Covalent versus ionic bonding in alkalimetal fluoride oligomers

Area of Science:

• Inorganic Chemistry
• Theoretical Chemistry
• Materials Science

Background:

• Alkali metal fluorides (FM) exhibit highly polar bonds.
• Previous research indicated significant covalent contributions in other polar bonds (C--M, H--M).
• The role of covalency in the F--M bond remained an open question.

Purpose of the Study:

• To investigate the covalent contribution to the F--M bond in alkali metal fluorides.
• To analyze the structural and thermochemical properties of alkali metal fluoride monomers (FM) and tetramers ((FM)4).
• To understand the influence of the metal atom (M=Li, Na, K, Rb) on F--M bond characteristics and oligomerization.

Main Methods:

• Density Functional Theory (DFT) calculations at the BP86/TZ2P level.
• Theoretical study of alkali metal fluoride monomers and tetramers.
• Analysis using Kohn-Sham molecular orbital theory, dipole moments, Voronoi deformation density, and Hirshfeld atomic charges.

Main Results:

• F--M bonds in alkali metal fluorides are confirmed to be extremely polar.
• Covalent stabilization contributes minimally (approx. -2 to -6 kcal/mol) to the total bond strength (approx. -114 to -136 kcal/mol).
• The F--M bond exhibits a predominantly ionic bonding mechanism.
• Polarity decreases upon tetramerization.
• Structural transition observed in tetramers: F4 tetrahedron larger than M4 for Li/Na, smaller for K/Rb.

Conclusions:

• The F--M bond in alkali metal fluorides is fundamentally ionic, with negligible covalent character.
• The metal atom significantly influences the structural and bonding properties of alkali metal fluorides.
• Tetramerization leads to reduced polarity and distinct structural arrangements depending on the alkali metal.