Lowest unoccupied molecular orbital. See

Abbreviations ave, average cot, cyclooctatetraene Cp, tT -cyclopentadienyl DME, 1,2-dimethoxyetbane DMF, A, iV-dimethylformamide dppe, Ph2PCH2CH2PPIi2 ESR, electron spin resonance Fc, ferrocenyl HOMO, highest occupied molecular orbital LUMO, lowest unoccupied molecular orbital see, saturated calomel electrode TCNQ, 7,7,8,8-tetracyano-p-quinodimethane THF, tetrahydrofiiran. 

LUMO Lowest Unoccupied Molecular Orbital (see HOMO) ... 

Chemical Properties. The chemistry of ketenes is dominated by the strongly electrophilic j/)-hybridi2ed carbon atom and alow energy lowest unoccupied molecular orbital (LUMO). Therefore, ketenes are especially prone to nucleophilic attack at Cl and to [2 + 2] cycloadditions. Less frequent reactions are the so-called ketene iasertion, a special case of addition to substances with strongly polarized or polarizable single bonds (37), and the addition of electrophiles at C2. For a review of addition reactions of ketenes see Reference 8. 

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. 

LUMO, see Lowest unoccupied molecular orbital Lyase, 1041-1042 Lycopene, structure of, 483 Lysergic acid diethylamide, structure of, 963... 

It is important to note here that both of the 5-exo radical cyclizations (133—>132—>131, Scheme 27) must proceed in a cis fashion the transition state leading to a strained mms-fused bicy-clo[3.3.0]octane does not permit efficient overlap between the singly occupied molecular orbital (SOMO) of the radical and the lowest unoccupied molecular orbital (LUMO) of the alkene. The relative orientation of the two side chains in the monocyclic radical precursor 134 is thus very significant because it dictates the relationship between the two outer rings (i. e. syn or anti) in the tricyclic product. The cis-anti-cis ring fusion stereochemistry of hirsutene would arise naturally from a cyclization precursor with trans-disposed side chain appendages (see 134). 

In absorption and fluorescence spectroscopy, two important types of orbitals are considered the Highest Occupied Molecular Orbitals (HOMO) and the Lowest Unoccupied Molecular Orbitals (LUMO). Both of these refer to the ground state of the molecule. For instance, in formaldehyde, the HOMO is the n orbital and the LUMO is the n orbital (see Figure 2.1). 

This type of probe, often called fluorescent photoinduced electron transfer (PET) sensors, has been extensively studied (for reviews, see Refs. 22 and 23). Figure 2.2 illustrates how a cation can control the photoinduced charge transfer in a fluoroiono-phore in which the cation receptor is an electron donor (e.g., amino group) and the fluorophore (e.g., anthracene) plays the role of an acceptor. On excitation of the fluorophore, an electron of the highest occupied molecular orbital (HOMO) is promoted to the lowest unoccupied molecular orbital (LUMO), which enables photoinduced electron transfer from the HOMO of the donor (belonging to the free cation receptor) to that of the fluorophore, causing fluorescence quenching of the latter. On... 

An alternative suggestion is that the carbonyl group rc-antibonding orbital which acts as the lowest unoccupied molecular orbital (LUMO) in the reaction has a greater density on the axial face.79 It is not entirely clear at the present time how important such orbital effects are. Most of the stereoselectivities which have been reported can be reconciled with torsional and steric effects being dominant.80 See Section 3.10 of Part A for further discussion of this issue. 

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