Molecules

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12 pages, 2021 KiB

Open AccessArticle

Insights on Stability Constants and Structures of Complexes between Coumarin Derivatives and Pb(II) in Aqueous Media

by Emilia Furia, Vincenzo Lettera, Anna Napoli and Donatella Aiello

Molecules 2024, 29(9), 1911; https://doi.org/10.3390/molecules29091911 - 23 Apr 2024

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In the frame of a systematic study on the sequestering ability of natural antioxidants towards metal cations, here the complexation of coumarin-3-carboxilic acid (HCCA) with Pb(II) and the overall stability constants of the resulting complexes, at 37 °C and in 0.16 M NaClO [...] Read more.

In the frame of a systematic study on the sequestering ability of natural antioxidants towards metal cations, here the complexation of coumarin-3-carboxilic acid (HCCA) with Pb(II) and the overall stability constants of the resulting complexes, at 37 °C and in 0.16 M NaClO₄, are discussed. Reaction of Pb(ClO₄)₂ with HCCA in an aqueous medium at a pH range from 2 to 6 and various ratios (1:1–1:10) yielded the Pb–CCA complexes, which were characterized spectrometrically by laser desorption ionization mass spectrometry (LD-MS). LD-MS has provided the composition and structure of Pb–CCA species according to the speciation model proposed on the basis of the potentiometric data. The graphic representation of the complex’s concentration curves is given by the distribution diagram, which provides a whole depiction of the species present in the solution at the selected pH ranges. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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21 pages, 3544 KiB

Open AccessArticle

A Study of the Micellar Formation of N-Alkyl Betaine Ethyl Ester Chlorides Based on the Physicochemical Properties of Their Aqueous Solutions

by Monika Geppert-Rybczyńska, Anna Mrozek-Wilczkiewicz, Patrycja Rawicka and Piotr Bartczak

Molecules 2024, 29(8), 1844; https://doi.org/10.3390/molecules29081844 - 18 Apr 2024

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Abstract

In this study, a series of four surface-active compounds—N-alkyl betaine ethyl ester chlorides, C_nBetC₂Cl—were synthesized and characterized in aqueous solutions. As with other alkyl betaines, these amphiphiles can be practically used, for example, as co-surfactants and/or solubility [...] Read more.

In this study, a series of four surface-active compounds—N-alkyl betaine ethyl ester chlorides, C_nBetC₂Cl—were synthesized and characterized in aqueous solutions. As with other alkyl betaines, these amphiphiles can be practically used, for example, as co-surfactants and/or solubility enhancers acting according to hydrotropic or micellar mechanisms, depending on the alkyl chain length in the amine. We focused on the representatives of the medium alkyl chain length (C₆–C₁₂) to find the dependence between the alkyl chain length in N-alkyl betaine ethyl ester chlorides and the surface, volumetric, acoustic, and viscometric properties of their solutions. Ethyl esters, the derivatives of amino acids, were chosen to increase functionality and take advantage of possible hydrolysis in solutions at higher pH, which is also a key parameter in biodegradability. The micellization parameters were calculated based on the physicochemical characteristics. We focused our interest on the ester with a dodecyl substituent since we can compare and discuss its properties with some other C₁₂ representatives that are available in literature. Surprisingly, its micellization characteristic is almost temperature-independent in the investigated temperature range, t = (15–45) °C. Particularly interesting are the results of dynamic light scattering (DLS), which show that the changes in physicochemical parameters of the C₁₂ homolog around the CMC are caused by the two types of micelles of different sizes present in solutions. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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15 pages, 2548 KiB

Open AccessArticle

Adsorption of O₂ on the Preferred -O-Au Sites of Small Gold Oxide Clusters: Charge-dependent Interaction and Activation

by Lulu Huang, Wen Liu and Xiaopeng Xing

Molecules 2024, 29(7), 1645; https://doi.org/10.3390/molecules29071645 - 06 Apr 2024

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Abstract

Decades of research have illuminated the significant roles of gold/gold oxide clusters in small molecule catalytic oxidation. However, many fundamental questions, such as the actual sites to adsorb and activate O₂ and the impact of charge, remain unanswered. Here, we have utilized [...] Read more.

