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Droughts are becoming more intense and frequent in the Brazilian semiarid because of El Niño and global climate changes. The Jaguaribe River estuary is a semiarid ecosystem that experiences a reduction in freshwater discharges due to droughts and river damming. The decrease in freshwater fluxes has increased metal availability through the water residence time increase in the Jaguaribe River estuary. Then, this study aimed to evaluate the dissolved organic matter quality and its interaction with metals in the Jaguaribe River estuary after a severe drought period. It was performed through carbon analyses, fluorescence spectroscopy, ultrafiltration technique, and determinations of metals by ICP-MS. Optical analysis showed that the dissolved organic carbon (DOC) was preponderantly composed of terrestrial-derived humic compounds, while the low ratio between the particulate organic carbon (POC) and chlorophyll-a indicated that POC was predominantly phytoplankton-derived. DOC and POC presented non-conservative removal during the estuarine mixing. DOM and dissolved elements were mostly distributed within the LMW fraction and presented a low percentage in the colloidal fraction. Li, Rb, Sr, Mo, and U showed conservative behavior, while Cu, Fe, Cr, and V had non-conservative behavior with a significant positive correlation with DOM, suggesting DOM as a relevant driver of metal availability at the Jaguaribe River estuary even during the rainy season.

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This study comprehensively analyzes the magnetically induced current density of polycyclic compounds labeled as "aromatic chameleons" since they can arrange their π-electrons to exhibit aromaticity in both the ground and the lowest triplet state. These compounds comprise benzenoid moieties fused to a central skeleton with 4n π-electrons and traditional magnetic descriptors are biased due to the superposition of local magnetic responses. In the S0 state, our analysis reveals that the molecular constituent fragments preserve their (anti)aromatic features in agreement with two types of resonant structures: one associated with aromatic benzenoids and the other with a central antiaromatic ring. Regarding the T1 state, a global and diatropic ring current is revealed. Our aromaticity study is complemented with advanced electronic and geometric descriptors to consider different aspects of aromaticity, particularly important in the evaluation of excited state aromaticity. Remarkably, these descriptors consistently align with the general features on the main delocalization pathways in polycyclic hydrocarbons consisting of fused 4n π-electron rings. Moreover, our study demonstrates an inverse correlation between the singlet-triplet energy difference and the antiaromatic character of the central ring in S0.

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Nowadays, an active research topic is the connection between Clar's rule, aromaticity, and magnetic properties of polycyclic benzenoid hydrocarbons. In the present work, we employ a meticulous magnetically induced current density analysis to define the net current flowing through any cyclic circuit, connecting it to aromaticity based on the ring current concept. Our investigation reveals that the analyzed polycyclic systems display a prominent global ring current, contrasting with subdued semi-local and local ring currents. These patterns align with Clar's aromatic π-sextets only in cases where migrating π-sextet structures are invoked. The results of this study will enrich our comprehension of aromaticity and magnetic behavior in such systems, offering significant insights into coexisting ring current circuits in these systems.

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When (4n +2) π-electrons are located in single planar ring, it conventionally qualifies as aromatic. According Hückel's rule, systems possessing ten π-electrons should be aromatic. Herein we report a series of D5h  Li6 E5 Li6 sandwich structures, representing the first global minima featuring ten π-electrons E5 10- ring (E=Si-Pb). However, these π-electrons localize as five π-lone-pairs rather than delocalized orbitals. The high symmetry structure achieved is a direct consequence of σ-aromaticity, particularly favored in elements from Si to Pb, resulting in a pronounced diatropic ring current flow that contributes to the enhanced stability of these systems.

