Yingzhe Liu

Structural characteristics of liquid nitromethane at the nanoscale confinement in carbon nanotubes

AbstractThe stability of energetic materials confined in the carbon nanotubes can be improved at ambient pressure and room temperature, leading to potential energy storage and controlled energy release. However, the microscopic structure of confined energetic materials and the role played by the confinement size are still fragmentary. In this study, molecular dynamics simulations have been performed to explore the structural characteristics of liquid nitromethane (NM), one of the simplest energetic materials, confined in a series of armchair single-walled carbon nanotubes (SWNTs) changing from (5,5) to (16,16) at ambient conditions. The simulation results show that the size-dependent ordered structures of NM with preferred orientations are formed inside the tubular cavities driven by the van der Waals attractions between NM and SWNT together with the dipole-dipole interactions of NM, giving rise to a higher local mass density than that of bulk NM. The NM dipoles prefer to align parallel along the SWNT axis in an end-to-end fashion inside all the nanotubes except the (7,7) SWNT where a unique staggered orientation of NM dipoles perpendicular to the SWNT axis is observed. As the SWNT radius increases, the structural arrangements and dipole orientations of NM become disordered as a result of the weakening of van der Waals interactions between NM and SWNT. Graphical AbstractOrdered structures of liquid nitromethane with preferred orientations are formed at the confinement in carbon nanotubes, which is dependent on the confinement size.

Theoretical study on the structure and stability of [1,2,5] oxadiazolo [3,4-e] [1,2,3,4]-tetrazine-4,6-Di-N-dioxide (FTDO)

Although many 1,2,3,4-tetrazine-1,3-dioxide derivates have been synthesized, [1,2,5] oxadiazolo [3,4-e] [1,2,3,4]-tetrazine-4,6-di-N-dioxide (FTDO) is the only one with high enthalpy of formation and high detonation velocity. Whereas, its stability has not been studied. In the present work, the structure of FTDO was investigated using density functional theory (DFT) method, and its stability was calculated by potential energy surface scanning and structure interconvert thermodynamics under different temperatures. The spontaneous isomerization of FTDO and its effect on the stability of FTDO were investigated. The dissociation of FTDO to N2, N2O and furoxan fragments was studied, and the possibility of synthetic route from FTDO to TTTO was discussed.

Understanding the growth morphology of explosive crystals in solution: insights from solvent behavior at the crystal surface

The structural, energetic, and dynamic properties at the interfaces between the five growth faces of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) crystal and acetone (AC) solvent were investigated by computational simulations with an aim to understand the RDX crystal morphology when grown from AC solvent at the molecular level. The simulation results showed that the solvent behavior, such as the mass density distribution, dipole orientation, interaction, and diffusion, was dependent on the structural features of each crystal surface. The binding sites for solvent incorporation at the crystal surface were found and were visually displayed on the basis of occupancy analysis. The typical binding motifs were extracted from the simulation trajectory, with the corresponding binding affinities estimated as (002) ≈ (210) > (200) > (020) > (111) at the M06-2X/6-31++G** level of theory. The theoretical prediction of the morphological importance of each growth face was in reasonable agreement with the experimental RDX morphology grown from AC solvent.

