A theoretical charge density study on nitrogen-rich 4,4′,5,5′-tetranitro-2,2′-bi-1H-imidazole (TNBI) energetic molecule

The bond topological and electrostatic properties of nitrogen-rich 4,4′,5,5′-tetranitro-2,2′-bi-1H-imidazole (TNBI) energetic molecule have been calculated from the DFT method with the basis set 6-311G** and the AIM theory. The optimized geometry of this molecule is almost matched with the experimental geometric parameters. The electron density at the bond critical point and the Laplacian of electron density of C–NO₂ bonds are not equal, one of them is much weaker than the other. Similar trend exists in the C–N bonds of the imidazole ring of the molecule. The ratio of the bond dissociation energy (BDE) of the weakest bond to the molecular total energy exhibits nearly a linear correlation with the impact sensitivity; its h ₅₀% value is ~32.01 cm. The electrostatic potential around both the nitro groups are found unequal; the NO₂ group of weakest C–NO₂ bond exhibits an extended electronegative region.     

Mechanism of the primary stages of decomposition of aliphatic nitro- and fluoronitronitramines

The primary stage of the decomposition of compounds RN(NO₂)CH₂C(NO₂)₂X is the homolytic cleavage of the C?NO₂ bond, at X=NO₂ and N?NO₂ bond at X=F. The inductive effect of substituents decreases the dissociation energies of the C?N and N?N bonds by 1–2 kcal mol^?1. Kinetic effects caused by the spatial interaction of groups and by stepwise decomposition of polyfunctional compounds are described.     

Effects of the simultaneous presence of nitro and amine substituents in cubane and some azacubanes

We have carried out an ab initio self-consistent-field molecular orbital study of the structures and bond orders (as measures of relative bond strengths) of nitro and aminonitro derivatives of cubane and some azacubanes. The NO₂ group is found to have, in general, a conformation-dependent effect on relative bond strengths. When the plane of the nitro group is perpendicular to that of an endocyclic bond, that bond is weakened. An aza nitrogen adjacent to C-NO₂, however, eliminates the bond-weakening due to a perpendicular nitro group, except when a C-NH₂ is also adjacent to the C-NO₂. The NH₂ group has a direction-specific bond-weakening effect on one [and only one] endocyclic bond; the particular bond undergoing this weakening is determined by the orientation of the amine nitrogen lone pair. The influences of the NO₂ and NH₂ are essentially independent; when these substituents are on adjacent carbons the effects are reinforcing. Thus, the combination of a perpendicular NO₂ and an NH₂ on adjacent carbons in cubane and azacubanes produces a marked bond weakening. Our findings suggest that strained cage systems that have alternating aza nitrogens and C-NO₂ groups should be further investigated as potential high-energy systems.     

Understanding organofluorine chemistry. An introduction to the C-F bond

Fluorine is the most electronegative element in the periodic table. When bound to carbon it forms the strongest bonds in organic chemistry and this makes fluorine substitution attractive for the development of pharmaceuticals and a wide range of speciality materials. Although highly polarised, the C-F bond gains stability from the resultant electrostatic attraction between the polarised C delta+ and F delta- atoms. This polarity suppresses lone pair donation from fluorine and in general fluorine is a weak coordinator. However, the C-F bond has interesting properties which can be understood either in terms of electrostatic/dipole interactions or by considering stereoelectronic interactions with neighbouring bonds or lone pairs. In this tutorial review these fundamental aspects of the C-F bond are explored to rationalise the geometry, conformation and reactivity of individual organofluorine compounds.     

Ruthenium-catalyzed arylation of fluorinated aromatic ketones via ortho-selective carbon-fluorine bond cleavage

We report here ruthenium-catalyzed arylation of fluorinated aromatic ketones via ortho-selective carbon-fluorine bond cleavage. In the presence of trimethylvinylsilane and cesium fluoride, ortho carbon-fluoride bonds of aromatic ketones were phenylated by 5,5-dimethyl-2-phenyl-1,3,2-dioxaborinane using RuH₂(CO)(PPh₃)₃ as a catalyst. Tandem C-F phenylation/C-H alkylation was observed for substrates bearing both one ortho hydrogen and one ortho fluorine atoms.     

