Partial positive charges on the surface of antimony compounds interact with partial negative charges. For example, Sb(C 6H 5) 3 has a higher LUMO energy (−0.55 eV) and weaker FIA (59 kcal/mol) than Sb(C 6F 5) 3 (−1.76 eV and 89 kcal/mol). Lowering the LUMO (σ*(Sb–X)) energy increases the Lewis acidity. According to Gabbaï et al., NBO analysis of the Sb(C 6F 5) 3P(O)Ph 3 adduct indicates a donor-acceptor interaction between lp(O) and σ*(Sb–C 6F 5). Since Lewis adducts are formed by dative bond between Lewis bases and Lewis acids, the orbital overlap between the Lewis base and σ*(Sb–X) orbital is the source of the acidity. The FIA of two popular strong Lewis acids, BF 3 and B(C 6F 5) 3, are 81 and 106 kcal/mol (340 and 440 kJ/mol) respectively. FIA is defined as the amount of energy released upon binding a fluoride ion in the gas phase. Both contributions are studied by calculations, and the acidities of theses compounds are quantified by the Gutmann–Beckett method, Hammett acidity function, p K a, and fluoride ion affinity (FIA). These two contributions to the Lewis acidity have been evaluated. Σ*(Sb–X), where X describes substituents on antimony, contributes to the Lewis acidity of antimony compounds in two ways: donor–acceptor orbital interaction and electrostatic interaction. HCP metals are not as ductile as FCC metals.NBO of Sb(C 6F 5) 3-P(O)Ph 3 adduct describing donor–acceptor interaction between lp(O) and σ*(Sb-C 6F 5). Metals containing HCP structures include beryllium, magnesium, zinc, cadmium, cobalt, thallium, and zirconium. However, unlike the fcc, it is not a Bravais lattice as there are two nonequivalent sets of lattice points. Hexagonal close packed (hcp) is one of the two simple types of atomic packing with the highest density, the other being the face centered cubic (fcc). The middle layer contains three atoms nestled between the atoms of the top and bottom layers, hence, the name close-packed. The top and bottom layers contain six atoms at the corners of a hexagon and one atom at the center of each hexagon. In a hexagonal close-packed (HCP) arrangement of atoms, the unit cell consists of three layers of atoms. These metals possess low strength and high ductility. Metals containing FCC structures include austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium. This structure, along with its hexagonal relative (hcp), has the most efficient packing (74%). In a face-centered cubic arrangement, a unit cell contains (8 corner atoms × ⅛) + (6 face atoms × ½) = 4 atoms.
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