Measurement of the rate coefficient of the reaction CH+O2 → products in the temperature range 2200 to 2600 K

The rate coefficient of the reaction CH+O₂ → products was determined by measuring CH-radical concentration profiles in shock-heated 100–150 ppm ethane/1000 ppm O₂ mixtures in Ar using cw, narrow-linewidth laser absorption at 431.131 nm. Comparing the measured CH concentration profiles to ones calculated using a detailed kinetics model, yielded the following average value for the rate coefficient independent of temperature over the range 2200–2600 K: The experimental conditions were chosen such that the calculated profiles were sensitive mainly to the reactions CH+O₂ → products and CH₃+M → CH+H₂+M. For the methyl decomposition reaction channel, the following rate-coefficient expression provided the best fit of the measured CH profiles: Additionally, the rate coefficient of the reaction CH₂+H→CH+ H₂ was determined indirectly in the same system: © 1997 John Wiley & Sons, Inc.     

Cavity-enhanced absorption spectroscopy for shocktubes: Design and optimization

Cavity-enhanced absorption spectroscopy (CEAS) has generated much interest in shocktube kinetics studies because of its recent success in achieving improved sensitivity and high time resolution with robust optical alignment. While recent progress demonstrated experimental schemes including off-axis scanned-wavelength approach and on-axis ps-pulsed laser approach, that both successfully suppressed the laser-cavity coupling noise, this paper develops a theoretical model to predict the CEAS sensor performance that can be used as a design tool applicable to more generalized cases. The method models the optical field in the cavity based on the decentered Gaussian beam model, from which the cavity transmission spectrum and the laser-cavity coupling noise can be numerically calculated. The simulation results predict sensor performance for different cavity configurations and laser characteristics, including various degrees of laser-cavity mode-matching, laser linewidths, scanning rates, and cavity filling conditions. Simulation with example wavelengths in the ultraviolet, near-infrared, and mid-infrared showed increasing mode-matched beam waist size for increasing wavelengths. An off-axis alignment scheme was found to be capable of suppressing the coupling noise by two orders-of-magnitude at a moderate laser linewidth of 1?GHz. Coupling noise level on the order of 1e-5 for scanned-wavelength off-axis alignment case with a narrowband mid-infrared laser was obtained by model calculation and agreed with experimental results within acceptable uncertainty range. The developed method can serve to guide future design and optimization of CEAS system in shocktube studies.     

Measurements of high-pressure CO2 absorption near 2.0 μm and implications on tunable diode laser sensor design

A tunable diode laser (TDL) is used to measure the absorption spectra of the R46 through R54 transitions of the 20012←00001 band of CO₂ near 2.0 μm (5000 cm⁻¹) at room temperature and pressures to 10 atm (densities to 9.2 amagat). Spectra are recorded using direct absorption spectroscopy and wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) in a mixture containing 11% CO₂ in air. The direct absorption spectra are influenced by non-Lorentzian effects including finite-duration collisions which perturb far-wing absorption, and an empirical χ-function correction to the Voigt line shape is shown to greatly reduce error in the spectral model. WMS-2f spectra are shown to be at least a factor of four less-influenced by non-Lorentzian effects in this region, making this approach more resistant to errors in the far-wing line shape model and allowing a comparison between the spectral parameters of HITRAN and a new database which includes pressure-induced shift coefficients. The implications of these measurements on practical, high-pressure CO₂ sensor design are discussed.     

Mid-infrared absorption measurements of liquid hydrocarbon fuels near

Quantitative absorption spectra for several hydrocarbon fuels in the liquid phase at are presented. Measurements of toluene, n-dodecane, n-decane, and three samples of gasoline were made over the spectral region 2700–3200 to support the development of mid-infrared laser-absorption diagnostics for measurements of fuel vapor in the presence of liquid films and aerosols. A procedure for quantitative Fourier transform infrared (FTIR) absorption measurements of strongly absorbing liquids is described and the resulting absorption spectra are compared with previously measured absorption spectra in the vapor phase. The measured absorption spectra for liquid gasoline are shown to scale with the volume percent of olefin, alkane, and aromatic hydrocarbons in each sample. Finally, the observed frequency shift of in the spectra of vapor and liquid hydrocarbons is discussed, including the potential for measurements of fuel vapor in the presence of liquid films.     

