Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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15 pages, 2640 KiB  
Article
Theoretical Study on the Structures, Electronic Properties, and Aromaticity of Thiophene Analogues of Anti-Kekulene
by Shingo Hashimoto and Kazukuni Tahara
Chemistry 2022, 4(4), 1546-1560; https://doi.org/10.3390/chemistry4040102 - 11 Nov 2022
Cited by 2 | Viewed by 2724
Abstract
We predict the geometries, electronic properties, and aromaticity of thiophene analogues of anti-kekulene with six to nine thiophene rings 14, together with those of cyclobutadithiophenes (CDTs) and anti-kekulene as reference compounds, using density functional theory calculations. Investigation of the simplest [...] Read more.
We predict the geometries, electronic properties, and aromaticity of thiophene analogues of anti-kekulene with six to nine thiophene rings 14, together with those of cyclobutadithiophenes (CDTs) and anti-kekulene as reference compounds, using density functional theory calculations. Investigation of the simplest reference compounds, CDTs, reveals that the local aromaticity of their thiophene rings is influenced by their fused position (b- or c-bond) to the four-membered ring (4MR). A thiophene ring fused at the b-position (b-TR) retains its aromatic character to some extent, whereas the aromatic character of one fused at the c-position is attenuated. The 4MR with two fused b-TRs retains a strong anti-aromatic character. Thiophene analogues of anti-kekulene with six to eight thiophene rings 13 favor bowl-shaped structures, in contrast to the planar structure of anti-kekulene, because of the shorter distances of the sulfur bridges. Compound 4, with nine thiophene rings, adopts a planar structure. The local aromaticity and anti-aromaticity of the thiophene ring and 4MR are significantly attenuated in 14 compared with the reference compounds, the CDTs and anti-kekulene. This can be attributed to the considerable contribution of the quinoidal electronic structure in 14. The present study provides new insight into the aromatic and electronic nature of systems containing cyclobutadienothiophene. Full article
(This article belongs to the Section Physical Chemistry and Chemical Physics)
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12 pages, 3035 KiB  
Article
Head vs. Tail Squaramide–Naphthalimide Conjugates: Self-Assembly and Anion Binding Behaviour
by Anthony A. Abogunrin, Stephen A. Healy, Orla Fenelon and Robert B. P. Elmes
Chemistry 2022, 4(4), 1288-1299; https://doi.org/10.3390/chemistry4040085 - 18 Oct 2022
Cited by 3 | Viewed by 2354
Abstract
The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to [...] Read more.
The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to the inclusion of a nitro-aromatic squaramide which efficiently quenches the emission of the naphthalimide. Both compounds were also shown to undergo self-aggregation as studied by 1H NMR and scanning electron microscopy (SEM). Furthermore, SN1 and SN2 gave rise to stark colourimetric changes in response to basic anions such as AcO, SO42− HPO42−, and F. The observed colour changes are thought to be due to deprotonation of a squaramide NH. The same basic anions also result in a further quenching of the naphthalimide emission. No colour change or emission modulations were observed in the presence of Cl; however, 1H NMR studies suggest that moderate H-bonding occurs between this anion and both SN1 and SN2. Full article
(This article belongs to the Special Issue Supramolecular Anion Recognition: Principles and Applications)
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24 pages, 5705 KiB  
Essay
Hypervalence: A Useful Concept or One That Should Be Gracefully Retired?
by Nicholas C. Norman and Paul G. Pringle
Chemistry 2022, 4(4), 1226-1249; https://doi.org/10.3390/chemistry4040082 - 8 Oct 2022
Cited by 11 | Viewed by 4939
Abstract
In this essay the origins of the term hypervalence and its application in p-block element chemistry are considered and it is argued that the term should now be consigned to the graveyard of concepts that no longer afford any discernible value or insight, [...] Read more.
