Lowest unoccupied molecular orbital organic molecules

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. 

In using the concept of molecular orbital theory to discuss the absorption of light by organic molecules, we concentrate on two molecular orbitals in particular. The highest occupied molecular orbital (HOMO) is the ground-state molecular orbital of highest energy with electrons in it and the lowest unoccupied molecular orbital (LUMO) is the... 

The concept of molecular orbitals (MOs) helps to explain the electron structure of ion-radicals. When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. HOMO is a bonding orbital. If one electron is introduced externally, it takes the lowest unoccupied molecular orbital (LUMO), and the molecule becomes an anion-radical. LUMO is an antibonding orbital. Depending on the HOMO or LUMO involved in the redox reaction, organic donors appear as n, a, or n species, whereas organic acceptors can be tt or a species. Sometimes, a combination of these functions takes place. 

For any candidate molecule, one must consider the relative location of the highest occupied molecular orbital (HOMO) of D and the lowest unoccupied molecular orbital (LUMO) of the A molecule. The "parent" neutral molecule and the "daughter" radical anion radical A- or cation radical D+ must be thermodynamically and kinetically stable, as well as energetically accessible. Since most organic synthesis is done in solutions, the species should not oxidize or reduce the solvent. In addition, the second ionization state (D2+ or A-) must also be stable, since an additional electron or a hole must also fleetingly reside on D+ or A-. There are, at present, four methods useful for stabilizing these systems ... 

In Chapter 4 we talked about the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of organic molecules. CH3 (like all radicals) has an orbital containing one electron, which we call a Singly Occupied Molecular Orbital (SOMO). 

The calculation of the exact band structure from first principles, however, is rather complex and requires considerable simplifications. The usual and very successful method to calculate the band structure of organic charge transfer salts is a tight-binding method, called extended Hiickel approximation. In this approximation, one starts from the molecular orbitals (MO) which are approximated by linear combinations of the constituent atomic orbitals. Each MO can be occupied by two electrons with antiparallel spins. These valence electrons are assumed to be spread over the whole molecule. Usually, only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are relevant and are, therefore, considered in most band-structure calculations [41]. 

As emphasized by Fukui, the mechanism of chemical reactions can often be understood in terms of frontier orbitals—the highest occupied molecular orbitals (HOMO S) and lowest unoccupied molecular orbitals (LUMO s) of reacting molecules. Ideally, the frontier orbitals of the reactants interact to form the MO s of the products. And it is in such transformations that orbital symmetry is conserved. We will consider two relevant examples from organic chemistry electrocyclic reactions and cycloadditions. 

Linear relationships between quantum chemical indexes (maximum negative (q ax) and positive (q x) charges) as well as energies of highest occupied molecular orbital (Ehomo = IP) where IP is the vertical ionization potential) and lowest unoccupied molecular orbital (Elumo) find the empirical donor-acceptor parameters of organic molecules were established in [49]. Parameters of the relationships are presented in Table 1. 

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