The nonaqueous microtitration of pentaerythritol tetranitrate (PETN)

Summary PETN has been determined by nonaqueous titration as an acid in pyridine and l, l,3,3-tetramethylguanidine. The titrant was tetrabutylammonium hydroxide. Endpoints were determined potentiometrically by a glass/modified calomel electrode system, or by two polarized platinum electrodes. The method may be applicable to other very weakly acidic compounds for which no other titrimetric methods are currently available.

---

Zusammenfassung Pentaeerythrit-Tetranitrat wurde als Säure gelöst in Pyridin und 1,1,3,3-Tetramethylguanidin mit Tetrabutylammoniumhydroxid titriert. Der Endpunkt wurde potentiometrisch mit dem System Glaselektrode/modifizierte Kalomelelektrode oder mit zwei polarisierten Platinelektroden bestimmt. Das Verfahren dürfte sich auch für andere sehr schwache Säuren eignen, für die andere maßanalytische Methoden zur Zeit nicht verfügbar sind.

---

---

Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore Laboratory under contract number W-7405-ENG-48.     

Spectrophotometric determination of pentaerythritol tetranitrate (PETN) in waste water from lead styphnate primer plants

An accurate spectrophotometric method is proposed for the determination of pentaerythritol tetranitrate (PETN) in waste water from lead styphnate primer plants by use of phenoldisulphonic acid. The waste water is filtered through a sintered glass crucible and the PETN is determined in the filtrate and the residue. In the determination of PETN in the filtrate, sodium hydroxide is added and the PETN is extracted with methylene chloride (in alkaline solution, styphnate and TNT are not extracted). The methylene chloride solution is then evaporated to dryness, the residue is treated with a solution of phenoldisulphonic acid in sulphuric acid, water and ammonia are added, and the yellow colour is measured. In the determination of PETN in the residue, the PETN is dissolved in acetone, an aliquot of the acetone solution is treated with water and sodium hydroxide, the PETN is extracted with methylene chloride and the colour is developed as above. Various factors affecting the determination were investigated. The solubility of PETN in water was studied.     

Orientation dependent far-infrared terahertz absorptions in single crystal pentaerythritol tetranitrate (PETN) using terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy (THZ-TDS) has been used to measure the absorption spectra in the range 7-100 cm(-1) (0.2-3 THz) of single crystal pentaerythritol tetranitrate (PETN). Absorption was measured in transmission mode as a function of incident polarization with the incident and transmitted wave vectors oriented along the crystallographic directions [100], <10(a/c)(2)>, and <110>. Samples were rotated with respect to the incident polarization while absorption was measured at both 300 and 20 K. Comparatively minor differences were observed among the three orientations. Two broad absorptions at 72 and >90 cm(-1), and several weaker absorptions at 36, 55, 80, and 82 cm(-1), have been observed at cryogenic temperatures.     

Terahertz normal mode relaxation in pentaerythritol tetranitrate

Normal vibrational modes for a three-dimensional defect-free crystal of the high explosive pentaerythritol tetranitrate were obtained in the framework of classical mechanics using a previously published unreactive potential-energy surface [J. Phys. Chem. B 112, 734 (2008)]. Using these results the vibrational density of states was obtained for the entire vibrational frequency range. Relaxation of selectively excited terahertz-active modes was studied using isochoric-isoergic (NVE) molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Dependence of the relaxation time on the initial modal excitation was considered for five excitation energies between 10 and 500 kT and shown to be relatively weak. The terahertz absorption spectrum was constructed directly using linewidths obtained from the relaxation times of the excited modes for the case of 10 kT excitation. The spectrum shows reasonably good agreement with experimental results. Dynamics of redistribution of the excited mode energy among the other normal modes was also studied. The results indicate that, for the four terahertz-active initially excited modes considered, there is a small subset of zero wave vector (k = 0) modes that preferentially absorb the energy on a few-picosecond time scale. The majority of the excitation energy, however, is transferred nonspecifically to the bath modes of the system.     

季戊四醇四硝酸酯的分子结构和热解机理

运用SCF-AM1-MO方法，在RHF和UHF水平上分别全优化计算了季戊四醇四硝酸酯(太安)的分子结构和热解机理。计算所得三种构象的能量相差很小、且相互转化的能垒也较小。其均裂O-NO~2键(生成双自由基)的热解引发反应活化能较小，而通过α-H转移产生RCHO和HONO的环消除反应需要较高活化能。     

Electric and sorption properties of controlled pore glasses

A series of controlled pore glasses (CPG) was made from the same raw glass by means of different chemical and thermal treatments. BET specific area, selectivity of cesium adsorption in relation to sodium, ionosorption capacity under dynamic conditions of these CPG were measured and potentiometric titrations carried out. ₀ vs. pH curves show i.e.p. and c.i.p. values similar to those for silica. KOH treatment leads to increase of BET specific surface but ionosorption capacity is decreased.     