Electrostatic potentials of strained systems: nitrocyclopropane

The presence of an NO₂ substituent is found to eliminate the negative electrostatic potentials associated with the “bent” CC bonds cyclopropane. This is presumably due in large part to a rearrangement of electronics charge caused by the nitro group, as is shown by electronic density difference plots. Bond path calculations reveal bent bonds in nitrocyclopropane that are very similar to those in cycloprophane.     

Organic fluorines as halogen bond donors: Theoretical study and crystallographic evidence

Organic fluorines usually cannot act as halogen bond donors, as a result of the extreme electronegativity and least polarizability of the fluorine atom. However, when the electronegative ability of the substituents bound to the carbon atom is very strong, organic fluorines do show positive electrostatic potentials (ESPs) along the C? F bond and thus can form halogen bonds with electron donors. In this work, the effects of six different substituents, i.e., NO₂, CF₃, CN, COOH, CHO, and CCH, on the ESPs of the F atom were studied using the M06‐2x method. When two or three substituents with very strong electron‐withdrawing ability, such as NO₂ and CN, are linked to the C atom, positive ESPs take place on the outermost portion of the F atom. However, the ESPs remain negative along the C? F bond with the introduction of relatively weak electron‐withdrawing substituents, irrespective of the number of the subsituents. In addition, some complexes between fluorinated molecules with positive ESPs on the F atom and NH₃ were calculated, to characterize the structural and energetic features of these specific halogen bonds. Finally, some crystal structures extracted from the Cambridge Structural Database were selected to provide experimental evidence of these interactions involving the C? F bond. The results presented in this work are very important for the modern definition of halogen bonding as well as for the development of pharmaceuticals and a wide range of functional material. © 2015 Wiley Periodicals, Inc.     

DFT Studies on Detonation Properties,Pyrolysis Mechanism,and Stability of the Nitro and Hydroxyl Derivatives of Benzene

Ninety‐one nitro and hydroxyl derivatives of benzene were studied at the B3LYP/6‐31G?? level of density functional theory. Detonation properties were calculated using the Kamlet‐Jacobs equation. Three candidates (pentanitrophenol, pentanitrobenzene, and hexanitrobenzene) were recommended as potential high energy density compounds for their perfect detonation performances and reasonable stability. The pyrolysis mechanism was studied by analyzing the bond dissociation energy (BDE) and the activation energy (E_a) of hydrogen transfer (H–T) reaction for those with adjacent nitro and hydroxyl groups. The results show that E_a is much lower than BDEs of all bonds, so when there are adjacent nitro and hydroxyl groups in a molecule, the stability of the compound will decrease and the pyrolysis will be initiated by the H–T process. Otherwise, the pyrolysis will start from the breaking of the weakest C–NO₂ bond, and only under such condition, the Mulliken population or BDE of the C–NO₂ bond can be used to assess the relative stability of the compound.     

Mechanistic investigation of vinylic carbon-fluorine bond activation of perfluorinated cycloalkenes using Cp*2ZrH2 and Cp*2ZrHF

Cp*₂ZrH₂ (1) (Cp*: pentamethylcyclopentadienyl) reacts with cyclic perfluorinated olefins to give Cp*₂ZrHF (2) and hydrodefluorinated products under very mild conditions. Initial C-F bond activation occurs selectively at the vinylic positions of the cycloolefin to exchange fluorine for hydrogen. Several mechanisms are discussed for this H/F exchange: (a) olefin insertion/β-fluoride elimination, (b) olefin insertion/α-fluoride elimination, and (c) hydride/fluoride σ-bond metathesis. Following H/F σ-bond metathesis exchange of both vinylic C-F bonds of perfluorocyclobutene, 1 then reacts with allylic C-F bonds by insertion/β-fluoride elimination. A similar sequence is observed with perfluorocyclopentene. Cp*₂ZrHF reacts selectively with vinylic C-F bonds of perfluorocyclobutene to give 3,3,4,4-tetrafluorocyclobutene and Cp*₂ZrF₂ without further hydrodefluorination occurring. In the presence of excess 1 and H₂, perfluorocyclobutene and perfluorocyclopentene are reduced to cyclobutane and cyclopentane in 46% and 16% yield, respectively. DFT calculations exclude the pathway by way of the olefin insertion/α-fluoride elimination and suggest that the pathway by way of hydride/fluoride σ-bond metathesis is preferred.     

