Lowest unoccupied molecular orbital ethylene

Lowest unoccupied molecular orbital (or formaldehyde (tapi and ethylene... 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

If we start with states of tt-symmetry (dashed lines) we find three distinct peaks in the XES spectra reflecting the occupied states. The 1 a2u and lelgTr-like orbitals are essentially intact from the gas phase, while the third state, labeled e2u, is not seen for the free molecule. Based on symmetry-selection rules, it can be shown that this state is derived from the lowest unoccupied molecular orbital (LUMO) e2utt -orbital that becomes slightly occupied upon adsorption. We anticipate a similar bonding mechanism as discussed in the previous section for adsorbed ethylene with the exception of a weaker rehybridization due to the extra stability in the -system from the aromatic character. 

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. 

The orbital in ethylene that receives these electrons is the lowest-energy orbital available, the Lowest Unoccupied Molecular Orbital (LUMO). In ethylene, the LUMO is the tt antibonding orbital. If the electrons in the HOMO of butadiene can flow smoothly into the LUMO of ethylene, a concerted reaction can take place. 

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... 

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. 

Bis(julolidinyl)-ethylene-l, 2-dithiolate Laser Disc Read Only Memory Ligand to Ligand Charge Transfer Ligand to Metal Charge Transfer Lowest Unoccupied Molecular Orbital Maleamide-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. 

According to the frontier orbital theory, the orbitals that control these reactions are the aforementioned HOMO of one reactant and the LUMO (lowest unoccupied molecular orbital) of the other reactant. So, for this reaction, we have two possible scenarios interaction between HOMO ij/2 of butadiene and LUMO (p of ethylene or that between HOMO ti of ethylene and LUMO of butadiene. As the following illustration indicates, both possibilities lead to bonding overlap between the interacting orbitals of the reactants so the reaction is allowed, as we aU know. In addition, as fotmd by theory, between these two possible scenarios, we favor the interaction between the HOMO of the electron-rich reactant (butadiene in this case) with the LUMO of the electron-poor reactant (ethylene). 

The lowest unoccupied molecular orbital (LUMO) of ethylene, an antibonding (m ) orbital, can be represented as a similar linear combination but with opposite sign ... 

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... 

A CH2 group has six valence electrons, so ethylene has 12. When we place 12 electrons into Figure 1.14, we see that the HOMO is the tt orbital. The lowest unoccupied molecular... 

These absorptions are ascribed to n-n transitions, that is, transitions of an electron from the highest occupied n molecular orbital (HOMO) to the lowest unoccupied n molecular orbital (LUMO). One can decide which orbitals are the HOMO and LUMO by filling electrons into the molecular energy level diagram from the bottom up, two electrons to each molecular orbital. The number of electrons is the number of sp carbon atoms contributing to the n system of a neuhal polyalkene, two for each double bond. In ethylene, there is only one occupied MO and one unoccupied MO. The occupied orbital in ethylene is p below the energy level represented by ot, and the unoccupied orbital is p above it. The separation between the only possibilities for the HOMO and LUMO is 2.00p. 

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