Synthesis and fluorescence of N-substituted-1,8-naphthalimides

A number of novel N-substituted-1,8-naphthalimides have been prepared and their fluorescence yields measured in water at pH 7.4. The type of substitutent and the substitution pattern on the naphthalimide nucleus produce markedly different fluorescence yields, (quantum efficiencies, ø varying from ø = 0-0037 for N-(3-N'-morpholino-1-propyl)-4-amino-3-methoxy-1,8-naphthalirnide (7) to ø = 0–77 for N-(3-bromopropyl)-4-acetamido-1,8-naphthalimide (31).     

Muonium in bulk fused quartz: Test case for RAHD relaxation theory

Muonium (μ ⁺ e ⁻) in bulk fused quartz is a unique system in that theμ ⁺ spin polarization (in the muonium state) relaxes almost entirely via random anisotropic hyperfine distortions (RAHD). As such, this system provides an excellent test case for a new RAHD spin relaxation theory. This theory is quantitatively compared to static zero field data and the functional characteristics in both the high field and dynamic limits are considered as well.     

Effects of fuel blending on first stage and overall ignition processes

The effects of blending ratio on mixtures of an alcohol-to-jet (ATJ) fuel and a conventional petroleum-derived fuel on first stage ignition and overall ignition delay are examined at engine-relevant ambient conditions. Experiments are conducted in a high-temperature pressure vessel that maintains a small flow of dry air at the desired temperature (825 K and 900 K) and pressure (6 MPa and 9 MPa) for fuel injections from a custom single-hole, axially-oriented injector, representing medium (7.5 mg) and high (10 mg) engine loading. Formaldehyde, imaged using planar laser-induced fluorescence, is measured at discrete time steps throughout the first and second stage ignition process and is used as a marker of unburned short-chain hydrocarbons formed after the initial breakdown of the fuel. The formaldehyde images are used to calculate the first stage ignition delay for each ambient and fuel loading condition. Chemiluminescence imaging of excited hydroxyl radical at 75 kHz is used to determine the overall ignition delay. At all conditions, increased volume fraction of ATJ resulted in longer, but non-linearly increasing, overall ignition delay. Across all of the blends, first stage ignition delay accounted for about 15% of the increase in overall ignition delay compared to the military's aviation kerosene, F-24, which is Jet A with additives, while extended first stage ignition duration accounted for 85% of the increase. It is observed that blends consisting of 0–60% by volume of the low cetane number ATJ fuel produced nearly identical first stage ignition delays. These results will inform the development of ignition models that can capture the non-linear effects of fuel blending on ignition processes.     

Impact of synthesis conditions on surface chemistry and structure of carbide-derived carbons

Carbide-derived carbons produced by chlorination of titanium carbide at 600, 800, or 1100 °C were subjected to a post-treatment at 600 °C in Ar, H₂, or NH₃ atmosphere. Experimental results suggest that the chlorination temperature influences the ordering of carbon in a manner that impacts specific surface area and porosity. Higher chlorination temperatures lead to higher total pore volume and increased ordering, but lower microporosity. The effect of post-treatments on surface chemistry is pronounced only for samples chlorinated at 600 °C; post-treatments in Ar are shown to be less effective for chlorine removal than those performed in H₂ or NH₃. Post-treatments in Ar result in a lower total pore volume compared to the ones in H₂ or NH₃ for the same chlorination temperature. Samples chlorinated at higher temperatures contained less oxygen functionalities than samples chlorinated at 600 °C, and showed correspondingly less desorption of H₂O, possibly due to diminished uptake of ambient water.     

Importance of palladium-carbon bond energies in direct arylation of polyfluorinated benzenes

Fagnou et al. reported direct arylation reactions that use palladium catalysts to couple Ar(1)-X to Ar(2)-H with the aid of a coordinated base. These reactions are particularly favourable for polyfluorinated arenes Ar(2)-H (see S. I. Gorelsky, D. Lapointe and K. Fagnou, J. Am. Chem. Soc. 2008, 130, 10848). In this paper, we show by means of a DFT analysis how the energetics and activation energies vary with fluorine substitution and examine the structures of intermediates and transition states. The reactant is modelled by Pd(OAc)(Ph)(PMe(3))(DMA) (DMA = dimethylacetamide). The sequence consists of (a) replacement of DMA by arene, (b) Concerted Deprotonation Metallation (CMD), (c) decoordination of AcOH, (d) reductive elimination of biaryl. Many of the variations are dominated by the number of fluorine substituents ortho to the C-H bond and fall into three groups labelled accordingly: Set0Fo, Set1Fo, and Set2Fo. In the first step a coordinated solvent is replaced by the arene. The arenes of Set0Fo and Set1Fo coordinate in a conventional η(2)-CH=CH mode, whereas the arenes of Set2Fo coordinate in an η(1)-CH mode assisted by an OH-C hydrogen bond from the coordinated acetate. Both the energy barriers to CMD and the product energies fall into the three typical sets with the highest barrier and highest product energy being for Set0Fo. They correlate more satisfactorily with the variations in Pd-C bond energies than with the C-H acidities. The barriers to reductive elimination from Pd(Ph)(Ar(F))(PMe(3))(AcOH) increase systematically from Set0Fo to Set2Fo as the Pd-C bond becomes stronger in a regular fashion from Set0Fo to Set2Fo. Again there is a strong correlation between the energy barriers to reductive elimination and the Pd-C bond energies. It is found overall that the key aspects of the reactions are: (a) the lowering of the energy of the CMD step by the ortho fluorine substituents, (b) the regioselective activation of C-H bonds ortho to fluorine which is also determined at the CMD step, (c) the decoordination of AcOH, which maintains the transition state for reductive elimination at low Gibbs free energy. The presence of fluorine increases the effectiveness of the reaction in the sense of points a and b via the increasing strength of the palladium-carbon bond.