HMX Molecules Research Articles

An Investigation of KS-DFT Electron Densities used in Atoms-in-Molecules Studies of Energetic Molecules

The atoms-in-molecules (AIM) theory has been proposed as a method to understand chemical stability through stationary properties of the electron density. To assess the applicability of this method for establishing such correlations with various performance and vulnerability properties of energetic materials, we calculated the Kohn-Sham density functional theory (KS-DFT) wavefunctions and their subsequent AIM data for representative materials, including hexanitrobenzene (HNB), pentaerythritol tetranitrate (PETN), pentanitroaniline (PNA), 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), ethylenedinitramine (EDNA), 1,1-diamino-2,2-dinitroethylene (FOX-7), 3-nitro-1,2,4-triazol-5-one (NTO), nitroguanidine (NQ), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and the TATB dimer using the B3LYP, PBE, and PW91 potentials as well as Hartree-Fock (HF). For the HNB and HMX molecules and the TATB dimer, the number of critical points in the low-density regions of the density gradient vector field varied, sometimes dramatically, with basis set and potential even at their individually optimized geometries. Adding ghost atoms in the low-density regions also affected the existence of critical points. The variation was seen in results generated with three separate AIM software packages, AIMPAC, AIMAll, and InteGriTy. This inconsistency implies that KS-DFT wave-functions can have significant variation in the topology of the electron density to such an extent that these calculations cannot be used to justify the existence or absence of low-density critical points. Therefore, predictions of the stability of a molecule based solely on properties of low-density bond critical points generated from a single DFT calculation are questionable.

Surface and Interface Spectroscopy of High Explosives and Binders: HMX and Estane

AbstractVibrational sum‐frequency generation spectroscopy (SFG) is used to characterize the surfaces of β‐HMX single crystals and Estane polymer binder, as well as the HMX‐Estane interface. SFG is a nonlinear vibrational spectroscopy that selectively probes vibrational transitions at surfaces and interfaces. On the HMX {011} surface, both CH‐ and NO2‐stretching transitions are observed. Compared to bulk HMX, the surface transitions are blueshifted and the splittings are larger. This effect is explained by surface HMX molecules having partially buried and partially free CH2 and NO2 groups. Estane is a diblock copolymer with both soft and hard segments. Comparison of Estane spectra with polymers having only the soft unit and with polymers having predominantly hard units indicate there is a preference for the hard unit on the surface. SFG spectra of the HMX‐Estane interface show smaller splittings of the HMX CH‐stretch transitions than at the HMX‐air interface, because the partially free surface groups are buried in Estane.

Thermochemical functions for gas-phase, 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), its condensed phases, and its larger reaction products

1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane (HMX) is a major component of many explosives and propellants. Chemical kinetic simulations of the reactions of HMX require knowledge of its enthalpy, entropy, and heat capacity in the gas-phase and condensed phases. While some experimental measurements are available, most available thermochemical information has been obtained by analogy with the smaller species, RDX, and its reaction products. Using the Hartree-Fock method with the 6-31G∗ basis set we have calculated structures, vibrational frequencies, and bond strengths for the isolated HMX molecule and its two major reaction products. This, with some available experimental data, allows us to calculate the relevant thermochemical parameters for these species. With this information and some published experimental measurements of sublimation pressure, phase change enthalpies, and heat capacities, we have also calculated some thermochemical data for three condensed phases of HMX: β-HMX, δ-HMX and liquid HMX. We present the thermochemical parameters in both the JANAF and CHEMKIN format. We also compare the current calculations with previous estimates.

Energetic Crystal-Lattice-Dependent Responses

AbstractThe occurrence of [100] direction slip on the (021) slip plane of orthorhombic RDX (cyclotrimethylenetrinitramine, (CH2·N·NO2]3) is shown to favor the formation of 1,3-dinitroso-5-nitro-l,3,5-triazacyclohexane, as detected in drop-weight impact tests at sensitivity height levels near to those measured for initiation. Also, the reported observation of deformation twinning in the chemically-related monoclinic HMX (cyclotetra-methylenetetranitramine, (CH2·N·NO2 ]4) crystal lattice is explained on the basis of the greater flexibility of the larger HMX molecule allowing a number of bond rotations that are required to produce a relatively unusual Type II deformation twinning structure. These examples give support to the consideration that on the molecular level involved in deformation-induced decompositions there may be a direct mechanical force aspect that is additional to the established importance of “hot spot” heating, say, as described for dislocation pile-up avalanches in RDX.

Electron Paramagnetic Resonance of Ultraviolet Irradiated HMX Single Crystals

Abstract The secondary explosive HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) undergoes thermal and photolytic decomposition to produce free radical products. This is firmly established by studies using electron paramagnetic resonance (EPR) spectroscopy. In this study paramagnetic *NO2 molecules are shown to form during ultraviolet photolysis of single crystals of HMX at 77K. The *No2 molecules are highly ordered in the monoclinic crystal lattice of HMX. EPR spectra of different alignments of the HMX crystal relative to the applied magnetic field direction indicate *N02 formation at the N(3)-N(4) bond location of the HMX molecule. This differs from our previous study of RDX (hexahydro-l,3,5-trinitro-s-triazine) which indicates *No2 formation at the axial and equatoríal N-N bond positions of the RDX molecule as a result of ultraviolet photolysis.

In situ characterization of the “melt” phase of RDX and HMX by rapid-scan FTIR spectroscopy

Direct information on the chemical composition of the “melt” phase of RDX and HMX has been obtained by using newly developed methods of Fourier transform infrared spectroscopy which allow for rapid acquisition of spectra. At a heating rate of about 150°C min −1 fresh RDX and HMX “melts” are found to be comprised mostly of intact RDX and HMX molecules. Thermal decomposition products (probably N-hydroxymethylformamide) appear in RDX as the “melt” ages. Decomposition of HMX below the melting temperature is evident with a formamidelike species being the principal residue in the condensed phase. The molecular structure of RDX and HMX in their respective “melts” is similar to that present in CD 3CN solution.