Correlation Between Energy Transfer Rate and Atomization Energy of Some Trinitro Aromatic Explosive Molecules

An assumptive theoretical relationship is suggested to describe the property of molecular atomization energy and energy transfer rate in the initiation of explosions. To investigate the relationship between atomization energy and energy transfer rate, the number of doorway modes of explosives is estimated by the theory of Dlott and Fayer in which the rate is proportional to the number of normal mode vibrations. It was evaluated frequencies of normal mode vibrations of eight molecules by means of density functional theory (DFT) at the b3p86/6-31G(d,p) level. It is found that the number of doorway modes shows a linear correlation to the atomization energies of the molecules, which were also calculated by means of the same method. A mechanism of this correlation is discussed. It is also noted that in those explosives with similar molecular structure and molecular weight, the correlation between the atomization energy and the number of doorway modes is higher.     

Theoretical studies on high energetic density nitramine explosives containing pyridine

The insensitive property of explosives containing pyridine is combined with the high energy of nitramine explosives,and the concept of new nitramine explosives containing pyridine is proposed,into which nitramine group with N N bonds is introduced as much as possible.Based on molecular structures of nitramine compounds containing pyridine,density functional theory(DFT) calculation method was applied to study designed molecules at B3LYP/6-31+G(d) level.The geometric and electronic structures,density,heats of formation(HOF),detonation performance and bond dissociation energies(BDE) were investigated and comparable to 1,3,5-trinitro-1,3,5-triazinane(RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane(HMX).The simulation results reveal that molecules B and D perform similarly to traditionally used RDX.Molecule E outperform RDX,with performance that approach that of HMX and may be considered as potential candidate of high energy density compound(HEDC).These results provide basic information for molecular design of novel high energetic density compounds.     

Infrared spectra of diphenylphthalide and polydiphenylenephthalide

The splitting of the ν(C=O) absorption band (AB) of about 12 cm⁻¹ is found in the IR spectra of diphenylphthalide (DPP) in the crystalline phase and CCl₄ solution. In the crystalline phase, this splitting is likely to be caused by the inequivalence of DPP molecules in the crystallographic cell, while in the solution, by the dimerization of DPP molecules via dipole-dipole and/or hydrogen bonds. A theoretical low-frequency shift of the ν(C=O) AB for a complex of two DPP molecules (in comparison with a single molecule) is 14 cm⁻¹ in the PBE/3ξ approximation, which is close to the experimentally observed splitting. In two quantum chemical approximations (B3LYP/6-311G(d,p) (I) and PBE/3ξ (II)) the optimal structure and vibrational spectrum of DPP are calculated. Approximation I better reproduces the intensities, whereas approximation II better reproduces the IR frequencies of the DPP spectrum. Almost all 48 ABs of the IR spectrum of DPP are assigned to theoretical normal vibrations (modes). Based on the potential energy distribution over natural coordinates and the visualization of vibrations, experimental ABs (and the corresponding modes) are assigned to the stretching and bending vibrations of certain bonds in the DPP molecule. In particular, ABs at 1107 cm⁻¹ and 970 cm⁻¹ are assigned to the ν(-OC-O-) and ν(-C-O-) stretching vibrations, respectively, of the DPP lactonic ring, which differs from the previously accepted assignment. The results of the interpretation of the DPP spectrum are used to assign a number of ABs in the IR spectrum of polydiphenylenephthalide (PDP), for which DPP is a model compound. According to the calculations in approximation II of the vibrational spectrum of a model valence-bonded dimeric molecule, the intense complex AB at 800–870 cm⁻¹ in the IR spectrum of PDP is mainly due to the out-of-plane bending vibrations of C-H bonds in the 1,4-substituted benzene rings of polymer biphenyl moieties and the bending vibrations of the lactonic ring.     

