Quadricyclane and Norbornadiene as High-Energy Aviation Fuels: A DFT Study

High-energy-density (HED) hydrocarbons, such as quadricyclane (QC) and norbornadiene (NBD), are gaining attention for advanced aerospace fuel applications due to their ability to store substantial strain energy. In this study, we employ high-level density functional theory (DFT) calculations to investigate the ground-state electronic structures of QC and NBD, focusing on their ionization potential (IP) spectra and outer valence Dyson orbitals in the context of energy storage. Our theoretical results are supported by recent high-resolution synchrotron-based photoelectron spectroscopy (PES) measurements. The data reveal that key C–C bonds in QC (specifically C(1)–C(2), C(1)–C(7), and the newly formed C(2)–C(6)) exhibit substantial bond strain, with bond lengths around 1.51 Å, compared to the standard 1.54 Å in ethane and unstrained systems. Significant differences in the binding energy spectra between the two isomers underscore their distinct electronic structures. Excess orbital energy spectra (EOES) further reveal pronounced disparities in electron configurations from the core levels to the valence region. Notably, NBD's outer valence orbitals (below 12 eV) exhibit a 1:1:2 pattern indicative of through-space interactions, while QC displays a 1:2:1 configuration characteristic of a restructured σ-framework. These differences highlight that the electronic reorganization accompanying the NBD-to-QC transformation involves fundamental shifts in the orbital character and electron density distribution. The total energy difference between the isomers reflects the strain energy stored in QC, underpinning its role as a high-energy fuel candidate. This study provides crucial insights into the electronic origins of strain energy storage in HED hydrocarbons, offering a foundation for the rational design of next-generation sustainable aviation fuels (SAFs).