SOMO singly occupied molecular potential

Radicals can be either reduced (to anions or organometallics) or oxidized to cations by formal single electron transfer (Scheme 11).50 Such redox reactions can be conducted either chemically or electro-chemically51 and the rates of electron transfer are usually analyzed by the Marcus theory and related treatments.50 These rates depend (in part) on the difference in reduction potential between the radical and the reductant (or oxidant). Thus a species such as an a-amino radical with high-lying singly occupied molecular orbital (SOMO) is more readily oxidized, while a species such as the malonyl radical with a low-lying SOMO is more readily reduced. The inherent difference in reduction potential of substituted radicals is an important control element in several kinds of reactions. 

Figure 20-7. B3LYP/6-31++G" calculated adiabatic and vertical potential energy surfaces (PESs) of C5 -05/ bond dissociation of 5 -dTMPH radical anion. Energies and distances are given in kcal/mol and angstroms (A), respectively. The singly occupied molecular orbital (SOMO) is also shown. (Reprinted with permission from ref. [209], J. Phys. Chem. (2007) American Chemical Society.)...

The behavior of coupling constants as a function of x is qualitatively very similar for the different radicals a(C) is always positive and increases with x due to the progressive contribution of carbon s orbitals to the singly occupied molecular orbital (SOMO). The effect is similar for a(F) and a(H), but since a(H) is negative for the planar conformation (due to first order spin polarization) the absolute value of a(H) decreases up to x 10° and next increases. This allows to discuss vibrational averaging effects simply in terms of the potential governing the out of plane motion. 

In this calculation, the potential energy surface (PES) for Cs -Os bond dissociation of 5 -dTMPH in vertical and adiabatic states was calculated using both compact (6-31G ) and diffuse (6-31++G ) basis sets. At each step on the PES, the singly occupied molecular orbital (SOMO)... 

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