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The understanding of the molecular- and colloidal-structure of asphaltenes has seen a major progress; however, there are still issues that require answer. One of them is the location of the heteroatoms in the polycyclic aromatic hydrocarbon (PAH) fused aromatic ring (FAR) region of asphaltenes. Therefore, the effect on the frontier molecular orbitals (HOMO-LUMO) energy-gap due to the addition of a heteroatom (N or S) to PAHs, which are candidates of the PAH region in asphaltenes, has been systematically analyzed by placing S or N in various sites of the PAH molecule. The S is introduced as a thiophenic ring in a bay region, while the N is introduced as a pyridinic-N, which are prevalent forms in the asphaltene-PAH. 174 PAHs are studied with five fused aromatic rings (5FAR) to 10FAR. The π-electron allocation in resonant π-sextets and isolated double bonds is obtained using the Y-rule. The frontier orbitals optical transition is calculated with the ZINDO/S method. Within a FAR family an increment of π-sextets produces and increase of the HOMO-LUMO energy-gap. There is a linear relationship between the Y-rule mapping (percentage of fraction of π-sextet bond divided by nFAR) and the HOMO-LUMO energy-gap. In addition, the effect on the frontier orbitals energy-gap and on the π-electronic allocation due to the presence of N and S is negligible; therefore, to reach conclusions related to the asphaltene-PAH based on conclusions reached for PAH systems, with no heteroatoms, is a reasonable approach.

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A low-lying structure is revealed for the CuB12 - cluster, which is bowl-shaped. It consists of a triangular CuB2 base and a B10 rim. Molecular dynamics simulations indicates its structural robustness; at an elevated temperature (600 K), the base rotates reversibly within the B10 perimeter. Chemical bonding analysis detects 2σ- and 3π-delocalized bonds, suggesting double aromaticity. This is also confirmed by two diatropic and concentric ring currents under an external magnetic field.

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Here, it is shown that the M3B12 (M = Cu-Au) clusters' global minima consist of an elongated planar B12 fragment connected by an in-plane linear M3 fragment. This result is striking since this B12 planar structure is not favored in the bare cluster, nor when one or two metals are added. The minimum energy structures were revealed by screening the potential energy surface using genetic algorithms and density functional theory calculations. Chemical bonding analysis shows that the strong electrostatic interactions with the metal compensate for the high energy spent in the M3 and B12 fragment distortion. Furthermore, metals participate in the delocalized π-bonds, which infers an aromatic character to these species.

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Vermicomposting is a biological process for efficient cattle manure treatment, but the vermicomposting time determines the quality of the vermicompost. The objective of this study was to evaluate the effect of cattle manure vermicomposting time on earthworm biomass and the changes in physical, chemical, and biological in properties of the vermicompost. The cattle manure was inoculated with Eisenia andrei earthworms and conducted vermicomposting for 0, 15, 30, 45, 60, and 120 days. The analysis of 44 chemical, physical, and biological properties allowed the vermicomposting process to be divided into initial (<45 days) and final (45-120 days) phases. The initial phase was characterized by high microbial activity and the final by high physical-chemical transformation of the vermicompost and an increase in earthworm density. The organic matter aromaticity increased until the 45th day, subsequently decreasing. Although 30 d of vermicompost are sufficient to obtain a high-quality organic fertilizer, 120 d are necessary for producing matrices.

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In the current work, the authors analyzed and compared the chemical behavior of some (anti)aromatic compounds. The species selected are benzene and cyclobutadiene as the aromatic and antiaromatic classical examples, respectively. Next, the anion Al 4 2 - , which is the first all-metal molecule catalogued as aromatic and its non-metallic isoelectronic analog, B 4 2 - were also analyzed. The antiaromatic clusters Al 4 4 - and B 4 4 - were studied in form of lithium salts. And the end, the non-planar B 12 boron cluster and its isoelectronic analogs ( B 11 Be - and B 11 C + ) were considered for being analyzed under the same criterium. The analysis was realized employing the following descriptors: molecular electrostatic potential and local hyper-softness to get insights concerning local reactivity when facing reagents leading to ionic or covalent interactions. The results show that all the molecules analyzed presented some specific variations in their respective local reactivity despite being labeled as aromatic compounds. This analysis provides a notion that the local reactivity is more based on the nature of the atoms and the molecular geometry than the aromatic character by itself.

