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In 1977 Weiss and Grimes, by means of mass spectrometry and 1 H and 11 B NMR spectroscopy, proposed two structures (I and II) for the ferraborane (η 5 -C 5 H 5 )Fe(B 5 H 10 ), isoelectronic with ferrocene. In this work, by means of high-level quantum-chemical computations, we confirm the experimental structures of the two isomers with their corresponding energies, and assign the reported 1 H and 11 B NMR chemical shifts. A striking result from this study is the planarization (3D→2D) of the B 5 H 10 - ligand - an unknown isolated anion, isoelectronic with aromatic cyclopentadienyl anion C 5 H 5 - - when attached to the (η 5 -C 5 H 5 )Fe + moiety, thus resulting in a more stable ferraborane isomer II., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Recently, Guha and co-workers (Sarmah, K.; Kalita, A.; Purkayastha, S.; Guha, A. K. Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride. Angew. Chem. Int. Ed. 2024 , e202318741) reported a highly intriguing bonding motif: planar pentacoordinate hydrogen (ppH) in Li 5 H 6 - , featuring C 2 v symmetry in the singlet state with two distinct H-Li (center-ring) bond distances. We herein revisited the potential energy surface of Li 5 H 6 - by using a target-oriented genetic algorithm. Our investigation revealed that the lowest-energy structure of Li 5 H 6 - exhibits a ppH configuration with very high D 5 h symmetry and a 1 A 1 ' electronic state. We did not find any electronic effect like Jahn-Teller distortion that could be responsible for lowering its symmetry. Moreover, our calculations demonstrated significant differences in the relative energies of other low-lying isomers. An energetically very competitive planar tetracoordinate hydrogen (ptH) isomer is also located, but it corresponds to a very shallow minimum on the potential energy surface depending on the used level of theory. Chemical bonding analyses, including AdNDP and EDA-NOCV, uncover that the optimal Lewis structure for Li 5 H 6 - involves H - ions stabilized by the Li 5 H 5 crown. Surprisingly, despite the dominance of electrostatic interactions, the contribution from covalent bonding is also significant between ppH and the Li 5 H 5 moiety, derived from H - (1s) → Li 5 H 5 σ donation. Magnetically induced current density analysis revealed that due to minimal orbital overlap and the highly polar nature of the H-Li covalent interaction, the ppH exhibits local diatropic ring currents around the H centers, which fails to result in a global aromatic ring current. The coordination of Li 5 H 6 - with Lewis acids, BH 3 and BMe 3 , instantly converts the ppH configuration to (quasi) ptH. These Lewis acid-bound ptH complexes show high electronic stability and high thermochemical stability against dissociation and, therefore, will be ideal candidates for the experimental realization.

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 S 0 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 T 1 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 S 0 ., (© 2024 Wiley-VCH GmbH.)

The quest for planar hypercoordinate atoms (phA) beyond six has predominantly focused on transition metals, with dodecacoordination being the highest reported thus far. Extending this bonding scenario to main-group elements, which typically lack d orbitals despite their larger atomic radius, has posed significant challenges. Intrigued by the potentiality of covalent bonding formation using the d orbitals of the heavier alkaline-earth metals (Ae = Ca, Sr, Ba), the so-called "honorary transition metals", we aim to push the boundaries of planar hypercoordination. By including rings formed by 12-15 atoms of boron-carbon and Ae centers, we propose a design scheme of 180 candidates with a phA. Further systematic screening, structural examination, and stability assessments identified 10 potential clusters with a planar hypercoordinate alkaline-earth metal (phAe) as the lowest-energy form. These unconventional structures embody planar dodeca-, trideca-, tetradeca-, and pentadecacoordinate atoms. Chemical bonding analyses reveal the important role of Ae d orbitals in facilitating covalent interactions between the central Ae atom and the surrounding boron-carbon rings, thereby establishing a new record for coordination numbers in the two-dimensional realm.

In this study, we delved into the structure of B 5 H 5 and questioned some of its accepted assumptions. By exploring the potential energy surface, we found a new three-dimensional structure as the global minimum. This finding is in contrast with the previously hypothesized planar and cage-like models. Our exploration extends to the kinetic stability of various B 5 H 5 isomers, offering insights into the dynamic behavior of these molecules.

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 D 5h  Li 6 E 5 Li 6 sandwich structures, representing the first global minima featuring ten π-electrons E 5 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., (© 2023 Wiley-VCH GmbH.)

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., (© 2023 Wiley-VCH GmbH.)

Using various exploration strategies, in this study, we investigated the potential energy surfaces (PES) of CBe 5 H 5 + and C n Be 3 n +2 H 2 n +2 2+ ( n = 2-4) clusters. Previous studies proposed that the planar pentacoordinate carbons (ppCs) were the global minima of these clusters. However, our study identified new putative global minima and competitive isomers, refuting some previous assignments. We employed several methods, including evolutive-inspired stochastic approaches guided by "chemical criteria", and ab initio molecular dynamics simulations at elevated temperatures. Our results showed that the size of the scanned population significantly affected the evolutive method and that constrained or guided procedures showed an advantage in identifying better minima for larger systems. This study demonstrated that using multiple complementary strategies can result in a wider variety of minima in a given energy range. Our findings provide valuable insights into exploring the potential energy surfaces of clusters, mainly medium-sized clusters, which could be the connections between small clusters and nanomaterials.

