Nuclear Quadrupole Resonance and Bonding in Solid Layer Type Metal Halides

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Abstract
Pure nuclear quadrupole resonance spectra have been reported for halogen nuclei in a number of metal halides crystallizing in layer structures of the CdI2 or related types. Because of the extreme smallness of the quadrupole coupling constants, particularly in the case of the dihalides, the possible appropriateness of a purely ionic model to account for the electric field gradients has been investigated. Detailed calculations of the ion‐lattice field gradient have been made for a number of compounds representing several types of infinite‐layer structure. In no case is the coupling due to the ion‐lattice alone sufficient to explain the observed interactions. In fact, comparison with the experimental data suggests that empirical values of the antishielding factor γ∞ about 3–5 times smaller than the theoretical values are appropriate to account for the ion‐lattice contribution to the coupling. Covalent bonding between the metal and halogen layers involving only small departures from pure p‐bond configurations at the halogens can account for the experimental results. Taking the ion‐lattice gradient into account causes in general a small increase in the values of s hybridization needed in the covalent bond model.