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Forest fires incorporate pyrogenic organic matter into the soil, affecting the characteristics of soil organic matter (SOM) due to its high aromaticity, increasing its renewal time. However, the factors that control the concentration of pyrogenic organic matter and its chemical composition and structure are still little known. Forest fragments dispersed in a savanna matrix of the northern Brazilian Amazon are frequently impacted by fires, which can affect the composition of SOM within the fragments. The aim of this study was to analyse the chemical composition of SOM in the border and interior of six forest fragments in the Roraima savanna, in the northern Brazilian Amazon. Soil samples were collected at 10-cm intervals up to 1 m in depth at the border and in the interior of each fragment. Soil organic material concentration was determined with 10% HF solution and its elemental composition, thermogravimetric index, and 13C CPMAS NMR spectroscopic analysis were determined. There was no significant difference in the aromaticity index between border and interior. The concentration of alkyl C structures between border (22 to 25%) and interior (19 to 29%) indicated the occurrence of medium-intensity fires in the study area. The thermogravimetric analysis showed no significant difference in the thermal stability of SOM between border and interior. Our results showed no pronounced difference in SOM quality up to 1 m depth between the border and the interior of the evaluated forest fragments, indicating that this compartment is stable throughout the fragments.(AU)

Incêndios florestais incorporam matéria orgânica pirogênica ao solo, afetando as características da matéria orgânica do solo (MOS), devido a sua alta aromaticidade. Porém, os fatores que controlam a composição química e a estrutura da matéria orgânica pirogência ainda são pouco conhecidos. Fragmentos florestais dispersos em uma matriz de savana na Amazônia setentrional brasileira são freqüentemente impactados por incêndios, que podem afetar a composição da MOS nos fragmentos. O objetivo deste estudo foi analisar a composição química da MOS na borda e no interior de seis fragmentos florestais situados na savana de Roraima, norte da Amazônia brasileira. Amostras de solo foram coletadas em intervalos de 10 cm até 1 m de profundidade na borda e no interior de cada fragmento. A concentração da MOS foi determinada com solução de HF a 10% e sua composição elementar, índice termogravimétrico e análise espectroscópica de 13C CPMAS RMN foram determinados. Não houve diferença significativa no índice de aromaticidade entre borda e interior. A concentração de estruturas alquil C entre a borda (22 a 25%) e o interior (19 a 29%) indicou a ocorrência de incêndios de média intensidade na área de estudo. A análise termogravimétrica não mostrou diferença significativa na estabilidade térmica da matéria orgânica do solo entre a borda e o interior. Nossos resultados não mostraram diferença pronunciada na qualidade da MOS até 1 m de profundidade entre a borda e o interior dos fragmentos florestais avaliados, indicando que este compartimento é estável ao longo dos fragmentos.(AU)

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The assertion made by Wu et al. that aromaticity may have considerable implications for molecular design motivated us to use nucleus-independent chemical shifts (NICS) as an aromaticity criterion to evaluate the antifungal activity of two series of indol-4-ones. A linear regression analysis of NICS and antifungal activity showed that both tested variables were significantly related (p < 0.05); when aromaticity increased, the antifungal activity decreased for series I and increased for series II. To verify the validity of the obtained equations, a new set of 44 benzofuran-4-ones was designed by replacing the nitrogen atom of the five-membered ring with oxygen in indol-4-ones. The NICS(0) and NICS(1) of benzofuran-4-ones were calculated and used to predict their biological activities using the previous equations. A set of 10 benzofuran-4-ones was synthesized and tested in eight human pathogenic fungi, showing the validity of the equations. The minimum inhibitory concentration (MIC) in yeasts was 31.25 µg·mL-1 for Candida glabrata, Candida krusei and Candida guilliermondii with compounds 15-32, 15-15 and 15-1. The MIC for filamentous fungi was 1.95 µg·mL-1 for Aspergillus niger for compounds 15-1, 15-33 and 15-34. The results obtained support the use of NICS in the molecular design of compounds with antifungal activity.