Decades of research have illuminated the significant roles of gold/gold oxide clusters in small molecule catalytic oxidation. However, many fundamental questions, such as the actual sites to adsorb and activate O₂ and the impact of charge, remain unanswered. Here, we have utilized an improved genetic algorithm program coupled with the DFT method to systematically search for the structures of Au_1–5O_x^−/+/0 (x = 1–4) and calculated binding interactions between Au_1–5O_x^−/+/0 (x = 1–2) and O₂, aiming to determine the active sites and to elucidate the impact of different charge states in gold oxide systems. The results revealed that the reactivity of all three kinds of small gold oxide clusters toward O₂ is strongly site-dependent, with clusters featuring an -O-Au site exhibiting a preference for adsorption. The charges on small gold oxide clusters significantly impact the interaction strength and the activation degree of adsorbed O₂: in the case of anionic cluster, the interaction between O₂ and the -O-Au sites leads to a chemical reaction involving electron transfer, thereby significantly activating O₂; in neutral and cationic clusters, the adsorption of O₂ on their -O-Au sites can be viewed as an electrostatic interaction. Pointedly, for cationic clusters, the highly concentrated positive charge on the Au atom of the -O-Au sites can strongly adsorb but hardly activate the adsorbed O₂. These results have certain reference points for understanding the gold oxide interfaces and the improved catalytic oxidation performance of gold-based systems in the presence of atomic oxygen species. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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23 pages, 7478 KiB

Open AccessArticle

Predicting Organometallic Intermediates in the Surface-Assisted Ullmann Coupling of Chrysene Isomers

by Jakub Lisiecki and Paweł Szabelski

Molecules 2024, 29(7), 1553; https://doi.org/10.3390/molecules29071553 - 30 Mar 2024

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Abstract

On-surface polymerization of functional organic molecules has been recently recognized as a promising route to persistent low-dimensional structures with tailorable properties. In this contribution, using the coarse-grained Monte Carlo simulation method, we study the initial stage of the Ullmann coupling of doubly halogenated [...] Read more.

On-surface polymerization of functional organic molecules has been recently recognized as a promising route to persistent low-dimensional structures with tailorable properties. In this contribution, using the coarse-grained Monte Carlo simulation method, we study the initial stage of the Ullmann coupling of doubly halogenated chrysene isomers adsorbed on a catalytically active (111) crystalline surface. To that end, we focus on the formation of labile metal-organic precursor structures preceding the covalent bonding of chrysene monomers. Four monomeric chrysene units with differently distributed halogen substituents were probed in the simulations, and the resulting precursor structures were compared and quantified. Moreover, the effect of (pro)chirality of chrysene tectons on the structure formation was elucidated by running separate simulations in enantiopure and racemic systems. The calculations showed that suitable manipulation of the halogen substitution pattern allows for the creation of diverse precursor architectures, ranging from straight and winded chains to cyclic oligomers with enantiopure, racemic, and nonracemic composition. The obtained findings can be helpful in developing synthetic strategies for covalent polymers with predefined architecture and functionality. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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16 pages, 6822 KiB

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Exploring the Influence of Morphology on Bipolaron–Polaron Ratios and Conductivity in Polypyrrole in the Presence of Surfactants

by Thaneeya Samwang, Nozomi Morishita Watanabe, Yukihiro Okamoto and Hiroshi Umakoshi

Molecules 2024, 29(6), 1197; https://doi.org/10.3390/molecules29061197 - 07 Mar 2024

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Abstract

This research aims to deepen the understanding of the relationship between conductivity and morphology in polypyrrole (PPy) via a comparison of the bipolaron to polaron ratios with a focus on the C-H deformation area. PPy samples were synthesized with different surfactants: sodium dodecyl [...] Read more.

This research aims to deepen the understanding of the relationship between conductivity and morphology in polypyrrole (PPy) via a comparison of the bipolaron to polaron ratios with a focus on the C-H deformation area. PPy samples were synthesized with different surfactants: sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and tween 80 (TW). This study revealed that SDS significantly altered the bipolaron and polaron in the C-H deformation region and showed higher conductivity than other surfactants. Notably, the morphological shifts to a sheet-like structure when using ammonium sulfate (APS) contrasted with the particle-like form observed with ferric chloride (FeCl₃). These results showed that if the oxidant changed, the bipolaron and polaron ratios in C-H deformation were unrelated to PPy morphology. However, this work showed a consistent relationship between SDS use, the bipolaron and polaron ratios in the C-H deformation, and the conductivity properties. Moreover, the natural positive charge of PPy and negatively charged SDS molecules may lead to an electrostatic interaction between PPy and SDS. This work assumes that this interaction might cause the transformation of polaron to bipolaron in the C–H deformation region, resulting in improved conductivity of PPy. This work offers more support for the future investigation of PPy characteristics. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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15 pages, 6279 KiB

Open AccessArticle

First-Principles Study of Bimetallic Pairs Embedded on Graphene Co-Doped with N and O for N₂ Electroreduction

by Haozhe Dong, Hao Sun, Guanru Xing, Shize Liu, Xuemei Duan and Jingyao Liu

Molecules 2024, 29(4), 779; https://doi.org/10.3390/molecules29040779 - 08 Feb 2024

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Abstract

The electrocatalytic nitrogen reduction reaction (NRR) is considered a viable alternative to the Haber–Bosch process for ammonia synthesis, and the design of highly active and selective catalysts is crucial for the industrialization of the NRR. Dual-atom catalysts (DACs) with dual active sites offer [...] Read more.

