Singly occupied molecular orbital , and

Fig. 9. Orbital pattern of singly occupied molecular orbital and Jahn-Teller active mode. Both SOMOs are antisymmetric with respect to the cr plane. For corannulene (top) the tangential direction along the pseudorotational path at the minimum corresponds to the antisymmetric vibration, while, for coronene (bottom), the symmetric vibration corresponds to the tangential direction at the minimum.

Electronic Structure—Singly Occupied Molecular Orbital and Interactions Through Bonds... 

When an electron is removed from a neutral oligomer, the distortion of molecular geometry (lattice relaxation) results in a raising of the former HOMO level (now denoted SOMO = singly occupied molecular orbital) and lowering of the LUMO level. The level below the former HOMO level (HOMO-1) forms the HOMO of the radical cation and dication. Two new optical transitions are allowed for the radical cation (polaron), from the new HOMO to SOMO and SOMO to LUMO. 

Fig. 12.4. PMO representation ofp-orbital interactions with C=C (a), C=0 (b), and OR (c) substituents. Form of SOMO (singly occupied molecular orbital) is ...

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). 

A radical has a singly occupied molecular orbital (SOMO). This is the frontier orbital. The SOMO interacts with HOMO and the LUMO of closed-shell molecules to stabilize the transition state (Scheme 27). The radical can be a donor toward a monomer with low LUMO or an acceptor toward one with high HOMO. 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

Table 1 Calculation of some molecular-based descriptors for BOA, DIMBOA and MBOA. Physicochemical descriptor like logP (partition coefficient between octanol and water) constitutional descriptors like the number of a specified atoms or bonds (number of carbons, hydrogens, oxygens, nitrogens, single and aromatic bonds, the total number of atoms and bonds) and molecular weight quantum-mechanical descriptors like HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital).

Figure 1 Molecular structures and singly occupied molecular orbitals for radicals 1-7...

Bock et al. studied the cation radicals of various organosilanes having a -system in the / position extensively and found that the charge is delocalized considerably into the silyl groups [9,10]. Kira and Sakurai et al. also carried out an ESR study of cation radicals of allylsilanes and revealed the large polarization of the SOMO (singly occupied molecular orbital) as a consequence of the a-% interaction [11],... 

Thus, the apparent paradox lies in the fact that radical and radical-ion electrocyclic reactions are all forbidden in the Woodward-Hoffinann sense because the symmetry of the singly occupied molecular orbital (SOMO) changes... 

Figure 6 Singly occupied molecular orbital (SOMO) of a propeller-like trimer radical anion of acetonitrile obtained using density functional theory. The structure was immersed in a polarizable dielectric continuum with the properties of liquid acetonitrile. Several surfaces (on the right) and midplane cuts (on the left) are shown. The SOMO has a diffuse halo that envelops the whole cluster within this halo, there is a more compact kernel that has nodes at the cavity center and on the molecules.

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