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Covalent versus ionic bonding in alkalimetal fluoride oligomers
Author(s) -
Bickelhaupt F. M.,
Solà M.,
Guerra C. Fonseca
Publication year - 2006
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.20547
Subject(s) - chemistry , ionic bonding , crystallography , covalent bond , bond length , density functional theory , computational chemistry , atom (system on chip) , chemical polarity , lithium (medication) , molecular geometry , dipole , crystal structure , molecule , ion , organic chemistry , medicine , computer science , embedded system , endocrinology
The most polar bond in chemistry is that between a fluorine and an alkalimetal atom. Inspired by our recent finding that other polar bonds (CM and HM) have important covalent contributions (i.e., stabilization due to bond overlap), we herein address the question if covalency is also essential in the FM 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., FM 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 FM 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 FM 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., FF ≫ MM). But in the potassium and rubidium fluoride tetramers, the F 4 tetrahedron is smaller than and inside the M 4 cluster (i.e., FF < MM). © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 238–250, 2007