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The shielding cone concept is one of the most characteristic aspects of planar aromatic species. Herein, we explored how two neighbor aromatic moieties behave as dual planar-planar, planar-spherical, and spherical-spherical species given by biphenyl, [PhCB11 H11 ]- , and [CB11 H11 ]2 2- , respectively. Our results show two separate aromatic regimes given by Clar's sextet and spherical aromatic states, with independent shielding cones sharing shielding regions of moderate strength, which can be extended for networks involving multiple independent aromatic states.

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Through delicate tuning of the electronic structure, we report herein a rational design of seventeen new putative global minimum energy structures containing a planar tetra- or pentacoordinate carbon atom embedded in an aromatic hydrocarbon. These structures are the result of replacing three consecutive hydrogen atoms of an aromatic hydrocarbon by less electronegative groups, forming a multicenter σ-bond with the planar hypercoordinate carbon atom and participating in the π-electron delocalization. This strategy that maximizes both mechanical and electronic effects through aromatic architectures can be extended to several molecular combinations to achieve new and diverse compounds containing planar hypercoordinate carbon centers.

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The textbook concept of shielding cone is one of the characteristic properties for planar aromatic species, which explains the nuclear shielding of neighbor molecules located above and below the molecular plane. Here, we explore its resemblance in spherical aromatic fullerenes, where particularly for C606-, it explains the record for the highest shielding observed via 3He-NMR for the endohedral 3He@C606- species. Our results compare the behavior for He@C60 and He@C606- denoting relevant changes in their magnetic response behavior, where an induced shielding cone was observed for the latter with its long-range shielding character. In contrast to planar aromatics which give rise to a shielding cone property only for a perpendicularly oriented field, it was found that the shielding cone in spherical aromatic fullerenes occurs to any orientation. Thus, the orientation dependence behavior of the shielding cone is accounted, unraveling a characteristic shielding cone pattern in He@C606- which further rationalizes its record on the most shielded He nucleus, upon rotation or tumbling from the aromatic ring about the applied field. For the C70 case, the opposite situation has been characterized via 3He-NMR experiments, which is explained in terms of the deshielding/shielding region inside the C70 cage. Graphical abstract.

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The induced current density (J(r)) provides useful information about the electronic structure of molecules under a magnetic field (B). However, the analysis of its topology is cumbersome because of its vectorial nature. We show that its tropicity (direction of rotation) and its strength can be compressed in the triple product B ⋅ ∇ × J(r) (tpJ(r)) that is a scalar field. The topology of tpJ(r) has clear similarities to the Laplacian of the electron density. Additionally, the topology of aromatic and antiaromatic compounds is notoriously different. The vorticity of J(r) is helpful to define the circulation of the current density, C, that contrary to other methods, can be easily defined for individual rings in polycyclic molecules. This allows tpJ(r) to clearly reproduce the Clar's structure of polycyclic aromatic hydrocarbons. © 2019 Wiley Periodicals, Inc.

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Extensive explorations of their potential energy surfaces, combined with high-level quantum chemical computations, strikingly show that the lowest energy structures of the (Li6 Si5 )2-5 systems consist of 2-5 Si5 6- aromatic units, surrounded by Li+ counterions, respectively. These viable gas-phase compounds are the pioneering reported cases of oligomers made by planar aromatic silicon rings. Based on the key evidence that these oligomers are energetically favored, and that their silicon rings aromaticity is thoroughly preserved, the Li6 Si5 cluster is proposed as a potential assembly unit to build silicon-lithium nanostructures, thus opening new paths to design Zintl compounds at the nanoscale level.

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This study reports the spin-orbit effects on the aromaticity of the N5-, P5-, As5-, Sb5-, Bi5-, and Mc5- anionic clusters via the magnetically induced current-density method. All-electron density functional theory (DFT) calculations were carried out using the four-component Dirac-Coulomb (DC) hamiltonian, including scalar and spin-orbit relativistic effects. The magnetic index of aromaticity was calculated by numerical integration over the current flow between two atoms in the pentagonal ring. These values were compared to the spin-free values (spin-orbit coupling switched off), in order to assess the spin-orbit effect on aromaticity. It was found that in the heavy anions, Bi5- and Mc5-, there is a significant influence of the spin-orbit coupling. © 2018 Wiley Periodicals, Inc.

