Implications for Advanced HED Fuel Design: Electronic and Spectroscopic Signatures of Methyl–Substituted Adamantanes

A systematic structure–property relationship is established for adamantane (Ada) and its bridgehead methylated derivatives using an integrated analysis of density functional theory (DFT)–calculated ionization potentials (IP), valence orbital energies, and 13 C NMR spectra. The first IP decreases monotonically from 9.94 eV (Ada) to 9.77 eV (teMAda), reflecting increased electron density with substitution. Consistent electronic trends are observed in the valence orbitals, while the 13 C NMR spectra provide experimentally accessible fingerprints, exhibiting increasing chemical shift dispersion and symmetry breaking with methylation. These correlated descriptors demonstrate that progressive substitution induces coherent electronic and spectroscopic responses across the diamondoid framework. Notably, secondary carbons show the strongest NMR sensitivity, while bridgehead sites govern the primary electronic perturbation. This enables direct linkage between molecular structure, electronic stability, and observable spectroscopic signatures. The results identify dMAda and tMAda as optimal candidates, exhibiting electronic characteristics consistent with reduced crystallinity and enhanced liquid-phase stability. The present study provides a physically interpretable descriptor-based framework for tuning diamondoid molecules, offering practical design guidance for high-energy–density (HED) fuel components relevant to sustainable aviation fuels (SAF).