Ultrafast shock compression and shock-induced decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene subjected to a subnanosecond-duration shock: an analysis of decomposition products

Shock compression studies of pressed and confined ultrafine 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) powder were conducted using ultrashort ~300 ps, ~50 GPa shock waves. The recovered decomposition products were characterized using X-ray photoelectron spectroscopy, infrared spectroscopy, and Raman spectroscopy. A substantial amount of shock-related chemistry was observed. Approximately 75% of the nitrogen atoms were liberated as gas-phase species, along with ~33% of the oxygen atoms, as a result of the applied shock. Furthermore, we observe C 1s binding energies suggesting the formation of sp(3) hybridized amorphous carbon. For comparison, a carbon nitride material was also prepared and characterized by thermally pyrolizing TATB. The shock-compressed TATB and the thermally pyrolized TATB are qualitatively different, suggesting that, carbon nitrides, a possible indicator of nitrogen-rich heterocycles precursors, are not a major product class for strongly overdriven shock conditions. These experimental conditions were, however, not detonation conditions, and the possible formation of nitrogen-rich heterocycles in actual detonations still exists.     

Low noise 80–115 GHz quasiparticle mixer with small Nb/Al-Oxide/Nb tunnel junctions

Millimeter-wave characterization of a heterodyne receiver using (2 m²) Nb/Al-Ox/Nb Superconducting-Insulator-Superconducting (SIS) junctions arrays is reported. The fabrication of the Nb/Al-Ox/Nb SIS junction arrays as a heterodyne mixer is described. The leakage current of these junctions is below 2A at 4.2K and unmeasurable at 2.5K. The receiver gave a noise temperature Double Side Band (DSB) between 63K and 187K over the frequency range 80 to 115 GHz at the first conversion peak. The results are comparable to those obtained with SIS receivers using well researched lead junctions. Contrary to the lead junctions, our mixer using all Nb junctions have proven remarkably stable with respect to thermal cycling, characteristics which are required for space applications. To our knowledge, this is the most reliable low noise receiver operating in this frequency range.     

A variational method for the calculation of spin-rovibronic energy levels of triatomic molecules with three interacting electronic states

A general method is presented for the evaluation of the spin rovibronic energy levels of triatomic molecules with up to three interacting potential energy surfaces. The full theory is outlined in detail, both for singlet and for doublet electronic states and the method is then applied to the benchmark example of C₂H. High quality multireference configuration interaction calculations have been carried out to generate the 3-dimensional near-equilibrium adiabatic potential energy surfaces of the three lowest 1²A'(X²Σ⁺), 2²A'(A²Π), 1²A''(A²Π) electronic states of C₂H, and the pair of interacting states of A 0 symmetry have then been diabatized. Results are presented for J up to 7/2 and compared with gas-phase high resolution experimental results for energies up to 5600 cm¹.     

Mechanism and stereoselectivity of a dual amino-catalyzed robinson annulation: rare duumvirate stereocontrol

Computational study of the mechanisms and stereoselectivities of a dual amino-catalyzed synthesis of cyclohexenones containing all-carbon γ-quaternary and ?-tertiary stereocenters is reported. Extensive conformational search with density functional theory optimizations, the high-accuracy SCS-MP2/cc-pV∞Z energies, and PCM solvation corrections were used to characterize all intermediates and transition states. Six mechanisms were considered, all consistent with available experiments. The reaction proceeds via sequential Michael and Mannich conjugate additions whereby the primary amine activates the aldehyde and the catalyst activates the pentenone. We have discovered a rare duumvirate stereocontrol: the Michael reaction sets the enantioselectivity, but both the Michael and the Mannich reactions control the diastereoselectivity.     

Quantum mechanical embedding theory based on a unique embedding potential

We remove the nonuniqueness of the embedding potential that exists in most previous quantum mechanical embedding schemes by letting the environment and embedded region share a common embedding (interaction) potential. To efficiently solve for the embedding potential, an optimized effective potential method is derived. This embedding potential, which eschews use of approximate kinetic energy density functionals, is then used to describe the environment while a correlated wavefunction (CW) treatment of the embedded region is employed. We first demonstrate the accuracy of this new embedded CW (ECW) method by calculating the van der Waals binding energy curve between a hydrogen molecule and a hydrogen chain. We then examine the prototypical adsorption of CO on a metal surface, here the Cu(111) surface. In addition to obtaining proper site ordering (top site most stable) and binding energies within this theory, the ECW exhibits dramatic changes in the p-character of the CO 4σ and 5σ orbitals upon adsorption that agree very well with x-ray emission spectra, providing further validation of the theory. Finally, we generalize our embedding theory to spin-polarized quantum systems and discuss the connection between our theory and partition density functional theory.     

Analysis of silica fume produced by zircon desilication

Novel chemical methods have been developed to allow for the determination of the components of silica fume produced by zircon desilication. Hitherto, no methods have been described for the analysis of this material. The amorphous silica is first removed by treatment with sodium hydroxide. The residue from the hydroxide treatment may then be subjected to a suite of reagents to determine the zircon, the total zirconia, the monoclinic zirconia and the tetragonal zirconia content of the fume. The zircon content of the fume is determined by treatment of the hydroxide residue with concentrated hydrofluoric acid (HF). The total zirconia content of the fume is determined by digestion of the hydroxide residue with fuming sulphuric acid (oleum), while the relative amounts of monoclinic and tetragonal zirconia may be found by treatment of the hydroxide residue with 10%w/v HF, which attacks the less stable tetragonal phase. Both X-ray diffraction and particle size analysis were used to validate the steps in the analytical procedure. An explanation of the presence of tetragonal zirconia in the fume is proposed. A greater understanding of the composition of the fume led to the installation of a separator in the company's production line to remove the zircon. Australian Fused Materials (AFM) now produces a vastly superior grade of fume marketed under the code SF-98.     

