NQCC

Other key properties of these complexes relevant for comparison with experiment are the nuclear quadrupole coupling constants. Theoretical NQCC values, as calculated via electric field gradients, are discussed exhaustively in several recent theoretical investigations [26,28,30,33,34] and in experimental work [4-11]. BH HLYP/aug-cc-pVTZ calculated NQCCs [30,34], as evaluated for XY- -NH3 complexes, are listed in Table 10. 

Because the atoms occupy highly symmetric lattice positions, anisotropic parameters such as chemical shift anisotropy (CSA) or nuclear quadrupole coupling constants (NQCC) are either zero or fairly small. 

The EFGs for both 69,7 Ga and 14N have been calculated using the WIEN-97 program, and the results compare more favorably with the experimental results than do those from a simple point charge model including Stemheimer antishielding factors (note that the experimental 14N NQCC value used in the comparison was somewhat larger than the accurate value reported above) [52]. 

When Wqi / Wq2 the magnetization recovery may appear close to singleexponential, but the time constant thereby obtained is misleading [50]. The measurement of 7) of quadrupolar nuclei under MAS conditions presents additional complications that have been discussed by comparison to static results in GaN [50]. The quadrupolar (two phonon Raman) relaxation mechanism is strongly temperature dependent, varying as T1 well below and T2 well above the Debye temperature [ 119]. It is also effective even in cases where the static NQCC is zero, as in an ideal ZB lattice, since displacements from equilibrium positions produce finite EFGs. 

Fig. 7 Identification of cubic ZB and hexagonal WZ polytypes of GaN by chemical shift (indicated) and NQCC differences based upon MAS-NMR of nuclei of both elements, (a) 71Ga MAS-NMR, with vertical expansions (red) showing ST reflecting NQCCs (reprinted from [60]). (b) 14N MAS-NMR at indicated spinning speeds, with measured T1 values (original spectra, data cited in [60])...

The ZB and WZ forms of ZnS could be distinguished on the basis of their isotropic 67Zn and 33S static NMR chemical shifts [63] however, estimates of small CSAs obtained from static 33S NMR of the WZ forms of ZnS and CdS were obtained by ignoring any small second-order quadrupole broadening [65], which may account for discrepancies when NQCCs are assumed to be responsible for the broadenings [65]. 

Earlier sections of this review have already discussed results for quadrupolar nuclei in certain connections for Knight shifts (Sects. 3.4.3 and 3.4.4), for electric-field (Stark) effects upon NQCCs (Sect. 3.1), for measurements of NQCCs in GaN by static NMR and the effects of strain upon NQCCs (Sect. 3.2.1), for obtaining exchange couplings by MAS-NMR (Sect. 3.2.2), and for characterizing polytypes and defects in cubic polytypes by chemical shifts and NQCCs obtained from MAS-NMR (Sect. 3.3.2). This section will give some further examples of information about semiconductors obtained from the NMR of quadrupolar nuclei (see also [18]). 

The study of local or long-range ordering in semiconductor alloys based upon the effects on NQCCs has been carried out in a number of cases. These studies are analogous to the study of ordering in In Ga P by means of the chemical shift interaction, as described in Sect. 3.3.1. 

From a very general outlook, one can argue that the NMR studies of quadrupolar nuclei present in nanoscale semiconductors should offer a more incisive look into the chemical and electronic structure than do studies of spin-1/2 nuclei. The rationale is that quadrupolar nuclei can report on the same chemical, hyperftne, or Knight shifts and dipolar or indirect couplings as observed for spin-1/2 nuclei, but also provide an additional dimension of information in terms of the NQCC and associated EFGs. Although not yet reported, DFT calculations of both chemical shifts and NQCC values for the same nuclei in nano-semiconductors should provide a more stringent comparison of theoretical and experimental results, particularly if the two parameters can be correlated experimentally, as seems feasible. 

The ability of NMR to distinguish poly types will be greatly aided, as it was in the recent case discussed of SiC, by theoretical calculations of chemical shifts and CS A values (and for quadrupolar nuclei, NQCC values). Also, MAS-NMR may succeed in identifying low levels of polytypes or new ones that have not been seen by diffraction methods. 

The calculated nuclear quadrupole coupling constants (NQCCs) of in artemisinin 9a and some of its derivatives and the effects of charge density due to the nature of ligands on the NQCC of were described. All calculations were performed at the HF/3-21G level <2005MI366>. 

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