Phase orbit

The characteristic feature of valence bond theory is that it pictures a covalent bond between two atoms in terms of an m phase overlap of a half filled orbital of one atom with a half filled orbital of the other illustrated for the case of H2 m Figure 2 3 Two hydrogen atoms each containing an electron m a Is orbital combine so that their orbitals overlap to give a new orbital associated with both of them In phase orbital overlap (con structive interference) increases the probability of finding an electron m the region between the two nuclei where it feels the attractive force of both of them... 

Keywords Chemical orbital theory. Electron delocalization. Frontier orbital. Orbital amplitude, Orbital energy, Orbital interaction. Orbital mixing rule, Orbital phase, Orbital phase continuity, Orbital phase environment. Orbital synunetry, Reactivity, Selectivity... 

Keywords Orbital mixing. Orbital amplitude. Orbital phase. Orbital polarization. Orbital deformation, Regioselectivity, Stereoselectivity, n Facial selectivity... 

In the presence of out-of-phase spiro interaction of the dienes, there was also a large subsidiary in-phase orbital interaction on the naphthalene moiety of 93a (94). The syn addition observed in the case of 93a can be accounted for in terms of the additional intervention of this SOI. 

Figure 2.48. Computed XES spectrum along the (110) direction, together with plots of the xz-projection of some of the corresponding molecular orbitals. The corresponding gas phase orbitals are also shown. From Ref. [96].

The stereochemistry is the result of a balance between the in-phase and out-of-phase orbital overlap between the nucleophile and the substrate LUMO. 

If a cycloaddition produces an overlap of positive-phase orbitals with negative-phase orbitals (destructive overlap), antibonding interactions are generated. Antibonding interactions raise the activation energy, so the reaction is classified as symmetry-forbidden. The thermal [2 + 2] cycloaddition of two ethylenes to give cyclobutane is a symmetry-forbidden reaction. 

The fl2" symmetry of the B 2p orbital matches that of LGO(5) and an in-phase orbital interaction gives rise to an MO that has 7r-bonding character delocalized over all three B—F interactions. 

The new a- bonds formed in a Diels-Alder reaction result from overlap of in-phase orbitals. 

A pericyclic reaction such as the Diels-Alder reaction can be described by a theory called the conservation of orbital symmetry. This simple theory says that pericyclic reactions occur as a result of the overlap of in-phase orbitals. The phase of the orbitals in Figure 8.4 is indicated by their color. Thus, each new a bond formed in a Diels-Alder reaction must be created by the overlap of orbitals of the same color. Because two new a bonds are formed, we need to have four orbitals in the correct place and with the correct color (symmetry). The figure shows that, regardless of which pair of HOMO and LUMO we choose, the overlapping orbitals have the same color. In other words, a Diels-Alder reaction occurs with relative ease. The Diels-Alder reaction and other cycloaddition reactions are discussed in greater detail in Section 29.4. 

The conservation of orbital symmetry theory states that in-phase orbitals overlap during the course of a pericyclic reaction. The conservation of orbital symmetry theory was based on the frontier orbital theory put forth by Kenichi Fukui in 1954. Although Fukui s theory was more than 10 years old, it had been overlooked because of its mathematical complexity and Fukui s failure to apply it to stereoselective reactions. 

Notice that a symmetry-allowed pathway is one in which in-phase orbitals overlap a symmetry-forbidden pathway is one in which out-of-phase orbitals would overlap. A synunetry-aUowed reaction can take place under relatively mild conditions. If a reaction is symmetry-forbidden, it caimot take place by a concerted pathway. If a symmetry-forbidden reaction takes place at all, it must do so by a nonconcerted mechanism. 

A symmetry-allowed pathway requires in-phase orbital overlap. 

Frontier orbital analysis of a [4 -I- 2] cycloaddition reaction shows that overlap of in-phase orbitals to form the two new a bonds requires suprafacial orbital overlap (Figure 29.5). This is tme whether we use the LUMO of the dienophile (a system with one TT bond Figure 29.1) and the HOMO of the diene (a system with two conjugated rr bonds Figure 29.2) or the HOMO of the dienophile and the LUMO of the diene. Now we can understand why Diels-Alder reactions occur with relative ease (Section 8.8). 

The two lobes of up orbital have opposite phases. When two in-phase atomic orbitals interact, a covalent bond is formed two out-of-phase orbitals interact to create a node. The conservation of orbital symmetry theory states that in-... 

Normal phase chromatography The chromatographic separation in which the stationary phase is more polar than the mobile phase Orbital An electron cloud associated with an atom/molecule within which there is approximately a 95% probability of finding the associated electrons... 

---