The electrocatalytic nitrogen reduction reaction (NRR) is considered a viable alternative to the Haber–Bosch process for ammonia synthesis, and the design of highly active and selective catalysts is crucial for the industrialization of the NRR. Dual-atom catalysts (DACs) with dual active sites offer flexible active sites and synergistic effects between atoms, providing more possibilities for the tuning of catalytic performance. In this study, we designed 48 graphene-based DACs with N₄O₂ coordination (MM′@N₄O₂-G) using density functional theory. Through a series of screening strategies, we explored the reaction mechanisms of the NRR for eight catalysts in depth and revealed the “acceptance–donation” mechanism between the active sites and the N₂ molecules through electronic structure analysis. The study found that the limiting potential of the catalysts exhibited a volcano-shaped relationship with the d-band center of the active sites, indicating that the synergistic effect between the bimetallic components can regulate the d-band center position of the active metal M, thereby controlling the reaction activity. Furthermore, we investigated the selectivity of the eight DACs and identified five potential NRR catalysts. Among them, MoCo@N₄O₂-G showed the best NRR performance, with a limiting potential of −0.20 V. This study provides theoretical insights for the design and development of efficient NRR electrocatalysts. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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14 pages, 5613 KiB

Open AccessArticle

Beryllium Dimer Reactions with Acetonitrile: Formation of Strong Be−Be Bonds

by Fei Cong, Liyan Cai, Juanjuan Cheng, Zhen Pu and Xuefeng Wang

Molecules 2024, 29(1), 177; https://doi.org/10.3390/molecules29010177 - 28 Dec 2023

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Abstract

Laser ablated Be atoms have been reacted with acetonitrile molecules in 4 K solid neon matrix. The diberyllium products BeBeNCCH₃ and CNBeBeCH₃ have been identified by D and ¹³C isotopic substitutions and quantum chemical calculations. The stabilization of the diberyllium [...] Read more.

Laser ablated Be atoms have been reacted with acetonitrile molecules in 4 K solid neon matrix. The diberyllium products BeBeNCCH₃ and CNBeBeCH₃ have been identified by D and ¹³C isotopic substitutions and quantum chemical calculations. The stabilization of the diberyllium species is rationalized from the formation of the real Be−Be single bonds with bond distances as 2.077 and 2.058 Å and binding energies as −27.1 and −77.2 kcal/mol calculated at CCSD (T)/aug-cc-pVTZ level of theory for BeBeNCCH₃ and CNBeBeCH₃, respectively. EDA-NOCV analysis described the interaction between Be₂ and NC···CH₃ fragments as Lewis “acid−base” interactions. In the complexes, the Be₂ moiety carries positive charges which transfer from antibonding orbital of Be₂ to the bonding fragments significantly strengthen the Be−Be bonds that are corroborated by AIM, LOL and NBO analyses. In addition, mono beryllium products BeNCCH₃, CNBeCH₃, HBeCH₂CN and HBeNCCH₂ have also been observed in our experiments. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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24 pages, 8708 KiB

Open AccessArticle

Compact Combustion Mechanisms of Typical n-Alkanes Developed by the Minimized Reaction Network Method

by Jiangtao Shentu, Yanrong Lu, Yiwei Li, Juanqin Li, Yebing Mao and Xiangyuan Li

Molecules 2023, 28(23), 7695; https://doi.org/10.3390/molecules28237695 - 21 Nov 2023

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Abstract

The existing combustion kinetic modeling method which aims at developing phenomenological combustion mechanisms characterized by multiple reactions confronts several challenges, including the conflicts between computing resources and mechanism scales during numerical simulation, etc. In order to address these issues, the minimized reaction network [...] Read more.

The existing combustion kinetic modeling method which aims at developing phenomenological combustion mechanisms characterized by multiple reactions confronts several challenges, including the conflicts between computing resources and mechanism scales during numerical simulation, etc. In order to address these issues, the minimized reaction network method for complex combustion system modeling based on the principle of simultaneous chemical equilibrium is proposed, which is aimed to develop combustion mechanisms with minimal reaction steps under a limited number of species. The concept of mechanism resolution is proposed in this method, and the reaction network with minimal reaction steps under a given mechanism resolution is constructed so that the scale of mechanisms is compressed greatly. Meanwhile, distinguishing from other mechanisms, the reversible form of elementary reactions is adopted and the classical two-parameter (A, E_a) Arrhenius equation fits the rate constants. Typical n-alkanes including n-butane, n-heptane, n-octane, n-decane, n-dodecane and n-hexadecane were taken as examples to indicate the development process of mechanisms and systematic kinetic validations were carried out. Results show that this method leads to very compact mechanisms with satisfactory accuracy, and it eliminates the process of mechanism reduction and is beneficial for mechanism optimization. This method and the derived kinetic mechanisms are hoped to contribute to combustion engineering applications. Full article

(This article belongs to the Special Issue Exclusive Feature Papers in Physical Chemistry, 2nd Edition)

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