Scattering of wavepackets by a flat plate in the vicinity of a turbulent jet

We present an investigation of the acoustic scattering due to the presence of a flat plate in the vicinity of a turbulent subsonic jet. Experiments have been performed to measure changes in the velocity and sound fields for Mach numbers ranging from 0.4 to 0.6, and for distances between the plate and the jet axis ranging from 1 to 2 jet diameters. Results show only very slight changes in the mean flow induced by the plate, and no differences in the velocity fluctuation amplitudes on the jet centreline, suggesting that wave-packet models derived for jets without installation effects may be representative of the installed case, at least for the jet–plate distances considered here. The acoustic results, on the other hand, include a significant increase in the low-frequency sound radiation, and phase opposition between the shielded and unshielded sides of the plate. There is an exponential decay of the scattered sound with increasing jet–plate distance, suggesting that low-frequency radiation is due to the scattering of evanescent hydrodynamic wavepackets in the jet near field. To model this phenomenon, we calculate sound generation from wave-packet sources in two ways: on one hand we use a tailored Green?s function that accounts for the presence of a semi-infinite, rigid flat plate; and, on the other, we solve numerically the Helmholtz equation, with boundary conditions representative of a finite flat plate, using a fast multipole boundary element method. In agreement with the experimental measurements, numerical calculations capture the phase opposition between shielded and unshielded sides, and the scattered sound depends exponentially on the position of the plate. This exponential dependence is related to non-compact effects associated with wavepackets, as compact sources would lead to an algebraic dependence. Acoustic pressure directivities computed for the finite and semi-infinite flat plates agree well where acoustic reflection and diffraction from the trailing edge of the plates are concerned. However, additional diffraction effects associated with the leading and lateral edges of the finite plate, and which take the form of multiple lobes in the directivity, are illustrated by the comparison. As the plate dimensions are increased, i.e. the Helmholtz number is increased, the solution approaches that obtained for the semi-infinite plate.     

Synthesis of Ni-Ru alloy nanoparticles and their high catalytic activity in dehydrogenation of ammonia borane

We report the synthesis and characterization of new Ni(x)Ru(1-x) (x = 0.56-0.74) alloy nanoparticles (NPs) and their catalytic activity for hydrogen release in the ammonia borane hydrolysis process. The alloy NPs were obtained by wet-chemistry method using a rapid lithium triethylborohydride reduction of Ni(2+) and Ru(3+) precursors in oleylamine. The nature of each alloy sample was fully characterized by TEM, XRD, energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). We found that the as-prepared Ni-Ru alloy NPs exhibited exceptional catalytic activity for the ammonia borane hydrolysis reaction for hydrogen release. All Ni-Ru alloy NPs, and in particular the Ni(0.74)Ru(0.26) sample, outperform the activity of similar size monometallic Ni and Ru NPs, and even of Ni@Ru core-shell NPs. The hydrolysis activation energy for the Ni(0.74)Ru(0.26) alloy catalyst was measured to be approximately 37?kJ?mol(-1). This value is considerably lower than the values measured for monometallic Ni (≈70?kJ?mol(-1)) and Ru NPs (≈49?kJ?mol(-1)), and for Ni@Ru (≈44?kJ?mol(-1)), and is also lower than the values of most noble-metal-containing bimetallic NPs reported in the literature. Thus, a remarkable improvement of catalytic activity of Ru in the dehydrogenation of ammonia borane was obtained by alloying Ru with a Ni, which is a relatively cheap metal.     

