Nuclear quadrupole coupling constants eQVzz/h have been measured for 14N (196 kHz) in NH4Al(SO4)2 · 12H2O and 87Rb (12.9±0.1 MHz) in RbAl(SO4)2 · 12H2O. If we assume regular tetrahedral symmetry for the NH4 ion, such that the intraionic charges produce, aside from the effect of antishielding, zero effective electric field gradient at the N nucleus, then the ratio of the coupling constants implies an antishielding factor of about −4.
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Bailey et al. (1974) studied this question.
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