Melting of solvents nanoconfined by polymers and networks

Thermoporosimetry is becoming increasingly used to study nanoscale heterogeneity and structure in polymer networks. The starting point for thermoporosimetry is the Gibbs-Thomson (GT) relation between melting point and inverse crystal size. In the case of polymers, the Flory-Huggins (FH) model also predicts that there is a depression of the melting point because of the mixing of the polymer and the solvent molecules, and this needs to be taken into account. The first step in analysis of size heterogeneity using thermoporosimetry and the GT equation requires that there be quantitative agreement between the FH theory and the melting points of the diluent in the uncrosslinked rubber. We find that both benzene and hexadecane exhibit excessive melting point depressions in uncrosslinked polyisoprene. This may imply that the uncrosslinked polymer is divided into ‘nanoheterogeneities’. We further find that the heat of fusion decreases as polymer concentration increases for the benzene, but not for the hexadecane. Finally, we compare pore size distributions obtained for a crosslinked polyisoprene as determined from the melting behavior of n-hexadecane as a diluent, using different references for how the uncrosslinked polymer behaves. While number average distribution is not very different between the different analyses, the weight average distribution is. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3475–3486, 2006     

Decomposition of pentaerythritol tetranitrate [C(CH2ONO2)4] following electronic excitation

We report the experimental and theoretical study of the decomposition of gas phase pentaerythritol tetranitrate (PETN) [C(CH(2)ONO(2))(4)] following electronic state excitation. PETN has received major attention as an insensitive, high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. The initial decomposition mechanism of PETN is explored with nanosecond energy resolved spectroscopy and quantum chemical theory employing the ONIOM algorithm at the complete active space self-consistent field (CASSCF) level. The nitric oxide (NO) molecule is observed as an initial decomposition product from PETN at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A (2)Σ(+) ← X (2)Π electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for PETN, which generates the NO product with a rotationally cold (~20 K) and a vibrationally hot (~1300 K) distribution. Potential energy surface calculations at the ONIOM(CASSCF:UFF) level of theory illustrate that conical intersections play an important role in the decomposition mechanism. Electronically excited S(1) PETN returns to the ground state through the (S(1)/S(0))(CI) conical intersection, and undergoes a nitro-nitrite isomerization to generate the NO product.     

Atoms-in-molecules study of intra- and intermolecular bonding in the pentaerythritol tetranitrate crystal

Chemical bonding in the pentaerythritol tetranitrate crystal based on the experimental electron density obtained from X-ray diffraction data at 100 K and theoretical calculations at the experimental molecular geometry have been analyzed in terms of the Quantum Theory of Atoms in Molecules. Features of the intra- and intermolecular bond critical points and the oxygen atom lone-pair locations are discussed. Numerous intermolecular bonding interactions, including O...H and O...O, have been found and characterized. Atomic charges and atomic energies were integrated and compared with those for similar compounds. The N-O topological bond orders have been calculated for the first time, and the PETN atomic valences have been estimated.     

Raman scattering studies of the high-pressure stability of pentaerythritol tetranitrate, C(CH2ONO2)4

High-pressure Raman scattering studies have been performed on a crystalline energetic material, pentaerythritol tetranitrate C(CH2ONO2)4 (PETN), an important secondary explosive. In situ, ambient-temperature investigations employed diamond anvil cell techniques and nitrogen as a quasi-hydrostatic-pressure-transmitting medium. The pressure-induced alterations in the profiles of the Raman lines, including positions, bandwidths, and intensities, were studied in a compression sequence up to about 31.3 GPa and in a subsequent decompression to ambient conditions. The observed changes of the Raman spectra implied that PETN gradually densified and compressed smoothly up to the highest investigated pressures. Compression below 12 GPa gradually shifted all Raman peaks to higher frequencies without significantly changing their relative intensities or bandwidths. At higher pressures, the peak intensities of the Raman spectra decreased considerably and the bands broadened significantly. The Raman spectrum of the material quenched from 31.3 GPa to ambient conditions indicated that no pressure-driven permanent reconstructive modification or decomposition of the PETN structure occurred. That is, the spectral changes were completely reversible upon compression and subsequent decompression to ambient conditions.     

