The Elusive Ketene (H2CCO) Channel in the Infrared Multiphoton Dissociation of Solid 1,3,5-Trinitro-1,3,5-Triazinane (RDX)

Understanding of the fundamental mechanisms involved in the decomposition of 1,3,5-trinitro-1,3,5-triazinane (RDX) still represents a major challenge for the energetic materials and physical (organic) chemistry communities mainly because multiple competing dissociation channels are likely involved and previous detection methods of the products are not isomer selective. In this study we exploited a microsecond pulsed infrared laser to decompose thin RDX films at 5 K under mild conditions to limit the fragmentation channels. The subliming decomposition products during the temperature programed desorption phase are detected using isomer selective single photoionization time-of-flight mass spectrometry (PI-ReTOF-MS). This technique enables us to assign a product signal at m/z=42 to ketene (H₂CCO), but not to diazomethane (H₂CNN; 42 amu) as speculated previously. Electronic structure calculations support our experimental observations and unravel the decomposition mechanisms of RDX leading eventually to the elusive ketene (H₂CCO) via an exotic, four-membered ring intermediate. This study highlights the necessity to exploit isomer-selective detection schemes to probe the true decomposition products of nitramine-based energetic materials.     

Gas Phase Formation of Methylgermylene (HGeCH3)

The methylgermylene species (HGeCH₃; X¹A′) has been synthesized via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with germane (GeH₄) under single collision conditions in crossed molecular beams experiments. Augmented by electronic structure calculations, this elementary reaction was found to proceed through barrierless insertion of the methylidyne radical in one of the four germanium-hydrogen bonds on the doublet potential energy surface yielding the germylmethyl (CH₂GeH₃; X²A′) collision complex. This insertion is followed by a hydrogen shift from germanium to carbon and unimolecular decomposition of the methylgermyl (GeH₂CH₃; X²A′) intermediate by atomic hydrogen elimination leading to singlet methylgermylene (HGeCH₃; X¹A′). Our investigation provides a glimpse at the largely unknown reaction dynamics and isomerization processes of the carbon-germanium system, which are quite distinct from those of the isovalent carbon system thus providing insights into the intriguing chemical bonding of organo germanium species on the most fundamental, microscopic level.     

Determination of phenolic compounds,in vitro antioxidant activity and characterization of secondary metabolites in different parts of Passiflora cincinnata by HPLC-DAD-MS/MS analysis

Abstract

Ethanol extracts of different parts of Passiflora cincinnata were obtained by maceration. The total phenolic and flavonoid contents were evaluated. The antioxidant activities were determined by β-carotene-linoleic acid bleaching test, 2,2-diphenyl-1-picrylhydrazil (DPPH), and 2,2’-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical scavenging. The crude ethanol stem extract showed the highest amount of total polyphenols (45.53?mg gallic acid equivalent/g) while the highest total flavonoid contents (1.42?mg of quercetin equivalent/g) were observed in the leaf extract. The lowest IC₅₀ (25.65?μg/ml) by the DPPH method was observed for the stem extract. The ABTS method showed a significant antioxidant activity for all investigated extracts. The secondary metabolite composition of ethanol extracts was assessed by HPLC-DAD-MS/MS analysis, leading to the identification of fourteen secondary metabolites in P. cincinnata extracts. These results showed the potentiality of this species as a source of phenolic compounds and antioxidants.     

Portable capillary liquid chromatography for pharmaceutical and illicit drug analysis

A newly developed portable capillary liquid chromatograph was investigated for the separation of various pharmaceutical and illicit drug compounds. The system consists of two high‐pressure syringe pumps capable of delivering capillary‐scale flow rates at pressures up to 10 000 psi. Capillary liquid chromatography columns packed with sub‐2 μm particles are housed in cartridges that can be inserted into the system and easily connected through high‐pressure fluidic contact points by simply applying a specific, predetermined torque rather than using standard fittings and less precise sealing protocols. Several over‐the‐counter analgesic drug separations are demonstrated, along with a simple online measurement of tablet dissolution. Twenty illicit drug compounds were also separated across six targeted drug panels. The results described in this study demonstrate the capability of this compact liquid chromatography instrument to address several important drug‐related applications while simplifying system operation, and greatly reducing solvent usage and waste generation essential for onsite analysis.     

