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

Many studies utilize the hypothesis that inhibition efficiency increases with increasing electron-donating ability (Delta N) if Delta N < 3.6. This co-called "Delta N < 3.6" rule is critically reevaluated and it is demonstrated to have no validity, firstly, because Delta N is practically always considerably smaller than 3.6, and secondly, because more accurate recalculation of data from where the "Delta N < 3.6" rule originated reveals that there is no correlation between inhibition efficiencies and Delta N. 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.