36 research outputs found
High-Pressure Synthesis of a Pentazolate Salt
The pentazolates, the last all-nitrogen members of the azole series, have
been notoriously elusive for the last hundred years despite enormous efforts to
make these compounds in either gas or condensed phases. Here we report a
successful synthesis of a solid state compound consisting of isolated
pentazolate anions N5-, which is achieved by compressing and laser heating
cesium azide (CsN3) mixed with N2 cryogenic liquid in a diamond anvil cell. The
experiment was guided by theory, which predicted the transformation of the
mixture at high pressures to a new compound, cesium pentazolate salt (CsN5).
Electron transfer from Cs atoms to N5 rings enables both aromaticity in the
pentazolates as well as ionic bonding in the CsN5 crystal. This work provides a
critical insight into the role of extreme conditions in exploring unusual
bonding routes that ultimately lead to the formation of novel high nitrogen
content species
Pressure-induced phase transition in the electronic structure of palladium nitride
We present a combined theoretical and experimental study of the electronic
structure and equation of state (EOS) of crystalline PdN2. The compound forms
above 58 GPa in the pyrite structure and is metastable down to 11 GPa. We show
that the EOS cannot be accurately described within either the local density or
generalized gradient approximations. The Heyd-Scuseria-Ernzerhof
exchange-correlation functional (HSE06), however, provides very good agreement
with experimental data. We explain the strong pressure dependence of the Raman
intensities in terms of a similar dependence of the calculated band gap, which
closes just below 11 GPa. At this pressure, the HSE06 functional predicts a
first-order isostructural transition accompanied by a pronounced elastic
instability of the longitudinal-acoustic branches that provides the mechanism
for the experimentally observed decomposition. Using an extensive Wannier
function analysis, we show that the structural transformation is driven by a
phase transition of the electronic structure, which is manifested by a
discontinuous change in the hybridization between Pd-d and N-p electrons as
well as a conversion from single to triple bonded nitrogen dimers. We argue for
the possible existence of a critical point for the isostructural transition, at
which massive fluctuations in both the electronic as well as the structural
degrees of freedom are expected.Comment: 9 pages, 12 figures. Revised version corrects minor typographical
error
A study of tantalum pentoxide Ta2O5 structures up to 28 GPa
Tantalum pentoxide Ta2O5 with the orthorhombic L-Ta2O5 structure has been experimentally studied up to 28.3 GPa (at ambient temperature) using synchrotron angle-dispersive powder X-ray diffraction (XRD). The ambient pressure phase remains stable up to 25 GPa where with increased pressure a crystalline to amorphous phase transition occurs. A detailed equation of state (EOS), including pressure dependent lattice parameters, is reported. The results of this study were compared with a previous high-pressure XRD study by Li et al. A clear discrepancy between the ambient-pressure crystal structures and, consequently, the reported EOSs between the two studies was revealed. The origin of this discrepancy is attributed to two different crystal structures used to index the XRD patterns
RHPS4 G-quadruplex ligand induces anti-proliferative effects in brain tumor cells
Background
Telomeric 3’ overhangs can fold into a four-stranded DNA structure termed G-quadruplex (G4), a formation which inhibits telomerase. As telomerase activation is crucial for telomere maintenance in most cancer cells, several classes of G4 ligands have been designed to directly disrupt telomeric structure.
Methods
We exposed brain tumor cells to the G4 ligand 3,11-difluoro-6,8,13-trimethyl-8H-quino[4,3,2-kl]acridinium methosulfate (RHPS4) and investigated proliferation, cell cycle dynamics, telomere length, telomerase activity and activated c-Myc levels.
Results
Although all cell lines tested were sensitive to RHPS4, PFSK-1 central nervous system primitive neuroectodermal cells, DAOY medulloblastoma cells and U87 glioblastoma cells exhibited up to 30-fold increased sensitivity compared to KNS42 glioblastoma, C6 glioma and Res196 ependymoma cells. An increased proportion of S-phase cells were observed in medulloblastoma and high grade glioma cells whilst CNS PNET cells showed an increased proportion of G1-phase cells. RHPS4-induced phenotypes were concomitant with telomerase inhibition, manifested in a telomere length-independent manner and not associated with activated c-Myc levels. However, anti-proliferative effects were also observed in normal neural/endothelial cells in vitro and ex vivo.
Conclusion
This study warrants in vivo validation of RHPS4 and alternative G4 ligands as potential anti-cancer agents for brain tumors but highlights the consideration of dose-limiting tissue toxicities
Deflagration Rates and Molecular Bonding Trends of Statically Compressed Secondary Explosives Deflagration Rates and Molecular Bonding Trends of Statically Compressed Secondary Explosives
Abstract. We discuss our measurements of the chemical reaction propagation rate as a function of pressure. Materials investigated have included CL-20, HMX, TATB, and RDX crystalline powders, LX-04, Comp B, and nitromethane. The anomalous correspondence between crystal structure, including in some instances isostructural phase transitions, on pressure-dependant RPRs of TATB, HMX, Nitromethane, CL-20, and PETN have been elucidated using micro-IR and -Raman spectroscopies. Here we specifically highlight pressure-dependent physicochemical mechanisms affecting the deflagration rate of nitromethane and epsilon-CL-20. We find that pressure induced splitting of symmetric stretch NO 2 vibrations can signal the onset of increasingly more rapid combustion reactions