High level ab initio calculations on ClFn− (n = 1–6): Recoupled pair bonding involving a closed-shell central ion

Abstract In this paper we complete our studies of the bonding in the ClF n +,0,− series, characterizing the structures and energetics of the ground and low-lying excited states of ClF n − ( n  = 1–6) via coupled cluster calculations with large correlation consistent basis sets and interpreting the results of the calculations using concepts from generalized valence bond (GVB) theory. The chlorine anion is isoelectronic with the argon atom, possessing three 3p lone pairs. However, the electrons in Cl − are much less tightly bound than those of Ar, and it is thus possible for the fluorine atom to form recoupled pair bonds and recoupled pair bond dyads with the chlorine anion even if this is not possible with the argon atom. The calculated dissociation energies, D 0 (F n −1 Cl F), for the anions are 28.9, 72.3, 12.5, 59.9, 4.5, and 38.8 kcal/mol for n  = 1–6, and the adiabatic electron affinities, EA 0 (ClF n ), are 2.25, 4.78, 3.29, 5.47, 3.80, and 5.43 eV, respectively [CCSD(T)/RCCSD(T)/AVQ + Z]. Unlike the cation series, the ClF n − anions have competing dissociation channels, ClF n −  → ClF n −1  + F − and ClF n −  → ClF n −1 −  + F, because of the similar electron affinities of the fluorine atom and some of the ClF n −1 species. The ClF 2 − and ClF 4 − anions dissociate to F − and (ClF, ClF 3 ), while the remaining members of the family dissociate to F and ClF n −1 − . However, the electronic structure of the ClF 2 − species at its equilibrium geometry is consistent with the addition of a fluorine atom to ClF − . In addition to the ground states of ClF n − , three previously unknown excited states were found for ClF 2 − , ClF 3 − , and ClF 4 − with excitation energies of Δ E 0  = 2.24, 2.02 and 2.11 eV, respectively. In contrast to the ground states, which possess the maximum number of recoupled pair bond dyads, the excited state species have one fewer dyad and two additional recoupled pair bonds.