Planar boron clusters have often been regarded as "π-analogous" to aromatic arenes because of their similar delocalized π-bonding. However, unlike arenes such as C 5 H 5 - and C 6 H 6 , boron clusters have not previously shown the ability to form sandwich complexes. In this study, we present the first sandwich complex involving beryllium and boron, B 7 Be 6 B 7 . The global minimum of this combination adopts a unique architecture having a D 6h geometry, featuring an unprecedented monocyclic Be 6 ring sandwiched between two quasi-planar B 7 motifs. The thermochemical and kinetic stability of B 7 Be 6 B 7 can be attributed to strong electrostatic and covalent interactions between the fragments. Chemical bonding analysis shows that B 7 Be 6 B 7 can be considered as a [B 7 ] 3- [Be 6 ] 6+ [B 7 ] 3- complex. Moreover, there is a significant electron delocalization within this cluster, supported by the local diatropic contributions of the B 7 and Be 6 fragments., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The presence of a delocalized π-bond is often considered an essential criterion for achieving planar hypercoordination. Herein, we show that σ-delocalization could be sufficient to make the planar configuration the most stable isomer in a series of planar pentacoordinate s-block metals. High-level ab initio computations reveal that the global minimum of a series of interalkali and interalkali-alkaline earth clusters (LiNa 5 , Li 5 Mg + , Na 5 Mg + , K 5 Ca + , CaRb 5 + , Rb 5 Sr + , and SrCs 5 + ) adopts a singlet D 5h structure with a planar pentacoordinate lithium or alkaline earth metal (AE = Mg, Ca, Sr). These clusters are unusual combinations to stabilize a planar pentacoordinate atom, as all their constituents are electropositive. Despite the absence of π-electrons, Hückel's rule is fulfilled by the six σ-electrons. Furthermore, the systems exhibit a diatropic ring current in response to an external magnetic field and a strong magnetic shielding, so they might be classified as σ-aromatic. Therefore, multicenter σ-bonds and the resulting σ-delocalization stabilize these clusters, even though they lack π-aromaticity., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The antioxidant activity of nine lichen substances, including methylatrarate (1), methyl haematommate (2), lobaric acid (3), fumarprotocetraric acid (4), sphaerophorin (5), subsphaeric acid (6), diffractaic acid (7), barbatolic acid (8) and salazinic acid (9) has been determined through cyclic voltammetry. The compounds 1-4 presented slopes close to the Nernst constant of 0.059 V, indicating a 2H + /2e - relation between protons and electrons, as long as the compounds 5, 6, 7, 8, and 9 present slopes between 0.037 V and 0.032 V, indicating a 1H + /2e - relation between protons and electrons. These results show a high free radical scavenging activity by means of the release of H + , suggesting an important antioxidant capacity of these molecules. Theoretical calculations of hydrogen bond dissociation enthalpies (BDE), proton affinities (PA), and Proton Transfer (PT) mechanisms, at M06-2x/6-311+G(d,p) level complement the experimental results. Computations support that the best antioxidant activity is obtained for the molecules (3, 4, 5, 6, 7 and 8), that have a carboxylic acid group close to a phenolic hydroxyl group, through hydrogen atomic transfer (HAT) and sequential proton loss electron transfer (SPLET) mechanisms. Additional computations were performed for modelling binding affinity of the lichen substances with CYPs enzymes, mainly CYP1A2, CYP51, and CYP2C9*2 isoforms, showing strong affinity for all the compounds described in this study., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

In this work, we explore, using high-level calculations, the ability of BH 4 + to interact with noble gases. The He system is energetically unstable, while the Ne system could only be observed at cryogenic temperatures. In the case of the Ar, Kr and Xe systems, all are energetically stable, even at room temperature. The different chemical bond descriptors reveal a covalent character between B and the noble gas from Ar to Rn. However, this interaction gradually weakens the multicentric bond between the boron atom and the H 2 fragment. Thus, although BH 4 Rn + exhibits a strong covalent bond, it tends to dissociate at room temperature into BH 2 Rn + +H 2 ., (© 2022 Wiley-VCH GmbH.)

