Quantum molecular dynamics simulations of the oxidation of aluminum-cyclopentadienyl clusters

The article of record as published may be located at http://dx.doi.org/10.1088/1742-6596/500/17/172001We report Car-Parrinello molecular dynamics simulations of the oxidation of aluminum-cyclopentadienyl clusters currently being considered as novel fuels or energetic materials. These clusters contain a small aluminum core surrounded by a single organic ligand layer. The aromatic cyclopentadienyl ligands form a very strong bond with surface Al atoms, giving rise to a stable organometallic cluster which crystallizes into a low-symmetry solid-state material. Our calculations of an isolated cluster in oxygen show minimal reaction between the ligand and oxygen molecules at simulation temperatures of 300 and 1000 K. Rather, in all cases O2 diffuses through the ligand barrier, splits into atomic oxygen upon contact with the aluminum, and forms an amorphous aluminum oxide core. Loss of aluminum-ligand units, as expected from bond strength calculations, is not observed except following significant oxidation. We present simple metrics to quantitatively compare the steric barrier of the outer ligands that limits the oxidation process.This work was funded by the Office of Naval Research

The Role of Ligand Steric Bulk in New Monovalent Aluminum Compounds

The article of record as published may be located at https://doi.org/10.1021/acs.jpca.7b02075The tetrameric Al(I) cyclopentadienyl compound Al4Cp*4 (Cp* = C5Me5) is a prototypical low-valence Al compound, with delocalized bonding between four Al(I) atoms and η5 ligands bound to the cluster exterior. The synthesis of new [AlR]4 (R = C5Me4Pr, C5Me4iPr) tetramers is presented. Though these systems failed to crystallize, comparison of variable-temperature 27Al NMR data with density functional theory (DFT) calculations indicate that these are Al4R4 tetramers analogous to Al4Cp*4 but with increased ligand steric bulk. NMR, DFT, and Atoms in Molecules analyses show that these clusters are enthalpically more stable as tetramers than the Cp* variant, due in part to noncovalent interactions across the bulkier ligand groups. Thermochemistry calculations for the low-valence metal interactions were found to be extremely sensitive to the DFT methodology used; the M06-2X functional with a cc-pVTZ basis set is shown to provide very accurate values for the enthalpy of tetramerization and 27Al NMR shifts. This computational method is then used to predict geometrical structures, noncovalent ligand interactions, and monomer/tetramer equilibrium in solution for a series of Al(I) cyclopentadienyl compounds of varying steric bulk

Searching for Dark Matter with Future Cosmic Positron Experiments

Dark matter particles annihilating in the Galactic halo can provide a flux of positrons potentially observable in upcoming experiments, such as PAMELA and AMS-02. We discuss the spectral features which may be associated with dark matter annihilation in the positron spectrum and assess the prospects for observing such features in future experiments. Although we focus on some specific dark matter candidates, neutralinos and Kaluza-Klein states, we carry out our study in a model independent fashion. We also revisit the positron spectrum observed by HEAT.Comment: 19 pages, 33 figure

Control of Biohazards: A High Performance Energetic Polycyclized Iodine-Containing Biocide

The article of record as published may be found at http://dx.doi.org/10.1021/acs.inorgchem.8b01600Biohazards and chemical hazards as well as radioactive hazards have always been a threat to human health. The search for solutions to these problems is an ongoing worldwide effort. In order to control biohazards by chemical methods, a synthetically useful fused tricyclic iodine-rich compound, 2,6-diiodo-3,5-dinitro-4,9-dihydrodipyrazolo [1,5a:5',1'-d][1,3,5]triazine (5), with good detonation performance was synthesized, characterized, and its properties determined. This compound which acts as an agent defeat weapon has been shown to destroy certain microorganisms effectively by releasing iodine after undergoing decomposition or combustion. The small iodine residues remaining will not be deleterious to human life after 1 month.Financial support of the Office of Naval Research (N00014-16- 1-2089), and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028) is gratefully acknowledged. The M. J. Murdock Charitable Trust (No. 2014120) is thanked for funds supporting the purchase of a 500 MHz NMR.Financial support of the Office of Naval Research (N00014-16- 1-2089), and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028) is gratefully acknowledged. The M. J. Murdock Charitable Trust (No. 2014120) is thanked for funds supporting the purchase of a 500 MHz NMR

Energy and Biocides Storage Compounds: Synthesis and Characterization of Energetic Bridged Bis(triiodoazoles)

The article of record as published may be found at https://doi.org/10.1021/acs.inorgchem.7b02277Energetic bridged triiodopyrazoles and triiodoimidazoles were designed and synthsized by reacting potassium triiodopyrazolate or triiodoimidazolate with corresponding dichloro compounds. All compounds were fully characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, and elemental analyses. The structure of compound 1 was further confirmed by single-crystal X-ray diffraction. All of the compounds exhibit good thermal stability with decomposition temperatures between 199 and 270 °C and high densities ranging from 2.804 to 3.358 g/cm3. The detonation performances and the detonation products were calculated by CHEETAH 7. Compound 3 (Dv = 4765 m s–1; P = 17.9 GPa) and compound 7 (Dv = 4841 m s–1; P = 18.5 GPa) show comparable detonation pressure to TNT, and high iodine content makes them promising as energy and biocides storage compounds.This work was supported by the Office of Naval Research (N00014-16-1-2089) and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028). The authors thank the M. J. Murdock Charitable Trust, Vancouver, WA (Reference No.: 2014120: MNL:11/20/2014), for funds to purchase a 500 MHz NMR spectrometer.

