SOMO-LUMO orbital interactions

Figure 1,4 SOMO-1IOMO and SOMO-LUMO orbital interaction diagrams.

SOMO-LUMO orbital interaction with electron-deficient olefins (Mode A)... 

In homolytic substitution reactions, the 2-position of thiophene is the preferred site of attack. This is easily rationalized in terms of frontier orbital theory (B-76MI31401). Because of symmetry, both HOMO and LUMO of thiophene have the same absolute values for the coefficients (as shown in 216). Thus it is immaterial whether the [SOMO (radical)-HOMO (thiophene)] or the [SOMO (radical)-LUMO (thiophene)] interaction determines the site of attack only the 2-position is the point at which the radical would attack. The same conclusion is iso reached by consideration of product development control (74AHC(16)123). Attack at the 2-position would result in a transition state with an allylic radical, which would be stabilized to a greater extent than the one arising from attack at position 3 (Scheme 57). 

S2N2 dimerizes rapidly to S4N4 in the presence of nucleophiles and this process can be understood in terms of the frontier orbital interactions between the singly occupied molecular orbital (SOMO) of an S2N2 radical anion and the LUMO of an S2N2 molecule. The polymerization of crystalline S2N2 is a nondiffusive process that produces a mixture of monoclinic (90%) and orthorhombic (10%) (SN). ... 

LUMO of the O — O bond i.e. the energy of the low lying CTg 0 orbital (Scheme-6). The energy effect associated with the SOMO-LUMO interaction becomes more favorable as the energy gap decreases. So tertiary radicals having a higher SOMO than secondary and primary radicals are more reactive. If the reaction were controlled by an orbital interaction between the SOMO and an occupied MO of the peracid group, i.e. the a0 0 MO, the reverse reactivity should have been observed the primary radical should have been more reactive than the tertiary one. 

Frontier Molecular Orbital (FMO) theory may also be applied to provide qualitative understanding. The frontier orbital of the radical is that bearing the free spin (the SOMO) and during radical addition this will interact with both the jt antibonding orbital (the LUMO) and the n-orbital (the HOMO) of the olefin. Both the SOMO-HOMO and the SOMO-LUMO interactions lead to a net drop in energy i.e. E respectively - Figure 1.4J. The dominant interaction and... 

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