Despite the central role of aromaticity in the chemistry of expanded porphyrins, the evaluation of aromaticity remains difficult for these extended macrocycles. The presence of multiple conjugation pathways and different planar and nonplanar π-conjugation topologies makes the quantification of global and local aromaticity even more challenging. In neutral expanded porphyrins, the predominance of the aromatic conjugation pathway passing through the imine-type nitrogens and circumventing the amino NH groups is established. However, for charged macrocycles, the question about the main conjugation circuit remains open. Accordingly, different conjugation pathways in a set of neutral, anionic, and cationic expanded porphyrins were investigated by means of several aromaticity indices rooted in the structural, magnetic, and electronic criteria. Overall, our results reveal the predominance of the conjugation pathway that passes through all nitrogen atoms to describe the aromaticity of deprotonated expanded porphyrins, while the outer pathway through the perimeter carbon atoms becomes the most aromatic in protonated macrocycles. In nonplanar and charged macrocycles, a discrepancy between electronic and magnetic descriptors is observed. Nevertheless, our work demonstrates AV min remains the best tool to determine the main conjugation pathway of expanded porphyrins., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Here, it is shown that the M 3 B 12 (M = Cu-Au) clusters' global minima consist of an elongated planar B 12 fragment connected by an in-plane linear M 3 fragment. This result is striking since this B 12 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 M 3 and B 12 fragment distortion. Furthermore, metals participate in the delocalized π-bonds, which infers an aromatic character to these species.

Bismuth complexes stabilized by carbon-based donor ligands are underserved by their instability, often due to facile ligand dissociation and deleterious protonolysis. Herein, we show that the ortho -bismuthination of hexaphenylcarbodiphosphorane enables a robust framework with geometrically constrained carbone-bismuth bonding interactions, which are highly tunable by cationization. The carbodiphosphorane bismuth halides ( 1 and 2 ) are remarkably air-stable and feature unprecedented trans carbone C-Bi-X ligation, resulting in highly elongated Bi-X bonds. In contrast to known carbone-bismuth complexes, hydrolytic activation of the carbone yields well-defined organobismuth complexes, and subsequent dehydrohalogenation is feasible using potassium bis(trimethylsilyl)amide or N-heterocyclic carbenes. The redox-flexibility of this framework was evaluated in the high catalytic activity of 1 and 2 for silylation of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) under mild conditions (50 °C, 24-96 h) and low catalyst loadings (5-10 mol %), which suggests the accessibility of short-lived hydridic and radical bismuth species. The reaction of 1 , PhSiH 3 , and tris(pentafluorophenyl)borane (BCF) yields the first crystallographically characterized bismuth hydridoborate complex as an ionic species ( 9 ), presumably by BCF-mediated hydride abstraction from an unobserved [Bi]-H intermediate. All isolated compounds have been characterized by heteronuclear NMR spectroscopy and X-ray crystallography, and the bonding situation in representative complexes ( 1 , 2 , 5 , and 9 ) were further evaluated using density functional theory.

It is now known that the heavier noble gases (Ng=Ar-Rn) show some varying degrees of reactivity with a gradual increase in reactivity along Ar-Rn. However, because of their very small size and very high ionization potential, helium and neon are the hardest targets to crack. Although few neon complexes are isolated at very low temperatures, helium needs very extreme situations like very high pressure. Here, we find that protonated BeO, BeOH + can bind helium and neon spontaneously at room temperature. Therefore, extreme conditions like very low temperature and/or high pressure will not be required for their experimental isolation. The Ng-Be bond strength is very high for their heavier homologs and the bond strength shows a gradual increase from He to Rn. Moreover, the Ng-Be attractive energy is almost exclusively originated from the orbital interaction which is composed of one Ng(s/p σ )→BeOH + σ-donation and two weaker Ng(p π )→BeOH + π-donations, except for helium. Helium uses its low-lying vacant 2p orbitals to accept π-electron density from BeOH + . Previously, such electron-accepting ability of helium was used to explain a somewhat stronger helium bond than neon for neutral complexes. However, the present results indicate that such π-back donations are too weak in nature to decide any energetic trend between helium and neon., (© 2022 Wiley-VCH GmbH.)

A systematic exploration of the potential energy surface reveals two global minima with three planar tetra coordinate carbons (ptCs) and two global minima with three quasi-ptCs for E 6 C 15 (E = Si-Pb) combinations. These consist of aromatic polycyclic templates suitable for further design of different materials without hindering the ptC texture.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr 3 and [( py CDC)PnBr 2 ][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr 3 ( 1 ) with one or two equivalents of AgNTf 2 (NTf 2 = bis(trifluoromethanesulfonyl)imide) or AgSbF 6 gives stibaalkene mono- and dications of the form [(CDC)SbBr 3- n ][A] n ( 2-4 ; n = 1,2; A = NTf 2 or SbF 6 ). The stibaalkene trication [(CDC) 2 Sb][NTf 2 ] 3 ( 5 ) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6 - 9 . With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene ( py CDC) was used as a ligand to access [( py CDC)PnBr 2 ][Br] ( 10 , 12 ) and trications [( py CDC)Pn][NTf 2 ] 3 (Pn = Sb ( 11 ), Bi ( 13 )), forgoing the need for a second CDC as used in the synthesis of 5 . The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf 2 ] 2 ( 4 ) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF 3 , and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF 5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.

A low-lying structure is revealed for the CuB 12 - cluster, which is bowl-shaped. It consists of a triangular CuB 2 base and a B 10 rim. Molecular dynamics simulations indicates its structural robustness; at an elevated temperature (600 K), the base rotates reversibly within the B 10 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., (© 2022 Wiley-VCH GmbH.)

