Stability of MX2−3 ions in the gas phase and when do ionic molecules have large ionization potentials

Abstract

The present work aims at theoretical consideration of the geometrical and electronic structures of a homologous series of free MXmq−-type alkali halides (M=Li, Na, K; X=F, Cl; m=1,2,3; q=0,1,2) in order to get insight into their kinetic and electronic stability. At the ab initio Hartree–Fock self-consistent field (HF-SCF) level of theory, the lowest energy fragmentation channel leading to the decomposition of the dianions MX32− into MX2− and X− has been investigated. The potential energy surface was found to exhibit a broad, but flat energy barrier to fragmentation. These findings have been confirmed using results from configuration interaction calculations and the molecular dianions are predicted to be long-lived species formally existing in a metastable state. The stability of the gas-phase MX32− dianions and of the MX2− fragmentation products with respect to autodetachment of an extra electron has been investigated using ab initio HF-SCF and Green’s function methods. The inclusion of many-body effects by the latter was found to decrease the vertical binding energy of the extra electrons with respect to the Koopmans’ theorem HF-SCF result, but the extra electrons remain bound. At the ab initio level, the compounds studied in this work are predicted to be extremely ionic species. The variation of the properties through a series of MXmq− species has been investigated. Motivated by the overall large ionic character—in particular of the MX32− molecules—the properties of the systems have been studied using a previously derived theoretical approach, referred to as ionic model, based on (classical) electrostatic terms. The versatility of the ionic model scheme has been extended here to obtain information on the tendency of an ionic molecule to accomodate an additional electron.