RESUMEN

The vibrational spectrum of liquid and solid nitroethane was measured as a function of pressure. Both Raman scattering and absorption IR spectroscopies were applied to samples of nitroethane, statically compressed at ambient temperature to a maximum pressure of 8.0 GPa and 16.9 GPa, respectively. A new amorphous to crystalline transition pressure was found to lie between 1.59-1.63 GPa. Davydov splitting of internal modes into two components suggests two molecules associated with the unit cell, which is consistent with the DFT predictions made in a previous study. For most bands below 1200 cm-1, pressure induced mode progression was consistent with DFT predictions. Conversely, observed mode shifts in the 2950-3100 cm-1 region were generally stiffer than their DFT counterparts. A discontinuity in mode evolution between 3.7-4.3 GPa was observed for a number of modes and shown to coincide with hydrogen bond rearrangement in this pressure region. Preferred orientation and crystallite strain might explain the increased scatter between the various pressure induced mode shift cycles. Time intervals on the order of ∼30 h may be required between spectra, in order to give the crystallites time to equilibrate their strain.

RESUMEN

Ambient temperature static high pressure compression of liquid nitroethane has been performed using the diamond anvil technique. The first transition to a crystalline powder around 4.2 GPa did not return reliable indexing solutions and could be the result of a mixture of phases. Subsequent cycling of the pressure apparatus with the same sample triggered a solid-solid transition around 4.9-4.3 GPa on the downstroke. Indexing returned a monoclinic structure with spacegroup P21 or P21/m which is consistent with the lowest enthalpy solution predicted from density functional theory (DFT) calculations performed in this study, namely, P21. Attempts to reproduce the first anomalous mixture of phases with other samples were not successful; rather, the numerically preferred monoclinic structure manifests itself after a liquid-solid transition around 4.3-3.6 GPa, a value consistent with the results of a previous study. The transition typically occurs more readily on the downstroke. Incomplete Debye rings resulting from preferred orientation complicates any Rietveld refinement, though the DFT simulations predict 2 molecules per unit cell. Unit cell volumes during the upstroke were within 3% of that predicted by DFT calculations. This departure increases to about 9% below that from DFT predictions during the downstroke suggesting the presence of residual stresses due to non-hydrostatic conditions. Finally, a sudden relief in d-spacing values around 3.7-4.3 GPa during the downstroke is thought to be due to the influence of intermolecular hydrogen bonding.

RESUMEN

The structures and properties of several energetic compounds based on a high-nitrogen-content anion, namely 2,3,5,6-tetra(1H-tetrazol-5-yl)pyrazine (H4 TTP) are reported here for the first time. These energetic salts were synthesized by reacting H4 TTP with various alkali metal hydroxides (sodium, potassium, rubidium, caesium) and N-based (ammonia, hydrazine, hydroxylamine, guanidine carbonate, aminoguanidine bicarbonate). The resulting materials were comprehensively characterized by multinuclear (1 H, 13 C) NMR spectroscopy, infrared spectroscopy, elemental analysis, DSC, as well as low-temperature single-crystal X-ray diffraction. Heats of formation for the metal-free species as well as detonation parameters were calculated. The presented energetic materials (EMs) show high thermal stability (207 °C≤Tdec ≤300 °C), while the metal-free ionic derivatives exhibit desirable properties such as detonation velocity (6873 m s-1 ≤VC-J ≤8364 m s-1 ), detonation pressure (14.3 GPa≤pC-J ≤24.9 GPa), and specific impulse (141.4≤Isp ≤192.5 s).

RESUMEN

The computation of s-type Gaussian pseudopotential matrix elements involving low powers of the distance from the pseudopotential center using Gaussian orbitals can be reduced to familiar integrals. They may be directly expressed as either simple three-center overlap integrals for even powers of the radial distance from the pseudopotential center or related to the three-center nuclear integrals of a Gaussian charge distribution for odd powers. Orbital angular momentum about each atom is added to these integrals by solid-harmonic differentiation with respect to its center. The solid-harmonic addition theorem allows all the integrals to be factored into products of invariant one-dimensional integrals involving the Gaussian exponents and angular factors that contain the azimuthal quantum numbers but are independent of all Gaussian exponents. Precomputing the angular factors allow looping over all Gaussian exponents about the three centers. The fact that solid harmonics are eigenstates of angular momentum removes the singularities seen in previous treatments of pseudopotential matrix elements.

RESUMEN

5,5'-bis(1H-tetrazolyl)amine (BTA), a nitrogen rich molecular solid has been investigated under compression at room temperature [corrected]. Powder x-ray diffraction using synchrotron radiation and micro-Raman spectroscopy were carried out to pressures up to 12.9 GPa. BTA conserves the crystalline structure of its room condition phase up to the highest pressure, i.e., an orthorhombic unit cell (Pbca). A fit of the isothermal compression data to the Birch-Murnaghan equation of state reveals the high compressibility of BTA. An analysis of the volume change with pressure yields a bulk modulus and its derivative similar to that of high-nitrogen content molecular crystals. Upon laser heating to approximately 1100 K, the sample decomposed while pressurized at 2.1 GPa, resulting in a graphitic compound. Finally, numerical simulations demonstrate that the minimum energy conformation is not experimentally observed since a higher energy conformation allows for a more stable dense packing of the BTA molecules.

