481 research outputs found
Measuring system value in the Ares 1 rocket using an uncertainty-based coupling analysis approach
Coupling of physics in large-scale complex engineering systems must be correctly accounted for during the systems engineering process to ensure no unanticipated behaviors or unintended consequences arise in the system during operation. Structural vibration of large segmented solid rocket motors, known as thrust oscillation, is a well-documented problem that can affect the health and safety of any crew onboard. Within the Ares 1 rocket, larger than anticipated vibrations were recorded during late stage flight that propagated from the engine chamber to the Orion crew module. Upon investigation engineers found the root cause to be the structure of the rockets feedback onto fluid flow within the engine. The goal of this paper is to showcase a coupling strength analysis from the field of Multidisciplinary Design Optimization to identify the major impacts that caused the Thrust Oscillation event in the Ares 1. Once identified an uncertainty analysis of the coupled system using an uncertainty based optimization technique is used to identify the likelihood of occurrence for these strong or weak interactions to take place
The meson at the physical point with Wilson twisted mass fermions
We present results for the eta prime meson and the topological susceptibility
in two flavour lattice QCD. The results are obtained using Wilson twisted mass
fermions at maximal twist with pion masses ranging from 340 MeV down to the
physical point. A comparison to literature values is performed giving a handle
on discretisation effects.Comment: Lattice 2017 proceeding contributio
Aryl dechlorination and defluorination with an organic super-photoreductant
Direct excitation of the commercially available super-electron donor tetrakis(dimethylamino)ethylene (TDAE) with light-emitting diodes at 440 or 390 nm provides a stoichiometric reductant that is able to reduce aryl chlorides and fluorides. The method is very simple and requires only TDAE, substrate, and solvent at room temperature. The photoactive excited state of TDAE has a lifetime of 17.3 ns in cyclohexane at room temperature and an oxidation potential of ca. â3.4 V vs. SCE. This makes TDAE one of the strongest photoreductants able to operate on the basis of single excitation with visible photons. Direct substrate activation occurs in benzene, but acetone is reduced by photoexcited TDAE and substrate reduction takes place by a previously unexplored solvent radical anion mechanism. Our work shows that solvent can have a leveling effect on the photochemically available redox power, reminiscent of the pH-leveling effect that solvent has in acidâbase chemistry
Stepwise Photoinduced Electron Transfer in a Tetrathiafulvalene-Phenothiazine-Ruthenium Triad
A molecular triad comprising a [Ru(bpy)3]2+ (bpy = 2,2â˛âbipyridine) photosensitizer, a primary phenothiazine (PTZ) donor and a secondary (extended) tetrathiafulvalene (exTTF) donor was synthesized and explored by UV/Vis transient absorption spectroscopy. Initial photoinduced electron transfer from PTZ to the 3MLCTâexcited [Ru(bpy)3]2+ occurs within less than 60 ps, and subsequently PTZ is regenerated by electron transfer from exTTF with a time constant of 300 ps. The resulting photoproduct comprising exTTF¡+ and [Ru(bpy)3]+ has a lifetime of 6100 ps in deâaerated CH3CN at room temperature. Additional oneâ and twoâpulse laser flash photolysis studies of the triad were performed in the presence of excess methyl viologen (MV2+), to explore the possibility of lightâdriven charge accumulation on exTTF. MV2+ clearly oxidized [Ru(bpy)3]+ and thereby reâinstated groundâstate [Ru(bpy)3]2+ in triads in which exTTF had been oxidized to exTTF¡+, but further excitation of the solution containing the exTTF¡+âPTZâ[Ru(bpy)3]2+ photoproduct did not provide evidence for exTTF2+. Nevertheless, it seems that the design principle of a covalent donorâdonorâsensitizer triad (as opposed to simpler donorâsensitizer dyads) is beneficial for lightâdriven accumulation of oxidation equivalents. These investigations are relevant in the greater context of multiâelectron photoredox chemistry and artificial photosynthesis
Prospectus, April 21, 1999
https://spark.parkland.edu/prospectus_1999/1013/thumbnail.jp
Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements
Photoredox chemistry with metal complexes as sensitizers and catalysts frequently relies on precious elements such as ruthenium or iridium. Over the past 5â
