The higher anti-human immunodeficiency virus activity of a symmetrical 2,2′-disubstitued derivative of diphenyl disulfide (DPDS) has been explained by the lower energy of the lowest unoccupied molecular orbital (LUMO), resulted from a better hydrogen bond stabilization of the σ*SS bond orbital (BO). This conclusion entails the participation of σ*SS BO in constructing the LUMO. The higher content of σ*SS BO, compared to π*CC BOs of phenyl groups, in LUMO of DPDS has been found through analysis of the LUMO of DPDS expanded in the BO space. The high content of σ*SS BO (%σ*SS) in the LUMO of DPDS has laid the foundation for the formation of σ-type radical anion intermediate in the stepwise reductive cleavage of disulfide bond in the symmetrical 4,4′-disubstitued DPDS derivatives. For the nine 4,4′-disubstituted DPDS-derivatives under reductive cleavage studies, the increasing %σ*SS in the LUMOs is parallel to the increasing value of inner reorganization energy.
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