Hasil untuk "Organic chemistry"

Mian Li, Dan Li, M. O'Keeffe et al.

Zhenqiang Wang, Seth M. Cohen

D. Farrusseng, S. Aguado, C. Pinel

G. Ulrich, R. Ziessel, A. Harriman

A. Corma, H. García

J. M. Beale, J. H. Block

B. Finlayson‐Pitts, J. Pitts

Stefan Bräse, C. Gil, K. Knepper et al.

K. Godula, D. Sames

D. Tranchemontagne, Z. Ni, M. O'Keeffe et al.

Tobias M. Dijkhuis, Thanja Lamberts, Serena Viti et al.

Astrochemical models are essential to bridge the gap between the timescales of reactions, experiments, and observations. Ice chemistry in these models experiences a large computational complexity as a result of the many parameters required for the modeling of chemistry occurring on these ices, such as binding energies and reaction energy barriers. Many of these parameters are poorly constrained, and accurately determining all would be too costly. We aim to find out which parameters describing ice chemistry have a large effect on the calculated abundances of ices for different prestellar objects. Using Monte Carlo sampled binding energies, diffusion barriers, desorption and diffusion prefactors, and reaction energy barriers, we determined the sensitivity of the abundances of the main ice species calculated with UCLCHEM, an astrochemical modeling code, on each of these parameters. We do this for a large grid of physical conditions across temperature, density, cosmic ray ionization rate and UV field strength. We find that, regardless of the physical conditions, the main sensitivities of abundances of the main ice species are the diffusion barriers of small and relatively mobile reactive species such as H, N, O, HCO, and CH$_3$. Thus, these parameters should be determined more accurately to increase the accuracy of models, paving the way to a better understanding of observations of ices. In many cases, accurate reaction energy barriers are not essential due to the treatment of competition between reactions and diffusion.

Idaira Pacheco-Fernández, V. Pino

Current trends in incorporating the principles of green chemistry in analytical methods have led to the design and usage of new solvents to replace conventional organic solvents, which characterize by their high volatility, flammability, and toxicity. Among the alternatives that have emerged, amphiphilic solvents, ionic liquids, and deep eutectic solvents are the most explored candidates in this research field. Taking advantage of the solvation properties of these new solvents, together with the synthetic versatility in the case of ionic liquids and deep eutectic solvents, a wide variety of applications of these solvents within green analytical chemistry appear in the recent literature. The aim of this article is to provide a quick summary of the state of the art on the usage of these new green solvents in analytical chemistry, particularly in liquid-phase microextraction methods (within sample preparation) and as additives or pseudostationary phases in liquid chromatography (within analytical separation methods).

Julia Senkina, Spencer Knapp

In order to improve the drug-likeness qualities, the antimalarial endochin-like quinolone (ELQ) scaffold has been modified by replacing the 4-(trifluoromethoxy)phenyl portion with an isoidide unit that is further adjustable by varying the distal O-substituents. As expected, the water solubilities of the new analogs are greatly improved, and the melting points are lower. However, the antimalarial potency of the new analogs is reduced to EC₅₀ > 1 millimolar, a result ascribable to the hydrophilic nature of the new substitution.

Ibukun O. Oresanya, Ilkay Erdogan Orhan, Julia Heil et al.

