Infrared monitoring of the Space Station environment

The measurement and monitoring of infrared emission in the environment of the Space Station has a twofold importance - for the study of the phenomena itself and as an aid in planning and interpreting Station based infrared experiments. Spectral measurements of the infrared component of the spacecraft glow will, along with measurements in other spectral regions, provide data necessary to fully understand and model the physical and chemical processes producing these emissions. The monitoring of the intensity of these emissions will provide background limits for Space Station based infrared experiments and permit the determination of optimum instrument placement and pointing direction. Continuous monitoring of temporal changes in the background radiation (glow) will also permit better interpretation of Station-based infrared earth sensing and astronomical observations. The primary processes producing infrared emissions in the Space Station environment are: (1) Gas phase excitations of Station generated molecules ( e.g., CO2, H2O, organics...) by collisions with the ambient flux of mainly O and N2. Molecular excitations and generation of new species by collisions of ambient molecules with Station surfaces. They provide a list of resulting species, transition energies, excitation cross sections and relevant time constants. The modeled spectrum of the excited species occurs primarily at wavelengths shorter than 8 micrometer. Emissions at longer wavelengths may become important during rocket firing or in the presence of dust

Cyclometalated Tantalum Diphenolate Pincer Complexes: Intramolecular C−H/M−CH_3σ-Bond Metathesis May Be Faster than O−H/M−CH_3 Protonolysis

A diphenol linked at the ortho positions to a benzene ring was metalated with TaCl_2(CH_3)_3. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzenediyl ring reveals an unexpected mechanism involving protonolysis of a methyl group, followed by C−H/Ta−CH_3 σ-bond metathesis, leading to cyclometalation of the linking ring and finally protonation of the cyclometalated group by the pendant phenol

Zirconium and titanium complexes supported by tridentate LX2 ligands having two phenolates linked to furan, thiophene, and pyridine donors: precatalysts for propylene polymerization and oligomerization

Zirconium and titanium complexes with tridentate bis(phenolate)-donor (donor = pyridine, furan and thiophene) ligands have been prepared and investigated for applications in propylene polymerization. The ligand framework has two X-type phenolates connected to the flat heterocyclic L-type donor at the 2,6- or 2.5- positions via direct ring-ring (sp^2-sp^2)linkages. The zirconium and titanium dibenzyl complexes have been prepared by treatment of the neutral bis(phenol)-donor ligands with M(CH_2Ph)_4 (M = Ti, Zr) with loss of 2 equiv of toluene. Titanium complexes with bis(phenolate)pyridine and -furan ligands and zirconium complexes with bis(phenolate)pyridine and -thiophene ligands have been characterized by single-crystal X-ray diffraction. The solid-state structures of the bis(benzyl)titanium complexes are roughly C_2 symmetric, while the zirconium derivatives display C_s and C^1 symmetry. The bis(phenolate)pyridine titanium complexes are structurally affected by the size of the substituents substituents (CMe_3 or CEt_3) ortho to the oxygens, the larger group leading to a larger C_2 distortion. Both titanium and zirconium dibenzyl complexes were found to be catalyst precursors for the polymerization of propylene upon activation with methylaluminoxane (MAO). The activities observed for the zirconium complexes are particularly notable, exceeding 10^6 g polypropylene/mol Zr center dot h in some cases. The bis(phenolate)pyridine titanium analogues are about 10^3 times less active, but generate polymers of higher molecular weight. When activated with MAO, the titanium bis(phenolate)furan and bis(phenolate)thiophene systems were found to promote propylene oligomerization

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Coherence in Microchip Traps

We report the coherent manipulation of internal states of neutral atoms in a magnetic microchip trap. Coherence lifetimes exceeding 1 s are observed with atoms at distances of $5-130 \mu$m from the microchip surface. The coherence lifetime in the chip trap is independent of atom-surface distance within our measurement accuracy, and agrees well with the results of similar measurements in macroscopic magnetic traps. Due to the absence of surface-induced decoherence, a miniaturized atomic clock with a relative stability in the $10^{-13}$ range can be realized. For applications in quantum information processing, we propose to use microwave near-fields in the proximity of chip wires to create potentials that depend on the internal state of the atoms.Comment: Revised version, accepted for publication in Phys. Rev. Lett., 4 pages, 4 figure

Synthesis and reactivity of tantalum complexes supported by bidentate X2 and tridentate LX2 ligands with two phenolates linked to pyridine, thiophene, furan, and benzene connectors: mechanistic studies of the formation of a tantalum benzylidene and insertion chemistry for tantalum-carbon bonds

