Lewis bases molecular orbital stabilization

The molecular orbital picture of benzene proposes that the six jt electrons are no longer associated with particular bonds, but are effectively delocalized over the whole molecule, spread out via orbitals that span all six carbons. This picture allows us to appreciate the enhanced stability of an aromatic ring, and also, in due course, to understand the reactivity of aromatic systems. There is an alternative approach based on Lewis structures that is also of particular value in helping us to understand chemical behaviour. Because this method is simple and easy to apply, it is an approach we shall use frequently. This approach is based on what we term resonance structures. 

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. 

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