On the alleged importance of the molecular electron-donating ability and the HOMO–LUMO gap in corrosion inhibition studies

Highlights: • Data from where the "ΔN < 3.6" rule originates were more accurately recalculated. • ΔN is practically always considerably smaller than 3.6. • There is no correlation between inhibition efficiency and ΔN. • The HOMO–LUMO gap alone is not sufficient to anticipate inhibition efficiency. Many studies utilize the hypothesis that inhibition efficiency increases with increasing electron-donating ability (ΔN) if ΔN < 3.6. This co-called "ΔN < 3.6" rule is critically reevaluated and it is demonstrated to have no validity, firstly, because ΔN is practically always considerably smaller than 3.6, and secondly, because more accurate recalculation of data from where the "ΔN < 3.6" rule originated reveals that there is no correlation between inhibition efficiencies and ΔN. As for the importance of HOMO–LUMO gap, it is shown that for the considered set of 22 heterocyclic organic corrosion inhibitors for iron in acidic solutions, the inhibition efficiency shows a weak volcano-type dependence on the HOMO–LUMO gap, but the correlation is too weak to be quantitative, hence a HOMO–LUMO gap alone is not sufficient to anticipate inhibition efficiency. This weak volcano dependence is at variance with the assumption, often adopted in the literature, that a smaller HOMO–LUMO gap indicates a better inhibitor.