Highest occupied molecular orbital ethylene

Highest occupied molecular orbital for formaldehyde (tapI and ethylene... 

The bonding orbitals and of ethylenes are combined in phase to be the lowest TZ molecular orbitals (tTj) of butadiene (Scheme 18). The out-of-phase combined orbital (jtj) is the highest occupied molecular orbital (HOMO). The in-phase combination of... 

In phenyl substituted ethylene, stilbene CH=CH, the central -system can conjugate with the phenyl -orbitals. The ground state (I) and the lowest excited state (II) MO s in transtilbene are indicated as HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) in Figure 7.6. 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion radicals and cation radicals. For the cyano vinylferrocene anion radical, the strong delocalization of an unpaired electron was observed see Section 1.2.2. This is accompanied by effective cis —> trans conversion (the barrier of rotation around the —C=C— bond is lowered). As for the cation radicals of the vinylferrocene series, a single electron remains in the highest molecular orbital formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the highest occupied molecular orbital is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without participation of the ethylenic fragment. The situation is quite different from the arylethylene radical cations, in which all ir-orbitals overlap. 

In the MO treatment of chemical reactivity, the overlap of the highest occupied molecular orbital (HOMO) of one reaction partner with the lowest unoccupied molecular orbital (LUMO) of the other partner plays an important role. In a very rough, qualitative manner we may represent the HOMO S and LUMO s of ethylene and substituted ethylenes as shown in Fig. 

As a reaction occurs between molecules, electrons are involved as the orbitals on one molecule interact with those on another. In a general way, the electrons in the highest occupied molecular orbital (generally called the HOMO) of a molecule become attracted to or shared with the lowest unoccupied molecular orbital (identified as the LUMO) on the other. These outer orbitals are often referred to as the frontier orbitals. As is known from other areas of chemistry, the outer orbitals must have matching symmetry for overlap to be effective. For example, in the ethylene molecule, the two combinations of the p orbitals not used in <7 bonding can be shown as... 

The Linear Combination of Atomic Orbitals (LCAO) approximation is fundamental to many of our current models of chemistry. Both the vast majority of the calculational programs that we use, be they ab initioy density functional, semiempirical molecular orbital, or even some sophisticated force-fields, and our qualitative understanding of chemistry are based on the concept that the orbitals of a given molecule can be built from the orbitals of the constituent atoms. We feel comfortable with the Ji-HOMO (Highest Occupied Molecular Orbital) of ethylene depicted as a combination of two carbon p-orbitals, as shown in Fig. 2.1, although this is not a very accurate description of the electron density of this Molecular Orbital (MO). The use of the Jt-Atomic Orbitals (AOs), however, makes it easier to understand both the characteristics of the MO itself and the transformations that it can undergo during reactions. 

Simple molecular orbital theory allows some general conclusions to be made. If four separate ethylenic double bonds approach each other symmetrically with p orbitals perpendicular to the plane new orbitals and energy levels are formed. Both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are illustrated schematically in Figure 1. Two important features emerge. 

Highest Occupied Molecular Orbital Julolidinyl-ethylene-1,2-dithiolate... 

To analyze whether these reactions are allowed or forbidden, chemists focus on the frontier molecular orbitals of the reactants. The frontier molecular orbitals consist of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). The terms highest and lowest refer to the energy of the orbitals, and the terms occupied and unoccupied refer to whether the orbital is populated with two electrons or is empty. For example, the HOMO and LUMO of butadiene would be orbitals 2 and 3 in Figure 20.2, respectively. The HOMO and LUMO of ethylene are simply the tt and tt orbitals given in Figure 1.21. 

The highest occupied molecular orbital (HOMO) of ethylene, a bonding n orbital, can be approximated reasonably well by a symmetric combination of carbon Ip orbitals centered on carbon atoms 1 and 2 and oriented so that their z-axes are parallel ... 

Notice that the transition of lowest energy in 1,3-butadiene, v 2 Ws > is a /r — r transition and that it has a lower energy than the corresponding transition in ethylene, yri — y/2. This result is general. As we increase the number of p orbitals making up the conjugated system, the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) has... 

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