Catalytic copolymerization of CO and ethylene with a charge neutral palladium(II) zwitterion

The synthesis of a zwitterionic Pd(II) complex supported by an anionic bis(phosphino)borate ligand, Ph(2)B(CH(2)PPh(2))(2) (abbreviated as [Ph(2)BP(2)]), is reported. The new complex, [Ph(2)BP(2)]PdMe(THF), is active for CO and ethylene copolymerization. The copolymerization activity and polyketone molecular weight for the neutral, zwitterionic system are compared with those for the cationic systems [R(2)E(CH(2)PPh(2))(2)PdMe(THF)][B(C(6)F(5))(4)] where ER(2) = SiPh(2) and CH(2). Surprisingly, the more electron rich zwitterionic system is a catalyst of activity comparable to that of the more conventional cationic systems.     

Streamlined ellipsometry procedure for characterizing physical aging rates of thin polymer films

Existing studies in the research literature showing conflicting changes in physical aging rates with decreasing film thickness in nanoconfined polymer films highlight the need for a single experimental technique to efficiently characterize physical aging rates in thin polymer films of varying chemical structure. To that end, we have developed a streamlined ellipsometry procedure to measure the structural relaxation of thin glassy polymer films. We evaluate different methods of calculating a physical aging rate β from the measured thickness h(t) and index of refraction n(t) data. We present extensive measurements of β as a function of aging temperature and aging time for polystyrene (PS) films supported on silicon, and determine that the physical aging rate β can be easily and reliably determined from β = −1/h₀ dh/d(log t), where h₀ is the initial measure of the film thickness at an aging time of 10 min. We have also carried out oxygen permeation studies on poly(methyl methacrylate) (PMMA) films from 800 μm down to 190 nm in thickness, and find no change in the permeability with film thickness or physical aging at room temperature for up to 65 days, which suggests that gas permeation may be insensitive to physical aging in such low free volume polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2509–2519, 2009     

A combined spectroscopic and computational study of a high-spin S = 7/2 diiron complex with a short iron-iron bond

The nature of the iron-iron bond in the mixed-valent diiron tris(diphenylforamidinate) complex Fe(2)(DPhF)(3), which was first reported by Cotton, Murillo et al. (Inorg. Chim. Acta 1994, 219, 7-10), has been examined using additional spectroscopic and theoretical methods. It is shown that the coupling between the two iron centers is strongly ferromagnetic, giving rise to an octet spin ground state. On the basis of M?ssbauer spectroscopy, the two iron centers, formally mixed-valent Fe(II)Fe(I), are completely equivalent with an isomer shift δ = 0.65 mm s(-1) and quadrupole splitting ΔE(Q) = +0.32 mm s(-1). A large, positive zero-field splitting D(7/2) = 8.2 cm(-1) has been determined from magnetic susceptibility measurements. Multiconfigurational quantum studies of the complete molecule Fe(2)(DPhF)(3) found one dominant configuration (σ)(2)(π)(4)(π*)(2)(σ*)(1)(δ)(2)(δ*)(2), which accounts for 73% of the ground-state wave function. By considering all the configurations, an estimated metal-metal bond order of 1.15 has been calculated. Finally, Fe(2)(DPhF)(3) exhibits weak electronic absorptions in the visible and near-infrared regions, which are assigned as d-d transitions from the doubly occupied metal-metal π molecular orbital to half-occupied π*, δ, and δ* orbitals.     

Automating data acquisition affects mass spectrum quality and reproducibility during bacterial profiling using an intact cell sample preparation method with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The use of matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (TOF MS) has emerged as a promising tool to rapidly profile bacteria at the genus and species level and, more recently, at the sub-species (strain) level. Recently, it has been proposed that the approach can be enhanced with regard to reproducibility and throughput by automating spectrum acquisition; however, effects of automating spectrum acquisition on spectrum quality and reproducibility have not been investigated. Using an intact cell-based sample preparation method, we directly compared the quality and reproducibility of spectra acquired in a fully automated fashion to those acquired manually by two operators with different levels of experience. While automation tended to increase base peak resolution, other measures of spectrum quality, including signal-to-noise (S:N) ratio, data richness, and reproducibility were reduced. Negative effects of automation on the performance of this approach to bacterial profiling may be particularly important during profiling of closely related strains of bacteria that yield very similar spectra.     