In this essay the origins of the term hypervalence and its application in p-block element chemistry are considered and it is argued that the term should now be consigned to the graveyard of concepts that no longer afford any discernible value or insight, certainly from an educational perspective. In contrast, the educational merits of the octet rule are also examined where it is concluded that this rule does have significant pedagogical value, albeit mostly within the ambit of introductory level explanations. For a few of the chosen exemplar compounds, a selection of orbital-based analyses, at different levels of sophistication, are also considered, and their values appraised together with a brief survey of some of the more general computational studies which have been employed in relation to this topic. Full article
(This article belongs to the Special Issue Chemical Bonding)
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16 pages, 3229 KiB  
Article
Sorption-Assisted Ultrafiltration Hybrid Method for Treatment of the Radioactive Aqueous Solutions
by Leon Fuks, Agnieszka Miśkiewicz and Grażyna Zakrzewska-Kołtuniewicz
Chemistry 2022, 4(3), 1076-1091; https://doi.org/10.3390/chemistry4030073 - 16 Sep 2022
Cited by 4 | Viewed by 2303
Abstract
The paper presents results of studies on the possibility of using the ultrafiltration method supported by sorption on low-cost, easily accessible aluminosilicates to purify water contaminated with radionuclides. An aqueous solution contaminated with radionuclides in the form of cations at different oxidation states—Cs(I)-137, [...] Read more.
The paper presents results of studies on the possibility of using the ultrafiltration method supported by sorption on low-cost, easily accessible aluminosilicates to purify water contaminated with radionuclides. An aqueous solution contaminated with radionuclides in the form of cations at different oxidation states—Cs(I)-137, Co(II)-60 and Am(III)-241—as well as pertechnetate anions—TcO4-99m—was treated by the proposed hybrid method. In the presented work, the influence of the important process parameters (i.e., pH, sorbent dosage, temperature and feed flow rate) on the removal efficiency of radionuclides was studied. The obtained results showed that hazardous impurities, both in the form of cations and anions, may be effectively removed from water by the application of sorption-assisted UF (SAUF) using the clay-salt slimes as a sorbent. As a final stage of the work, we treated the simulated liquid radioactive waste using the SAUF method, also showing satisfactory results in its purification efficiency. Full article
(This article belongs to the Section Radiochemistry)
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18 pages, 3456 KiB  
Article
Unveiling the Chemistry of Higher-Order Cycloaddition Reactions within the Molecular Electron Density Theory
by Luis R. Domingo, Mar Ríos-Gutiérrez and Patricia Pérez
Chemistry 2022, 4(3), 735-752; https://doi.org/10.3390/chemistry4030052 - 26 Jul 2022
Cited by 6 | Viewed by 2421
Abstract
The higher-order cycloaddition (HOCA) reaction of tropone with cyclopentadiene (Cp) has been studied within the Molecular Electron Density Theory. The Electron Localization Function (ELF) analysis of the electronic structure of tropone and Cp characterizes the structural behaviors of the two conjugated unsaturated systems, [...] Read more.
The higher-order cycloaddition (HOCA) reaction of tropone with cyclopentadiene (Cp) has been studied within the Molecular Electron Density Theory. The Electron Localization Function (ELF) analysis of the electronic structure of tropone and Cp characterizes the structural behaviors of the two conjugated unsaturated systems, while the conceptual DFT reactivity indices classify tropone as a strong electrophile and Cp as a strong nucleophile participating in polar cycloaddition reactions of reverse electron density flux. Eight competitive reaction paths have been characterized for this cycloaddition reaction. The most favorable one allowing the formation of the formal out [6 + 4] cycloadduct has an activation enthalpy of 16.2 kcal·mol−1, and the reaction is exothermic by −21.4 kcal·mol−1. This HOCA reaction, which takes place through a non-concerted two-stage one-step mechanism, presents high stereo-, pseudocyclic- and regioselectivities, explaining the exclusive formation of the experimental [6 + 4] cycloadduct. While the most favorable nucleophilic attack of Cp on most electrophilic C2 positions of tropone accounts for regioselectivities, the favorable electrostatic interactions present between the Cp framework and the negatively charged O8 oxygen of tropone account for the stereo- and pseudocyclic selectivities. Despite the symmetry of the two reagents, this HOCA reaction takes place via a highly asynchronous transition state structure as a consequence of the most favorable two-center interactions taking place between the electrophilic C2 center of tropone and the nucleophilic C9 center of Cp. Full article
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26 pages, 7274 KiB  
Review
Metal–Organic Cages: Applications in Organic Reactions
by Shangjun Chen and Li-Jun Chen
Chemistry 2022, 4(2), 494-519; https://doi.org/10.3390/chemistry4020036 - 18 May 2022
Cited by 12 | Viewed by 5346
Abstract
Supramolecular metal–organic cages, a class of molecular containers formed via coordination-driven self-assembly, have attracted sustained attention for their applications in catalysis, due to their structural aesthetics and unique properties. Their inherent confined cavity is considered to be analogous to the binding pocket of [...] Read more.