Using either alkane elimination or salt metathesis methods, tantalum complexes have been prepared with new ligand systems with tridentate bis(phenolate)donor (donor = pyridine, furan, and thiophene) or bidentate bis(phenolate)benzene arrangements. The ligand framework has two X-type phenolates connected to the flat heterocyclic L-type donor at the 2,6- or 2,5- positions or to the 2,6- positions of benzene via direct ring−ring (sp_2−sp_2) linkages. Solid-state structures of these complexes show that in all cases the ligands bind in a mer fashion, but with different geometries of the LX_2 frameworks. The pyridine-linked system binds in a Cs-fashion, the furan-linked system in a C2_v-fashion, and the thiophene-linked system in a C_1-fashion. A bis(phenolate)pyridine tantalum tribenzyl species (7), upon heating in the presence of dimethylphenylphosphine, generates a stable benzylidene complex by α-hydrogen abstraction with loss of toluene and PMe_2Ph trapping. This process was found to be independent of PMe_2Ph concentration with ΔH = 31.3 ± 0.6 kcal·mol−1 and ΔS = 3 ± 2 cal·mol−1·K−1, and the kinetic isotope effect kH/kD = 4.9 ± 0.4, consistent with a mechanism involving rate determining α-hydrogen abstraction with loss of toluene, followed by fast phosphine coordination to the resulting benzylidene species. An X-ray structure determination reveals that the benzylidene π-bond is oriented perpendicular to the oxygen−oxygen vector, in accord with the prediction of DFT calculations. Tantalum alkyl complexes with the benzene-linked bis(phenolate) ligand (Ta(CH_3)2[(OC_6H_2-tBu_2)2C_6H_3] (16), Ta(CH_2Ph)2[(OC_6H_2-tBu_2)2C_6H_3] (17), and TaCl_2CH_3[(OC_6H_2-tBu_2)2C_6H_4] (18)) are obtained with (to afford pincer complexes) or without cyclometalation at the ipso-position. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzene-diyl ring reveals an unexpected mechanism for the metalation of bis(phenol)benzene with TaC_l2(CH_3)3 to generate 18. This process involves protonolysis of a methyl group, followed by C-H/Ta-CH_3 σ bond metathesis leading to cyclometalation of the linking ring, and finally protonation of the cyclometallated group by the pendant phenol. TaCl_2CH_3[(OC_6H_2-tBu_2)2C_6H_4] was found to undergo σ bond metathesis at temperatures over 90 °C to give the pincer complex TaCl_2[(OC_6H_2-tBu_2)2C_6H_3] (1_9) and methane (ΔH = 27.1 ± 0.9 kcal·mol−1; ΔS≠ = −2 ± 2 cal·mol^1·K^1; k_H/k_D = 1.6 ± 0.2 at 125 °C). Ta(CH_3)_2[(OC_6H_2-tBu_2)_2C_6H_3] (16) was found to react with tBuNC to insert into the Ta-CH_3 bonds and generate an imino-acyl species (23). Reaction of 16 with Ph_2CO or PhCN leads to insertion into the Ta-Ph bond to give 21 and 22. Complexes 6, 7, 10, 11-P, 12, 13, 17, 18, 19-OEt_2, 21, 22, and 23 have been structurally characterized by single crystal X-ray diffraction, and all show a mer binding mode of the diphenolate ligands, but the ligand geometry varies leading to C_2v-, pseudo-C_s-, pseudo-C_2-, and C_1-symmetric structures

Gating NO Release from Nitric Oxide Synthase

We have investigated the kinetics of NO escape from Geobacillus stearothermophilus nitric oxide synthase (gsNOS). Previous work indicated that NO release was gated at position 223 in mammalian enzymes; our kinetics experiments include mutants at that position along with measurements on the wild type enzyme. Employing stopped-flow UV–vis methods, reactions were triggered by mixing a reduced enzyme/N-hydroxy-l-arginine complex with an aerated buffer solution. NO release kinetics were obtained for wt NOS and three mutants (H134S, I223V, H134S/I223V). We have confirmed that wt gsNOS has the lowest NO release rate of known NOS enzymes, whether bacterial or mammalian. We also have found that steric clashes at positions 223 and 134 hinder NO escape, as judged by enhanced rates in the single mutants. The empirical rate of NO release from the gsNOS double mutant (H134/I223V) is nearly as rapid as that of the fastest mammalian enzymes, demonstrating that both positions 223 and 134 function as gates for escape of the product diatomic molecule

Arene C-H Amination at Nickel in Terphenyl–Diphosphine Complexes with Labile Metal–Arene Interactions

The meta-terphenyl diphosphine, m-P_2, 1, was utilized to support Ni centers in the oxidation states 0, I, and II. A series of complexes bearing different substituents or ligands at Ni was prepared to investigate the dependence of metal–arene interactions on oxidation state and substitution at the metal center. Complex (m-P_2)Ni (2) shows strong Ni^0–arene interactions involving the central arene ring of the terphenyl ligand both in solution and the solid state. These interactions are significantly less pronounced in Ni^0 complexes bearing L-type ligands (2-L: L=CH_3CN, CO, Ph_2CN_2), Ni^IX complexes (3-X: X=Cl, BF_4, N_3, N_3B(C_6F_5)_3), and [(m-P_2)Ni^(II)Cl_2] (4). Complex 2 reacts with substrates, such as diphenyldiazoalkane, sulfur ylides (Ph_2=CH_2), organoazides (RN_3: R=para-C_6H_4OMe, para-C_6H_4CF_3, 1-adamantyl), and N_2O with the locus of observed reactivity dependent on the nature of the substrate. These reactions led to isolation of an η^1-diphenyldiazoalkane adduct (2-Ph_2CN_2), methylidene insertion into a Ni-P bond followed by rearrangement of a nickel-bound phosphorus ylide (5) to a benzylphosphine (6), Staudinger oxidation of the phosphine arms, and metal-mediated nitrene insertion into an arene C-H bond of 1, all derived from the same compound (2). Hydrogen-atom abstraction from a Ni^I–amide (9) and the resulting nitrene transfer supports the viability of Ni–imide intermediates in the reaction of 1 with 1-azido-arenes

Extreme Covariant Quantum Observables in the Case of an Abelian Symmetry Group and a Transitive Value Space

We represent quantum observables as POVMs (normalized positive operator valued measures) and consider convex sets of observables which are covariant with respect to a unitary representation of a locally compact Abelian symmetry group $G$. The value space of such observables is a transitive $G$-space. We characterize the extreme points of covariant observables and also determine the covariant extreme points of the larger set of all quantum observables. The results are applied to position, position difference and time observables.Comment: 23 page