Electronic Reactivity Index (ERI): A Heuristic Composite Descriptor for Screening Green Corrosion Inhibitors for Marine Steel

Most quantum-chemical screens of organic corrosion inhibitors rank molecules from either EHOMO or the HOMO–LUMO gap, ΔE. That split is convenient, but chemically incomplete. Adsorption on steel requires both an accessible occupied frontier orbital and enough electronic softness to accommodate interfacial charge redistribution. To capture this coupled requirement, we define the Electronic Reactivity Index, ERI = EHOMO / ΔE, as a heuristic first-pass descriptor for green inhibitor candidates for marine steel. In CDFT terms, ERI scales with (1/2) EHOMO × S, where S is global softness. With the negative orbital-energy convention used here, more negative ERI values mark molecules that combine accessible occupied orbitals with narrower gaps. Twenty-two organic molecules from five structural classes were evaluated using PM7 with implicit aqueous solvation, epsilon = 78.4. Quercetin gave the strongest ERI response (-1.165), followed by luteolin and caffeic acid. The highest-ranked region was dominated by conjugated flavonoids and phenolics, consistent with extended pi-delocalisation and oxygen-centred donor sites. Literature-reported inhibition behaviour followed the same broad chemical trend, although it does not provide statistical validation. SwissADME profiling of five selected high-ranked candidates showed no Lipinski rule-of-five violations. ERI is best viewed as a low-cost electronic filter before chloride-media electrochemical testing.