Deciphering the electronic and NMR fingerprints of JP-10 fuel: a DFT study of exo-chair versus exo-boat isomers†

The rational design of high-energy-density (HED) sustainable aviation fuels (SAFs) often relies on understanding the electronic structures of fuel hydrocarbons. This study uses density functional theory (DFT) to investigate the exo-chair and exo-boat isomers of Jet Propellant 10 (JP-10) fuel. The exo-chair isomer is found to be more stable by 1.83 kcal mol⁻¹, with an energy barrier of 3.65 kcal mol⁻¹ separating the two using the B3LYP/cc-pVTZ method. Despite small geometric differences, significant changes in the shape, dipole moments (0.0156 Debye for the exo-chair and 0.0426 Debye for the exo-boat) and NMR chemical shifts at the flag carbon (C5), the boat tip carbon (C10) and the hydrogens bonding with them. The B3PW91/cc-pVTZ method produces more accurate carbon NMR chemical shifts (RMSD (δ_C) = 1.48 ppm) than the B3LYP/cc-pVTZ method (RMSD (δ_C) = 3.21 ppm), whereas the reverse holds for the proton-NMR chemical shifts, RMSD (δ_H) = 0.33 ppm and 0.31 ppm, in agreement with early studies. The NMR trajectories during the chair and boat transition reveal the most significant changes at the transition state (TS). In addition, the carbon atoms engaging larger strain (e.g. junction carbons and the flag carbon) exhibit apparent deshielding. Excess orbital energy spectrum (EOES) analysis further identifies key inner valence orbital changes during the isomerization, indicating the role of bonding interactions in stabilizing the exo-chair isomer. These findings offer valuable insights into the electronic structural factors that influence the stability of multicyclic hydrocarbons, aiding the future design of more efficient SAFs.