ASCA and contemporaneous ground-based observations of the BL Lacertae objects 1749+096 and 2200+420 (BL Lac)

We present ASCA observations of the radio-selected BL Lacertae objects 1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 Sept and Nov, respectively. The ASCA spectra of both sources can be described as a first approximation by a power law with photon index Gamma ~ 2. This is flatter than for most X-ray-selected BL Lacs observed with ASCA, in agreement with the predictions of current blazar unification models. While 1749+096 exhibits tentative evidence for spectral flattening at low energies, a concave continuum is detected for 2200+420: the steep low-energy component is consistent the high-energy tail of the synchrotron emission responsible for the longer wavelengths, while the harder tail at higher energies is the onset of the Compton component. The spectral energy distributions from radio to gamma-rays are consistent with synchrotron-self Compton emission from a single homogeneous region shortward of the IR/optical wavelengths, with a second component in the radio domain related to a more extended emission region. For 2200+420, comparing the 1995 Nov state with the optical/GeV flare of 1997 July, we find that models requiring inverse Compton scattering of external photons provide a viable mechanism for the production of the highest (GeV) energies during the flare. An increase of the external radiation density and of the power injected in the jet can reproduce the flat gamma-ray continuum observed in 1997 July. A directly testable prediction of this model is that the line luminosity in 2200+420 should vary shortly after (~1 month) a non-thermal synchrotron flare.Comment: 28 pages,6 figures, 5 tables; LaTeX document. accepted for publication in the Astrophysical Journa

Templated Growth of a Spin-Frustrated Cluster Fragment of MnBr2 in a Metal−Organic Framework

17 USC 105 interim-entered record; under review.The article of record as published may be found at https://doi.org/10.1021/acs.inorgchem.1c01345The metal−organic framework Zr6O4(OH)4(bpydc)6 (bpydc2− = 2,2′-bipyridine-5,5′-dicarboxylate) is used to template the growth of a cluster fragment of the two-dimensional solid MnBr2, which was predicted to exhibit spin frustration. Single-crystal and powder X-ray diffraction analyses reveal a cluster with 19 metal ions arranged in a triangular lattice motif. Static magnetic susceptibility measurements indicate antiferromagnetic coupling between the high-spin (S = 5/2) MnII centers, and dynamic magnetic susceptibility data suggest population of low-lying excited states, consistent with magnetic frustration. Density functional theory calculations are used to determine the energies for a subset of thousands of magnetic configurations available to the cluster. The Yamaguchi generalized spin-projection method is then employed to construct a model for magnetic coupling interactions within the cluster, enabling facile determination of the energy for all possible magnetic configurations. The confined cluster is predicted to possess a doubly degenerate, highly geometrically frustrated ground state with a total spin of STotal = 5/2.This research was supported through a Multidisciplinary University Research Initiatives Program funded by the U.S. Department of Defense, Office of Naval Research, under Award N00014-15-1-2681. Single-crystal X-ray diffraction experiments were performed at Beamline 11.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC02-05CH11231

Superior High-Energy-Density Biocidal Agent Achieved with a 3D Metal−Organic Framework

17 USC 105 interim-entered record; under review.The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.0c12251.|17 USC 105 interim-entered record; under review.The article of record as published may be foun|d at https://doi.org/A significant number of challenges are encountered when developing biocidal agents with high throwing capacity for biosafety applications. Now a three-dimensional metal−organic framework (3D MOF) {MOF (2), [Cu(atrz)(IO3)2]n (atrz = 4,4′-azo-1,2,4-triazole)} was obtained using a postsynthetic method from MOF (1) {[Cu(atrz)3(NO3)2]n}. Benefitting from the oxygen-rich and small volume of the iodate (IO3) ligands (2.73 Å) in MOF (2) compared to the atrz ligand (7.70 Å) in MOF (1), the density of MOF (2) is 3.168 g cm−3, nearly twice that of its precursor. Its detonation velocity of 7271 ms−1 exceeds that of TNT (trinitrotoluene) and its detonation pressure of 40.6 GPa is superior to that of HMX (cyclotetramethylenetetranitramine) (1,3,5,7- tetranitro-1,3,5,7-tetrazoctane, 39.2 Gpa), which are the highest detonation properties for a biocidal agent. Its superior detonation performance results in its main product, I2, being distributed over a wide area, markedly reducing the diffusion of harmful microorganisms. This study offers novel insight not only for high-energy-density materials but also for huge potential applications as biocidal agents

Have Atmospheric Cerenkov Telescopes Observed Dark Matter?

Two ground-based experiments have recently independently detected TeV $\gamma$-rays from the direction of the Galactic center. The observations made by the VERITAS and CANGAROO collaborations are unexpected, although not impossible to interpret in terms of astrophysical sources. Here we examine in detail whether the observed $\gamma$-rays may arise from the more exotic alternative of annihilations of dark matter particles clustered in the center of the Galaxy.Comment: 19 pages, 7 figure