Cyclization-Modulated Structure and Oxidative Behavior of Methoxy-Substituted Chalcones: Implications for Diesel-Biodiesel Additive Design.

The oxidative instability of diesel-biodiesel blends remains a critical challenge for fuel quality, motivating the search for new antioxidant candidates. In this study, a novel methoxy-substituted cyclic chalcone 1 was synthesized to investigate how intramolecular cyclization influences supramolecular assembly, electronic properties, and oxidative-response behavior in comparison with a structurally related open-chain analog 2. Single-crystal X-ray diffraction revealed that cyclization induces pronounced conformational rigidity and alters the torsion angles governing intermolecular interactions in the solid state. Density functional theory calculations showed that the cyclic chalcone exhibits a larger HOMO-LUMO gap, greater chemical hardness, and lower electrophilicity, consistent with a more rigid and electronically less perturbed structure. Under oxidative stress conditions, compound 1 was less affected than 2, indicating greater resistance to oxidative transformation. Rancimat experiments showed that the addition of compound 1 to diesel-biodiesel blends resulted in a slight increase in the induction period during storage, consistent with its greater oxidative persistence, although the stabilization effect remained modest under the evaluated conditions. Machine-learning predictions of hydroxyl-radical reaction rate coefficients revealed distinct oxidative-response profiles for the cyclic and open-chain systems, with predicted rate constants exceeding those of representative diesel-biodiesel components and commercial antioxidants such as butylhydroxytoluene, tert-butylhydroquinone, and butylhydroxyanisole. The results demonstrate that cyclization modulates the balance between molecular reactivity and oxidative persistence, providing new insights into structure-property relationships relevant to the rational design of future antioxidant additives for diesel-biodiesel blends.