4-Oxo- or 1-oxo-N7O+? A computational and experimental study

In a previous paper [Inorg. Chem., 2010, 49, 1245], we studied the reaction of F2NO+ with an excess of HN3 which led to the quantitative formation of N5+ and N2O. Based on 15N-labeling experiments and theoretical calculations, the formation of a 4-oxo-N7O+ intermediate with a decomposition energy barrier of about 40 kcal mol−1 was proposed. Since this relatively high barrier disagreed with our failure to experimentally observe this cation, a thorough theoretical study of the isomerization, dissociation and formation pathways of N4FO+ and N7O+ was carried out at the B3LYP and G3B3 levels at 240 K. It was found that the self-decomposition of 4-oxo-N7O+ to NO+ and N2 has a considerably lower barrier of only 19.6 kcal mol−1 and, therefore, would be more likely than a self-decomposition to N5+ and N2O. Additional calculations also showed that alternate reaction pathways between the stable and well-characterized z-N4FO+ intermediate product and HN3 involving 7- or 9-membered cyclic transition states, can lead to the observed N5+ and N2O products with the observed 15N distribution and barriers as low as 20.7 kcal mol−1. The transition states for these reactions contain a 1-oxo-N7O+ component which can decompose without a barrier to N5+ and N2O. These alternative pathways involving an unstable 1-oxo-N7O+ cation are in better agreement with experiment than the one involving 4-oxo-N7O+. The correctness of this re-interpretation was experimentally verified by a 15N-labeling experiment between α- and γ-15N-labeled HN3 and unlabeled N4FO+ which resulted exclusively in unlabeled N2O and α- and γ-15N-labeled N5+. Therefore, we conclude that in the reaction of NF2O+ with excess HN3 the experimental and theoretical evidence supports only the formation of an unstable 1-oxo-N7O+ cation, and that for the preparation of the symmetric 4-oxo-N7O+ cation different synthetic approaches will be required.

Face-Dependent Solvent Adsorption: A Comparative Study on the Interfaces of HMX Crystal with Three Solvents

To understand the crystal-solvent interfacial interactions on the molecular scale, the interfaces between three solvents, that is, acetone, γ-butyrolactone, and cyclohexanone, and three growth faces of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) crystal have been investigated with the aid of theoretical chemistry. The results show that the structural features of crystal faces play a critical role in the energetic, structural, and dynamic properties at the interfaces. For each solvent, the same change trend of some properties among the three faces of HMX crystal is observed, including adsorption affinity, local mass density, and solvent diffusion. For example, the rate of solvent diffusion at the three faces ranks as (011) > (110) > (020) regardless of solvent species. This can be attributed to the similar adsorption sites for solvent incorporation at the same face, which are concentrated at the cavities formed by surficial HMX molecules.

Adsorption behavior of acetone solvent at the HMX crystal faces: A molecular dynamics study

Molecular dynamics simulations have been performed to understand the adsorption behavior of acetone (AC) solvent at the three surfaces of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctan (HMX) crystal, i.e. (011), (110), and (020) faces. The simulation results show that the structural features and electrostatic potentials of crystal faces are determined by the HMX molecular packing, inducing distinct mass density distribution, dipole orientation, and diffusion of solvent molecules in the interfacial regions. The solvent adsorption is mainly governed by the van der Waals forces, and the crystal-solvent interaction energies among three systems are ranked as (020)≈(110)>(011). The adsorption sites for solvent incorporation at the crystal surface were found and visualized with the aid of occupancy analysis. A uniform arrangement of adsorption sites is observed at the rough (020) surface as a result of ordered adsorption motif.

Theoretical study of the effect of N-oxides on the performances of energetic compounds

AbstractIn order to study the effects of N-oxide on structure and performance, six categories of energetic compounds were systemically investigated. The results indicated that the C–C bonds in the rings were shortened, and the C–N bonds close to the N → O bond were elongated when N atoms was oxidized to form N → O bonds. N → O bonds can increase the densities of most categories of compounds, and the increment will increase with the number of N → O bonds. As to their detonation performances, almost all categories of compounds had an increased trend, except for some NO2-, NHNO2- and ONO2-substituted compounds. The contribution of 1,2,3,4-tetrazine and 1,2,4,5-tetrazine to performances was better than that of pyrazine and [1,2,5] oxadiazolo [3,4-b] pyrazine on the whole, and the groups, especially energetic groups, made a huge contribution to performance. When R was a NH2 or ONO2 group, all compounds had lower impact sensitivities, and thus represent candidates for novel energetic compounds. However, other than the sixth category of compounds, all compounds had higher impact sensitivities when R was a NO2 or NHNO2 group, and have little significance in application. Graphical abstractTo study the effects of N-oxide on the structure and performance of energetic compounds, and to propose theoretical guidance for the design of novel compounds, the six categories (94 species) listed in the figure were investigated systemically by density functional theory methods and some empirical formulae

Ordered and layered structure of liquid nitromethane within a graphene bilayer: toward stabilization of energetic materials through nanoscale confinement.