Biological activities of six under-utilized medicinal leafy vegetable plants indigenous to Africa, <i>i.e</i>., <i>Basella alba</i>, <i>Crassocephalum rubens</i>, <i>Gnetum africanum</i>, <i>Launaea taraxacifolia, Solanecio biafrae</i>, and <i>Solanum macrocarpon</i>, were investigated via two independent techniques. The total phenolic content (TPC) was determined, and six microtiter plate assays were applied after extraction and fractionation. Three were antioxidant <i>in vitro</i> assays, <i>i.e</i>., ferric reducing antioxidant power (FRAP), cupric reduction antioxidant capacity (CUPRAC), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging, and the others were enzyme (acetylcholinesterase, butyrylcholinesterase, and tyrosinase) inhibition assays. The highest TPC and antioxidant activity from all the methods were obtained from polar and medium polar fractions of <i>C. rubens, S. biafrae,</i> and <i>S. macrocarpon</i>. The highest acetyl- and butyrylcholinesterase inhibition was exhibited by polar fractions of <i>S. biafrae</i>, <i>C. rubens,</i> and <i>L. taraxacifolia</i>, the latter comparable to galantamine. The highest tyrosinase inhibition was observed in the <i>n</i>-butanol fraction of <i>C. rubens</i> and ethyl acetate fraction of <i>S. biafrae. In vitro</i> assay results of the different extracts and fractions were mostly in agreement with the bioactivity profiling via high-performance thin-layer chromatography–multi-imaging–effect-directed analysis, exploiting nine different planar assays. Several separated compounds of the plant extracts showed antioxidant, <i>α-</i>glucosidase, <i>α-</i>amylase, acetyl- and butyrylcholinesterase-inhibiting, Gram-positive/-negative antimicrobial, cytotoxic, and genotoxic activities. A prominent apolar bioactive compound zone was tentatively assigned to fatty acids, in particular linolenic acid, via electrospray ionization high-resolution mass spectrometry. The detected antioxidant, antimicrobial, antidiabetic, anticholinesterase, cytotoxic, and genotoxic potentials of these vegetable plants, in particular <i>C. rubens, S. biafrae</i>, and <i>S. macrocarpon</i>, may validate some of their ethnomedicinal uses.

Eric R. Willis, Drew A. Christianson, Robin T. Garrod

We present a chemical kinetics model of the solid-phase chemical evolution of a comet, beginning with a long period of cold-storage in the Oort Cloud, followed by five orbits that bring the comet close to the Sun. The chemical model is based on an earlier treatment that considered only the cold-storage phase, and which was based on the interstellar ice chemical kinetics model MAGICKAL. The comet is treated as 25 chemically distinct layers. Updates to the previous model includes: (i) Time- and depth-dependent temperature profiles according to heliocentric distance; (ii) a rigorous treatment of back-diffusion for species capable of diffusing through the bulk-ice layers; (iii) adoption of recent improvements in the kinetic treatment of nondiffusive chemical reaction rates. Starting from an initially simple ice composition, interstellar UV photons drive a rapid chemistry in the upper micron of material, but diminished by absorption of the UV by the dust component. Galactic cosmic rays (GCRs) drive a much slower chemistry in the deeper ices over the long cold-storage period down to 10 m. The first solar approach drives off the upper layers of ice material via thermal desorption and/or dissociation, bringing closer to the surface the deeper material that previously underwent long-term processing by GCRs. Subsequent orbits are more uniform in their chemical behavior. Loss of molecular material leads to concentration of the dust in the upper layers. Substantial quantities of complex organic molecules are formed in the upper 10 m during the cold storage phase, with some of this material released during solar approach; however, their abundances with respect to water appear too low to account for the observed gas-phase values for comet Hale-Bopp, indicating that the majority of complex molecular material observed, at least in comet Hale-Bopp, is an inheritance of primordial material.

Craig R. Walton, Jessica K. Rigley, Alexander Lipp et al.

Earth's surface is deficient in available forms of many elements considered limiting for prebiotic chemistry. In contrast, many extraterrestrial rocky objects are rich in these same elements. Limiting prebiotic ingredients may, therefore, have been delivered by exogenous material; however, the mechanisms by which exogeneous material may be reliably and non-destructively supplied to a planetary surface remains unclear. Today, the flux of extraterrestrial matter to Earth is dominated by fine-grained cosmic dust. Although this material is rarely discussed in a prebiotic context due to its delivery over a large surface area, concentrated cosmic dust deposits are known to form on Earth today due to the action of sedimentary processes. Here we combine empirical constraints on dust sedimentation with dynamical simulations of dust formation and planetary accretion to show that localized sedimentary deposits of cosmic dust could have accumulated in arid environments on early Earth, in particular glacial settings that today produce cryoconite sediments. Our results challenge the widely held assumption that cosmic dust is incapable of fertilizing prebiotic chemistry. Cosmic dust deposits may have plausibly formed on early Earth and acted to fertilize prebiotic chemistry.

Zigeng Huang, Zhen Guo, Changsu Cao et al.

