Electric field gradient tensors in CuBr2

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Abstract
The NQR spectrum of polycrystalline CuBr2 has been obtained at 20°C. Absorption was detected for 65Cu (28.18±0.01 MHz), 63Cu (30.37±0.01 MHz), 81Br (70.10±0.05 MHz), and 79Br (83.98±0.05 MHz). The halogen resonances are discussed in the light of the theory of Townes and Dailey without neglecting the contribution of ionic charges and antishielding. In this approach the electric field gradient tensor is seen to depend on the ionicity of the Cu−Br bonds and on the value of the antishielding factor of the Br− ions. The relation between the two parameters for the cases, where the experimental frequencies are reproduced, is derived. With this relation the antishielding factor calculated by Sternheimer would indicate an ionicity of 90%, while in the purely ionic limit an antishielding factor of 93.8 must be assumed. The presence of finite amounts of covalency in the bonds is evidenced by the copper resonances. Thus, the observed coupling strength for the copper ions is only about 50% of the value observed for a number of octahedrally coordinated Cu2+ compounds.