Tubular boron clusters represent a class of extremely unusual geometries that can be regarded as a key indicator for the 2D-to-3D boron structural evolution as well as the embryos for boron nanotubes. While a good number of pure boron or metal-doped boron tubular clusters have been reported so far, most of them are two-ring tubular structures, and their higher-ring analogues are very scarce. We report herein the first example of a four-ring tubular boron motif in the cagelike global minimum of Be 2 B 24 + . Global-minimum searches of MB 24 q and M 2 B 24 q (M = alkali/alkaline-earth metals; q = 1+, 0, 1-) reveal that the most stable structure of Be 2 B 24 + is a C 2 v -symmetric cage having a four-ring tubular boron moiety, whereas it is a high-lying isomer for those having a two/three-ring tubular boron motif for all other systems. The B 24 framework in Be 2 B 24 + can be viewed as consisting of two two-ring B 12 tubular structures linked together at one side of the B 6 rings along the high-symmetry axis and two offside B 6 rings capped by two Be atoms. The Be 2 -B 24 bonding is best described as Be 2 2+ in an excited triplet state, forming two highly polarized covalent bonds with B 24 - in a quartet spin state. The unique ability of beryllium to make strong covalent and electrostatic interactions makes the Be 2 B 24 + cluster stable in such an unusual geometry.

Though search algorithms are appropriate tools for identifying low-energy isomers, fixing several constraints seems to be a fundamental prerequisite to successfully running any structural search program. This causes some potential setbacks as far as identifying all possible isomers, close to the lowest-energy isomer, for any elemental composition. The number of explored candidates, the choice of method, basis set, and availability of CPU time needed to analyze the various initial test structures become necessary restrictions in resolving the issues of structural isomerism reasonably. While one could arrive at new structures through chemical intuition, reproducing or achieving those exact same structures requires increasing the number of variables in any given program, which causes further constraints in exploring the potential energy surface in a reasonable amount of time. Thus, it is emphasized here that an integrated approach between search algorithms and chemical intuition is necessary by taking the C 12 O 2 Mg 2 system as an example. Our initial search through the AUTOMATON program yielded 1450 different geometries. However, through chemical intuition, we found eighteen new geometries within 40.0 kcal mol -1 at the PBE0-D3/def2-TZVP level. These results indirectly emphasize that an integrated approach between search algorithms and chemical intuition is necessary to further our knowledge in chemical space for any given elemental composition.

After exploring the potential energy surfaces of M m CE 2 p (E=S-Te, M=Li-Cs, m=2, 3 and p=m-2) and M n CE 3 q (E=S-Te, M=Li-Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE 2 2- and Y-shaped CE 3 2- dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals., (© 2021 Wiley-VCH GmbH.)

We computationally explore an alternative to stabilize one-dimensional (1D) silicon-lithium nanowires (NWs). The Li 12 Si 9 Zintl phase exhibits the NW [ Li 6 Si 5 ] ∞ 1 , combined with Y-shaped Si 4 structures. Interestingly, this NW could be assembled from the stacking of the Li 6 Si 5 aromatic cluster. The [ Li 6 Si 5 ] ∞ 1 @CNT nanocomposite has been investigated with density functional theory (DFT), including molecular dynamics simulations and electronic structure calculations. We found that van der Waals interaction between Li's and CNT's walls is relevant for stabilizing this hybrid nanocomposite. This work suggests that nanostructured confinement (within CNTs) may be an alternative to stabilize this free NW, cleaning its properties regarding Li 12 Si 9 solid phase, i.e., metallic character, concerning the perturbation provided by their environment in the Li 12 Si 7 compound., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Orellana, Pino-Rios, Yañez, Vásquez-Espinal, Peccati, Contreras-García, Cardenas and Tiznado.)

Here, we introduce a hybrid method, named Kick-Fukui, to explore the potential energy surface (PES) of clusters and molecules using the Coulombic integral between the Fukui functions in the first screening of the best individuals. In the process, small stable molecules or clusters whose combination has the stoichiometry of the explored species are used as assembly units. First, a small set of candidates has been selected from a large and stochastically generated (Kick) population according to the maximum value of the Coulombic integral between the Fukui functions of both fragments. Subsequently, these few candidates are optimized using a gradient method and density functional theory (DFT) calculations. The performance of the program has been evaluated to explore the PES of various systems, including atomic and molecular clusters. In most cases studied, the global minimum (GM) has been identified with a low computational cost. The strategy does not allow to identify the GM of some silicon clusters; however, it predicts local minima very close in energy to the GM that could be used as the initial population of evolutionary algorithms.

It is here reported that the NaBH 3 - cluster exhibits a Na⋅B one-electron bond, a well-established type of electron-deficient bonding in the literature. The topological analysis of the electron localization function, at the correlated level, reveals that Na - , when approaching the bonding distance, fairly distributes its valence electron pair between two lobes. One of these electrons is used to bond with BH 3 , which participates through its boron empty p-orbital. Furthermore, the bonding situation of LiBH 3 - , KBH 3 - , MgBH 3 , and CaBH 3 global minima structures are similar to that of NaBH 3 - , extending the family of these new one-electron bond systems with biradicaloid character., (© 2021 Wiley-VCH GmbH.)

