Hasil untuk "Organic chemistry"

Wen Zhang, R. Xiong

B. Ervens, B. Turpin, R. Weber

Abstract. Progress has been made over the past decade in predicting secondary organic aerosol (SOA) mass in the atmosphere using vapor pressure-driven partitioning, which implies that SOA compounds are formed in the gas phase and then partition to an organic phase (gasSOA). However, discrepancies in predicting organic aerosol oxidation state, size and product (molecular mass) distribution, relative humidity (RH) dependence, color, and vertical profile suggest that additional SOA sources and aging processes may be important. The formation of SOA in cloud and aerosol water (aqSOA) is not considered in these models even though water is an abundant medium for atmospheric chemistry and such chemistry can form dicarboxylic acids and "humic-like substances" (oligomers, high-molecular-weight compounds), i.e. compounds that do not have any gas phase sources but comprise a significant fraction of the total SOA mass. There is direct evidence from field observations and laboratory studies that organic aerosol is formed in cloud and aerosol water, contributing substantial mass to the droplet mode. This review summarizes the current knowledge on aqueous phase organic reactions and combines evidence that points to a significant role of aqSOA formation in the atmosphere. Model studies are discussed that explore the importance of aqSOA formation and suggestions for model improvements are made based on the comprehensive set of laboratory data presented here. A first comparison is made between aqSOA and gasSOA yields and mass predictions for selected conditions. These simulations suggest that aqSOA might contribute almost as much mass as gasSOA to the SOA budget, with highest contributions from biogenic emissions of volatile organic compounds (VOC) in the presence of anthropogenic pollutants (i.e. NOx) at high relative humidity and cloudiness. Gaps in the current understanding of aqSOA processes are discussed and further studies (laboratory, field, model) are outlined to complement current data sets.

Feng Wang, Xiaogang Liu

R. Cioc, Eelco Ruijter, R. Orru

R. Ramakrishnan, M. Rupp, Pavlo O. Dral et al.

Chemically accurate and comprehensive studies of the virtual space of all possible molecules are severely limited by the computational cost of quantum chemistry. We introduce a composite strategy that adds machine learning corrections to computationally inexpensive approximate legacy quantum methods. After training, highly accurate predictions of enthalpies, free energies, entropies, and electron correlation energies are possible, for significantly larger molecular sets than used for training. For thermochemical properties of up to 16k isomers of C7H10O2 we present numerical evidence that chemical accuracy can be reached. We also predict electron correlation energy in post Hartree-Fock methods, at the computational cost of Hartree-Fock, and we establish a qualitative relationship between molecular entropy and electron correlation. The transferability of our approach is demonstrated, using semiempirical quantum chemistry and machine learning models trained on 1 and 10% of 134k organic molecules, to reproduce enthalpies of all remaining molecules at density functional theory level of accuracy.

A. Narita, Xiao-Ye Wang, Xinliang Feng et al.

A. Goldstein, I. Galbally

D. Tomalia, A. M. Naylor, W. Goddard

J. Kendall

A. de la Hoz, Á. Díaz‐Ortiz, Andrés Moreno

F. Fehsenfeld, J. Calvert, R. Fall et al.

G. Cravotto, P. Cintas

M. Movsisyan, E. Delbeke, Jan Berton et al.

Xiao‐Ming Chen, M. Tong

Y. Lim, Y. Tan, M. Perri et al.

Abstract. There is a growing understanding that secondary organic aerosol (SOA) can form through reactions in atmospheric waters (i.e., clouds, fogs, and aerosol water). In clouds and wet aerosols, water-soluble organic products of gas-phase photochemistry dissolve into the aqueous phase where they can react further (e.g., with OH radicals) to form low volatility products that are largely retained in the particle phase. Organic acids, oligomers and other products form via radical and non-radical reactions, including hemiacetal formation during droplet evaporation, acid/base catalysis, and reaction of organics with other constituents (e.g., NH4+). This paper provides an overview of SOA formation through aqueous chemistry, including atmospheric evidence for this process and a review of radical and non-radical chemistry, using glyoxal as a model precursor. Previously unreported analyses and new kinetic modeling are reported herein to support the discussion of radical chemistry. Results suggest that reactions with OH radicals tend to be faster and form more SOA than non-radical reactions. In clouds these reactions yield organic acids, whereas in wet aerosols they yield large multifunctional humic-like substances formed via radical-radical reactions and their O/C ratios are near 1.

Catherine Kiefer, Steffen Schwarz, Nima Naderi et al.