On the molecular origin of the sensitivity to impact of cyclic nitramines

Nitramine explosives can combine relative insensitivity to initiation and great energy content. In this work, based on a previous approach developed for nitroaromatic explosives, we propose four mathematical models to correlate impact sensitivity, given by the h₅₀ value, to molecular charge properties. Fourteen cyclic nitramines were studied using Density Functional Theory (DFT). Six molecules of the set have measured h₅₀ values, which were used to evaluate the sensitivity models. Converged DFT charge densities of the molecules were partitioned and analyzed according to the distributed multipole analysis (DMA) atom-centered method. The sensitivity models were based on the DMA electric multipole values. The electron withdrawing role of the nitro group and the strong polarization of the charges of the nitrogen atom in the amine group were clearly identified. The influence of the electronic properties on the sensitivity of the explosives was characterized by including in the sensitivity models the charge values of the nitro or the nitramine groups and electron delocalization, the latter quantified by the DMA quadrupole values of the ring atoms. Inclusion of electron delocalization effects can improve the prediction of h₅₀ values for two out of the five strained-ring nitramines in the set. The charge values of the nitramine groups are the most important molecular property affecting the impact sensitivity. The h₅₀ values of eight nitramine explosives of the set not available experimentally were computed.     

The very far-infrared spectra of energetic materials and possible confusion materials using terahertz pulsed spectroscopy

We have measured the terahertz absorption spectra of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), pentaerythritol tetranitrate (PETN), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), 2,4,6-trinitrotoluene (TNT), the plastic explosives Semtex H, SX2, and Metabel, and a number of confusion materials using terahertz pulsed transmission spectroscopy. Spectral fingerprints were obtained from 3 to 133 cm⁻¹. The spectra of the plastic explosives are dominated by the spectral signatures of their explosive components due to low frequency vibrations and crystalline phonon modes. Importantly, the terahertz spectra of the confusion materials show no resemblance to the explosives spectra. The refractive indices obtained for the plastic explosives and confusion materials allowed us to derive reflectance spectra, which appear distinct and so suggest that terahertz reflection spectroscopy is a suitable tool for the detection of concealed explosives in security applications.     

Raman spectroscopic investigation of the antimalarial agent mefloquine

The antimalarial agent mefloquine was investigated using Fourier transform near-infrared (FT NIR) Raman and FT IR spectroscopy. The IR and Raman spectra were calculated with the help of density functional theory (DFT) and a very good agreement with the experimental spectra was achieved. These DFT calculations were applied to unambiguously assign the prominent features in the experimental vibrational spectra. The calculation of the potential energy distribution (PED) and the atomic displacements provide further valuable insight into the molecular vibrations. The most prominent NIR Raman bands at 1,363 cm⁻¹ and 1,434 cm⁻¹ are due to C=C stretching (in the quinoline part of mefloquine) and CH₂ wagging vibrations, while the most intense IR peaks at 1,314 cm⁻¹; 1,147 cm⁻¹; and 1,109 cm⁻¹ mainly consist of ring breathings and δCH (quinoline); C–F stretchings; and asymmetric ring breathings, C–O stretching as well as CH₂ twisting/rockings located at the piperidine moiety. Since the active agent (mefloquine) is usually present in very low concentrations within the biological samples, UV resonance Raman spectra of physiological solutions of mefloquine were recorded. By employing the detailed non-resonant mode assignment it was also possible to unambiguously identify the resonantly enhanced modes at 1,619 cm⁻¹, 1,603 cm⁻¹ and 1,586 cm⁻¹ in the UV Raman spectra as high symmetric C=C stretching vibrations in the quinoline part of mefloquine. These spectroscopic results are important for the interpretation of upcoming in vitro and in vivo mefloquine target interaction experiments.     

Infrared spectra of cyclic ethers and their derivatives

Data for the characteristic bands of cyclic ethers are reviewed. The infrared spectra of a number of 2-mono- and 2, 5-di-substituted derivatives of tetrahydrofuran are investigated. Absorption bands at about 900 cm⁻¹ are related to pulsation vibrations, and those at about 1200 cm⁻¹ to antisymmetric skeletal vibrations, of the tetrahydrofuran ring. It is shown that to confirm the presence of a tetrahydrofuran ring in a molecule, it is necessary to take into account not only the band of valence antisymmetric vibrations of the group C-O-C (ν _C-O-C ^as 1075 cm⁻¹), but also bands due to ring pulsation vibrations (ring symmetric valence vibrations ν _sk ^s ∼ 900 cm¹).     

