![]() Sb(C 6F 5) 3 ( 3) has three σ*(Sb–C 6F 5) orbitals and three Lewis acidic sites. 3-coordinate Sb(III) Īlthough stibanes have a lone pair electrons, their antibonding orbitals with electron-withdrawing substituents renders them Lewis acidic. For example, Sb(C 6F 5) 3( o-O 2C 6Cl 4) has a more positively charged site than Sb(C 6F 5) 3 as shown in its electrostatic potential map, corresponding to higher Lewis acidity (the FIA of Sb(C 6F 5) 3( o-O 2C 6Cl 4) and Sb(C 6F 5) 3 are 116 and 89 kcal/mol, respectively). 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. 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).
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