Expanding on the work of Pelizzetti et al., Yamabe and colleagues demonstrated that the reduction potentials of substituted benzene-diols are correlated with the electron donating or withdrawing character of the substituents (X) on the aryl group. Specifically, they showed that the HOMO of substituted catechols is composed of two types of interactions between the molecular orbitals (MO) of the parent compound (benzene-1,2-diol, P) and substituent (X). The first interaction, known as HOMOP -HOMOX, occurs between HOMO of the parent molecule (HOMOP) and the HOMO of the substituent (HOMOX).