Orbitals molecular, singly occupied

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 ...

Diradial. A molecule which is characterized by two singly-occupied molecular orbitals. It may either be a Singlet or a Triplet. 

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. 

It is well known that Hund s rule is applicable to atoms, but hardly so to the exchange coupling between two singly occupied molecular orbitals (SOMOs) of a diradical with small overlap integrals. Several MO-based approaches were then developed. Diradicals were featured by a pair of non-bonding molecular orbitals (NBMOs), which are occupied by two electrons [65-67]. Within the framework of Hiickel MO approximation, the relationship between the number of NBMOs,... 

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... 

EDT-TTF Ethylenedithio-tetrathiafulvalene SOMO Singly occupied 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... 

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