The density functional theory evaluation of the heats of formation of some aromatic compounds through the isodesmic approach

Theoretical studies on the structures, material properties, and IR spectra of polymorphs of 3,4-bis(1H-5-tetrazolyl)furoxan

Theoretical studies on the structures, densities, and heats of formation of conformational isomers of 3,4-bis(1H-5-tetrazolyl)furoxan (H₂BTF) were performed based on density functional theory (DFT) calculations. Two stable planar conformational isomers, the face-to-back and the back-to-face conformers, and one stable slightly twisted conformer, the back-to-back conformer, were predicted for H₂BTF at the M06-2X/6-311 + G(d,p) level of theory. The face-to-back conformer was calculated to be the most stable conformational isomer on the potential energy surface. No stable face-to-face conformer, whether planar or tilted, was identified in the calculations. The Vienna Ab Initio Simulation Package (VASP) was used in combination with molecular dynamics simulation to explore the stable crystal forms and the densities of the stable conformational isomers. Two of them exhibited high densities: the face-to-back conformer with P2₁ symmetry (2.01 g/cm³) and the back-to-back conformer with Pna2₁ symmetry (2.05 g/cm³). Their heats of formation were also predicted to be high when calculated at the same DFT level. The detonation pressures and velocities of these polymorphs, as calculated using the EXPLO5 program, are well above those of many advanced high energy density materials, pointing to the potential use of these conformers as novel explosives with good detonation performance. Also, IR spectra are shown to be able to distinguish these denser polymorphs of H₂BTF. This study suggests that it could be worth investigating whether denser polymorphs of H₂BTF can be grown.

Comparative theoretical investigation of the structures, energetics, and stabilities of C7N5H11cages

Carbon-nitrogen cages are the focus of much research due to their potential use as high energy density materials (HEDMs). Several such cage isomers of C7N5H11, created by modifying the most stable N12 cage, were examined by performing theoretical calculations to evaluate their suitability as potential HEDMs. Calculations were carried out with density functional theory and Møller-Plesset perturbation theory (MP2) using the basis sets 6-31+G(d,p) and cc-pvdz. The relative thermodynamic stabilities of the isomers were explored in two ways: (1) the thermodynamic stability of one isomer was compared to that of another isomer based on their relative energies; (2) the kinetic stabilities of the isomers were determined by calculating the corresponding bond-breaking energies.

DFT studies on a high energy density cage compound 4-trinitroethyl-2,6,8,10,12-pentanitrohezaazaisowurtzitane

Polynitro cage compound 4-trinitroethyl-2,6,8,10,12-pentanitrohexaazaisowurtzitane has the same framework with but higher stability than CL-20 and is a potential new high energy density compound (HEDC). In this paper, the B3LYP/6-31G(d,p) method of density functional theory (DFT) has been used to study its heat of formation, IR spectrum, and thermodynamic properties. The stability of the compound was evaluated by the bond dissociation energies. The calculated results show that the first step of pyrolysis is the rupture of the N-NO(2) bond in the side chain and verify the experimental observation that the title compound has better stability than CL-20. The crystal structure obtained by molecular mechanics belongs to the P2(1)2(1)2(1) space group, with lattice parameters a = 12.59 Å, b = 10.52 Å, c = 12.89 Å, Z = 4, and ρ = 2.165 g·cm(-3). Both the detonation velocity of 9.767 km·s(-1) and the detonation pressure of 45.191 GPa estimated using the Kamlet-Jacobs equation are better than those of CL-20. Considering that this cage compound has a better detonation performance and stability than CL-20, it may be a superior HEDC.