Exploring the physical,chemical and thermal characteristics of a new potentially insensitive high explosive RX-55-AE-5

Current work at Lawrence Livermore National Laboratory (LLNL) includes both understanding properties of old explosives and measuring properties of new ones. The necessity to know and understand the properties of energetic materials is driven by the need to improve performance and enhance stability to various stimuli, such as thermal, friction and impact insult. This paper will concentrate on the physical properties of RX-55-AE-5, which is formulated from heterocyclic explosive, 2,6-diamino-3,5-dinitropyrazine-1-oxide, LLM-105, and 2.5% Viton A. Differential scanning calorimetry (DSC) was used to measure a specific heat capacity, C _p, of≈0.950 J g⁻¹ °C⁻¹, and a thermal conductivity, κ, of≈0.475 W m⁻¹ °C⁻¹. The LLNL kinetics modeling code Kinetics05 and the Advanced Kinetics and Technology Solutions (AKTS) code thermokinetics were both used to calculate Arrhenius kinetics for decomposition of LLM-105. Both obtained an activation energy barrier E≈180 kJ mol⁻¹ for mass loss in an open pan. Thermal mechanical analysis, TMA, was used to measure the coefficient of thermal expansion (CTE). The CTE for this formulation was calculated to be ≈61 μm m⁻¹ °C⁻¹. Impact, spark, friction are also reported.     

On the imaging of land mines using e+−e− pair production

The problem of disposing of abandoned land mines is very serious in many countries. Anti-personnel land mines (APM's) contain as little as 50 gram or less of explosive, which is enough to take off an adult's foot, or to kill a child. Anti-tank mines (ATM's), designed to penetrate the armour on the bottom of a tank, are much larger. Current techniques of finding them are not adequate. All practical high explosives contain 20% or more of nitrogen, which has a thermal neutron cross section of 75 mbarn, producing γ's of up to 10.8 MeV. The idea of using this property to detect explosives has been tested by others, but because of backgrounds is unable to find anything less than several hundred grams of explosive. The refinement proposed here is to convert the γs, track the resulting e⁺−e⁻ pairs in MWPC's, and use the information to locate the γ source, i.e. the mine. The directional information provided should reduce the backgrounds considerably. Result of an experimental test are presented, and possibilities for the future discussed.     

Vibrational spectra and stereochemistry of mono- and polysaccharides. IV. α-methyl-d-glucoside, α-methyl-d-galactopyranoside,and α-methyl-d-mannopyranoside

The IR and Raman spectra of α-methyl-D-glucoside (αMDG), α-methyl-D-galactoside (αMDGal), and α-methyl-D-mannoside (αMDM) are compared. The main distinctions between these spectra have been interpreted using experimental and theoretical data on the frequencies and modes of normal vibrations and on the potential energy distributions over particular bonds and atomic groups of the substances under investigation. Spectral characteristics that are determined by different configurations of C−O (CH) groups attached to C2 and C4 atoms and different conformations of C6H₂O6H fragments have been revealed. It has been established that replacement of hydroxyls at C1 with methoxy groups significantly increases the number of frequencies of normal vibrations that are localized mostly in the C1C2 bonds (particularly in αMDGal). For the αMDGal molecules with axial C4O4 groups, the number of frequencies of normal vibrations that show predominant contributions of these groups of PED is smaller. For the αMDM molecules, in which the conformation of CH₂OH groups (with respect to rotation around the C5C6 bonds) differs from that in αMDG and αMDGal molecules, most vibrations are localized in the C6O6 bonds rather than in the C5O5 bonds. This fact is very important in correlating the vibrational spectra and structural properties of mono- and polysaccharides. B.I. Stepanov Institute of Physics, Belarus Academy of Sciences. V. Tshebyatoski Institute of Low Temperatures and Structural Studies, Polish Academy of Sciences. Translated fromZhurmal Struktumoi Khimii, Vol. 36, No. 3, pp. 456–466, May–June, 1995. Translated by I. Izvekova     

