Polymer Degradation through Chemical Change: A Quantum-based Test of Inferred Reactions in Irradiated Polydimethylsiloxane

Chemical reaction schemes are key conceptual tools for interpreting the results of experiments and simulations, but often carry implicit assumptions that remain largely unverified for complicated systems. Established schemes for chemical damage through crosslinking in irradiated silicone polymers comprised of polydimethylsiloxane (PDMS) date to the 1950's and correlate small-molecule off-gassing with specific crosslink features. In this regard, we use a somewhat reductionist model to develop a general conditional probability and correlation analysis approach that tests these types of causal connections between proposed experimental observables to reexamine this chemistry through quantum-based molecular dynamics (QMD) simulations. Analysis of the QMD simulations suggests that the established reaction schemes are qualitatively reasonable, but lack strong causal connections under a broad set of conditions that would enable making direct quantitative connections between off-gassing and crosslinking. Further assessment of the QMD data uncovers a strong (but nonideal) quantitative connection between exceptionally hard-to-measure chain scission events and the formation of silanol (Si-OH) groups. Our analysis indicates that conventional notions of radiation damage to PDMS should be further qualified and not necessarily used ad hoc. In addition, our efforts enable independent quantum-based tests that can inform confidence in assumed connections between experimental observables without the burden of fully elucidating entire reaction networks.

Identification and Modelling of Chlorine Decay Mechanisms in Reclaimed Water Containing Ammonia

Keeping an effective disinfectant residual concentration in reclaimed water is still a challenge, due to its high levels of ammonia and organic matter when compared with those in drinking water. This research proposes the integration of the reaction schemes of monochloramine auto-decomposition with an empirical kinetic mechanism accounting for reactive chlorine species decay in the presence of organic matter, for which three mechanisms were hypothesized and tested. A parallel second order mechanism, where monochloramine reacts both with fast and slow organic matter reactive fractions, was identified as the most suitable. The model, comprising two rate constants and two fictive concentrations of organic matter as parameters, was further successfully calibrated with real reclaimed waters with two initial free chlorine doses of 8.01 × 10−5 M (≈5 mg/L) and 2.67 × 10−4 M (≈20 mg/L). The proposed model is believed to support future studies aiming to predict and manage chlorine decay in reclaimed water distribution systems.

An Overview of Synthesis and Biological Activity of Dihydropyrimidine Derivatives

: Dihydropyrimidine derivatives are most important scaffolds due to structure similarities with natural products it is a hetrocyclic compound. The chemistry of Dihydropyrimidine is a blossoming field. Various reaction schemes for the preparation of Dihydropyrimidines and produce different biological effect and offer vast scope in the field of medicinal chemistry. This article goal to analysis the work reported the recent chemistry and pharmacological activities of dihydropyrimidine derivatives.

A two-step strategy for the selective conversion of ethanol to propene and hydrogen

Simultaneous production of propene and hydrogen from ethanol is the promising way of a renewable feedstock conversion into valuable products. Propene is used for the production of polypropene plastics, acrylonitrile, propene oxide, and many other manufactures; hydrogen is considered the most viable energy carrier for the future. Conventional reaction schemes of the direct one-step catalytic conversion of ethanol into propene include the reaction of the isopropanol dehydration to propene. However, the dehydration ability of a catalyst inevitably gives rise to the ethanol dehydration to ethene that diminishes the propene yield. To avoid ethanol dehydration to ethene, the two-step process is composed of the ethanol conversion to acetone in the first step and the acetone conversion to propene in the second step. The thermodynamic analysis of the known reaction pathways for the ethanol conversion to propene shows that a 74.6% yield of propene can be achieved even at a low temperature of 200oC. According to the literature data, possible catalysts can be Cu/La2Zr2O7 or Fe3O4 for the first step, and the mixed Ag–In/SiO2 and K3PW12O40 catalyst for the second step. We speculate that the propene yield may reach 72% using these catalysts in the two-step process.

Understanding the nature of NH3-coordinated active sites and the complete reaction schemes for NH3-SCR using Cu-SAPO-34 catalysts

Cu-SAPO-34 zeolite catalysts show excellent NH3-SCR performance at low temperature, which is due to the catalytic capacity of copper species.

Prebiotic Chemistry in the Wild: How Geology Interferes with the Origins of Life

A plausible explanation for the origin of life must satisfy constraints imposed by both organic chemistry and early Earth geochemistry. However, the full scope of geochemical parameter space is rarely considered by either theoretical or experimental models of abiogenesis. Here we propose a novel approach, which can make maximum use of available data. We posit that constructive and destructive geochemical interferences with proposed prebiotic reaction schemes can be used to restrict plausible environmental parameter space for the origin of life. Our approach is demonstrated by exploring parameter space for dehydration reactions. Such reactions are universally important in extant biochemistry and all proposed prebiotic reaction schemes, yet challenging to perform under plausible conditions. We specifically explore a minimal pathway for RNA synthesis (formaldehyde; ribose; ribose phosphate; adenosine monophosphate; RNA). Based on assembled thermodynamic and geochemical constraints, we identify that low water activity is a key constructive interference in prebiotic chemistry. Critically, the manner in which low water activity is achieved can strongly discriminate between different environmental scenarios. Exploring interference chemistry is hence an effective means of discriminating between competing origin of life scenarios.