Ab initio all-electron periodic Hartree-Fock study of hydrostatic compression of pentaerythritol tetranitrate

We present a computational study of hydrostatic compression effects on the pentaerythritol tetranitrate (PETN) energetic material up to 22.7 GPa by means of the ab initio all-electron periodic Hartree-Fock quantum mechanical method with the STO-3G Gaussian basis set. We fitted the calculated volume-energy relation to the energy SJEOS polynomial function from which we obtained the compression dependence of the pressure (P), the bulk modulus (B), and its pressure derivative (B'). We also fitted the experimental volume-pressure relation to the pressure SJEOS polynomial function, which allowed us to calculate the experimental bulk modulus (B(exp)) and its pressure derivative (). Our calculated values, B = 6.73 GPa and B' = 24.63, are in reasonable agreement with the values B(exp) = 8.48 GPa and = 14.42 from our fit to the experimental X-ray data and with the value B(exp) = 9.8 GPa that was derived from the experimental elastic constants. In addition, we present a discussion on how the lattice vectors and the internal coordinates (i.e., bond lengths, bond angles, and torsion angles) of the C(CH(2)ONO(2))(4) molecules in the PETN lattice change during hydrostatic compression of the crystal. Our calculated results suggest that the C(CH(2)ONO(2))(4) molecules cannot be considered as being rigid but are in fact flexible, accommodating lattice compression through torsions, bendings in their bond angles, and contractions in their bond lengths. At pressures higher than about 8 GPa, however, both the C(CH(2)ONO(2))(4) molecules and the c lattice vector seem to stiffen somewhat. The a lattice vector does not exhibit this stiffening. As a consequence, the pressure dependence of the c/a ratio shows a minimum at about 8 GPa.     

Raman spectra of shock compressed pentaerythritol tetranitrate single crystals: anisotropic response

To gain insight into the anisotropic sensitivity of shocked pentaerythritol tetranitrate (PETN) single crystals, single-pulse Raman spectroscopy was used to examine the response of crystals shocked along the [100] (insensitive) and [110] (sensitive) orientations. High-resolution Raman spectra revealed several orientation-dependent features under shock compression: (i) substantially different stress dependence of the Raman shift for the CH(2) and NO(2) stretching modes for the two orientations, (ii) discontinuity in the stress dependence of the Raman shift for the CH(2) stretching modes above 4 GPa for the [110] orientation, and (iii) large broadening for the CH(2) and NO(2) asymmetric stretching modes for stresses above 4 GPa for the [110] orientation. The present data in combination with previous static pressure results provide support for conformational changes in PETN molecules for shock compression along the [110] (sensitive) orientation. Implications of the present results for the anisotropic sensitivity of shocked PETN are discussed.     

H and H NMR studies of cyclohexane nanocrystals in controlled pore glasses

The formation and behaviour of cyclohexane and cyclohexane-d₁₂ nanocrystals in mesoporous solids of well-defined dimensional constraints are studied by ¹H and ²H NMR. The NMR line widths, spin–spin relaxation times (T₂), spin–lattice relaxation times (T₁) and diffusivities (D) were measured as a function of temperature, and the results are discussed with reference to the values obtained for the bulk materials. The confined solids exhibit substantial changes in the phase behaviour and molecular dynamics. Thus, the line-shape measurements reveal a two-phase system consisting of a highly mobile component at the surface of the pore and a plastically crystalline phase in the centre of the pore. The liquid-like surface layer in the mesopores is observable well below the reduced transition temperature of the confined cyclohexane. However, the T₂ and diffusion measurements show that the mobile phase also embraces a minor component attributed to non-frozen liquid in pockets or offshoots.     

Polymer support oligonucleotide synthesis-XV: Synthesis of oligodeoxynucleotides on controlled pore glass (CPG) using phosphate and a new phosphite triester approach

Controlled pore glass (CPG) has been functionalized with suitably protected deoxynucleosides and used to synthesize d(TTTATT) and d(CATCAGGAAGT) following the phosphate triester approach with 1-(mesitylene-2-sulfonyl)-3-nitro-1,2,4-triazole (MSNT) as condensing agent and ZnBr₂; as detritylating agent. The efficiency of monomer and dimer additions is discussed. The nonamer d(AACCAGCAC) has been synthesized by a modified phosphite triester approach using suitably protected methyl deoxynucleoside-N-morpholino-phosphoamidites as new active phosphitylating nucleotides. The sequence has been built up by monomer additions after activation of the phosphoamidites with tetrazole, capping with acetic anhydride/DMAP and detritylation with either ZnBr₂; (purine nucleotides) or trichloroacetic acid (pyrimidine nucleotides). Deprotection are performed by treatment with thiophenol, aqueous concentrated ammonia and tert. butylamine and the nonamer characterized by reversed phase hplc, polyacrylamide gel electrophoresis and sequence determination. The favourable properties of CPG as carrier for oligonucleotide syntheses are discussed.     