Asunto(s)

RESUMEN

A three-stage bonding pathway towards high-pressure chemical transformations from molecular precursors or intermediate states has been identified by first-principles simulations. With the evolution of principal stress tensor components in the response of chemical bonding to compressive loading, the three stages can be defined as the van der Waals bonding destruction, a bond breaking and forming reaction, and equilibrium of new bonds. The three-stage bonding pathway leads to the establishment of a fundamental principle of chemical bonding under compression. It reveals that during high-pressure chemical transformation, electrons moving away from functional groups follow anti-addition, collision-free paths to form new bonds in counteracting the local stress confinement. In applying this principle, a large number of molecular precursors were identified for high-pressure chemical transformations, resulting in new materials.

RESUMEN

Density-functional and coupled cluster calculations suggest that the stability, against unimolecular dissociation, of the cyclic D(3h) trimer of CO2, 1,3,5-trioxetanetrione, is greater than all but one other chemically bound oligomer of CO2. It requires far less energy to produce, on a per CO2 basis, than the low-symmetry cyclic 1,2 dioxetanedione dimer, but its kinetic stability against unimolecular dissociation is much lower. The extreme stability of the dimer, which makes it an excellent intermediate in chemiluminescence, is caused by an extreme range of geometric change to its transition state leading to a trapezoidal potential energy surface. The thermodynamically more stable trimer affords a low pressure pathway from molecular carbon dioxide to the extended covalent structure at high pressure.

RESUMEN

A nitrogen-rich ligand bis(1H-tetrazol-5-yl)amine (H(3)bta) was employed to isolate a new Fe(III) complex, Na(2)NH(4)[Fe(III)(Hbta)(3)]·3DMF·2H(2)O (1). Single crystal X-ray diffraction revealed that complex 1 consists of Fe(III) ions in an octahedral environment where each metal ion is coordinated by three Hbta(2-) ligands forming the [Fe(III)(Hbta)(3)](3-) core. Each unit is linked to two one-dimensional (1-D) Na(+)/solvent chains creating a two-dimensional (2-D) network. In addition, the presence of multiple hydrogen bonds in all directions between ammonium cation and ligands of different [Fe(III)(Hbta)(3)](3-) units generates a three-dimensional (3-D) network. Magnetic measurements confirmed that the Fe(III) center undergoes a Spin Crossover (SCO) at high temperature (T(1/2) = 460(10) K).

RESUMEN

High pressure ab initio evolutionary structure searches resulted in a hydronitrogen solid with a composition of (NH)(4). The structure searches also provided two molecular isomers, ammonium azide (AA) and trans-tetrazene (TTZ) which were previously discovered experimentally and can be taken as molecular precursors for high pressure synthesis of the hydronitrogen solid. The computed pressure versus enthalpy diagram showed that the transformation pressure to the hydronitrogen solid is 36 GPa from AA and 75 GPa from TTZ. Its metastability was analyzed by the phonon dispersion spectrum and room-temperature vibrational density of state together with the transformation energy barrier back to molecular phases at 298 K. The predicted energy barrier of 0.21 eV/atom means that the proposed hydronitrogen solid should be very stable at ambient conditions.

Asunto(s)

RESUMEN

On the basis of first-principles theory calculations, a nitrogen-rich C(3)N(12) solid was presented through a transformation from a molecular precursor, cyanuric triazide (C(3)N(3))(N(3))(3), under high pressure and temperature. The transformation mechanism is mainly governed by azide-tetrazole chain-ring tautomerism leading to the sp(2) to sp(3) orbital activation of all carbon atoms. The phase diagram and the equation of state were calculated together with the ambient metastability of the new C(3)N(12) solid that has a material density of 2.926 g cm(-3) and an energy density of 15.56 kJ g(-1).

RESUMEN

We present a theoretical study of a new hybrid material, nanostructured polymeric nitrogen, where a polymeric nitrogen chain is encapsulated in a carbon nanotube. The electronic and structural properties of the new system are studied by means of ab initio electronic structure and molecular dynamics calculations. Finite temperature simulations demonstrate the stability of this nitrogen phase at ambient pressure and room temperature using carbon nanotube confinement. This nanostructured confinement may open a new path towards stabilizing polynitrogen or polymeric nitrogen at ambient conditions.

RESUMEN

We present a novel method for time-dependent density functional theory calculations on dynamic linear response and electron density evolution in the real-time domain with the finite basis expansion approach of conventional quantum chemistry. To demonstrate the validity and efficiency of this method, dynamic polarizabilities of a water chain and diphenylene molecules are computed by employing the Chebyshev interpolation algorithm, which was developed by Baer and co-workers. The calculated dynamic polarizabilities show good agreement with those obtained from conventional linear response calculations. The density evolution in the real-time domain with application of a long-duration electric field gives electronic conduction in molecules, where a dynamic process of charge transfer is observed with the snapshots of response density in real time. Charge transfer oscillating with the frequency of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap is shown in a diphenylene molecule while there is little change in time for a water chain.