Rational Design and Structure–Activity Relationship Analysis of Decalin-Based High-Energy-Density Fuel Molecules

Decalin, accessible through the hydrogenation of coal-tar-derived naphthalene, is a saturated bicyclic hydrocarbon consisting of two fused cyclohexane rings, and provides a promising platform for high-energy-density fuel (HEDF) design. A systematic library of alkyl- and cycloalkyl-substituted trans-decalin derivatives was constructed to investigate structure–property relationships involving density, net heat of combustion (NHOC), specific impulse, viscosity, flash point, and thermal/oxidative stability. Minimum C–C and C–H bond dissociation enthalpies were used to comparatively assess thermal and oxidative stability by reflecting initial C–C homolysis and H abstraction tendencies, respectively. Density and volumetric NHOC were governed mainly by molecular compactness and packing efficiency, whereas gravimetric NHOC and specific impulse depended primarily on the H/C ratio and ring strain. The flash point was mainly associated with molecular mass, while viscosity was influenced by molecular mass and molecular architecture. Under the engineering-oriented constraints of viscosity ≤ 16 mPa·s and flash points ≥ 360 K, multi-objective screening identified the spiro four-membered-ring motif, particularly α-S-Cycle4, as providing the best overall property balance. In contrast, the fused three-membered-ring motif showed higher specific impulse but greater susceptibility to initial bond activation, suggesting its potential use as a blending component. This work provides quantitative structure-based guidelines for the rational design of HEDFs.