We report the global minima structures of Li 8 Si 8 , Li 10 Si 9 , and Li 12 Si 10 systems, in which silicon moieties maintain structural and chemical bonding characteristics similar to those of their building blocks: the aromatic clusters T d -Li 4 Si 4 and C 2v -Li 6 Si 5 . Electron counting rules, chemical bonding analysis, and magnetic response properties verify the silicon unit's aromaticity persistence. This study demonstrates the feasibility of assembling silicon-based nanostructures from aromatics clusters as building blocks., (© 2021 Wiley-VCH GmbH.)

The exhaustive exploration of the potential energy surfaces of CE 2 M 2 (E = Si-Pb; M = Li and Na) revealed seven global minima containing a planar tetracoordinate carbon (ptC). The design, based on a π-localization strategy, resulted in a ptC with two double bonds forming a linear or a bent allene-type E═C═E motif. The magnetic response of the bent E═C═E fragments support a σ-aromaticity. The bonding analysis indicated that the ptCs form C-E covalent bonds and C-M electrostatic interactions.

Herein, the first global minima containing a planar hexacoordinate carbon (phC) atom are reported. The fifteen structures belong to the CE 3 M 3 + (E=S-Te and M=Li-Cs) series and satisfy both geometric and electronic criteria to be considered as a true phC. The design strategy consisted of replacing oxygen in the D 3h  CO 3 Li 3 + structure with heavy and less electronegative chalcogens, inducing a negative charge on the C atom and an attractive electrostatic interaction between C and the alkali-metal cations. The chemical bonding analyses indicate that carbon is covalently bonded to three chalcogens and ionically connected to the three alkali metals., (© 2021 Wiley-VCH GmbH.)

The pandemic that started in Wuhan (China) in 2019 has caused a large number of deaths, and infected people around the world due to the absence of effective therapy against coronavirus 2 of the severe acute respiratory syndrome (SARS-CoV-2). Viral maturation requires the activity of the main viral protease (M pro ), so its inhibition stops the progress of the disease. To evaluate possible inhibitors, a computational model of the SARS-CoV-2 enzyme M pro was constructed in complex with 26 synthetic ligands derived from coumarins and quinolines. Analysis of simulations of molecular dynamics and molecular docking of the models show a high affinity for the enzyme ( ∆E binding between -5.1 and 7.1 kcal mol -1 ). The six compounds with the highest affinity show K d between 6.26 × 10 -6 and 17.2 × 10 -6 , with binding affinity between -20 and -25 kcal mol -1 , with ligand efficiency less than 0.3 associated with possible inhibitory candidates. In addition to the high affinity of these compounds for SARS-CoV-2 M pro , low toxicity is expected considering the Lipinski, Veber and Pfizer rules. Therefore, this novel study provides candidate inhibitors that would allow experimental studies which can lead to the development of new treatments for SARS-CoV-2., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Yañez, Osorio, Uriarte, Areche, Tiznado, Perez-Donoso, García-Beltrán and González-Nilo.)

The relativistic effects on the aromaticity of a set of benzene analogues, E3M3H3 (E = C-Pb; M = N-Bi) heterocycles, using magnetically induced current density (MICD) and the NICSzz component of the conventional nucleus independent chemical shift (NICS), is hereby examined. The relativistic effects were evaluated by means of four-component relativistic MICD, and two-component NMR relativistic shielding tensor methods. MICD and NICS were also computed in a non-relativistic fashion, to assess the influence of scalar-relativistic and spin-orbit effects. Most of the studied compounds exhibit a net diatropic ring current (aromatic), excluding the nitrogen-containing compounds which are non-aromatic (except for C3N3H3), in agreement with their higher E-N electronegativity difference. Only in the case of bismuth compounds, E3Bi3H3, aromaticity is substantially decreased when relativistic effects are included (mainly due to the spin-orbit contribution). The larger the mass of the system, the larger the magnitude of this change, in line with the expected relativistic effects for heavier elements. The analysis based on the NICSzz computations agrees with that of the MICD, thus supporting both the magnetic behavior and the aromatic character of these compounds.

The aromaticity in porphyrinoids results from the π conjugation through two different annular perimeters: the macrocyclic ring and the local heterocyclic rings appended to it. Analyses, based on aromatic stabilization energies (ASE), indicate that the local circuits (6π) are responsible for the significant aromatic stabilization of these systems. This local aromaticity can be coupled with the one from 4n + 2π macrocyclic circuit. It can either compensate for the destabilization due to a 4n π macrocyclic circuit, or be the only source of aromatic stabilization in porphyrinoids with macrocycles without π-conjugated bonds. This "multifaceted" aromatic character of porphyrinoids makes it challenging to analyze their aromaticity using magnetic descriptors because of the intricate interaction of local versus macro-cyclic circulation. In this contribution, we show that the analysis of the bifurcation of the induced magnetic field, Bind, allows clear identification and quantification of both local, and macrocyclic aromaticity, in a representative group of porphyrinioids. In porphyrin, bifurcation values accurately predict the local and macrocyclic contribution rate to overall aromatic stabilization determined by ASE.