Desulfurization of phosphorothioate oligonucleotides via the sulfur-by-oxygen replacement induced by the hydroxyl radical during negative electrospray ionization mass spectrometry

While the occurrence of desulfurization of phosphorothioate oligonucleotides in solution is well established, this study represents the first attempt to investigate the basis of the unexpected desulfurization via the net sulfur-by-oxygen (S-O) replacement during negative electrospray ionization (ESI). The current work, facilitated by quantitative mass deconvolution, demonstrates that considerable desulfurization can take place even under common negative ESI operating conditions. The extent of desulfurization is dependent on the molar phosphorothioate oligonucleotide-to-hydroxyl radical ratio, which is consistent with the corona discharge-induced origin of the hydroxyl radical leading to the S-O replacement. This hypothesis is supported by the fact that an increase of the high-performance liquid chromatography (HPLC) flow rate and the on-column concentration of a phosphorothioate oligonucleotide, as well as a decrease of the electrospray voltage reduce the degree of desulfurization. Comparative LC-tandem mass spectrometry (MS/MS) sequencing of a phosphorothioate oligonucleotide and its corresponding desulfurization product revealed evidence that the S-O replacement occurs at multiple phosphorothioate internucleotide linkage sites. In practice, the most convenient and effective strategy for minimizing this P?=?O artifact is to increase the LC flow rate and the on-column concentration of phosphorothioate oligonucleotides. Another approach to mitigate possible detrimental effects of the undesired desulfurization is to operate the ESI source at a very low electrospray voltage to diminish the corona discharge; however this will significantly compromise sensitivity when analyzing the low-level P?=?O impurities in phosphorothioate oligonucleotides.     

Long distance transmission of SC-FDMA signals by directly-modulated OIL-VCSEL

We demonstrate the long distance transmission of single-carrier frequency division multiple address signals by directly-modulated optically injection-locked vertical-cavity surface-emitting laser. Transmission distance as long as 50 km is achieved at 5 Gb/s (2.5 Gb/s for each user) through data pattern inversion and higher frequency response gain under optical injection locking.     

Solid-phase microextraction of N-nitrosamines

A method was developed for the extraction of seven N-nitrosamine compounds from water by solid-phase microextraction (SPME). The method developed requires a total analysis time of only 1.25 h for both extraction and detection (versus 3-20 h for other isolation techniques). Three gas chromatography (GC) detection systems were tested with the SPME method, nitrogen chemiluminesence detection (NCD), nitrogen-phosphorus detection (NPD) and chemical ionization mass spectrometry (CI-MS), with method detection limits (MDLs) found in the ng/L range. This method was used to analyze wastewater samples and showed excellent selectivity of extraction. The detection limits of this method for N-nitrosodimethylamine (NDMA) range from 30 to 890 ng/L as a function of detector type. The excellent selectivity of SPME in addition to the fast analysis time would make this method ideal for general surveys, wastewater analysis and laboratory studies (e.g. degradation kinetics or formation potential).     

Comparison of new microemulsion prepared "Pt-in-Ceria" catalyst with conventional "Pt-on-Ceria" catalyst for water-gas shift reaction

New "Pt-in-CeO(2)" catalyst prepared by microemulsion method is shown to give higher activity for a water-gas shift reaction but with no formation of CH(4), the side product from hydrogenation of carbon oxides using a hydrogen-rich reformate as compared to conventional "Pt-on-CeO(2)" catalysts. Detailed characterization by DRIFT analysis and temperature programmed reduction presented in this work clearly suggest the ceria coverage on Pt inhibits the metal from forming a strong CO adsorption.     

Novel copolymers of styrene. 7. Some ring-disubstituted butyl 2-cyano-3-phenyl-2-propenoates

Novel trisubstituted ethylenes, ring-disubstituted butyl 2-cyano-3-phenyl-2-propenoates, RPhCH?C(CN)CO₂C₄H₉ (where R is 2-bromo-5-methoxy, 3-bromo-4-methoxy, 5-bromo-2-methoxy, 2-chloro-3-methoxy, 3-chloro-4-methoxy, 2-chloro-6-methyl, 3-chloro-4-methyl, 2-fluoro-4-methoxy) were prepared and copolymerized with styrene. The monomers were synthesized by the piperidine catalyzed Knoevenagel condensation of ring-substituted benzaldehydes and butyl cyanoacetate, and characterized by CHN analysis, IR, ¹H and ¹³C-NMR. All the ethylenes were copolymerized with styrene in solution with radical initiation (ABCN) at 70°C. The compositions of the copolymers were calculated from nitrogen analysis and the structures were analyzed by IR, ¹H and ¹³C-NMR. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 200-500ºC range with residue (1-6% wt.), which then decomposed in the 500–800ºC range.     

Developmental changes in hematopoietic stem cell properties

Hematopoietic stem cells (HSCs) comprise a rare population of cells that can regenerate and maintain lifelong blood cell production. This functionality is achieved through their ability to undergo many divisions without activating a poised, but latent, capacity for differentiation into multiple blood cell types. Throughout life, HSCs undergo sequential changes in several key properties. These affect mechanisms that regulate the self-renewal, turnover and differentiation of HSCs as well as the properties of the committed progenitors and terminally differentiated cells derived from them. Recent findings point to the Lin28b-let-7 pathway as a master regulator of many of these changes with important implications for the clinical use of HSCs for marrow rescue and gene therapy, as well as furthering our understanding of the different pathogenesis of childhood and adult-onset leukemia.