Structure of Plastic Crystalline Succinonitrile: High‐Resolution in situ Powder Diffraction

The temperature dependent (150–290 K) crystal structure of the low‐temperature α‐phase, and high temperature β‐phase, of succinonitrile has been determined by high resolution in situ powder diffraction. The α‐phase has a monoclinic unit cell that contains four gauche molecules and belongs to the P2₁/a space group. The crystal undergoes a reversible first‐order phase transition at 233 K into the high temperature β‐phase. The lattice parameters increase with temperature and the phase transition leads to an abrupt 6.7 % increase in volume. The β‐phase crystallizes into a bcc‐structure that belongs to the space group. The high temperature phase; however, is a highly disordered plastic crystal at room temperature that contains both gauche and trans molecules. The non‐linearity in the overall isotropic temperature‐factor indicates other possible phase transitions in the temperature range of 233–250 K.     

Halogen Bonding between an Isoindoline Nitroxide and 1,4‐Diiodotetrafluorobenzene: New Tools and Tectons for Self‐Assembling Organic Spin Systems

Radical assembly : Halogen bonding has been observed for the first time between an isoindoline nitroxide and an iodoperfluorocarbon (see figure), which cocrystallize to form a discrete 2:1 supramolecular compound in which N? O^.???I halogen bonding is the dominant intermolecular interaction. This illustrates the potential use of halogen bonding and isoindoline nitroxide tectons for the assembly of organic spin systems.

     

Synthesis of 7-(E)-alkenyl-4-amino-3-quinolinecarbonitriles via Pd-mediated Heck, Stille, and Suzuki reactions

A regio- and stereoselective synthesis of 7-(E)-alkenyl-4-amino-3-quinolinecarbonitriles via Pd-mediated coupling reactions was developed. The comparison and optimization of stereoselectivity of the Heck, Stille, and Suzuki reactions of 7-bromo or 7-triflate-3-quinolinecarbonitrile are described. Compound 7 and 10 were potent inhibitors of Src kinase and Raf/Mek activity, respectively.     

Efforts Toward the Direct Experimental Characterization of Enzyme Microenvironments: Tyrosine100 in Dihydrofolate Reductase

State secrets : Site‐specific deuteration and FTIR studies reveal that Tyr100 in dihydrofolate reductase plays an important role in catalysis, with a strong electrostatic coupling occuring between Tyr100 and the charge that develops in the hydride‐transfer transition state (see picture, NADP⁺ purple, Tyr100 green). However, relaying correlated motions that facilitate catalysis from distal sites of the protein to the hydride donor may also be involved.

     

Development of an Automated High Throughput LCP-FRAP Assay to Guide Membrane Protein Crystallization in Lipid Mesophases

Crystallization in lipidic mesophases (in meso) has been successfully used to obtain a number of high-resolution membrane protein structures including challenging members of the human G protein-coupled receptor (GPCR) family. Crystallogenesis in arguably the most successful mesophase, lipidic cubic phase (LCP), critically depends on the ability of protein to diffuse in the LCP matrix and to form specific protein-protein contacts to support crystal nucleation and growth. The ability of an integral membrane protein to diffuse in LCP is strongly affected by the protein aggregation state, the structural parameters of LCP, and the chemical environment. In order to satisfy both requirements of diffusion and specific interactions, one must balance multiple parameters, such as identity of LCP host lipid, composition of precipitant solution, identity of ligand, and protein modifications. Screening within such multi-dimensional crystallization space presents a significant bottleneck in obtaining initial crystal leads. To reduce this combinatorial challenge, we developed a pre-crystallization screening assay to measure the diffusion characteristics of a protein target in LCP. Utilizing the Fluorescence Recovery After Photobleaching (FRAP) technique in an automated and high throughput manner, we were able to map conditions that support adequate diffusion in LCP using a minimal amount of protein. Data collection and processing protocols were validated using two model GPCR targets: the β(2)-adrenergic receptor and the A(2A) adenosine receptor.