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This paper presents a theoretical study of the effects of substituents (F, Cl, Br, CH3, and CN) on the aromaticity of borazine (B3N3H6), using density functional theory (DFT) and the Hartree-Fock (HF) method. The calculations to optimize the geometries, structural properties, and vibrational frequencies were performed using the same 6-311G(d,p) and 6-311++G(d,p) basis sets, comparing the methods with experimental results. In the analysis of the NICSZZ values, it was found that that replacing the hydrogen atoms by halogen atoms (F, Cl, and Br) and CH3 reduced the aromaticity of the borazine molecule, while use of the CN group resulted in NICSZZ values (0.9-2.0 Å) very close to those of borazine, presenting the following order of increasing aromaticity: B3N3H3-(Br)3 < B3N3H3-(Cl)3 < B3N3H3-(F)3 < B3N3H3-(CH3)3 < B3N3H6 ~ B3N3H3-(CN)3. All the spectra of the compounds showed only the presence of transition peaks distant from the UV region, reflecting the large energy difference between the HOMO and LUMO orbitals. After the substitution of the borazine ring, all the compounds presented an intensification of the spectrum, with a shift of the maximum absorbance toward red, indicative of a bathochromic effect. There was a direct inverse relation between the energy gap and the maximum wavelength of the compounds.

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Although there are a multitude of aromaticity indexes, only a few have a widespread usage. All famous aromaticity indexes are limited: HOMA and FLU are reference-dependent; ELF is π-bond-dependent; PDI is structurally dependent and NICS is ring size dependent. These limitations stimulate the continuous search for better (i.e., having no dependency), more flexible (i.e., applied to any aromatic system) and more effective (i.e., with excellent correlations with other indexes) aromaticity indexes. The D3BIA was our first topological aromaticity index. It is flexible, reference-independent and effective for planar and caged aromatic molecules. However, one of its terms, the degree of degeneracy (δ), is arbitrary and difficult to carry out for new users. Thus, in this work, we show that D2BIA-an improved version of D3BIA-is a good candidate to be used widely, since it retains the strong points of D3BIA while avoiding its weak point. In particular cases where all studied systems have δ = 1 (e.g., for acenes), then D2BIA equals D3BIA. For our recent study with acenes, D3BIA (and, as a consequence, D2BIA) has (have) an excellent correlation with FLU according to the MP3 method. In this work, by using DFT calculations for a series involving several six-membered and five-membered heteroaromatic rings, only D2BIA and NICS have very good correlation. All other well known aromaticity indexes used in this work (FLU, HOMA and ELF) gave poor correlations. As to homoaromatic systems, only D2BIA vs NICS and D2BIA vs FLU plots have excellent correlations. HOMA has the worst results in this series. Thus, D2BIA proved to be flexible and effective for the analysis of heteroaromatic rings of different sizes and for caged homoaromatic systems. Moreover, D2BIA has better correlations than D3BIA for planar aromatic systems, and same correlations for caged-homoaromatic systems. Graphical abstract D2BIA-an effective and improved version of the D3BIA aromaticity index.

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Recently, it has been shown that the superatom concept is intimately connected to relevant tools of great chemical significance, such as the Lewis structure model and the VSEPR theory, which has been employed to understand hybridized and dimeric-like molecules. This suggests a potential rational construction of superatomic clusters mimicking more complex structures. Here, we extend another well-employed concept to the superatomic clusters, to construct a novel Au42 isomer with resemblance to cyclic aromatic molecules. It is shown that the Hückel (4n+2)π rule is ready to be applied, predicting aromatic behavior latterly supported by the favorable evaluation of the induced shielding cone formation. The D6 h isomer of Au42 described here exhibits inherent characteristics mimicking aromatic hydrocarbon rings, displaying π-superatomic orbitals and related properties. This new cluster is the first member of the superatomic clusters family to exhibit an aromatic π-electron system.