The SURF model and the curvature effect for PBX 9502

SURF is a high explosive burn model based on the ignition & growth concept of hot-spot reaction. For the TATB based explosive PBX 9502, the model has been calibrated to shock-to-detonation transition experiments. To apply the SURF model for propagating detonation waves, the rate has to be extended to a higher pressure regime than is sampled by shock initiation experiments. The experimentally measured curvature effect – detonation speed as a function of front curvature or D _n(κ) – provides the appropriate data for calibrating the propagation regime. The calibration to the curvature effect is based on the ODEs for the reaction zone profile of a detonation wave in conjunction with a shooting algorithm to determine the rate model parameters, for a given κ, needed to obtain a specified detonation speed. A complication for calibrating PBX 9502 rate models arises from the kink in the experimentally measured D _n(κ) curve. This results from the fast and slow reactions that TATB exhibits. To account for this, we use an extension of the SURF model that utilises a sequence of two reactions. The first, with a fast rate, is due to molecular decomposition and is described by the original SURF formulation. The second, with a slow rate, is due to carbon clustering and is used to contribute additional energy from the formation of carbon bonds. The wave profile equations are generalised to the SURF-plus model. Model parameters are then determined for the propagation regime to fit the curvature effect data. The extended model is applicable to both the shock initiation regime and the propagating detonation wave regime.     

Structural,optical and electrical properties of tin oxide thin films by electrostatic spray deposition

Tin oxide (SnO₂) thin films were deposited by electrostatic spray deposition (ESD). The structural, optical and electrical properties of the films for different solvents were studied. The morphology of the deposited thin films was investigated by scanning electron microscopy. The optical transmission spectra of the films showed 66–75% transmittance in the visible region of spectrum. The electrical resistivity of thin films deposited using the different solvents ranged 1.08 × 10^?3–1.34 × 10^?3 Ω-cm. Overall, EG and PG were good solvents for depositing SnO₂ thin films by the ESD technique with stable cone jet.     

Hydrothermal decomposition of esters under high pressure

Abstract

The hydrothermal decomposition of two esters (methyl formate and ethyl formate) was studied in the absence of oxygen over the temperature range of 373 – 673 K at pressures from 15 to 35MPa for residence times between 72 and 600s. At hydrothermal conditions, the esters were hydrolyzed to formic acid and the corresponding alcohols in equilibrium yields of more than 90%, and the disappearance rate of ester was more promoted with increasing temperature than with increasing pressure. In supercritical water, the conversion of the esters reached 100% at short times. From reaction experiments with various catalysts, it was found that the hydrolysis of alkyl formates was accelerated by acid, while retarded by base in a manner different from that in usual water. In addition, a delayed reaction start observed in ester disappearance. profiles suggested strongly a catalytic action of formic acid produced by hydrolysis.     

Synthesis and characterization of a new acceptor (n‐type) fluorinated and terminal‐functionalized polythiophene

Donor‐ or acceptor‐substituted polythiophenes have many potential applications in optoelectronics. Fluorinated polythiophenes are particularly attractive because of the presence of fluorine, which can withdraw electrons and also improve polymer chemical stability. Because of the promising future of these polymers, there has been much interest in identifying favorable synthetic routes to new fluorinated monomers and polymers. In this study, the monomer had an electron‐withdrawing fluorinated ester and was derived from 3‐thiophene carboxylic acid and 2,2,3,3,4,4,4‐heptafluoro‐1‐butanol. The synthesis of an n‐type fluorinated and terminal‐functionalized polythiophene was accomplished with the Ullmann coupling reaction. A polymer soluble in tetrahydrofuran was obtained with a molecular weight of approximately 15,000 g/mol. In solution, it exhibited a band gap of 2.4 eV, and the photoluminescent excitation and emission maxima were 370 nm and 555 nm, respectively. All peaks were bathochromically shifted when they were measured in the solid state. The glass‐transition and decomposition (in air) temperatures were 129 and 493 °C, respectively. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4280–4287, 2005     

Regioregularity and solar cell device performance of poly(3‐dodecylthienylenevinylene)

Influence of side chain regioregularity on photovoltaic performance has been investigated in bulk heterojunction solar cells based on a series of poly(3‐dodecylthienylenevinylene)s (C₁₂‐PTV) and 6,6‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM). Performance of each C₁₂‐PTV is optimized for fair comparison. It is found that regiorandomness has no detrimental effect on device performance, in sharp contrast to poly(3‐hexylthiophene) (P3HT). Fully regioregular C₁₂‐PTV performs slightly poorer than less regioregular ones mainly due to its fast crystallization behavior. The results suggest that introduction of side chain regiorandomness is an effective strategy to enhance processability of certain types of polymers without a reduction in photovoltaic performance. The better polymer:PCBM weight ratio, found to be 3:7 for all C₁₂‐PTVs, and improved device performance, as compared with the literature work on the same polymer synthesized by a different method, demonstrate again the importance of the integrity of polymer main chain structure. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Pep-Calc.com: a set of web utilities for the calculation of peptide and peptoid properties and automatic mass spectral peak assignment

The ability to calculate molecular properties such as molecular weights, isoelectric points, and extinction coefficients is vital for scientists using and/or synthesizing peptides and peptoids for research. A suite of two web utilities: Peptide Calculator and Peptoid Calculator, available free at http://www.pep-calc.com, are presented. Both tools allow the calculation of peptide/peptoid chemical formulae and molecular weight, ChemDraw structure file export and automatic assignment of mass spectral peaks to deletion sequences and metal/protecting group adducts. Peptide Calculator also provides a calculated isoelectric point, molar extinction coefficient, graphical peptide charge summary and β-strand contiguity profile (for aggregation-prone sequences), indicating potential regions of synthesis difficulty. In addition to the unique automatic spectral assignment features offered across both utilities, Peptoid Calculator represents a first-of-a-kind resource for researchers in the field of peptoid science. With a constantly expanding database of over 120 amino acids, non-natural peptide building blocks and peptoid building blocks, it is anticipated that Pep-Calc.com will act as a valuable asset to those working on the synthesis and/or application of peptides and peptoids in the biophysical and life sciences fields.     