The main characteristics of the large number of coffee species are differences in aroma and caffeine content. Labeled blends of <i>Coffea arabica</i> (<i>C. arabica</i>) and <i>Coffea canephora</i> (<i>C. canephora</i>) are common to broaden the flavor profile or enhance the stimulating effect of the beverage. New emerging species such as <i>Coffea liberica</i> (<i>C. liberica</i>) further increase the variability in blends. However, significant price differences between coffee species increase the risk of unlabeled blends and thus influence food quality and safety for consumers. In this study, a prototypic hyphenation of trapped headspace-gas chromatography-ion mobility spectrometry-quadrupole mass spectrometry (THS-GC-IMS-QMS) was used for the detection of characteristic compounds of <i>C. arabica</i>, <i>C. canephora</i>, and <i>C. liberica</i> in green and roasted coffee samples. For the discrimination of coffee species with IMS data, multivariate resolution with multivariate curve resolution–alternating least squares (MCR-ALS) prior to partial least squares–discriminant analysis (PLS-DA) was evaluated. With this approach, the classification accuracy, as well as sensitivity and specificity, of the PLS-DA model was significantly improved from an overall accuracy of 87% without prior feature selection to 92%. As MCR-ALS preserves the physical and chemical properties of the original data, characteristic features were determined for subsequent substance identification. The simultaneously generated QMS data allowed for partial annotation of the characteristic volatile organic compounds (VOC) of roasted coffee.

Silvana M. Azcarate, Maria P. Segura-Borrego, Rocío Ríos-Reina et al.

The authentication of organic extra virgin olive oils (OEVOOs) is crucial for quality control and fraud prevention. This study applies proton-nuclear magnetic resonance (¹H-NMR) spectroscopy combined with chemometric analysis as a non-destructive, untargeted approach to differentiate EVOOs based on cultivation method (organic vs. conventional) and variety (<i>Hojiblanca</i> vs. <i>Picual</i>). Principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) demonstrated well-defined sample differentiation, while the variable importance in projection (VIP) selection and Tukey’s test identified key spectral regions responsible for classification. The results showed that sterols and lipid-related compounds played a major role in distinguishing organic from conventional oils, whereas fatty acids and phenolic compounds were more relevant for cultivar differentiation. These findings align with known metabolic differences, where <i>Picual</i> oils generally exhibit higher polyphenol content, and a distinct fatty acid composition compared to <i>Hojiblanca</i>. The agreement between chemometric classification models and statistical tests supports the potential of ¹H-NMR for OEVOO authentication. This method provides a comprehensive and reproducible metabolic fingerprint, enabling differentiation based on both agronomic practices and genetic factors. These findings suggest that ¹H-NMR spectroscopy, coupled with multivariate analysis, could be a valuable tool for quality control and fraud detection in the olive oil industry.

Bianca Flexa-Ribeiro, Manoel D. N. Garcia, Ana Carolina de J. Silva et al.

Nano-emulsions of essential oils (EO) and their chemical constituents are promising raw materials for the ecological control of <i>Tribolium castaneum</i>. <i>Curcuma longa</i> L. is a plant known for the properties of its rhizome, which is used in food, health, and hygiene products. Although its leaves are considered by-products with no commercial value, they produce an essential oil rich in bioactive monoterpenoids. This study aims to evaluate the repellency of nano-emulsions containing the EO from leaves of <i>C. longa</i> or its three main chemical constituents against <i>T. castaneum</i>. The representative mixture of EO extracted in four different months showed <i>p</i>-cymene (26.0%), 1,8-cineole (15.1%), and terpinolene (15.5%) as major compounds. Nano-emulsions of EO (HLB 16.7), terpinolene (HLB 15.0), 1,8-cineole (HLB15.0), and <i>p</i>-cymene (HLB 15.0) were repellent at concentrations of 11 μg/cm² (EO, terpinolene, and <i>p</i>-cymene) and 1.1 μg/cm² (1,8-cineole). The EO nano-emulsion droplet size increased linearly over time, remaining below 300 nm for 35 days. The EO nano-emulsion proved to be a green alternative to synthetic pesticides, as it was safe against the bioindicator <i>Chlorella vulgaris</i>. Furthermore, its main constituents were able to inhibit in silico the enzyme telomerase of <i>T. castaneum</i>, which is an enzyme essential for life. This study provides ideas for the utilization of EO from leaves of <i>C. longa</i> as raw material for new environmentally friendly plant-derived nanobiopesticides.

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).

T. Bennett, Y. Yue, Peng Li et al.

Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of 3D MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic nonmetallic, organic, and metallic), and retain the basic metal-ligand connectivity of crystalline MOFs, which connects their mechanical properties to their starting chemical composition. The transfer of functionality from crystal to glass points toward new routes to tunable, functional hybrid glasses.