Predictive simulation of surface chemistry is of paramount importance for progress in fields from catalysis to electrochemistry and clean energy generation. Ab-initio quantum many-body methods should be offering deep insights into these systems at the electronic level, but are limited in their efficacy by their steep computational cost. In this work, we build upon state-of-the-art correlated wavefunctions to reliably converge to the `gold standard' accuracy in quantum chemistry for application to extended surface chemistry. Efficiently harnessing graphics processing unit acceleration along with systematically improvable multiscale resolution techniques, we achieve linear computational scaling up to 392 atoms in size. These large-scale simulations demonstrate the importance of converging to these extended system sizes, achieving a validating handshake between simulations with different boundary conditions for the interaction of water on a graphene surface. We provide a new benchmark for this water-graphene interaction that clarifies the preference for water orientations at the graphene interface. This is extended to the adsorption of carbonaceous molecules on chemically complex surfaces, including metal oxides and metal-organic frameworks, where we consistently achieve chemical accuracy compared to experimental references, and well inside the scatter of traditional density functional material modeling approaches. This pushes the state of the art for simulation of molecular adsorption on surfaces, and marks progress into a post-density functional era for more reliable and improvable approaches to first-principles modeling of surface problems at an unprecedented scale and accuracy using ab-initio quantum many-body methods.

C. Newcomb, N. Qafoku, J. Grate et al.

Long residence times of soil organic matter have been attributed to reactive mineral surface sites that sorb organic species and cause inaccessibility due to physical isolation and chemical stabilization at the organic–mineral interface. Instrumentation for probing this interface is limited. As a result, much of the micron- and molecular-scale knowledge about organic–mineral interactions remains largely qualitative. Here we report the use of force spectroscopy to directly measure the binding between organic ligands with known chemical functionalities and soil minerals in aqueous environments. By systematically studying the role of organic functional group chemistry with model minerals, we demonstrate that chemistry of both the organic ligand and mineral contribute to values of binding free energy and that changes in pH and ionic strength produce significant differences in binding energies. These direct measurements of molecular binding provide mechanistic insights into organo–mineral interactions, which could potentially inform land-carbon models that explicitly include mineral-bound C pools. Most molecular scale knowledge on soil organo–mineral interactions remains qualitative due to instrument limitations. Here, the authors use force spectroscopy to directly measure free binding energy between organic ligands and minerals and find that both chemistry and environmental conditions affect binding.

Arthur Omran, Asbell Gonzalez, Cesar Menor-Salvan et al.

The formose reaction is a plausible prebiotic chemistry, famed for its production of sugars. In this work, we demonstrate that the Cannizzaro process is the dominant process in the formose reaction under many different conditions, thus necessitating a catalyst for the formose reaction under various environmental circumstances. The investigated formose reactions produce primarily organic acids associated with metabolism, a protometabolic system, and yield very little sugar left over. This is due to many of the acids forming from the degradation and Cannizaro reactions of many of the sugars produced during the formose reaction. We also show the heterogeneous Lewis-acid-based catalysis of the formose reaction by mineral systems associated with serpentinization. The minerals that showed catalytic activity include olivine, serpentinite, and calcium, and magnesium minerals including dolomite, calcite, and our Ca/Mg-chemical gardens. In addition, computational studies were performed for the first step of the formose reaction to investigate the reaction of formaldehyde, to either form methanol and formic acid under a Cannizzaro reaction or to react to form glycolaldehyde. Here, we postulate that serpentinization is therefore the startup process necessary to kick off a simple proto metabolic system—the formose protometabolic system.

Yuqi Chen, Richard G. Compton

Chloride and bromide are two of the most abundant anions found in seawater, and knowledge of their concentrations is essential for environmental monitoring. However, the analysis of chloride and bromide in seawater is challenging due to the complex nature of the seawater matrix. From an electrochemical perspective, we investigate the suitability of three types of electrode (Au, glassy carbon and Pt) for the analysis of Cl⁻ and/or Br⁻ in seawater. With the understanding of their electrochemical behaviours in artificial seawater (ASW), optimal voltammetric procedures for their detection are developed. The results show that the Au electrode is unsuitable for use as a Cl⁻ and/or Br⁻ sensor due to its dissolution and passivation in ASW. The use of glassy carbon resulted in poorly defined chloride and bromide signals. Finally, platinum was found to be a good candidate for chloride detection in artificial seawater using square wave voltammetry, and the results obtained in natural seawater via electrochemical measurement were in good agreement with those obtained via ion chromatography. Platinum electrodes are also recommended for bromide analysis.