A study of dipole-broadened NMR powder lineshapes in the presence of second-order quadrupole effects with applications to polycrystalline borates

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Computer simulations of NMR powder patterns for the m = 1 2 ⇋ m = − 1 2 transition broadened in second order by a quadrupole interaction having a nonzero electric field gradient asymmetry parameter are discussed. The dependence of measurable features of the derivative spectrum on the theoretical parameters is considered. It is found that the frequency dependence of the derivative linewidth is insensitive to relatively large changes in the asymmetry parameter. Thus, methods for determining the quadrupole coupling parameters from the frequency dependence of the NMR linewidth are subject to large uncertainties. In the special case when the dipole broadening is small relative to the quadrupole broadening, it is shown that reference to a library of standard, computer-simulated spectra allows precise determination of the coupling constant and asymmetry parameter without the need for computation of the specific experimental spectrum. This is illustrated with examples of 11B NMR spectra from several polycrystalline borates.