Probing delocalisation across highly ethynylated mono and dinuclear Pt(II) tethers containing nitrogroups and organic models as redox active probes: X-ray crystal structure of trans-[Pt(CC-C6H4NO2)2(PPh3)2]

The electrochemical study on Pt(II) organometallic and organic ethynylated compounds bearing nitro substituents in the phenyl groups has been performed, demonstrating that the presence of severe chemical irreversibility accompanying the electron transfer, preclude an evaluation of the electronic communication between redox active centres. The X-ray structure of the complex trans-[Pt(CC-C₆H₄NO₂)₂(PPh₃)₂] is showed.     

A computational analysis of the electrostatic potentials and relative bond strengths of hydrazine and some of its 1,1-dimethyl derivatives

We have carried out a computational study of hydrazine and five of its 1,1-dimethyl derivatives, focusing on their electrostatic potentials and relative bond strengths. Our approach has involved the calculation of ab initio self-consistent-field molecular orbital wave functions and molecular properties using the GAUSSIAN 82 system of programs. The electrostatic potentials of the hydrazines possess negative regions of varying sizes and strengths associated with the nitrogens of the α-diamino linkages. Through an analysis of the positions of the most negative potentials of these regions, we have obtained directly the dihedral angles between the nitrogen lone pairs in these systems. Our use of the electrostatic potential to obtain these angles is a direct and general approach, in contrast to indirect procedures used in the past. We find this dihedral angle to be close to 90° in hydrazine, with variations in the substituted hydrazines that depend on the nature of the substituents. A highly polar structure is found for 1-chloromethyl-1-methylhydrazine, which involves a delocalization of electronic charge from the substituted nitrogen towards the CH₂Cl group. We find that substituents able to withdraw significant amounts of electronic density from the central nitrogen lone pair regions, either through resonance or by induction, have a slight bond strengthening effect on the central N-N bond. This is attributed to a decrease in the repulsion between the weakened nitrogen lone pair regions. The difficulties encountered in seeking the controlled oxidation of hydrazine to nitro derivatives may be due, in part, to the fact that two factors which would favor this, highly negative nitrogen potentials and strong N-N bonds, are opposing in nature.     

Theoretic design of 1,2,3,4-tetrazine-1,3-dioxide-based high-energy density compounds with oxygen balance close to zero

Density functional theory method was used to study the heats of formation (HOFs), electronic structure, energetic properties, and thermal stability for a series of 1,2,3,4-tetrazine-1,3-dioxide derivatives with different substituents and bridge groups. It is found that the groups –NO₂, –C(NO₂)₃, and –N=N– play a very important role in increasing the HOFs of the derivatives. The effects of the substituents on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and HOMO–LUMO gaps are coupled to those of different substituents and bridges. The calculated detonation velocities and pressures indicate that the group –NO₂, –NF₂, –ONO₂, –C(NO₂)₃, or –NH– is an effective structural unit for enhancing the detonation performance for the derivatives. An analysis of the bond dissociation energies for several relatively weak bonds indicates that incorporating the groups –NO₂, –NF₂, –ONO₂, –C(NO₂)₃, and –N=N– into parent ring decreases their thermal stability. Considering the detonation performance and thermal stability, 18 compounds may be considered as the target compounds holding the greatest potential for synthesis and use as high-energy density compounds. Among them, the oxygen balances of four compounds are equal to zero. These results provide basic information for the molecular design of the novel high-energy compounds.     

