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In the current work, we introduce a novel class of molecules termed carbo -metallabenzenes, and their aromaticity has been comprehensively analyzed. The molecules were strategically designed with the insertion of acetylene (C≡C or C 2 ) units in some selected metallabenzenes. Furthermore, if a second metallic unit is inserted (replacing a sp 2 carbon) in the carbo -metallabenzenes rings, a new family of carbo -mers is generated, and this second group has been named as carbo -dimetallabenzenes. The primary objective of this work is to ascertain, through various methodologies, whether these newly proposed molecules retain the aromatic characteristics observed in carbo -benzene. The methodologies employed for bond analysis and aromaticity exploration include the analysis of the molecular orbitals, energy decomposition analysis, electron density of delocalized bonds, magnetically induced current density, and the induced magnetic field ( B ind ). This study sheds light on that the insertion of the metallic centers reduces the electronic delocalization and their aromaticity is, in some cases, comparable with the electronic delocalization of the inorganic iminobora -borazine and also provides valuable insights into their electronic structure through a multifaceted analysis., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

A new algorithm called Automatic Johnson Cluster Generator (AJCG) is presented, which, as its name indicates, allows the definition of the desired Johnson polyhedron to subsequently carry out all the possible permutations between the atoms that form this polyhedron. This new algorithm allows the exhaustive study of the structures' potential energy surface (PES). In addition, the AJCG algorithm is helpful for the study of three-dimensional compounds such as boranes or Zintl clusters and their structural derivatives with two or more different atoms. The automatic filling of vertices is particularly useful in mixed compounds because of the possibility of taking into account all possible configurations in the structure. As a test system, we investigated the arachno -type E 6 M 2 4- (E = Si, Ge, Sn; M = Sb, Bi) structure which has eight vertices and complies with Wade-Mingos rules. Initially, we defined a bipyramidal structure (10 vertices), and filled the vertices with the atoms in all possible configurations. Since the selected system has eight atoms, the two remaining vertices were filled with pseudo atoms to complete the structure. After re-optimizing the initial population generated with AJCG, a large number of isomers with energy below 10 kcal mol -1 are identified. These results show that the most stable isomers possess homonuclear M-M bonds, except Sn 6 Bi 2 4- . Although the overall putative minima differ at the PBE0-D3 and DLPNO-CCSD(T) levels, they are always competitive minima. In addition to using high-precision methodologies to correctly study relative energies, applying solvent effects in highly charged systems becomes mandatory. The aromatic character of these studied systems was demonstrated qualitatively with two- and three-dimensional mapping and quantitatively by calculating the value of the z-component of the induced magnetic field at the cage center, including scalar and spin-orbit correction for relativistic effects. The compounds studied have a high degree of aromaticity, which allows us to establish that despite structural modifications ( i.e. , from closo to arachno ), the aromaticity is preserved., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Orbital contributions to the magnetic response depend on the method used to compute them. Here, we show that dissecting nuclear magnetic shielding tensors using natural localized molecular orbitals (NLMOs) leads to anomalous core contributions. The arbitrariness of the assignment might significantly affect the interpretation of the magnetic response of nonplanar molecules such as C 60 or [14]helicene and the assessment of their aromatic character. We solve this problem by computing the core- and σ-components of the induced magnetic field (and NICS) and the magnetically induced current density by removing the valence electrons (RVE). We estimate the core contributions to the magnetic response by performing calculations on the corresponding highly charged molecules, such as C 6 H 6 30+ for benzene, using gauge-including atomic orbitals and canonical molecular orbitals (CMOs). The orbital contributions to nuclear magnetic shielding tensors are usually estimated by employing a natural chemical shielding (NCS) analysis in NLMO or CMO bases. The RVE approach shows that the core contribution to the magnetic response is small and localized at the nuclei, contrary to what NCS calculations suggest. This may lead to a completely incorrect interpretation of the magnetic σ-orbital response of nonplanar structures, which may play a major role in the overall magnetic shielding of the system. The RVE approach is thus a simple and inexpensive way to determine the magnetic response of the core- and σ-electrons.

