Phosphate radicals in DNA

3 Ionization potentials of base, deoxyribose and phosphate portions of DNA 

The vertical electron affinity (EA) of neutral radicals can be approximated by the Koopmans ionization potentials (IP) of the anionic states in the geometry of the neutral radical species as shown below  

This representation shows that the HOMO energy of the anion in the geometry of the neutral radical gives the vertical EA whereas the LUMO energy of the cation in the geometry of the neutral radical gives the vertical IP. This method must be employed since the Koopmans theorem applies only to closed shell systems. 

Calculations [145] were performed at the ROHF/6-31G and ROHF/D95v//6-31G levels on several model radicals and scaled to the known experimental electron affinities. This scaling was subsequently used in estimating the electron affinities of the DNA base radical intermediates. The 

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