Halogenotrinitromethanes: A Combined Study in the Crystalline and Gaseous Phase and Using Quantum Chemical Methods

The halogenotrinitromethanes FC(NO₂)₃ ( 1 ), BrC(NO₂)₃ ( 2 ), and IC(NO₂)₃ ( 3 ) were synthesized and fully characterized. The molecular structures of 1 – 3 were determined in the crystalline state by X‐ray diffraction, and gas‐phase structures of 1 and 2 were determined by electron diffraction. The Hal?C bond lengths in F?, Cl?, and Br?C(NO₂)₃ in the crystalline state are similar to those in the gas phase. The obtained experimental data are interpreted in terms of Natural Bond Orbitals (NBO), Atoms in Molecules (AIM), and Interacting Quantum Atoms (IQA) theories. All halogenotrinitromethanes show various intra‐ and intermolecular non‐bonded interactions. Intramolecular N ??? O and Hal ??? O (Hal=F ( 1 ), Br ( 2 ), I ( 3 )) interactions, both competitors in terms of the orientation of the nitro groups by rotation about the C?N bonds, lead to a propeller‐type twisting of these groups favoring the mentioned interactions. The origin of the unusually short Hal?C bonds is discussed in detail. The results of this study are compared to the molecular structure of ClC(NO₂)₃ and the respective interactions therein.     

Theoretical study on the interactions of halogen‐bonds and pnicogen‐bonds in phosphine derivatives with Br2, BrCl,and BrF

The MP2 ab initio quantum chemistry methods were utilized to study the halogen‐bond and pnicogen‐bond system formed between PH₂X (X = Br, CH₃, OH, CN, NO₂, CF₃) and BrY (Y = Br, Cl, F). Calculated results show that all substituent can form halogen‐bond complexes while part substituent can form pnicogen‐bond complexes. Traditional, chlorine‐shared and ion‐pair halogen‐bonds complexes have been found with the different substituent X and Y. The halogen‐bonds are stronger than the related pnicogen‐bonds. For halogen‐bonds, strongly electronegative substituents which are connected to the Lewis acid can strengthen the bonds and significantly influenced the structures and properties of the compounds. In contrast, the substituents which connected to the Lewis bases can produce opposite effects. The interaction energies of halogen‐bonds are 2.56 to 32.06 kcal·mol^?1; The strongest halogen‐bond was found in the complex of PH₂OH???BrF. The interaction energies of pnicogen‐bonds are in the range 1.20 to 2.28 kcal·mol^?1; the strongest pnicogen‐bond was found in PH₂Br???Br₂ complex. The charge transfer of lp(P) ? σ*(Br? Y), lp(F) ? σ*(Br? P), and lp(Br) ? σ*(X? P) play important roles in the formation of the halogen‐bonds and pnicogen‐bonds, which lead to polarization of the monomers. The polarization caused by the halogen‐bond is more obvious than that by the pnicogen‐bond, resulting in that some halogen‐bonds having little covalent character. The symmetry adapted perturbation theory (SAPT) energy decomposition analysis showes that the halogen‐bond and pnicogen‐bond interactions are predominantly electrostatic and dispersion, respectively.     

Trigger bond analysis of nitroaromatic energetic materials using wiberg bond indices

The identification of trigger bonds, bonds that break to initiate explosive decomposition, using computational methods could help direct the development of novel, “green” and efficient high energy density materials (HEDMs). Comparing bond densities in energetic materials to reference molecules using Wiberg bond indices (WBIs) provides a relative scale for bond activation (%ΔWBIs) to assign trigger bonds in a set of 63 nitroaromatic conventional energetic molecules. Intramolecular hydrogen bonding interactions enhance contributions of resonance structures that strengthen, or deactivate, the C NO₂ trigger bonds and reduce the sensitivity of nitroaniline‐based HEDMs. In contrast, unidirectional hydrogen bonding in nitrophenols strengthens the bond to the hydrogen bond acceptor, but the phenol lone pairs repel and activate an adjacent nitro group. Steric effects, electron withdrawing groups and greater nitro dihedral angles also activate the C NO₂ trigger bonds. %ΔWBIs indicate that nitro groups within an energetic molecule are not all necessarily equally activated to contribute to initiation. %ΔWBIs generally correlate well with impact sensitivity, especially for HEDMs with intramolecular hydrogen bonding, and are a better measure of trigger bond strength than bond dissociation energies (BDEs). However, the method is less effective for HEDMs with significant secondary effects in the solid state. Assignment of trigger bonds using %ΔWBIs could contribute to understanding the effect of intramolecular interactions on energetic properties. © 2018 Wiley Periodicals, Inc.     

