Spectral simulation, quadrupolar interactions

Since nuclear spin interactions are anisotropic, the observed NMR frequency depends on the position of the principal axis systems of 170 EFG and CS tensors with respect to the external magnetic field. In the following, 170 NMR frequencies under quadrupolar and/or CS interactions that will be used for spectral simulations are described in detail. It is convenient to separate the cases into static solid-state NMR techniques and MAS experiments. First, the static conditions are explained. 

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).

Correlations between 2H quadrupole interaction parameters and hydrogen bond geometry have also been considered for situations other than 0-H---0 hydrogen bonds. For example, solid state 2H NMR spectra of 2H labelled amino acids, peptides and polypeptides were measured over a wide temperature range [74]. From spectral simulations based on dynamic 2H NMR theory, parameters such as the 2H quadrupolar coupling constant and asymmetry parameter were determined, and relationships between these NMR parameters and the hydrogen... 

Representative quadrupolar nuclei and some quadrupolar interaction data are listed in Tab. 7.3. Simulated spectra for several of these sites are shown in Fig. 7.10 as if the specta were acquired on a 9.4 T (400 MHz) NMR spectrometer with a probe having a fantastically wide spectral width. In practice, the a-alumina spectrum can barely be acquired and the andalusite and nitro spectra are essentially unobservable. Even the H spectrum can be difficult to acquire without distortion from probe ringdown effects. 

To overcome possible errors in the derived parameters when fitting NMR spectra of powdered samples with large quadrupolar interactions, Perras et al. developed and implemented an exact treatment of the quadrupolar and Zeeman interactions for stationary powdered samples [9]. The software, QUEST (QUadrupolar Exact SofTware), is freely available and produces spectral simulations in a fraction of a second on a modem computer [10]. The importance of this exact treatment was demonstrated in a SSNMR study of a series of organic compounds featuring covalent... 

---