Pi-stacked interactions in explosive crystals: buffers against external mechanical stimuli

The pi-stacked interactions in some explosive crystal packing are discussed. Taking a typical pi-stacked explosive 2,4,6-trinitrobenzene-1,3,5-triamine (TATB) as a sample and using molecular simulations, we investigated the nature of the pi-stacked interactions versus the external mechanical stimuli causing possible slide and compression of explosives. As a result, between the neighbor layers in the TATB unit cell, the electrostatic attraction decreases with a little decrease of vdW attraction when its top layer slides, whereas the vdW attraction increases with a decrease of electrostatic attraction when TATB crystal is compressed along its c axis. Meanwhile, we studied the correlation between the pi-stacked structures and the impact sensitivities of explosives by means of three representatives including TATB with typical planar pi-stacked structures, 2,2-dinitroethylene-1,1-diamine (Fox-7) with wavelike pi-stacked structures, and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) without pi-stacked structure. The results showed that pi-stacked structures, particularly planar layers, can effectively buffer against external mechanical stimuli. That is, pi-stacked structures can partly convert the mechanical energy acting on them into their intermolecular interaction energy, to avoid the increase of the molecular vibration resulting in the explosive decomposition, the formation of hot spots, and the final detonation. This is another reason for the low mechanical sensitivity of pi-stacked explosives besides their stable conjugated molecular structures.     

Molecular structure of phthalocyaninato lanthanide LnPc(OAc) complexes derived from the FTIR and FT Raman studies

Room temperature Fourier transform IR and Raman spectra in the range 30–4000 cm⁻¹ and 80–4000 cm⁻¹ of Dy, Ho, Er and Lu phthalocyanide PcLn(OAc)-type complexes have been measured, respectively. The assignment of the bands observed has been made on the literature data. The molecular structure of the PcLnX-type derivatives has been discussed on the basis of the group theory taking into account the shape and number of the bands corresponding to the stretching and bending vibrations of the LnN₄O coordination polyhedron as well as whole PcLn(OAc) complex.     

Phase transitions,conformational lability,and intermolecular interactions in alkoxycyanobiphenyls

Vibrational spectroscopy methods (IR absorption, Raman scattering, calculations) were used to study changes in molecular structures of alkoxycyanobiphenyls during phase transitions. The spectra were measured in the 33–3500 cm⁻¹ region at temperatures of 100–450 K. The temperature dependences of the IR bands that correspond to the noncharacteristic vibrations of molecular fragments between the phenyl rings and the alkyl radicals point to the conformational polymorphism of these molecules. An analysis of the Raman bands corresponding to the characteristic vibrations of the C−H bonds of alkyl radicals [q(CH)], the C−H and C−C bonds of phenyl rings [q(CH) and Q(CC)], and the CN bonds of the cyano groups [Q(CN)] suggests significant intermolecular interactions. The conformational lability and intermolecular interactions are associated with differences in molecular packing in the substances of this homologous series. Saratov State University. Institute of Solid State Physics, Rostov State University. Institute of Physics, Uzbekistan Academy of Sciences. Samarkand State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 5, pp. 814–822, September–October, 1995. Translated by I. Izvekova     

Investigation of selective molecular interactions using two-dimensional Fourier transform IR spectroscopy

