Comprehensive structural insights into nitro-substituted azines as potential antioxidant additives for biodiesel

Abstract

Fossil fuels remain the primary global energy source, but their finite nature and environmental impact drive the search for renewable alternatives. Biodiesel is a promising candidate, though its oxidative instability limits widespread adoption. This work provides a comprehensive structural and computational analysis of two nitro-substituted azine derivatives to evaluate their potential as biodiesel additives. Single-crystal X-ray diffraction and Hirshfeld surface analysis revealed supramolecular stabilization through C–H⋯O, C–H⋯N, and C–H⋯π interactions, highlighting distinct packing motifs associated with nitro substitution. Topological and electronic descriptors showed that additional nitro and methyl groups reduced reactivity in the gas phase, while in the solid state the molecular energy gap (HOMO–LUMO) remained comparable. Also, a non-centric azine molecular structure exhibited an exceptionally high second-order nonlinear optical response, more than 30-fold higher than centric azine. Machine learning models were employed to predict the oxidation rate constants in the presence of ˙OH radicals and to predict the optical activity parameters. The results indicated a better absorption and emission response for azine with asymmetric electronic distribution and high dipole moment. Predictions of the oxidation rate in the presence of ˙OH radicals indicate superior antioxidant performance for the azine with the fewest nitro groups, with reaction rates comparable to those observed in diesel and the main components of biodiesel. These findings demonstrate that crystal packing, molecular symmetry, and substitution patterns govern both solid-state properties and antioxidant performance, underscoring the value of molecular-based approaches in designing next-generation biodiesel stabilizers.