Metal-catalyzed reduction of HCONR'2, R' = Me (DMF), Et (DEF), by silanes to produce R'2NMe and disiloxanes: a mechanism unraveled

We demonstrate that using Mo(CO)(6), Mo(CO)(5)NMe(3), and (η(5)-C(5)H(5))Mn(CO)(3) as catalysts for the silane, R(3)SiH, reduction of N,N-dimethylformamide (DMF), and N,N-diethylformamide (DEF), we can observe, intercept, and isolate, the important siloxymethylamine intermediates, R(3)SiOCH(2)NR'(2), R' = Me, Et, for the first time. In the presence of excess DMF such intermediates thermally react with a variety of silanes to form the corresponding disiloxanes in the absence of a metal catalyst. We also show that the germanium hydrides, Et(3)GeH and Bu(3)GeH, also reduce DMF to form trimethylamine and the corresponding digermoxane but observe no intermediates R(3)GeOCH(2)NMe(2). Bu(3)SnH reduces DMF, but along with the low yields of Bu(3)SnOSnBu(3) (but no Bu(3)SnOCH(2)NMe(2)) significant side products are obtained including (Bu(3)Sn)(2) and Bu(4)Sn. In the absence of DMF the siloxymethylamines can undergo metal-catalyzed reactions with silanes, germanes and stannanes to form disiloxanes, and R(3)SiOER(3) E = Ge, Sn, respectively. To date, the most efficient catalyst for this latter process is (η(5)-C(5)H(5))Mo(CO)(3)CH(3) via a photochemical reaction.     

Nonlinear modeling of equilibrium sorption of selected anionic adsorbates from aqueous solutions on cellulosic substrates: part 2: experimental validation

The equilibrium sorption of CI Direct Blue 1, as a model anionic adsorbate, from aqueous solution on cotton fabric was experimentally studied. The equilibrium sorption data were employed to test the performance of a proposed nonlinear sorption model in describing the sorption behavior of anionic adsorbates on cellulosic substrates. The analysis of data based on the nonlinear least-squares curve fitting showed excellent correlation against experimental values as indicated by a high coefficient of determination (R ²). The V and −Δμ° parameters were determined based on the nonlinear model and employed to represent the internal accessible volume of cotton to CI Direct Blue 1 and the standard affinity of sorption. It was shown that variations in temperature and concentration of electrolyte (e.g. NaCl) in the sorption system result in slight variations in V. The findings extend the scope of the previous study and indicate that the nonlinear sorption model can be used to accurately determine the thermodynamic properties of a sorption system, such as the affinity (−Δμ°), enthalpy (ΔH°), and entropy (ΔS°), for anionic adsorbates on cellulosic substrates.     

Experiment and theory of low-pressure nitrogen adsorption in organic layers supported or grafted on inorganic adsorbents: toward a tool to characterize surfaces of hybrid organic/inorganic systems

We report experimental nitrogen adsorption isotherms of organics-coated silicas, which exhibit a low-pressure desorption branch that does not meet the adsorption branch upon emptying of the pores. To address the physical origin of such a hysteresis loop, we propose an equilibrium thermodynamic model that enables one to explain this phenomenon. The present model assumes that, upon adsorption, a small amount of nitrogen molecules penetrate within the organic layer and reach adsorption sites that are located on the inorganic surface, between the grafted or adsorbed organic molecules. The number of accessible adsorption sites thus varies with the increasing gas pressure, and then we assume that it stays constant upon desorption. Comparison with experimental data shows that our model captures the features of nitrogen adsorption on such hybrid organic/inorganic materials. In particular, in addition to predicting the shape of the adsorption isotherm, the model is able to estimate, with a reasonable number of adjustable parameters, the height of the low-pressure hysteresis loop and to assess in a qualitative fashion the local density of the organic chains at the surface of the material.     

Coherent spectral enhancement of carrier-envelope-phase stable continua with dual-gas high harmonic generation

Attosecond science is enabled by the ability to convert femtosecond near-infrared laser light into coherent harmonics in the extreme ultraviolet spectral range. While attosecond sources have been utilized in experiments that have not demanded high intensities, substantially higher photon flux would provide a natural link to the next significant experimental breakthrough. Numerical simulations of dual-gas high harmonic generation indicate that the output in the cutoff spectral region can be selectively enhanced without disturbing the single-atom gating mechanism. Here, we summarize the results of these simulations and present first experimental findings to support these predictions.     