Supramolecular networks supported by the anion…π linkage of Keggin‐type heteropolyoxotungstates

Anion…π interactions are newly recognized weak supramolecular forces which are relevant to many types of electron‐deficient aromatic substrates. Being less competitive with respect to conventional hydrogen bonding, anion…π interactions are only rarely considered as a crystal‐structure‐defining factor. Their significance dramatically increases for polyoxometalate (POM) species, which offer extended oxide surfaces for maintaining dense aromatic/inorganic stacks. The structures of tetrakis(caffeinium) μ₁₂‐silicato‐tetracosa‐μ₂‐oxido‐dodecaoxidododecatungsten trihydrate, (C₈H₁₁N₄O₂)₄[SiW₁₂O₄₀]·3H₂O, (1), and tris(theobrominium) μ₁₂‐phosphato‐tetracosa‐μ₂‐oxido‐dodecaoxidododecatungsten ethanol sesquisolvate, (C₇H₉N₄O₂)₃[PW₁₂O₄₀]·1.5C₂H₅OH, (2), support the utility of anion…π interactions as a special kind of supramolecular synthon controlling the structures of ionic lattices. Both caffeinium [(HCaf)⁺ in (1)] and theobrominium cations [(HTbr)⁺ in (2)] reveal double stacking patterns at both axial sides of the aromatic frameworks, leading to the generation of anion…π…anion bridges. The latter provide the rare face‐to‐face linkage of the anions. In (1), every square face of the metal–oxide cuboctahedra accepts the interaction and the above bridges yield flat square nets, i.e. {(HCaf⁺)₂[SiW₁₂O₄₀]^4?}_n. Two additional cations afford single stacks only and they terminate the connectivity. Salt (2) retains a two‐dimensional (2D) motif of square nets, with anion…π…anion bridges involving two of the three (HTbr)⁺ cations. The remaining cations complete a fivefold anion…π environment of [PW₁₂O₄₀]^3?, acting as terminal groups. This single anion…π interaction is influenced by the specific pairing of (HTbr)⁺ cations by double amide‐to‐amide hydrogen bonding. Nevertheless, invariable 2D patterns in (1) and (2) suggest the dominant role of anion…π interactions as the structure‐governing factor, which is applicable to the construction of noncovalent linkages involving Keggin‐type oxometalates.     

A three-layered minimizer in R2 for a variational problem with a symmetric three-well potential

Let W be a potential on R ² which is equivariant by the symmetry group of the equilateral triangle and has three minima. We show that the elliptic system possesses a nontrivial smooth solution U : R ² → R ². Here DW(U)^T is the transpose of the derivative DW( U ). The natural energy of the problem is unbounded and compactness techniques cannot be applied. The proof depends on careful energy estimates and asymptotics for several one-dimensional problems and for two-dimensional problems on bounded domains. © 1996 John Wiley & Sons, Inc.     

Self-assembly phenomena in multicomponent systems composed of polyelectrolytes and ionic surfactants

The ability of anionic surfactants to expel fluorescencetagged poly(methacrylate) polyanion ( PMA ^*) from nonstoichiometric water-soluble interpolyelectrolyte complex ( NIPEC ) formed by PMA^* and polycation-quencher poly(N-alkyl-4-vinylpyridinium) ( PAlkVP ) and to form polyelectrolyte-surfactant complex ( PESC ): NIPEC(PMA^*-PAlkVP)+surfactant ⇔ PESC(PAlkVP-surfactant)+PMA^* was revealed and studied by fluorescence quenching technique and high-speed sedimentation. It was found that the transfer of poly(N-ethyl-4-vinylpyridinium) from NIPEC to PESC proceeded up to complete conversion providing a self-assembly of both PESC and NIPEC particles in the course of the reaction. The concentration of the surfactant and the “tail” length of the surfactant molecule, as well as the structure of Alk groups, temperature, pH, ionic strength, and the nature of added low-molecular-weight electrolyte proved to be the factors of the reaction equilibrium control.     

Demixing in polystyrene/methylcyclohexane solutions

Cloudpoint data for polystyrene/methylcyclohexane solutions extending over moderate ranges of pressure and molecular weight are available in the literature. Those data are supplemented with new results from this laboratory to fill in gaps and extend the MW range (to 761 ≤ MW/amu ≤ 2 × 10⁷). The resulting data net is discussed and reasons to extend studies to higher pressure are presented. © 1996 John Wiley & Sons, Inc.     

Nitrous Oxide Adsorption and Decomposition on Zeolites and Zeolite-like Materials

Decomposition of N₂O on modified zeolites, crystalline titanosilicalites, and related amorphous systems is studied by the catalytic and spectroscopic methods. Zinc-containing HZSM-5 zeolites and titanosilicalites with moderate Ti/Si ratios are shown to exhibit a better catalytic performance in N₂O decomposition as compared with conventionally used Cu/HZSM-5 zeolites and amorphous Cu-containing catalysts. Dehydroxylation of the HZSM-5 zeolite by calcination at 1120 K results in an enhancement of the N₂O conversion. The mechanism of the reaction and the role of coordinatively unsaturated cations and Lewis acid sites in N₂O decomposition are discussed on the basis of the spectroscopic data.