Observation of an Organic Acid Mediated Spin State Transition in a Co(II)-Schiff Base Complex: An EPR, HYSCORE, and DFT Study

The interactions of a weak organic acid (acetic acid, HOAc) with a toluene solution of the Co(II)-Schiff base type complex, (R,R')-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino Co(II) (labeled [Co(1)]), was investigated using EPR, HYSCORE, and DFT computations. This activated [Co(II)(1)] system is extremely important within the context of asymmetric catalysts (notably the hydrolytic kinetic resolution of epoxides) despite the lack of detailed structural information about the nature of the paramagnetic species present. Under anaerobic conditions, the LS [Co(II)(1)] complex with a |yz, (2)A(2)? ground state is converted into a low-spin (LS) and a high-spin (HS) complex in the presence of the acid. The newly formed LS state is assigned to the coordinated [Co(II)(1)]-(HOAc) complex, possessing a |z(2), (2)A(1)? ground state (species A; g(x) = 2.42, g(y) = 2.28, g(z) = 2.02, A(x) = 100, A(y) = 120, A(z) = 310 MHz). The newly formed HS state is assigned to an acetate coordinated [Co(II)(1)]-(OAc(-)) complex, possessing an S = (3)/(2) spin ground state (species B, responsible for a broad EPR signal with g ≈ 4.6). These spin ground states were confirmed with DFT calculations using the hybrid BP86 and B3LYP functionals. Under aerobic conditions, the LS and HS complexes (species A and B) are not observed; instead, a new HS complex (species C) is formed. This complex is tentatively assigned to a paramagnetic superoxo bridged dimer (AcO(-))[Co(II)(1)···O(2)(-)Co(III)(1)](HOAc), as distinct from the more common diamagnetic peroxo bridged dimers. Species C is characterized by a very broad HS EPR signal (g(x) = 5.1, g(y) = 3.9, g(z) = 2.1) and is reversibly formed by oxygenation of the LS [Co(II)(1)]-(HOAc) complex to the superoxo complex [Co(III)(1)O(2)(-)](HOAc), which subsequently forms the association complex C by interaction with the HS [Co(II)(1)](OAc(-)) species. The LS and HS complexes were also identified using other organic acids (benzoic and propanoic acid). Thermal annealing-quenching experiments revealed the additional presence of [Co(III)(1)O(2)(-)](HOAc) adducts, corroborating the presence of species C and the presence of diamagnetic dimer complexes in the solution, such as the EPR silent (HOAc)[Co(III)(1)(O(2)(2-))Co(III)(1)](HOAc). Overall, it appears that a facile interconversion of the [Co(1)] complex, possessing a LS ground state, occurs in the presence of acetic acid, producing both HS and LS Co(II) states, prior to formation of the oxidized active form of the catalyst, [Co(III)(1)](OAc(-)).     

Eicosatetraenehydroxamates: inhibitors of 5-lipoxygenase

The syntheses of 5-hydroxamyl and 5-hydroxamylmethyl-6,8,11,14,-eicosatetraenoic acids ($\text{3}$ and $\text{4}$), which possess potent 5-lipoxygenase inhibitory activity, are described.     

In situ Raman studies on lithiated single-wall carbon nanotubes in liquid ammonia

The charge transfer induced lithiation of single-wall carbon nanotubes (SWNTs) was investigated by in situ monitoring by Raman spectroscopy as lithium was added incrementally to a dispersion of SWNTs in liquid ammonia. Charge transfer from liquid ammonia solvated lithium to the SWNTs led to intercalation of lithium into the SWNT ropes, as well as to the semi-covalent lithiation of the SWNTs. Raman spectra of the SWNTs recorded as lithium was added showed a 30 wavenumber downshift of the G band (1594 cm⁻¹) with the concomitant appearance of a new peak at 1350 cm⁻¹ that was assigned as the signature of the lithiated SWNTs. Addition of 1-iodododecane to the lithiated SWNTs resulted in the covalent attachment of dodecyl groups. The intercalation of lithium throughout the SWNT ropes led to complete dodecylation of all individual SWNTs.     

Development and evaluation of a detailed mechanism for the atmospheric reactions of isoprene and NOx

A detailed photochemical mechanism for the atmospheric reactions of isoprene and its major oxidation products in the presence of NO_x, which incorporates the most recent laboratory results and our current understanding of the system, is described. It is evaluated by comparing its predictions against results of NO_x-air irradiations of isoprene and its two major products, methacrolein, and methyl vinyl ketone (MVK), in five different types of environmental chambers at two different laboratories. In most cases it simulated experimental results within the uncertainty of the data and the chamber and run characterization model. However, the photodecomposition quantum yields of methacrolein and MVK and the organic nitrate yield from the OH + isoprene reaction had to be adjusted to obtain satisfactory simulations of the data. The major discrepancy observed was that the model tended to underpredict PAN by ca. 40% in the isoprene experiments, despite the fact that the model predicted PAN from methacrolein and MVK reasonably well. The uncertainties and additional data needed to completely characterize the isoprene atmospheric photooxidation system are discussed. © 1996 John Wiley & Sons, Inc.