Theoretical investigations of a high density cage compound 10-(1-nitro-1, 2, 3, 4-tetraazol-5-yl)) methyl-2, 4, 6, 8, 12-hexanitrohexaazaisowurtzitane

A new polynitro cage compound with the framework of HNIW and a tetrazole unit, i.e., 10-(1-nitro-1, 2, 3, 4-tetraazol-5-yl)) methyl-2, 4, 6, 8, 12-hexanitrohexaazaisowurtzitane (NTz-HNIW) has been proposed and studied by density functional theory (DFT) and molecular mechanics methods. Properties such as IR spectrum, heat of formation, thermodynamic properties, and crystal structure were predicted. The compound belongs to the Pbca space group, with the lattice parameters a = 15.07 Å, b = 12.56 Å, c = 18.34 Å, Z = 8, and ρ = 1.990 g·cm(-3). The stability of the compound was evaluated by the bond dissociation energies and results showed that the first step of pyrolysis is the rupture of the N-NO(2) bond in the side chain. The detonation properties were estimated by the Kamlet-Jacobs equations based on the calculated crystal density and heat of formation, and the results were 9.240 km·s(-1) for detonation velocity and 40.136 GPa for detonation pressure. The designed compound has high thermal stability and good detonation properties and is probably a promising high energy density compound (HEDC).

Theoretical studies on nitrogen rich energetic azoles

Different nitro azole isomers based on five membered heterocyclics were designed and investigated using computational techniques in order to find out the comprehensive relationships between structure and performances of these high nitrogen compounds. Electronic structure of the molecules have been calculated using density functional theory (DFT) and the heat of formation has been calculated using the isodesmic reaction approach at B3LYP/6-31G* level. All designed compounds show high positive heat of formation due to the high nitrogen content and energetic nitro groups. The crystal densities of these energetic azoles have been predicted with different force fields. All the energetic azoles show densities higher than 1.87 g/cm(3). Detonation properties of energetic azoles are evaluated by using Kamlet-Jacobs equation based on the calculated densities and heat of formations. It is found that energetic azoles show detonation velocity about 9.0 km/s, and detonation pressure of 40GPa. Stability of the designed compounds has been predicted by evaluating the bond dissociation energy of the weakest C-NO(2) bond. The aromaticity using nucleus independent chemical shift (NICS) is also explored to predict the stability via delocalization of the π-electrons. Charge on the nitro group is used to assess the impact sensitivity in the present study. Overall, the study implies that all energetic azoles are found to be stable and expected to be the novel candidates of high energy density materials (HEDMs).

Computational Methods in Organic Thermochemistry. 1. Hydrocarbon Enthalpies and Free Energies of Formation

Standard state enthalpies and free energies of formation can be computed with reasonable accuracy (usually within 4 and often 2 kJ/mol) using high level model chemistries. A comparison set of nearly 300 organic compounds ranging from 1 to 10 carbon atoms having a variety of functional groups for which enthalpy and free energy literature values are available has been examined using G2, G2MP2, G3, G3MP2, G3B3, G3MP2B3, CBS-QB3, and density functional (B3LYP/6-311+G(3df,2p)) model chemistries. G3 gives an average mean absolute deviation of 3.0 and 13.4 kJ/mol for the enthalpies and free energies, respectively, using the atomization method and 3.1 and 3.7 kJ/mol when bond separation reactions are employed. G3 and G3B3 are the most accurate overall; the related G3MP2 and G3MP2B3 are nearly as accurate and can compute larger molecules. CBS-QB3 was also found to be accurate but is more limited in the size of molecules that can be computed. The density functional energies were found to have large deviations from the literature values using either the atomization or the bond separation method. Regardless of the model employed, the free energies are increasingly underestimated by computation as the size of the molecule increases. A series of corrections applied to the aliphatic hydrocarbons is presented, which usually reduces the deviations to less than 4 kJ/mol regardless of the size of the molecule.

Correlations between theoretical and experimental determination of heat of formation of certain aliphatic nitro compounds

Heats of formation of energetic materials were calculated by Dewar's AM1 and Stewart's PM3 methods. In order to compare the theoretical results with the experimental ones, some correlation models were proposed in this study. Correlations were evaluated by multivariable linear regression method, considering the number of nitro groups and the use of quadratic relations involving the number of carbon, hydrogen, nitrogen, and oxygen atoms. Results indicated very precise correlations. Based on these correlations, heats of formation of some aliphatic nitro compounds can be predicted at 95% predictive interval without experimental analysis.