Covalent versus ionic bonding in alkalimetal fluoride oligomers

✍ Scribed by F. M. Bickelhaupt; M. Solà; C. Fonseca Guerra

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 276 KB
• Volume: 28
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.20547

✦ Synopsis

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 F~4~ tetrahedron is larger than and surrounds the M~4~ cluster (i.e., FF ≫ MM). But in the potassium and rubidium fluoride tetramers, the F~4~ tetrahedron is smaller than and inside the M~4~ cluster (i.e., FF < MM). © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 238–250, 2007