Thermoresponsive Poly(2‐oxazine)s

The monomers 2‐methyl‐2‐oxazine (MeOZI), 2‐ethyl‐2‐oxazine (EtOZI), and 2‐n‐propyl‐2‐oxazine (nPropOZI) were synthesized and polymerized via the living cationic ring‐opening polymerization (CROP) under microwave‐assisted conditions. pEtOZI and pnPropOZI were found to be thermoresponsive, exhibiting LCST behavior in water and their cloud point temperatures (T_CP) are lower than for poly(2‐oxazoline)s with similar side chains. However, comparison of poly(2‐oxazine) and poly(2‐oxazoline)s isomers reveals that poly(2‐oxazine)s are more water soluble, indicating that the side chain has a stronger impact on polymer solubility than the main chain. In conclusion, variations of both the side chains and the main chains of the poly(cyclic imino ether)s resulted in a series of distinct homopolymers with tunable T_CP.     

The glycosyl C1′—N9 bond of deoxyadenosine and deoxyguanosine: response to electrophilic attacks on the purinic nitrogen atoms

Self-consistent-field computations shed light on two relevant conformations of deoxyadenosine (dA) and deoxyguanosine (dG): one with a pseudoequatorial C_1′N₉ glycosyl bond and the other, a slightly more stable one, with its C_1′N₉ bond in a bisectional orientation. In dA, both the N₃ and N₇ nitrogens are plausible sites for electrophilic attack, but only N₇ is a plausible site in dG. The addition of H⁺, CH₃ ⁺, C₂H₅ ⁺ or tert-C₄H₉ ⁺ onto N₇ does not provoke notable structural modifications and leaves the base of dA and dG in an antiperiplanar (or nearly antiperiplanar) position with respect to the sugar C_1′O_4′ bond, but N₃ additions cause the base to adopt a synperiplanar or strongly chiral position. This produces strong interactions between the purine and deoxyribose moieties, whose relief could aid the eventual cleavage of the glycosyl bond of dA. Addition of a radical cation onto N₇ reduces the dissociation energy of the glycosyl bond by an estimated 8 kcal mol⁻¹ in dA and 4 kcal mol⁻¹ in dG – a bond weakening likely to concur to a depurination of DNA induced by radical cations. Received: 13 September 1999 / Accepted: 3 February 2000 / Published online: 21 June 2000     

Multireference perturbation CI IV. Selection procedure for one-electron properties

We present in this paper a modification of the selection procedure, within the CIPSI algorithm, specifically intended for the calculation of one-electron properties. In this new procedure the determinants are selected following their contribution to a certain one-electron property. As test cases we report the calculation of the electric dipole of carbon monoxide and of the hyperfine coupling constants of the CH₃ radical. Received: 20 August 2000 / Accepted: 2 October 2000 / Published online: 21 December 2000     

[N,N′-(4-Methyl-4-azaheptane-1,7-diyl-κN)bis(4-methoxysalicylideniminato-κ4O,N,N′,O′)]cobalt(II) ethanol hemisolvate

The title compound, [Co(C₂₃H₂₉N₃O₄)]·0.5C₂H₆O or [Co^II{(4-MeO-sal)₂Medpt}]·0.5CH₃CH₂OH [(4-MeO-sal)₂Medpt is N,N′-(4-methyl-4-aza­heptane-1,7-diyl)­bis(4-methoxysalicyl­iden­iminate)], obtained through the reaction of H₂[(4-MeO-sal)₂Medpt] and Co(CH₃COO)₂ in refluxing ethanol under an atmosphere of ultrapure nitro­gen, has the usual pseudo-trigonal-bipyramidal coordination arrangement previously found for this class of (sal)₂Rdpt compounds. The O—Co—O bond angle [120.4 (1)°] is significantly smaller than the corresponding values previously found for most non-O₂-bound [Co^II{(sal)₂Medpt}]-type mol­ecules (observed range 126.9–138.6°), whereas the equatorial Co—N bond [2.185 (3) Å] is relatively long.