[n]Cycloparaphenylenes (or “carbon nanohoops”) are cyclic fragments of carbon nanotubes that consist of n para linked benzene rings. These strained, all sp2 hybridized macrocycles, have size-dependent optical and electronic properties that are the most dynamic at the smallest size regime where n = 5–12. This review highlights the unique physical phenomena surrounding this class of polycyclic aromatic hydrocarbons, specifically emphasizing the novel structural, optical, and electronic properties of [5]–[12]CPPs.

The dynamic, size-dependent properties of [5]–[12]cycloparaphenylenes

Kyle F. Biegasiewicz,† Justin R. Griffiths,‡ G. Paul Savage, John Tsanaktsidis, and Ronny Priefer* †Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, United States ‡Department of Chemistry, University at Buffalo, Buffalo, New York 14260-1660, United States Ian Wark Laboratory, CSIRO Manufacturing Flagship, Bayview Avenue, Clayton, Victoria 3168, Australia College of Pharmacy, Western New England University, Springfield, Massachusetts 01119, United States

Cubane: 50 years later.

Highly nitrated cage molecules constitute a new class of energetic materials that have received a substantial amount of interest. Among them 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is a powerful explosive with poor impact and friction characteristics. In the present study we aim to design novel energetic materials by tailoring the molecular structure of CL-20. Important characteristics such as the heat of formation and density have been predicted using density functional theory and packing calculations, respectively. Sensitivity correlations have been established for model compounds by analyzing the charge on the nitro groups. Molecules IDX1, IDX4, and IDX7 have been found to have comparable performance with better insensitivity characteristics and may be explored as CL-20 substitutes in defense applications.

Quantum-chemical studies on hexaazaisowurtzitanes.

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–NO2 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 P212121 space group, with lattice parameters a = 12.59 A, b = 10.52 A, c = 12.89 A, 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 t...

DFT Studies on a High Energy Density Cage Compound 4-Trinitroethyl-2,6,8,10,12-pentanitrohezaazaisowurtzitane

Computational study of imidazole derivative as high energetic materials.

Strain energies of cubane derivatives with different substituent groups.

Density functional theory study of piperidine and diazocine compounds

Computations by density functional theory (DFT) method were performed on a series of pyrazole derivatives The heats of formation (HOFs) were predicted using B3LYP and B3P86 functionals with aug-cc-pVDZ and 6-311++G ∗∗ basis sets via designed isodesmic reactions In the isodesmic reactions the pyrazole was chosen as a reference compound The general trend is that the HOFs increase with increasing number of –NH 2 and –N 3 groups And the HOFs initially decrease then increase as the number –NO 2 and –NF 2 groups increasing The HOF of 3,5-didifluoroaminopyrazole is the smallest (1344 kJ/mol), and the HOF of 3,4,5-triazidopyrazole is the largest (12406 kJ/mol) at the B3LYP/aug-cc-pVDZ level The position of the group also affects the HOFs Judged by the HOF values, the 4- and 3-monosubstituted isomers are the most stable monosubstituted isomers when the electron pulling and pushing groups are attached to the pyrazole ring, respectively The 3,5-isomers are the most stable di-substituted isomers The values of HOFs decrease as the –NO 2 groups being replaced by the –NH 2 and –NF 2 , but increase dramatically as the –NO 2 groups being replaced by –N 3 with the average increment of 3320 kJ/mol The relative stability of the title compounds was evaluated based on the calculated HOFs and the energy gaps between the frontier orbits Thermal stabilities were evaluated via bond dissociation energies (BDE) at the UB3LYP/aug-cc-pVDZ level The value of C–N 3 bond dissociation energy, 5245 kJ/mol in average, is relatively larger than other out-of-ring C–N bonds The BDE ZPE value of C–NO 2 bond is smaller than all the other C–N bonds, which is 2707 kJ/mol in average These results provide basic information for the molecular design of novel high energetic density materials

A density functional study of substituted pyrazole derivatives

The heats of formation (HOFs) were calculated for a series of neopentyl difluoroamino compounds by using density functional theory (DFT) method with 6-311G∗∗ basis set, as well as semi-empirical methods. In the isodesmic reactions designed for the computation of HOF, the neopentane was chosen as a reference compound. The HOFs increase smoothly as –ONO2 groups being replaced by –NF2 and –NO2. However, HOFs increase dramatically as –ONO2 groups being replaced by –N(NO2)CH3 and –N3 groups. The variations of HOFs are different with different substituents. The semi-empirical MO (MNDO, AM1, and PM3) methods did not produce accurate and reliable results for the HOFs of the title compounds. The relative stability of the title compounds was evaluated based on the calculated HOFs, the energy gaps between the frontier orbitals and the bond order of C–R bond (C–NF2, C–ONO2, C–NO2 and C–N3). The magnitudes of intramolecular group interactions were predicted through the disproportionation energies. Thermal stabilities were evaluated via bond dissociation energies (BDE) at the B3LYP/6-311G∗∗ level. These results provide basic information for the molecular design of novel high energetic density materials.

Theoretical studies on heats of formation, group interactions, and bond dissociation energies in neopentyl difluoroamino compounds

Homodesmotic reactions were designed for the computation of strain energies (SE) for four nitro-substituted 1,3,5,7-tetraazacubane derivatives. Total energies of the optimized geometric structures at the DFT-B3LYP/6-31G* and DFT-B3LYP/6-311G** levels were used to derive the SE. The variation of SE with respect to the number of substituents is similar with both basis sets. The SE value is 237.32 kcal/mol at the B3LYP/6-311G** level for 2,4,6,8-tetranitro-1,3,5,7-tetraazacubane, which is unexpectedly much larger than that of its cubane analogue. The SE increases remarkably with more nitro groups being attached to the cage skeleton of tetraazacubane. The ‘bending’ of the bonds within the cubic skeleton attributes to the increase of strains as the attached number of nitro groups increases.

Xiao-Wei Fan

Papers

Strain energies of cubane derivatives with different substituent groups.

Density functional theory study of piperidine and diazocine compounds

A density functional study of substituted pyrazole derivatives

Theoretical studies on heats of formation, group interactions, and bond dissociation energies in neopentyl difluoroamino compounds

Cage strain in nitro-substituted 1,3,5,7-tetraazacubanes