LUMO lowest energy unoccupied

The most important molecular- orbitals are the so-called frontier molecular- orbitals. These are the highest (energy) occupied molecular- orbital (HOMO), and lowest (energy) unoccupied molecular- orbital (LUMO). The following picture shows the LUMO surface for the hydrogen molecule, H2. The LUMO consists of two separate surfaces, a red... 

Figure 16.1 The chemical hardness of an atom, molecule, or ion is defined to be half. The value of the energy gap between the bonding orbitals (HOMO—highest orbitals occupied by electrons), and the anti-bonding orbitals (LUMO—lowest orbitals unoccupied by electrons). The zero level is the vacumn level, so I is the ionization energy, and A is the electron affinity, (a) For hard molecules the gap is large (b) it is small for soft molecules. The solid circles represent valence electrons. Adapted from Atkins (1991).

The lowest energy unoccupied orbital (LUMO - 27e) is anti-bonding with respect to the Rh-Rh bonds. As a consequence, the addition of electrons to [Rh4(CO)i2] would cause destruction of the molecular frame (see Chapter 8, Section 2.2). 

The highest-energy occupied molecular orbital (HOMO) and the lowest-energy unoccupied molecular orbital (LUMO) of a particular molecular entity. The HOMO can be completely or partially filled whereas the LUMO can be completely or partially vacant. The frontier orbital method allows one to interpret reaction behavior by studying HOMO and LUMO overlaps between reacting entities. 

Excitation in general consists in promotion of an electron from one of the highest-energy occupied molecular orbitals (HOMOs) to achieve one of the lowest-energy unoccupied molecular orbitals (LUMOs) of the ground-state molecule. This may be expressed, for example, by the terms (n, 7i ), (n, a ), (n, tt ), or (n, a ), whereas the multiplicity is shown as a prefix, eg 1(n, if ). Population of the antibonding excited states thus usually results in bond weakening. 

The orbitals must also have a similar energy. For the greatest interaction, the two orbitals should have the same energy. Only the highest-energy occupied orbitals (or HOMOs) of the nucleophile are likely to be similar in energy to only the lowest-energy unoccupied orbitals (or LUMOs) of the electrophile. 

LUMO stands for lowest energy unoccupied mbital... 

FIGURE 9.15 In this reaction, the HOMO-LUMO interaction is between a filled nonbonding orbital on chloride and the lowest energy unoccupied orbital of allyl, in this case, 4>2-... 

Calculated band structures show a series of allowed and forbidden bands. The highest energy occupied allowed band is called the valence band (VB), and the lowest energy unoccupied one the conduction band (CB). Between these bands lies a forbidden band, the width of which defines the energy gap of the solid. Note that in a molecular solid, a one to one correspondence can be made between the VB and the CB on the one hand, and the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of the isolated molecule on the other hand. 

Lowest Energy Unoccupied Molecular Orbital (LUMO)... 

It is now possible to "see" the spatial nature of molecular orbitals (10). This information has always been available in the voluminous output from quantum mechanics programs, but it can be discerned much more rapidly when presented in visual form. Chemical reactivity is often governed by the nature of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Spectroscopic phenomena usually depend on the HOMO and higher energy unoccupied states, all of which can be displayed and examined in detail. 

Another aspect of qualitative application of MO theory is the analysis of interactions of the orbitals in reacting molecules. As molecules approach one another and reaction proceeds, there is a mutual perturbation of the orbitals. This process continues until the reaction is complete and the new product (or intermediate in a multistep reaction) is formed. PMO theory incorporates the concept of frontier orbital control. This concept proposes that the most important interactions will be between a particular pair of orbitals. These orbitals are the highest filled oihital of one reactant (the HOMO, highest occupied molecular oihital) and the lowest unfilled (LUMO, lowest unoccupied molecular oihital) orbital of the other reactant. The basis for concentrating attention on these two orbitals is that they will be the closest in energy of the interacting orbitals. A basic postulate of PMO... 

LUMO (Section 10.13) The orbital of lowest energy that contains none of a molecule s electrons the lowest unoccupied molecular orbital. 

Alkenes absorb ultraviolet (UV) light and use the absorbed energy to excite an electron from the HOMO (highest-occupied molecular orbital) to the LUMO (lowest-unoccupied molecular orbital). 

In more detail, the interaction energy between donor and acceptor is determined by the ionisation potential of the donor and the electron affinity of the acceptor. The interaction energy increases with lowering of the former and raising of the latter. In the Mulliken picture (Scheme 2) it refers to a raising of the HOMO (highest occupied molecular orbital) and lowering of the LUMO (lowest unoccupied molecular orbital). Alternatively to this picture donor-acceptor formation can be viewed in a Born-Haber cycle, within two different steps (Scheme 3). 

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