AbstractThe structural characteristics involving thermal stabilities of liquid nitromethane (NM)—one of the simplest energetic materials—confined within a graphene (GRA) bilayer were investigated by means of all-atom molecular dynamics simulations and density functional theory calculations. The results show that ordered and layered structures are formed at the confinement of the GRA bilayer induced by the van der Waals attractions of NM with GRA and the dipole–dipole interactions of NM, which is strongly dependent on the confinement size, i.e., the GRA bilayer distance. These unique intermolecular arrangements and preferred orientations of confined NM lead to higher stabilities than bulk NM revealed by bond dissociation energy calculations. Graphical AbstractLiquid nitromethane within a graphene bilayer

Roads to pentazolate anion: a theoretical insight

The formation mechanism of pentazolate anion (PZA) is not yet clear. In order to present the possible formation pathways of PZA, the potential energy surfaces of phenylpentazole (PPZ), phenylpentazole radical (PPZ-R), phenylpentazole radical anion (PPZ-RA), PPZ and m-chloroperbenzoic acid (m-CPBA), p-pentazolylphenolate anion (p-PZPolA) and m-CPBA, and p-pentazolylphenol (p-PZPol) and m-CPBA were calculated by the computational electronic structure methods including the hybrid density functional, the double hybrid density functional and the coupled-cluster theories. At the thermodynamic point of view, the cleavages of C–N bonds of PPZ and PPZ-R need to absorb large amounts of heat. Thus, they are not feasible entrance for PZA formation at ambient condition. But excitation of PPZ and deprotonation of PPZ-RA probably happen before cleavage of C–N bond of PPZ at high-energy condition. As to the radical anion mechanism, the high accuracy calculations surveyed that the barrier of PZA formation is probably lower than that of dinitrogen evolution, but the small ionization potential of PPZ-RA gives rise to the unstable ionic pair of sodium PPZ at high temperature. In respect of oxidation mechanism, except for PPZ, the reactions of p-PZPolA and p-PZPol with m-CPBA can form PZA and quinone. The PZA formations have the barriers of about 20 kcal mol−1 which compete with the dinitrogen evolutions. The stabilities of PZA in both solid and gas phases were also studied herein. The proton prefers to transfer to pentazolyl group in the (N5)6(H3O)3(NH4)4Cl system which leads to the dissociation of pentazole ring. The ground states of M(N5)2(H2O)4 (M = Co, Fe and Mn) are high-spin states. The pentazolyl groups confined by the crystal waters in the coordinate compounds can improve the kinetic stability. As to the reactivity of PZA, it can be persistently oxidized by m-CPBA to oxo-PZA and 1,3-oxo-PZA with the barriers of about 20 kcal mol−1.

Structural Rearrangement of Energetic Materials under an External Electric Field: A Case Study of Nitromethane

As a significant stimulus, the external electric field (EEF) can change the decomposition mechanism and energy release of energetic materials (EMs). Hence, understanding the response of EMs to an EEF is greatly meaningful for their safe usage. Herein, the structural arrangement, a crucial factor in the impact sensitivity and detonation performance of EMs, under the EEF ranging from 0.0 to 0.5 V/Å was investigated via molecular dynamics simulation. Nitromethane (NM) was taken as a case study due to the simple structure. The simulation results show that there exists a critical EEF strength between 0.2 and 0.3 V/Å, which can induce the transition of NM molecules from relatively disordered distribution to solidlike ordered and compacted arrangement with a large density. In this ordered structure, NM dipoles are aligned in a head-to-tail pattern parallel to the EEF direction because of the favored dipole-dipole interactions and weak C-H···O hydrogen bonds. As the EEF strength is enhanced, the potential energy and cohesive energy density of the NM system gradually decrease and increase, respectively, indicative of high thermodynamics stability of ordered arrangement. The results reported here also shed light on the potential of the EEF to induce the nucleation and crystallization to explore new polymorphs of EMs.