Figure 6.

Aqueous base (B₁) ionization energies and orbital diagrams for 5′-dGMP⁻, 5′-dAMP⁻, 5′-dCMP⁻, 5′-dTMP⁻, 2′-deoxythymidine and uridine. The small difference between the ionization energies of 5′-dCMP⁻ and 5′-dTMP⁻ makes the ordering uncertain; however, in the gas phase, the B₁ ionization potential of the model compound 1-MeC is less than that of 1-MeT. Results are also given for 2-deoxyribose and H₂PO₄⁻. All ionization energies were obtained from Eq. 1. For H₂PO₄⁻, the gas-phase P1 ionization potential was calculated at the MP2/6–31+G* level (ref. 31), and ΔΔG_HYD was evaluated by using H₂PO₄⁻ and H₂PO₄^• charge distributions from 3–21G SCF calculations. For 5′-dGMP⁻ and 5′-dAMP⁻, uncorrected B₁ gas-phase IPs were obtained from 3–21G SCF calculations, and values of ΔΔG_HYD were the same as those used in earlier investigations of nucleotide electron donating properties (see refs. 13 and 17). For 2′-deoxythymidine, uridine, and 2-deoxyribose, aqueous ionization energies were obtained with the same method used to obtain 5′-dCMP⁻ and 5′-dTMP⁻ ionization energies. 3-OH-THF was used as a model compound to correct the gas-phase IP of 2-deoxyribose. 1,9-Dimethylguanine, 9-methyladenine, and 1-methyluracil with experimental adiabatic IPs of 7.71, 7.97, and 8.75 eV, respectively, were used as model compounds to correct the gas-phase B₁ ionization potentials of 5′-dGMP⁻, 5′-dAMP⁻, and uridine, respectively (see refs. 11, 13, 17, and 30).