A novel method of studying molecular interactions is introduced. It is a method based on the framework of a two-dimensional (2D) infrared (IR) correlation spectroscopy technique with a new data pretreatment strategy. In this method, an additional external perturbation stimulates the system to cause some selective changes in the state, order, and surroundings of system constituents. The overall response of the stimulated system to the applied external perturbation leads to distinctive changes in the measured spectrum, and a series of perturbation-induced dynamic spectra are collected in a systematic manner. Such a set of dynamic spectra are then transformed into a set of 2D correlation spectra by cross-correlation analysis. Temperature was chosen as an external perturbation, and the molecular interaction between 4-aminopyridine (Apy) and methacrylic acid (MAA) was investigated by 2D IR correlation spectroscopy. Synchronous cross peaks exist between the stretching vibration of the C–O group of MAA at 1,298 and 1,202 cm⁻¹ and the C=N group of Apy at 1,531 cm⁻¹, and between the carbonyl group of MAA at 1,705 cm⁻¹ and the amino group of Apy at 3,382 and 3,212 cm⁻¹. The synchronous cross peaks are from orientation of MAA and Apy vibrations generated at the same time; the synchronization of microstructure movements in the molecules indicates that there exists strong interactions between MAA and Apy. According to 2D correlation rules, static electricity and hydrogen-bonding interactions exist between Apy and MAA. Such results were further verified by ¹H-NMR spectroscopy. The successful application demonstrates that 2D IR correlation spectroscopy may be a convenient and effective method in the study of molecular interactions.     

DFT calculation of α-cyclodextrin dimers. contribution of hydrogen bonds to the energy of formation

The structures and energies of formation of α-cyclodextrin (α-CD) dimers formed according to the “head-to-head” (HH), “head-to-tail” (HT), and “tail-to-tail” (TT) modes, harmonic vibrational frequencies, and intensities of IR bands of the IR transitions were calculated by the DFT/PBE density functional method with full geometry optimization without symmetry restrictions. The spectral data were transformed into spectral patterns. An α-CD molecule can exist in two isomeric forms close in energy, namely, α-CD⁻ and α-CD⁺, with different directions of the ring intramolecular hydrogen bonds. Among the three α-CD⁻ dimers, the highest dimerization energy (E _d/kcal mol⁻¹) belongs to the HH⁻ (68.9), TT⁻ (43.4), and HT⁻ (24.8) dimers. The strength of the α-CD⁺ dimers decreases in the series: TT⁺ (56.7), HT⁺ (49.4), and HH⁺ (42.4). The energies E _H of hydrogen bonds were calculated from the low-frequency shifts of bands of stretching vibrations of the OH groups involved in the formation of these hydrogen bonds. The E _H value for each dimer correlates with E _d. A possibility of formation of intermolecular hydrogen bonds is a driving force of association of α-CD molecules in aqueous solutions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1289–1296, August, 2006.     

Lewis acid–base interactions enhance explosives sensing in silacycle polymers

The high sensitivity of silole- and silafluorene-containing polymers for detecting organic nitro, nitrate, and nitramine explosives cannot be solely attributed to favorable analyte–polymer hydrophobic interactions and amplified fluorescence quenching due to delocalization along the polymer chain. The Lewis acidity of silicon in conjugated poly(silafluorene-vinylene)s is shown to be important. This was established by examining the ²⁹Si NMR chemical shifts (Δ) for the model trimer fragment of the polymer CH₃–silafluorene–(trans-C₂H₂)–silafluorene–(trans-C₂H₂)–silafluorene–CH₃. The peripheral and central silicon resonances are up-field from a TMS reference at −9.50 and −18.9 ppm, respectively. Both resonances shift down-field in the presence of donor analytes and the observed shifts (0 to 1 ppm) correlate with the basicity of a variety of added Lewis bases, including TNT. The most basic analyte studied was acetonitrile and an association constant (K _a) of 0.12 M⁻¹ was calculated its binding to the peripheral silicon centers using the Scatchard method. Spin-lattice relaxation times (T ₁) of 5.86(3) and 4.83(4) s were measured for the methyl protons of acetonitrile in benzene-d ₆ at 20 °C in the absence and presence of the silafluorene trimer, respectively. The significant change in T ₁ values further supports a binding event between acetonitrile and the silafluorene trimer. These studies as well as significant changes and shifts observed in the characteristic UV–Vis absorption of the silafluorene group support an important role for the Lewis acid character of Si in polymer sensors that incorporate strained silacycles. The nitro groups of high explosives may act as weak Lewis-base donors to silacycles. This provides a donor–acceptor interaction that may be crucial for orienting the explosive analyte in the polymer film to provide an efficient pathway for inner-sphere electron transfer during the electron-transfer quenching process. Figure   Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Secondary electrospray ionization (SESI) of ambient vapors for explosive detection at concentrations below parts per trillion