Inspired by the double-aromatic (σ and π) C 6 H 3 + , C 6 I 6 2+ , and C 6 (SePh) 6 2+ ring-shaped compounds, herein we theoretically study their borazine derivative analogues. The systems studied are the cation and dications with formulas B 3 N 3 H 3 + , B 3 N 3 Br 6 2+ , B 3 N 3 I 6 2+ , B 3 N 3 (SeH) 6 2+ , and B 3 N 3 (TeH) 6 2+ . Our DFT calculations indicate that the ring-shaped planar structures of B 3 N 3 H 3 + , B 3 N 3 I 6 2+ , and B 3 N 3 (TeH) 6 2+ are more stable in the singlet state, while those of B 3 N 3 Br 6 2+ and B 3 N 3 (SeH) 6 2+ prefer the triplet state. Besides, exploration of the potential energy surface shows that the ring-shaped structure is the putative global minimum only for B 3 N 3 I 6 2+ . According to chemical bonding analysis, B 3 N 3 H 3 + , B 3 N 3 I 6 2+ , and B 3 N 3 (TeH) 6 2+ have σ and π delocalized bonds. The number of delocalized σ/π electrons is 2/6 for the first, and 10/6 for the second and third, similar to what their carbon analogs exhibit. Finally, the analysis of the magnetically induced current density allows B 3 N 3 H 3 + , B 3 N 3 I 6 2+ , and B 3 N 3 (TeH) 6 2+ to be classified as strongly σ aromatic, and poorly π aromatic compounds., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The aromaticity and dynamics of a set of recently proposed neutral 5- and 6-membered heterocycles that are closed by dative (donor-acceptor) or multi-center σ bonds, and have resonance forms with a Hückel number of π-electrons, are examined. The donors and acceptors in the rings include N, O, and F, and B, Be, and Mg, respectively. The planar geometry of the rings, coupled with evidence from different measures of aromaticity, namely the NICSzz, and NICSπzz components of the conventional nucleus independent chemical shifts (NICS), and ring current strengths (RCS), indicate non-trivial degrees of aromaticity in certain cases, including the cyclic C3B2OH6 and C3BOH5 isomers, both with three bonds to the O site in the ring. The former is lower in energy by at least 17.6 kcal mol-1 relative to linear alternatives obtained from molecular dynamics simulations in this work. Some of the other systems examined are best described as non-aromatic. Ring opening, closing, and isomerization are observed in molecular dynamics simulations for some of the systems studied. In a few cases, the ring indeed persists.

An alternative response function, based on the dual descriptor in terms of Koopmans' approximation, is hereby proposed for the description of chemical reactivity in systems with (quasi-) degenerate frontier molecular orbitals. This descriptor is constructed from Fukui functions that include contributions from different orbitals, i.e., orbital-weighted Fukui functions. The methodology is applied to three case studies: the first case consists of a series of benchmark organic and inorganic molecules from which the dual descriptor, based only on frontier orbitals, is not appropriate to describe their reactivity. The second case deals with the proper description of chemical reactivity in Diels-Alder reactions between fullerene C 60 and cyclopentadiene (CP), revealing the importance of considering secondary orbital interactions for an adequate regioselectivity description. The third, and last case, consists of a series of polycyclic aromatic hydrocarbons (PAHs) possessing molecular orbital degeneracy. By means of analyzing of this descriptor, an alternative approach to the description of aromaticity is proposed. In all cases, the proposed index called "orbital-weighted dual descriptor" has proven to accurately describe the chemical reactivity and aromaticity of the studied systems.

It was recently shown that, when tested in cellular systems, quercetin oxidized products (Qox) have significantly better antioxidant activity than quercetin (Q) itself. The main Qox identified in the experiments are either 2,5,7,3',4'-pentahydroxy-3,4-flavandione (Fl) or its tautomer, 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2 H )-benzofuranone (Bf) . We have now performed a theoretical evaluation of different physicochemical properties using density functional theory (DFT) calculations on Q and its main Qox species. The most stable structures (for Q and Qox) were identified after a structural search on their potential energy surface. Since proton affinities (PAs) are much lower than the bond dissociation enthalpies (BDEs) of phenolic hydrogens, we consider that direct antioxidant activity in these species is mainly due to the sequential proton loss electron transfer (SPLET) mechanism. Moreover, our kinetic studies, according to transition state theory, show that Q is more favored by this mechanism. However, Qox have lower PAs than Q , suggesting that antioxidant activity by the SPLET mechanism should be a result of a balance between proclivity to transfer protons (which favors Qox) and the reaction kinetics of the conjugated base in the sequential electron transfer mechanism (which favors Q ). Therefore, our results support the idea that Q is a better direct antioxidant than its oxidized derivatives due to its kinetically favored SPLET reactions. Moreover, our molecular docking calculations indicate a stabilizing interaction between either Q or Qox and the kelch-like ECH-associated protein-1 (Keap1), in the nuclear factor erythroid 2-related factor 2 (Nrf2)-binding site. This should favor the release of the Nrf2 factor, the master regulator of anti-oxidative responses, promoting the expression of the antioxidant responsive element (ARE)-dependent genes. Interestingly, the computed Keap1-metabolite interaction energy is most favored for the Bf compound, which in turn is the most stable oxidized tautomer, according to their computed energies. These results provide further support for the hypothesis that Qox species may be better indirect antioxidants than Q , reducing reactive oxygen species in animal cells by activating endogenous antioxidants., (Copyright © 2019 Vásquez-Espinal, Yañez, Osorio, Areche, García-Beltrán, Ruiz, Cassels and Tiznado.)

