at different magnetic field strengths using a projection-reconstruction method called chemical shift-quadrupolar projection-reconstruction of one-dimensional spectra (CQ-PRODI). 

It has also been shown that the technique referred to as multiple-quantum variable angle spinning (MQVAS) can be used for the purpose of separating the CSA and quadrupolar lineshapes [155]. In this experiment, a judicious choice of the rotor orientation is fjRL = 70.12°, which leads to cancelation of the fourth-rank term of the second-order interaction (Fig. 2). Thus, the refocusing ratio k for the quadrupolar interaction depends on the ratio of the second-rank terms... 

The ZnC>4 site of ZnO is axially symmetric, which is manifest in the quadrupolar lineshape with 77 = 0. Commercial ZnS samples are often mixtures... 

The Na MAS NMR spectra of a number of simple sodium compounds show narrow resonances resulting from the very symmetrical Na environments with xq values which are virtually zero (Table 7.1). Other compounds with less symmetrical Na environments show well-defined Na second-order quadrupolar lineshapes which can readily be simulated to give the quadrupolar interaction parameters (Table 7.1). The... 

Figure 7.2. A. MAS NMR spectra of NaAlSi30g (albite) at various temperatures. B. Na isotropic chemical shifts of albite derived from the second-order quadrupolar lineshapes and plotted as a function of temperature. The solid line is the shift predicted from the bond length relationship...

Figure 7.3. MAS NMR spectrum of NaZr03 acquired at 11.7 T, with the simulated spectrum composed of three constituent second-order quadrupolar lineshapes combined in the ratio 2 1 1. Note that the relative intensities of the individual components as shown are not those used in the final simulation. From Bastow et at. (1994), by pennission of the copyright owner.

The B mas and static NMR spectra of a series of boron carbides show a broad major resonance at about 1.3 to - 4.6 ppm, the peak position varying almost linearly with carbon content (Figure 7.10A). This resonance has been assigned to boron in the B-rich icosahedral units which are bonded together both directly and via three-atom chains (Kirkpatrick etal. 1991). A small additional shoulder on the major resonance of the static B spectra (Figure 7.10B) which increases in intensity with decreasing C content and can be simulated as a second-order quadrupolar lineshape has been assigned to the boron site in the centre of the various possible C-B-C chains (Kirkpatrick et al. 1991). 

Figure 7.10. A. Change in the B isotropic chemical shift of boron carbides with carbon content. B. Observed and simulated static B NMR spectra of B4C, with the simulated components. Note the small shoulder at about 130 ppm, simulated by a quadrupolar lineshape arising from B in the chain sites. From Kirkpatrick et al. (1991), by permission of the copyright owner.

The Mg MAS spectrum of Mg(OH)2 (brucite) is broad (Figure 8.10B), but shows a quadrupolar lineshape from which 8iso and Xq values of 14.1 ppm and 3.15 MHz were determined (MacKenzie and Meinhold 1993). These values are in reasonable agreement with a single crystal study of Mg(OH)2 in which the linewidth was found to... 

Figure 8.13. MAS NMR spectra of hydrotalcite (Mg6Al2(0H)i6C03.4H20) taken at various stages of its thermal decomposition sequence. Note the loss of the hydrotalcite quadrupolar lineshape and the progressive shift of the resonance line towards the position of MgO.

The 2 polymorphs of CaCOs, aragonite and calcite, can readily be distinguished on the basis of their " Ca MAS spectra (Figure 8.28B). Calcite shows a characteristic second-order quadrupolar lineshape from which the NMR parameters can he extracted by spectral simulation. The narrower Ca MAS resonance from aragonite shows no discernible structure, but the corresponding static aragonite spectrum is about 20 times broader than under MAS conditions. This suggests that CSA is a major contributor to the static linewidth, which is confirmed by satisfactory simulation of the spectrum... 

---