years, important progress towards the use of complexes made from earthâabundant elements in photoredox catalysis has been made. This review summarizes the advances made with photoactive CrIII, FeII, CuI, ZnII, ZrIV, Mo0, and UVI complexes in the context of synthetic organic photoredox chemistry using visible light as an energy input. Mechanistic considerations are combined with discussions of reaction types and scopes. Perspectives for the future of the field are discussed against the background of recent significant developments of new photoactive metal complexes made from earthâabundant elements
Long-Lived, Strongly Emissive, and Highly Reducing Excited States in Mo(0) Complexes with Chelating Isocyanides
Newly discovered tris(diisocyanide)molybdenum(0) complexes are Earth-abundant isoelectronic analogues of the well-known class of [Ru(Îą-diimine)3]2+ compounds with long-lived 3MLCT (metal-to-ligand charge transfer) excited states that lead to rich photophysics and photochemistry. Depending on ligand design, luminescence quantum yields up to 0.20 and microsecond excited state lifetimes are achieved in solution at room temperature, both significantly better than those for [Ru(2,2â˛-bipyridine)3]2+. The excited Mo(0) complexes can induce chemical reactions that are thermodynamically too demanding for common precious metal-based photosensitizers, including the widely employed fac-[Ir(2-phenylpyridine)3] complex, as demonstrated on a series of light-driven arylâaryl coupling reactions. The most robust Mo(0) complex exhibits stable photoluminescence and remains photoactive after continuous irradiation exceeding 2 months. Our comprehensive optical spectroscopic and photochemical study shows that Mo(0) complexes with diisocyanide chelate ligands constitute a new family of luminophores and photosensitizers, which is complementary to precious metal-based 4d6 and 5d6 complexes and represents an alternative to nonemissive Fe(II) compounds. This is relevant in the greater context of sustainable photophysics and photochemistry, as well as for possible applications in lighting, sensing, and catalysis
Circular Photoinduced Electron Transfer in a Donor-Acceptor- Acceptor Triad
An electronâdonorâacceptorâacceptor (DâA1âA2) triad has been developed that provides the first proofâofâconcept for a photoinitiated molecular circuit. After photoexcitation into an optical chargeâtransfer transition between D and A1, subsequent thermal electronâtransfer from A1.â to A2 is followed by geometric rearrangement in the D.+âA1âA2.â chargeâseparated state to form an ionâpair contact. This facilitates âforwardâ charge recombination between A2.â and D.+ to complete the molecular circuit with an estimated quantum efficiency of 4â% in toluene at 298â
K
Shortcuts for Electron-Transfer through the Secondary Structure of Helical Oligo-1,2-Naphthylenes
Atropisomeric 1,2ânaphthylene scaffolds provide access to donorâacceptor compounds with helical oligomerâbased bridges, and transient absorption studies revealed a highly unusual dependence of the electronâtransfer rate on oligomer length, which is due to their wellâdefined secondary structure. Close noncovalent intramolecular contacts enable shortcuts for electron transfer that would otherwise have to occur over longer distances along covalent pathways, reminiscent of the behavior seen for certain proteins. The simplistic picture of tubeâlike electron transfer can describe this superposition of different pathways including both the covalent helical backbone, as well as noncovalent contacts, contrasting the wireâlike behavior reported many times before for more conventional molecular bridges. The exquisite control over the molecular architecture, achievable with the configurationally stable and topologically defined 1,2ânaphthyleneâbased scaffolds, is of key importance for the tubeâlike electron transfer behavior. Our insights are relevant for the emerging field of multidimensional electron transfer and for possible future applications in molecular electronics
Four different emissions from a Pt(Bodipy)(PEt3)(2)(S-Pyrene) dyad
The Pt(bodipy)-(mercaptopyrene) dyad BPtSPyr shows four different emissions: intense near-infrared phosphorescence (ÎŚph up to 15%) from a charge-transfer state pyrSË+-Pt-BDPËâ, additional fluorescence and phosphorescence emissions from the 1ĎĎ* and 3ĎĎ* states of the bodipy ligand at r.t., and phosphorescence from the pyrene 3ĎĎ* and the bodipy 3ĎĎ* states in a glassy matrix at 77 K.publishe
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