The Mahler measure of parametrizable polynomials

Our aim is to explain instances in which the value of the logarithmic Mahler measure m(P) of a polynomial P∈Z[x,y] can be written in an unexpectedly neat manner. To this end we examine polynomials defining rational curves, which allows their zero-locus to be parametrized via x=f(t), y=g(t) for f,g∈C(t). As an illustration of this phenomenon, we prove the equality

     

Equatorial Perturbation Driven Reaction Bifurcation in Non-Heme Iron Complexes for Chlorite Oxidation

Chlorine oxyanions have various applications, such as bleaching and oxidizers in rocket fuels. However, their high solubility in water and long environmental lifetimes have led to ecological concerns, especially regarding drinking water quality. This study focuses on the conversion of chlorite to chlorine dioxide, which is of significant interest as it exhibits superior antimicrobial activity and generates less harmful byproducts for water treatment. Two nonheme iron(II) complexes capable of producing chlorine dioxide from chlorite at room temperature and pH 5.0 are presented. These complexes oxidize chlorite through high-valent iron (IV)-oxo intermediates formed in-situ. The study establishes second-order rate constants for chlorite oxidation and investigates the effects and mechanisms involved by substituting a methyl group in the secondary coordination sphere of the Fe^IV(O)(N4Py) system. By employing kinetic analysis and spectroscopic investigations, the crucial elements for the reaction mechanism in chlorite oxidation are identified. These findings pave the way for future advancements in this field.     

Is the μ‐Oxo‐μ‐Peroxodiiron Intermediate of a Ribonucleotide Reductase Biomimetic a Possible Oxidant of Epoxidation Reactions?

Density functional calculations on a mu-oxo-mu-peroxodiiron complex (1) with a tetrapodal ligand BPP (BPP=N,N-bis(2-pyridylmethyl)-3-aminopropionate) are presented that is a biomimetic of the active site region of ribonucleotide reductase (RNR). We have studied all low-lying electronic states and show that it has close-lying broken-shell singlet and undecaplet (S=0, 5) ground states with essentially two sextet spin iron atoms. In strongly distorted electronic systems in which the two iron atoms have different spin states, the peroxo group moves considerably out of the plane of the mu-oxodiiron group due to orbital rearrangements. The calculated absorption spectra of (1,11)1 are in good agreement with experimental studies on biomimetics and RNR enzyme systems. Moreover, vibrational shifts in the spectrum due to (18)O(2) substitution of the oxygen atoms in the peroxo group follow similar trends as experimental observations. To identify whether the mu-oxo-mu-1,2-peroxodiiron or the mu-oxo-mu-1,1-peroxodiiron complexes are able to epoxidize substrates, we studied the reactivity patterns versus propene. Generally, the reactions are stepwise via radical intermediates and proceed by two-state reactivity patterns on competing singlet and undecaplet spin state surfaces. However, both the mu-oxo-mu-1,2-peroxodiiron and mu-oxo-mu-1,1-peroxodiiron complex are sluggish oxidants with high epoxidation barriers. The epoxidation barriers for the mu-oxo-mu-1,1-peroxodiiron complex are significantly lower than the ones for the mu-oxo-mu-1,2-peroxodiiron complex but still are too high to be considered for catalytic properties. Thus, theory has ruled out two possible peroxodiiron catalysts as oxidants in RNR enzymes and biomimetics and the quest to find the actual oxidant in the enzyme mechanism continues.     

Racemic versus enantiopure alanine on Cu(110): an experimental study

The adsorption of racemic alanine on the Cu(110) surface has been compared to that of enantiopure alanine using low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). No evidence of chiral resolution at the surface was observed for the racemic system, indicating that the formation of separate enantiopure areas is not preferred. Also, in contrast to the enantiopure system, no chirally organized phase was observed for the racemic system. LEED shows that both systems display a common (3 x 2) phase at high coverage. However, the pathway and kinetic barriers to this phase differ markedly for the racemic and the enantiopure systems, with the racemic (3 x 2) appearing at a temperature that is more than 100 K below that required for the enantiopure system. In addition, we report intriguing complexities for the (3 x 2) LEED structure that is ubiquitous in amino acid/Cu(110) systems. First, a common (3 x 2) pattern with a zigzag distortion can be associated with both the racemic and enantiopure systems. For the racemic system, the coverage can be increased further to give a "true" (3 x 2) LEED pattern, which is a transformation that is impossible to enact for the enantiopure system. Most importantly, STM images of the "distorted" and "true" (3 x 2) structures created in the racemic system show subtle differences with neither arrangement being fully periodic over distances greater than a few molecules. Thus, the (3 x 2) phase appears to be more complicated than at first indicated and will require more complex models for a full interpretation.