The synthesis and structural characterization of the dimer [(Sn 6 Ge 2 Bi) 2 ] 4- raise the possibility of obtaining a broad variety of analogous compounds with different Sn/Ge/Bi proportions. Several combinations of nine atoms have been detected by electrospray mass spectrometry as potential assembly units. However, [(Sn 6 Ge 2 Bi) 2 ] 4- remains as the unique experimentally characterized species in this series. This fact has motivated us to explore its potential energy surface, as well as its monomers' [Sn 6 Ge 2 Bi] 3-/2- , in an effort to gain insight into the factors that might be privileging the experimental viability of this species. Our results show that the lowest-energy [Sn 6 Ge 2 Bi] 3- structure remains in its oxidized product [Sn 6 Ge 2 Bi] 2- , which corresponds to that identified in the dimer [(Sn 6 Ge 2 Bi) 2 ] 4- . Additionally, local minima, very close in energy to the lowest-energy monomer, are chiral mixtures that dimerize into diverse structures with a probable energetic cost, making them noncompetitive isomers. Finally, the global minimum of the dimer [(Sn 6 Ge 2 Bi) 2 ] 4- presents the most stable monomers as assembly units. These results show the importance of considering the simultaneity of all of these conditions for the viability of these types of compounds.

Extensive explorations of their potential energy surfaces, combined with high-level quantum chemical computations, strikingly show that the lowest energy structures of the (Li 6 Si 5 ) 2-5 systems consist of 2-5 Si 5 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 Li 6 Si 5 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Herein we report the systematic exploration of the potential energy surfaces of a series of clusters with formula E 5 M 7 + (E=C-Pb and M=Li-Cs). Fifteen of these combinations adopt a D 5h three-dimensional seven-pointed star-like structure in a singlet state, where M atoms interact electrostatically with the E 5 ring. The determining factors in the relative preference of having the D 5h structure over the most competitive isomer or vice-versa are analyzed. These star-shaped systems satisfy the 4n+2 Hückel's rule and exhibit a strong diatropic (σ and π) response to an external magnetic field., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Theoretical studies are essential for the structural characterization of clusters, when it comes to rationalize their unique size-dependent properties and composition. However, the rapid growth of local minima on the potential energy surface (PES), with respect to cluster size, makes the candidate identification a challenging undertaking. In this article, we introduce a hybrid strategy to explore the PES of clusters. This proposal involves the use of a biased initial population of a genetic algorithm procedure. Each individual in this population is built by assembling small fragments, according to the best matching of the Fukui function. The performance of a genetic algorithm procedure. The performance of the method is assessed on the PES exploration of medium-sized Si n clusters (n = 12-20). The most relevant results are: (a) the method converges at almost half of the time used by the canonical version of the GA and, (b) in all the studied cases, with the exception of Si 13 and Si 16 , the method allowed to identify the global minimum (GM) and other important low-lying structures. Additionally, the apparent deficiency of the proposal to identify the GM was corrected when a Si atom, or other low-lying isomers, were considered to build the clusters. © 2017 Wiley Periodicals, Inc., (© 2017 Wiley Periodicals, Inc.)

In this paper, we provide solid evidence to show that among an overwhelming structural diversity, alkaline earth metals (Ca, Sr, Ba) have the ability to form inverted sandwich compounds with C6H6, C7H7(+), and C8H8(2+) of Dnh symmetry and general formula [E-(CnHn)-E](n-6) (n = 6-8) with planar 10-π-electron aromatic cores by virtue of transferring two electrons per metal atom to the ring. However, the origin of the orbital interaction between the metals and the carbon ring is quite different; while [E-(C6H6)-E] complexes are dominated by δ-interactions, both π- and δ-interactions are important in [E-(C7H7)-E](+) and [E-(C8H8)-E](2+) complexes.

