Lewis acids molecular orbital stabilization

In addition, Diels-Alder adducts are formed through two types of approaches that lead to endo or exo isomers. The endo isomer is usually favored over the exo isomer, although the exo isomer is generally the thermodynamically preferred product. This is known as the Alder, or endo, rule and can be attributed to the additional stability gained by secondary molecular orbital overlap during the cycloaddition.22 26 27 Again, the use of a Lewis acid catalyst can alter the endo/exo ratio and has even been shown to give the thermodynamic exo adduct as the major product.28... 

The nitrogen molecule is strongly adsorbed on Lewis acid sites by interaction between the 5o electron pair and the vacant molecular orbital of the Lewis site [137]. The stabilization of the N—N bond in addition to the stabilizahon by adsorption on the acid sites is responsible for the large heat of N2 adsorption. The acid sites on SW/S are of Brpnsted type [138], and the acidity on H-Mor is also regarded as Brpnsted type, though a small number of Lewis sites are generated by treatment at high temperatures [139]. This is consistent with the formation of isobutane from pentane. Therefore it is concluded that the acidity of sulfated zirconia is of Lewis type. 

Why is such a trend observed Actually, the reason that hard acids and bases prefer to interact with each other is different than the reason that soft acids and bases prefer to interact. To see this, let s examine some mathematics that is meant to model the interaction between Lewis acids and bases in an early stage of their interaction. The analysis derives from pertur-bational molecular orbital theory (PMOT), which was briefly introduced in Chapter 1, and is explored in more depth in Chapter 14. In essence, three forces are considered to mediate the energy of interaction (Ej) between the acid and base as they approach each other in space (Eq. 5.28). One is the electrostatic repulsion between the electron clouds of the two entities, referred to as Ecore/ a positive destabilizing term. The second and third factors are both attractive and stabilizing. An electrostatic attraction between an acid and base occurs due to opposite charges on the acid and base this is called E s- Lastly, a term called Eoveriap/ which is related to the net overlap of the nucleophilic and electrophilic orbitals, is found to lower the energy of the system as the nucleophilic electrons delocalize into the empty electrophilic orbital. 

Reactions such as these that involve polar molecules are best understood in terms of Highest Occupied Molecular Orbital—Lowest Unoccupied Molecular Orbital (HOMO-LUMO) orbital interactions. As we saw in Section 1.7, p. 41, when a filled occupied orbital overlaps an empty orbital, the two electrons are stabilized in the new, lower energy molecular orbital. The words Lewis bases react with Lewis acids are essentially equivalent to saying, The interaction of a filled and empty orbital is stabilizing. Indeed, this notion is one of the central unifying themes of organic reactivity, as essentially all reactions involving polar molecules can be understood this way. 

Likewise, the number of available surfactants (Lewis bases) are also limited, since not all of them have available orbitals to form molecular orbitals with Lewis acid TM centers [74,76,82]. Typical surfactants used in the LAT method are alkyl phosphates and alkyl amines [72,74,76,82]. The second major drawback is the thermal stability of the formed mesostructure. Due to the strong S-I interaction, conventional solvent extraction methods are not enough to remove surfactant from mesostructured TM oxides to form mesoporous oxide materials. In addition, low metal to surfactant ratios ( 1) and essentially high temperature treatments (>500 °C) to remove hydrophobic alkyl chains, which make these materials thermally unstable. 

---