We determine the sensitivity of several commercial atmospheric pressure ionization mass spectrometers towards ambient vapors, ionized by contact with an electrospray of acidified or ammoniated solvent, a technique often referred to as secondary electrospray ionization (SESI). Although a record limit of detection of 0.2 × 10⁻¹² atmospheres (0.2 ppt) is found for explosives such as PETN and 0.4 ppt for TNT (without preconcentration), this still implies the need for some 10⁸–10⁹ vapor molecules/s for positive identification of explosives. This extremely inefficient use of sample is partly due to low charging probability (∼10⁻⁴), finite ion transmission, and counting probability in the mass spectrometer (1/10 in quadrupoles), and a variable combination of duty cycle and background noise responsible typically for a 10³ factor loss of useful signal.     

Competition between reactivity and relaxation in free electron attachment to molecules

The reactivity of the C₆F₅X (X=F, Cl, Br, I) molecules following low energy (0–15 eV) electron attachment is studied in the gas phase under single collision conditions, free molecular clusters and condensed molecules by means of crossed beams and surface experiments. All four molecules exhibit a very prominent resonance for low energy electron attachment (<1 eV, attachment cross section >10⁻¹⁴ cm²). Under collision free conditions thermal electron capture generates long lived molecular parent anions C₆F₅X^−*. Along the line Cl, Br, I dissociation into X⁻+C₆F₅ and X+C₆F₅-increasingly competes until for X=1 only chemical fragmentation is observed on the mass spectrometric time scale. In free molecular clusters chemical fragmentation is quantitatively quenched at low energies in favour of associative attachment yielding undissociated, relaxed ions (C₆F₅X)⁻ _n,n≥1. A further dissociative resonance at 6.5 eV in C₆F₅Cl is considerably enhanched in clusters. If these molecules are finally condensed on a solid surface, one observes a prominent Cl⁻ desorption resonance at 6.5 eV. While the quantitative quenching of the chemical reactivity at low energies is due to the additional possibilities of energy dissipation under aggregation, the enhanched reactivity at 6.5 eV is interpreted by the conversion of a core excited open channel resonance in single molecules into a closed channel (Feshbach) resonance when it is coupled to environmental molecules.     

An application of near-infrared and mid-infrared spectroscopy to the study of selected tellurite minerals: xocomecatlite,tlapallite and rodalquilarite

Near-infrared and mid-infrared spectra of three tellurite minerals have been investigated. The structures and spectral properties of copper bearing xocomecatlite and tlapallite are compared with an iron bearing rodalquilarite mineral. Two prominent bands observed at 9,855 and 9,015 cm⁻¹ are assigned to ²B_1g → ²B_2g and ²B_1g → ²A_1g transitions of Cu²⁺ ion in xocomecatlite. The cause of spectral distortion is the result of many cations of Ca, Pb, Cu and Zn in the tlapallite mineral structure. Rodalquilarite is characterised by ferric ion absorption in the range 12,300–8,800 cm⁻¹. Three water vibrational overtones are observed in xocomecatlite at 7,140, 7,075 and 6,935 cm⁻¹ whereas in tlapallite bands are shifted to lower wavenumbers at 7,135, 7,080 and 6,830 cm⁻¹. The complexity of rodalquilarite spectrum increases with the number of overlapping bands in the near-infrared. The observation of intense absorption feature near 7,200 cm⁻¹ confirms hydrogen bonding water molecules in xocomecatlite. Weak bands observed near 6,375 and 6,130 cm⁻¹ in tellurites are attributed to the hydrogen bonding between (TeO₃)²⁻ and H₂O. A number of overlapping bands at low wave numbers 4,800–4,000 cm⁻¹ are caused by combinational modes of tellurite ion. (TeO₃)²⁻ stretching vibrations are characterised by three main absorptions at ~1,070, 780 and 665 cm⁻¹. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.