Bestimmung des kleinsten Ionenabstandes in ionischen Schmelzen mittels Kernresonanz

Abstract

At constant magnetic field the resonance frequencies of nuclei in ionic salts increase linearly with temperature in both the crystalline and molten states. The linear temperature-dependence of the chemical shift is ascribed to the excitation of higher paramagnetic electronic states which accompanies the increasing overlap of cation and anion wave functions; both are proportional to the linear increase in the mean squared vibrational amplitude of ions with temperature. The magnitude of the break in the chemical shift at the melting point, coupled with a knowledge of the change in the coordination number and the variation in the wave function overlap with internuclear separation leads to an estimate of the distance of closest approach of neighboring ions in the molten state. For an assumed value of F=0.9, where F= (ZL/Zs)/(ΔEʟ/ΔES), the distance of closest cation-anion approach decreases by 0.18 Å as crystalline TlCl melts to form the liquid salt. The overlap of Tl+ and Cl- ions in crystalline TlCl at room temperature is 0.012 on the basis of the NMR frequency of 205Τl+ and is estimated to be 0.017 from machine calculations based on analytic wave functions for the ground states of the ions.