Table 2. Calculated ΔG° between Double and Single PT Oxidized States^a.

oxidized species	ΔG° (kcal/mol)
2: BIP-COOH•+	15.2
3: BIP-COOMe•+	14.9
4: BIP-CONH2•+	7.1
5: BIP-CONEt2•+	5.6
6: BIP-CH2NH2•+	–4.2
7: BIP-CH2NEt2•+	–6.5
9: BIP-PhHimine•+	–2.9
10: BIP-PhMeimine•+	–3.1
11: BIP-PhClimine•+	–2.1

^aThe reported values are the free energy differences between the oxidized state with and without the second proton transfer. The double proton transfer corresponds to proton transfer from the phenol to the proximal imidazole N and proton transfer from the distal imidazole NH to the R group, while the single proton transfer corresponds to only the first proton transfer. Negative values for ΔG° indicate that the double proton transfer is more thermodynamically favorable than the single proton transfer. No values are reported for 1 and 8 because only a single proton transfer is possible. Using the experimental pK_as (vide supra)²⁹⁻³¹ for the benzimidazole and exocyclic amines in acetonitrile we estimated ΔG° = −3.9 kcal/mol for 6 and −6.5 kcal/mol for 7, in good agreement with the calculated values.