Structure and stability of the X3- systems (X = fluoride, chloride, bromide, iodide) and their interaction with cations

Abstract

The molecular and electronic structure of the trihalide anions as well as their relative stability toward dissociation into a halogen molecule and a halide ion has been studied by full ab initio and pseudopotential methods including the electronic correlation at the MP2 and CIPSI level. In all cases, the H/sub 3//sup /minus// anions present a D/sub infinityh/ symmetry in their ground state and are stable against dissociation to X/sup /minus// and X/sub 2/ when the electronic correlation is included. Their relative stability with the exception of F/sub 3//sup /minus// increases along the group in agreement with the experimental trends. The electronic structure of these systems can be explained by the formation of a three-center bond. Such a model gives a good explanation of the variation of the dissociation energy of X/sub 3//sup /minus// and other properties of these systems. The interaction of the X/sub 3//sup /minus// anions with a cation is studied for KBr/sub 3/. The results show that this ionic interaction is directional, resulting in a stable structure with the cation in a side-on position where the X/sub 3//sup /minus// anion has lost the D/sub infinityh/ symmetry and one of the Br-Br bonds is longer than the other bymore » 0.5 /Angstrom/. Both the position and the asymmetry of the anion are in good agreement with various experimental crystal structures.« less