Supramolecular metal–organic cages, a class of molecular containers formed via coordination-driven self-assembly, have attracted sustained attention for their applications in catalysis, due to their structural aesthetics and unique properties. Their inherent confined cavity is considered to be analogous to the binding pocket of enzymes, and the facile tunability of building blocks offers a diverse platform for enzyme mimics to promote organic reactions. This minireview covers the recent progress of supramolecular metal–organic coordination cages for boosting organic reactions as reaction vessels or catalysts. The developments in the utilizations of the metal–organic cages for accelerating the organic reactions, improving the selectivity of the reactions are summarized. In addition, recent developments and successes in tandem or cascade reactions promoted by supramolecular metal–organic cages are discussed. Full article
(This article belongs to the Special Issue Metal-Organic Cages and Their Applications)
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15 pages, 2178 KiB  
Article
Valence-, Dipole- and Quadropole-Bound Electronically Excited States of Closed-Shell Anions Formed by Deprotonation of Cyano- and Ethynyl-Disubstituted Polycyclic Aromatic Hydrocarbons
by Marie E. Strauss, Taylor J. Santaloci and Ryan C. Fortenberry
Chemistry 2022, 4(1), 42-56; https://doi.org/10.3390/chemistry4010004 - 24 Jan 2022
Cited by 5 | Viewed by 3249
Abstract
Dicyano-functionalized benzene and naphthalene anion derivatives exhibit a relatively rich population of electronically excited states in stark contrast to many assumptions regarding the photophysics of anions in general. The present work has quantum chemically analyzed the potential electronically excited states of closed-shell anions [...] Read more.
Dicyano-functionalized benzene and naphthalene anion derivatives exhibit a relatively rich population of electronically excited states in stark contrast to many assumptions regarding the photophysics of anions in general. The present work has quantum chemically analyzed the potential electronically excited states of closed-shell anions created by replacing hydrogen atoms with valence-bound lone pairs in benzene and naphthalene difunctionalized with combinations of -CN and -C2H. Dicyanobenzene anion derivatives can exhibit dipole-bound excited states as long as the cyano groups are not in para position to one another. This also extends to cyanoethynylbenzene anions as well as deprotonated dicyano- and cyanoethynylnaphthalene anion derivatives. Diethynyl functionalization is less consistent. While large dipole moments are created in some cases for deprotonation on the -C2H group itself, the presence of electronically excited states beyond those that are dipole-bound is less consistent. Beyond these general trends, 2-dicyanonaphthalene-34 gives strong indication for exhibiting a quadrupole-bound excited state, and the 1-cyanoethynylnaphthalene-29 and -36 anion derivatives are shown to possess as many as two valence-bound excited states and one dipole-bound excited state. These photophysical properties may have an influence on regions where polycyclic aromatic hydrocarbons are known to exist such as in various astrochemical environments or even in combustion flames. Full article
(This article belongs to the Special Issue 2021 Profile Papers by Chemistry' Editorial Board Members)
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11 pages, 3895 KiB  
Article
Photoelectron Circular Dichroism as a Probe of Chiral Hydrocarbons
by Piero Decleva
Chemistry 2022, 4(1), 31-41; https://doi.org/10.3390/chemistry4010003 - 13 Jan 2022
Cited by 3 | Viewed by 2703
Abstract
The sensitivity of Photoelectron Circular Dichroism (PECD) in the angular distribution of photoelectrons, a recent chiral technique, to detect chirality in pure hydrocarbons is investigated in a number of benchmark molecules. It is found that a very large chiral signal is expected, surpassing [...] Read more.
The sensitivity of Photoelectron Circular Dichroism (PECD) in the angular distribution of photoelectrons, a recent chiral technique, to detect chirality in pure hydrocarbons is investigated in a number of benchmark molecules. It is found that a very large chiral signal is expected, surpassing most current examples, giving a sure fingerprint of absolute configuration. On the other hand, the sensitivity to specific isomers or closely related molecules is relatively modest. Full article
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