title: Covalent versus ionic bonding in alkalimetal fluoride oligomers.
abstract: The most polar bond in chemistry is that between a fluorine and an alkalimetal atom. Inspired by our recent finding that other polar bonds (C--M and H--M) have important covalent contributions (i.e., stabilization due to bond overlap), we herein address the question if covalency is also essential in the F--M bond. Thus, we have theoretically studied the alkalimetal fluoride monomers, FM, and (distorted) cubic tetramers, (FM)4, with M=Li, Na, K, and Rb, using density functional theory at the BP86/TZ2P level. Our objective is to determine how the structure and thermochemistry (e.g., F--M bond lengths and strengths, oligomerization energies, etc.) of alkalimetal fluorides depend on the metal atom, and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital theory. The analyses confirm the extreme polarity of the F--M bond (dipole moment, Voronoi deformation density and Hirshfeld atomic charges), and they reveal that bond overlap-derived stabilization (ca. -6, -6, and -2 kcal/mol) contributes only little to the bond strength (-136, -112, and -114 kcal/mol) and the trend therein along Li, Na, and K. According to this and other criteria, the F--M bond is not only strongly polar, but also has a truly ionic bonding mechanism. Interestingly, the polarity is reduced on tetramerization. For the lithium and sodium fluoride tetramers, the F4 tetrahedron is larger than and surrounds the M4 cluster (i.e., F--F>>M--M). But in the potassium and rubidium fluoride tetramers, the F4 tetrahedron is smaller than and inside the M4 cluster (i.e., F--F<M--M).
authors.affiliation: Afdeling Theoretische Chemie, Scheikundig Laboratorium der Vrije Universiteit, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands. FM.Bickelhaupt@few.vu.nl
authors.forename: F M
authors.initials: FM
authors.lastname: Bickelhaupt
authors.forename: M
authors.initials: M
authors.lastname: Solà
authors.forename: C Fonseca
authors.initials: CF
authors.lastname: Guerra