The most stable forms of E(5)Li(7)(+) (E = Ge, Sn, and Pb) have been explored by means of a stochastic search of their potential-energy surfaces by using the gradient embedded genetic algorithm (GEGA). The preferred isomer of the Ge(5)Li(7)(+) ion is a slightly distorted analogue of the D(5h) three-dimensional seven-pointed starlike structure adopted by the lighter C(5)Li(7)(+) and Si(5)Li(7)(+) clusters. In contrast, the preferred structures for Sn(5)Li(7)(+) and Pb(5)Li(7)(+) are quite different. By starting from the starlike arrangement, corresponding lowest-energy structures are generated by migration of one of the E atoms out of the plane with the a corresponding rearrangement of the Li atoms. To understand these structural preferences, we propose a new energy decomposition analysis based on isomerizations (isomerization energy decomposition analysis (IEDA)), which enable us to extract energetic information from isomerization between structures, mainly from highly charged fragments., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The potential energy surfaces of the Na(Si(5-n)(BH)(n))(-) and Si(5-n)(BH)(n)(2-) (n = 0-5) systems have been explored in detail. We established that all the global minimum structures of anionic and dianionic systems can be obtained by substitution of one or more silicon atoms of the global minima of NaSi(5)(-) and Si(5)(2-) for B-H units. The conservation of the overall structure upon isoelectronic substitution was shown to be due to the preservation of the chemical bonding pattern. Theoretical VDEs were calculated for all of the sodiated Na(Si(5-n)(BH)(n))(-) (n = 0-5) systems to facilitate their experimental detection.

We performed global minimum searches for the B(n) H(n+2) (n=2-5) series and found that classical structures composed of 2c-2e B-H and B-B bonds become progressively less stable along the series. Relative energies increase from 2.9 kcal mol(-1) in B(2) H(4) to 62.3 kcal mol(-1) in B(5) H(7). We believe this occurs because boron atoms in the studied molecules are trying to avoid sp(2)  hybridization and trigonal structure at the boron atoms, as in that case one 2p-AO is empty, which is highly unfavorable. This affinity of boron to have some electron density on all 2p-AOs and avoiding having one 2p-AO empty is a main reason why classical structures are not the most stable configurations and why multicenter bonding is so important for the studied boron-hydride clusters as well as for pure boron clusters and boron compounds in general., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Can relativistic effects modify the NICS and the B(ind) values? In this manuscript we evaluate the relativistic corrections incorporated via the zeroth-order regular approximation to the calculations of nucleus-independent chemical shifts and the induced magnetic field (B(ind)) in the E12(2-) spherenes (E = Ge, Sn, Pb). We found that both electron delocalization descriptors are strongly affected by the relativistic corrections. For instance, for plumbaspherene, the difference in values from the nonrelativistic to the relativity-included calculation is almost 40 ppm! Our results show that the changes observed in the NICS and B(ind) values in the title cages are a consequence of the treatment of the relativistic effects. If these effects are included as scalar or spin-orbit calculations, then we can establish the next trend: Ge12(2-) is a nonaromatic species, Sn12(2-) is a low aromatic species, and Pb12(2-) is strongly aromatic, according to calculated NICS and B(ind) values. Thus, any prediction of electron delocalization in molecules containing heavy elements without considering an adequate treatment for relativistic effects may lead to an erroneous chemical interpretation.

A theoretical study of two series of small clusters, Si3On(-) and Si6On(-) (n = 1-6), has been carried out. The minimum energy structures were produced adding an electron to neutral species followed by relaxation at the B3LYP-6-311G(2d) level. The vertical ionization energies (VIEs) were computed using the electron propagator theory (EPT) in two approximations, Unrestricted Outer Valence Green Functions (UOVGF) and partial third-order approximation (P3). In the series Si3On(-) the theoretical VIEs of the minimum energy structures agree well with experimental data. For the second series there are not experimental VIEs, and the theoretical results are predictions. The performance of EPT methodologies in conjunction with all-electron or pseudopotentials (PP) calculations is analyzed. The conjunction of P3 and PP approximation proves to be the most efficient and economical methodology to calculate the VIEs of small anionic silicon oxide clusters. In the series Si6On(-) different channels of fragmentation have been calculated. The results suggest that the fragments do not have drastic geometric changes and the anionic fragment corresponds to the atoms where the spin density of the initial large cluster is localized. The Fukui function calculated over selected optimized fragments predicts adequately the interaction between them to form large stable clusters.

A recently proposed local Fukui function is used to predict the binding site of atomic hydrogen on silicon clusters. To validate the predictions, an extensive search for the more stable SinH (n=3-10) clusters has been done using a modified genetic algorithm. In all cases, the isomer predicted by the Fukui function is found by the search, but it is not always the most stable one. It is discussed that in the cases where the geometrical structure of the bare silicon cluster suffers a considerable change due to the addition of one hydrogen atom, the situation is more complicated and the relaxation effects should be considered.

A new method based on frontier orbital theory has been used to investigate the binding site of molecular oxygen to neutral and anion copper clusters. It has been shown that one can make useful predictions of the binding sites based on the knowledge of the donor local reactivity of the cluster using the condensed Fukui function, f(-)(Ff). In this way, it was found that Cu(3), Cu(5), and Cu(5)(-) have the highest reactivity toward molecular oxygen.