Molecular design of trinitromethyl-substituted nitrogen-rich heterocycle derivatives with good oxygen balance as high-energy density compounds

Density functional theory method was used to study the heats of formation (HOFs), electronic structure, energetic properties, and pyrolysis mechanism of a series of trinitromethyl-substituted heterocycle (including triazole, tetrazole, furazan, tetrazine, and fused heterocycles) derivatives. It is found that the fused ring, tetrazine, and tetrazole are effective structural units for increasing the HOFs of the derivatives. The substitution of the combination of nitro and trinitromethyl is very useful for improving their HOFs. The calculated energetic properties indicate that the combination of the nitro and trinitromethyl is very helpful for improving their detonation properties and oxygen balances (OB). Most of the title compounds have a good OB over zero. The OB of six compounds are very high and over 22. An analysis of the bond dissociation energies for several relatively weak bonds suggests that the N–O bond in the ring is a trigger bond for BIII-1, CI-3, and CI-4, and the ring–NO₂ and (NO₂)₂C–NO₂ bond cleavage is likely to happen in thermal decomposition for the remaining compounds. Considering the detonation performance and thermal stability, seven compounds could be regarded as potential candidates for high-energy compounds. Four compounds may be used as the novel high-energy oxidizers.     

Hydrofluoroarylation of alkynes with fluoroarenes

A combination of Ni(cod)(2) and PCyp(3) is found to be an effective catalyst for chemoselective activation of the C-H bond of fluoroarenes over C-F bonds followed by insertion of alkynes to allow direct alkenylation of the electron-deficient arenes. The characteristics of the reactions are: a C-H bond ortho to a fluorine substituent is selectively activated; the reactivity of fluorobenzenes is roughly proportional to the number of fluorine atoms. The reaction conditions tolerate a broad range of both alkynes and fluoroarenes containing both electron-withdrawing and -donating groups, thus allowing efficient synthesis of a variety of substituted ethenes containing a fluoroaryl motif in high regio- and stereoselective manners. Mechanistic studies including both labeling experiments and stoichiometric reactions reveal that oxidative addition of C-H bonds in fluoroarenes to nickel(0) is kinetically highly facile whereas that of C-F bonds is thermodynamically favoured.     

Synthesis,structure and properties of fluorinated graphite

Fluorine-graphite intercalation compounds, C₂F to C₁₆F were synthesized by various methods. C-F bonds range from ionic to semi-covalent. These properties of C-F bonding give to fluorinated graphite metallic conductivity, higher hydrophilicity than graphite and high reduction potential. The c-axis and in-plane structures are governed by C-F bonding, fluorine intercalation rate and host graphites.     

Nature of MoH···I bonds in Cp2Mo(L)H···I‐C≡C‐R Complexes (L=H,CN, PPh2, C(CH3)3; R=NO2, Cl,Br, H,OH, CH3, NH2)

The nature of the MoH···I bond in Cp₂Mo(L)H···I‐C≡C‐R (L= H, CN, PPh₂, C(CH₃)₃; R=NO₂, Cl, Br, H, OH, CH₃, NH₂) was investigated using electrostatic potential analysis, topological analysis of the electron density, energy decomposition analysis and natural bond orbital analysis. The calculated results show that MoH···I interactions in the title complexes belong to halogen‐hydride bond, which is similar to halogen bonds, not hydrogen bonds. Different to the classical halogen bonds, the directionality of MoH···I bond is low; Although electrostatic interaction is dorminant, the orbital interactions also play important roles in this kind of halogen bond, and steric interactions are weak; the strength of H···I bond can tuned by the most positive electrostatic potential of the I atom. As the electron‐withdrawing ability of the R substituent in the alkyne increases, the electrostatic potential maximum of the I atom increases, which enhances the strength of the H···I halogen bond, as well as the electron transfer.     

Reactions of (difluoroamino)trinitromethane with nucleophilic reagents

Reactions of F₂NC(NO₂)₃ with metal fluorides (KF and CsF) in DMF yield a substitution product of the fluorine atom for one nitro group, F₂NC(NO₂)₂F. The reaction of F₂NC(NO₂)₃ with LiBr in ethanol or DMF affords Br(NO₂)C=NF rather than the expected bromo derivative F₂NC(NO₂)₂Br.