HIGH TEMPERATURE NMR STUDIES OF BROMINE AND IODINE CHEMICAL SHIFTS IN ALKALI BROMIDES AND IODIDES AND IN THALLIUM BROMIDE

The Br-81 and 1-127 chemical shifts of the alkali halides and of TlBr were studied as a function of temperature. The room temperature results are in close agreement with previous workers, and more information is deduced. A consistent explanation can be obtained by assuming Kondo-Yamashita's (K-Y) overlap model with due consideration of next-nearest-neighbor (NNN) interactions between halide ions. The possibility of a slight covalent effect in the heavy metal end of the alkali halides is discussed. The effect of covalency is manifested by TlBr, whose measurements can be successfully explained by the covalent model of Yosida-Moriya (Y-M). A correct way of averaging the effect of thermal vibronic motion on chemical shift in the K-Y overlap model is formulated, and a general formula is derived for the temperature variation of chemical shift. For the alkali halides, the thermal dilation of the lattice together with the thermal-vibration-induced enhancement of overlap account for the downfield shift with increasing temperature. The results are analyzed quantitatively, and a physically meaningful factor F has been calculated to indicate the selective degree of importance of NN interactions. As for TlBr, similar explanations for the observed results are deduced in the light of the Y-M covalent model. NNN is important to account for the difference in sign of the chemical shifts at the transition temperature from solid to molten states of TlBr as observed. © 1969.