Terahertz spectrum and normal-mode relaxation in pentaerythritol tetranitrate: effect of changes in bond-stretching force-field terms

Terahertz (THz) active normal-mode relaxation in crystalline pentaerythritol tetranitrate (PETN) was studied using classical molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Two modifications to the fully flexible non-reactive force field due to Borodin et al. [J. Phys. Chem. B 112, 734 (2008)] used in a previous study of THz-active normal-mode relaxation in PETN [J. Chem. Phys. 134, 014513 (2011)] were considered to assess the sensitivity of the earlier predictions to details of the covalent bond-stretching terms in the force field. In the first modification the harmonic bond-stretching potential was replaced with the Morse potential to study the effect of bond anharmonicity on the THz-region mode relaxation. In the second modification the C-H and nitro-group N-O bond lengths were constrained to constant values to mimic lower quantum occupation numbers for those high-frequency modes. The results for relaxation times of the initially excited modes were found to be insensitive to either force-field modification. Overall time scales for energy transfer to other modes in the system were essentially unaffected by the force-field modifications, whereas the detailed pathways by which the energy transfer occurs are more complicated for the Morse potential than for the harmonic-bond and fixed-bond cases. Terahertz infrared absorption spectra constructed using calculated normal-mode frequencies, transition dipoles, and relaxation times for THz-active modes were compared to the spectra obtained from the Fourier transform of the dipole-dipole time autocorrelation function (DDACF). Results from the two approaches are in near agreement with each other and with experimental results in terms of main peak positions. Both theoretical methods yield narrower peaks than observed experimentally and in addition predict a weaker peak at ω ～ 50 cm(-1) that is weak or absent experimentally. Peaks obtained using the DDACF approach are broader than those obtained from the normal-mode relaxation method.     

Thermodynamic analysis of pure and impurity doped pentaerythritol tetranitrate crystals grown at room temperature

Pentaerythritol tetranitrate (PETN) powders are used to initiate other explosives. During long-term storage, changes in powder properties can cause changes in the initiation performance. Changes in the morphology and surface area of aging powders are observed due to sublimation and growth of PETN crystals through coarsening mechanisms, (e.g. Ostwald ripening, sintering, etc.). In order to alleviate the sublimation of PETN crystals under service conditions, stabilization methods such as thermal cycling and doping with certain impurities during or after the crystallization of PETN have been proposed. In this report we present our work on the effect of impurities on the morphology and activation energy of the PETN crystals. The pure and impurity doped crystals of PETN were grown from supersaturated acetone solution by solvent evaporation technique at room temperature. The difference in the morphology of the impurity-doped PETN crystal compared to pure crystal was examined by optical microscopy. The changes in the activation energies and the evaporation rates are determined by thermogravimetry (TG). Our activation energies of evaporation agree with earlier reported enthalpies of vaporization. The morphology and activation energy of PETN crystals doped with Ca, Na, and Fe cations are similar to that for pure PETN crystal, whereas the Zn-ion-doped PETN crystals have different morphology and decreased activation energy.     

Cavitation and pore blocking in nanoporous glasses

In gas adsorption studies, porous glasses are frequently referred to as model materials for highly disordered mesopore systems. Numerous works suggest that an accurate interpretation of physisorption isotherms requires a complete understanding of network effects upon adsorption and desorption, respectively. The present article deals with nitrogen and argon adsorption at different temperatures (77 and 87 K) performed on a series of novel nanoporous glasses (NPG) with different mean pore widths. NPG samples contain smaller mesopores and significantly higher microporosity than porous Vycor glass or controlled pore glass. Since the mean pore width of NPG can be tuned sensitively, the evolution of adsorption characteristics with respect to a broadening pore network can be investigated starting from the narrowest nanopore width. With an increasing mean pore width, a H2-type hysteresis develops gradually which finally transforms into a H1-type. In this connection, a transition from a cavitation-induced desorption toward desorption controlled by pore blocking can be observed. Furthermore, we find concrete hints for a pore size dependence of the relative pressure of cavitation in highly disordered pore systems. By comparing nitrogen and argon adsorption, a comprehensive insight into adsorption mechanisms in novel disordered materials is provided.     

Surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses: curvature effects observed by 1H NMR

We have measured the thickness of the pre-molten surface layer that appears at the interface of octamethylcyclotetrasiloxane (OMCTS) to the matrix in controlled pore glasses with pore diameters ranging 7.5-73 nm. Except for the glass with the largest pores, the layer thickness data for different pore diameters fall on a single master curve when plotted versus Tm - T, where Tm is the size-dependent volume melting point of the pore-confined OMCTS. Hence, at a single temperature, the surface layer thickness depends strongly on the curvature of the pore wall and therefore that of the solid-liquid interface. For temperatures where it exceeds two monolayers, the layer thickness depends logarithmically on Tm - T; for the glass with the largest pores, this turns into a power law with the exponent -1/2. The results are interpreted in terms of a continuous model of the solid-liquid interface with an arbitrary curvature. Because OMCTS is a weakly polar molecule with close to spherical shape, our data also lend themselves to Lennard-Jones type simulations.