Bond Energies and Thermochemical Properties of Ring-Opened Diradicals and Carbenes of exo-Tricyclo[5.2.1.0^2,6]decane

Abstract

Exo-tricyclo[5.2.1.0(2,6)]decane (TCD) or exo-tetrahydrodicyclopentadiene is an interesting strained ring compound and the single-component high-energy density hydrocarbon fuel known as JP-10. Important initial reactions of TCD at high temperatures could cleave a strained carbon-carbon (C-C) bond in the ring system creating diradicals also constrained by the remaining ring system. This study determines the thermochemical properties of these diradicals (TCD-H2 mJ-nJ where m and n correspond to the cleaved carbons sites) including the carbon-carbon bond dissociation energy (C-C BDE) corresponding to the cleaved TCD site. Thermochemical properties including enthalpies (ΔH°f298), entropies (S(T)), heat capacities (Cp(T)), and C-H and C-C BDEs for the parent (TCD-H2 m-n), radical (TCD-H2 mJ-n and m-nJ), diradical (TCD-H2 mJ-nJ), and carbene (TCD-H2 mJJ-n and m-nJJ) species are determined. Structures, vibrational frequencies, moments of inertia, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) level of theory. Standard enthalpies of formation in the gas phase for the TCD-H2 m-n parent and radical species are determined using the B3LYP density functional theory and the higher level G3MP2B3 and CBS-QB3 composite methods. For singlet and triplet TCD diradicals and carbenes, M06-2X, ωB97X-D, and CCSD(T) methods are included in the analysis to determine ΔH°f298 values. The C-C BDEs are further calculated using CASMP2(2,2)/aug-cc-pvtz//CASSCF(2,2)/cc-pvtz and with the CASMP2 energies extrapolated to the complete basis set limit. The bond energies calculated with these methods are shown to be comparable to the other calculation methods. Isodesmic work reactions are used for enthalpy analysis of these compounds for effective cancelation of systematic errors arising from ring strain. C-C BDEs range from 77.4 to 84.6 kcal mol(-1) for TCD diradical singlet species. C-H BDEs for the parent TCD-H2 m-n carbon sites range from 93 to 101 kcal mol(-1) with a similar range seen for loss of the second hydrogen to generate the diradical singlet species. A wider range for C-C BDEs is seen for the carbenes from about 77 to 100 kcal mol(-1) as compared to the diradicals. Results from the DFT methods for the parents, radicals, diradicals, and carbenes are in good agreement with results from the composite methods using our sets of work reactions.

High-Pressure Thermal Decomposition of Tetrahydrotricyclopentadiene (THTCPD) and Binary High-Density Hydrocarbon Fuels of JP-10/THTCPD in a Tubular Flowing Reactor

“…It can be explained by the molecular structure that the greater the number of rings in the compound structures, the greater the repulsion between the atoms and the ring tension; meanwhile, they will be more unstable and more prone to cracking under high-temperature conditions. Another reasonable explanation has been reported from some studies that the C–C bond dissociation energy of JP-10 (322.17–418.4 kJ/mol) is higher than that of THTCPD (17.42–200.53 kJ/mol), which further confirms our results. In addition, the initial conversion of the JP-10/THTCPD mixtures were almost the same, because of it being the only reaction of THTCPD self-dissociation.…”

“…There is no significant difference in the decomposition rate of JP-10/THTCPD and that of THTCPD at the initial decomposition stage (before 600 °C), because the only reaction is THTCPD self-dissociation. At the initial stage of the binary fuel pyrolysis, THTCPD is first cracked through the breaking of the C−C bond, because the C−C bond dissociation energy of JP-10 (322.17−418.4 kJ/mol) 44 is higher than that of THTCPD (17.42−200.53 kJ/mol). 24 However, as the reaction continues, the free radicals generated by THTCPD are more likely to react with JP-10 via H-abstraction reaction, because of the high molar ratio of JP-10 (59.77% in 50% JP-10/50% THTCPD) in the binary fuels, which can significantly promote the JP-10 pyrolysis.…”

A Theoretical Study of Pyrolysis of exo-Tetrahydrodicyclopentadiene and Its Primary and Secondary Unimolecular Decomposition Products

“…The single-component hydrocarbon fuel Jet Propellant-10 (JP-10) is used in detonation engines, missiles, and supersonic combustion ramjets, where a fuel is required to have high thermal stability, high-energy density, and low freezing point. Because of its broad applications, the principal constituent of JP-10, tricyclo­[5.2.1.0 , ]­decane ( exo -tetrahydrodicyclopentadiene; exo -TCD; C 10 H 16 ; Figure ), has attracted significant attention of researchers, and numerous experimental, theoretical, and modeling studies of its pyrolysis and combustion have been reported in the literature. − A detailed overview on the JP-10 studies was provided in our recent publication, and here we only briefly reiterate its key points.…”

Extensive Theoretical Study of the Thermochemical Properties of Unsaturated Hydrocarbons and Allylic and Super-Allylic Radicals: The Development and Optimization of Group Additivity Values

“…Ritter and Bozzelli developed THERM (THermo Estimation for Radicals and Molecules) based on Benson’s GA method, which can be used as illustrated in Figure . Bozzelli and co-workers − have also studied the thermochemistry of different classes of species including sulfurated, nitridated, fluorinated, oxygenated, and saturated hydrocarbons, Table . Enthalpies of formation, entropies, and heat capacities have been compiled for certain classes of species using density functional theory (DFT) and composite compound methods, from which bond dissociation energies (BDEs) of similar type of bonds and GAVs were systematically reported and compared.…”