Table salt and other alkali metal chloride oligomers: structure, stability, and bonding

We have investigated table salt and other alkali metal chloride monomers, ClM, and (distorted) cubic tetramers, (ClM)(4), with M = Li, Na, K, and Rb, using density functional theory (DFT) at the BP86/TZ2P level. Our objectives are to determine how the structure and thermochemistry (e.g., Cl-M bond lengths and strengths, oligomerization energies, etc.) of alkali metal chlorides depend on the metal atom and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The analyses confirm the high polarity of the Cl-M bond (dipole moment, VDD, and Hirshfeld atomic charges). They also reveal that bond overlap derived stabilization (approximately -26, -20, and -8 kcal/mol), although clearly larger than in the corresponding F-M bonds, contributes relatively little to the (trend in) bond strengths (-105, -90, and -94 kcal/mol) along M = Li, Na, and K. Thus, the Cl-M bonding mechanism resembles more closely that of the even more ionic F-M bond than that of the more covalent C-M or H-M bonds. Tetramerization causes the Cl-M bond to expand, and it reduces its polarity.