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., (© 2021 Wiley Periodicals LLC.)

In the current work, the analysis of the electronic delocalization of some metallacycles, based on borazine, was realized by employing magnetic criteria, such as the induced magnetic field and magnetically induced current densities, and electronic criteria, such as adaptative natural density partitioning and the analysis of molecular orbitals. The current metallaborazines were generated from isoelectronic substitutions. The main question is whether the electronic delocalization increases or decreases. The results showed that metal-N bonded borazines could be cataloged as delocalized compounds. On the other hand, the metal-B bonded borazines could be cataloged as nonaromatic (or weak aromatic) compounds based on the results of this analysis., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

In the current work, some metallabenzenes with one and several fused rings were analyzed in terms of their electronic delocalization. These fused-ring metallabenzenes are known as metallabenzenoids, and their aromatic character is not free of controversy. The systems of the current work were designed from crystallographic data of some synthesized molecules, and their electronic delocalization (aromaticity) was computationally examined in terms of the molecular orbital analysis (Hückel's rule), the induced magnetic field, and ring currents. The computational evidence allows us to understand if these molecules are or are not aromatic compounds., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

In the current work, a new family of isoelectronic analogues to B12 is reported. The construction of this family was performed through the isoelectronic substitution principle to generate species such as B11C+, B11Be-, B10BeC, B10C22+, B10Be22- B9Be2C-, and B9BeC2+. The search for the global minimum was realized by utilizing genetic algorithms, while the induced magnetic field, electronic localization function, magnetic current densities, and multicenter aromaticity criteria were calculated to understand their electronic delocalization. Our results show that, in general, C atoms avoid hypercoordination, whereas we have found species with Be atoms located in hypercoordinated positions that are relatively stable. Our analysis of aromaticity indicates that B12 has double σ and π disk aromaticity. Mono, double or triple substitution of B by C+ or Be- reduces somewhat the aromaticity of the clusters, but less in the case of Be- substitution.

In this work, the induced magnetic field is analyzed for a series of substituted benzenes dications with formula C 6 R 6 2+ (R=I, At, SeH, SeCH 3 , TeH, TeCH 3 ), presumably exhibiting concentric aromaticity. Previous studies concluded that in the carbon skeleton, just π-electrons are delocalized. However, our results support that both the σ- and π-electrons are delocalized in the carbon skeleton, combined with a σ-delocalization in the external ring. The role of the relativistic effects in these dications is discussed in detail., Competing Interests: The authors declare no conflict of interest., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

The mechanism for the walk rearrangement in Dewar thiophenes has been clarified theoretically by studying the evolution of chemical bonds along the intrinsic reaction coordinates. Substituent effects on the overall mechanism are assessed by using combinations of the ring (R = H, CF 3 ) and traveling (X = S, S = O, and CH 2 ) groups. The origins of fluxionality in the S-oxide of perfluorotetramethyl Dewar thiophene are uncovered in this work. Dewar rearrangements are chemical processes that occur with a high degree of synchronicity. These changes are directly related to the activation energy.

The potential energy surfaces (PESs) of Lin(BH)5(n-6) systems (where n = 5, 6, and 7) were explored using the gradient embedded genetic algorithm (GEGA) program, in order to find their global minima conformations. This search predicts that the lowest-energy isomers of Li6(BH)5 and Li7(BH)5(+) contain a (BH)5(6-) pentagonal fragment, which is isoelectronic and structurally analogous to the prototypical aromatic hydrocarbon anion C5H5(-). Li7(BH)5(+), along with Li7C5(+), Li7Si5(+) and Li7Ge5(+), joins a select group of clusters that adopt a seven-peak star-shape geometry, which is favored by aromaticity in the central five-membered ring, and by the preference of Li atoms for bridging positions. The theoretical analysis of chemical bonding, based on magnetic criteria, supports the notion that electronic delocalization is an important stabilization factor in all these star-shaped clusters.

The global minimum structure of borospherene (B40) is a cage, comprising two hexagonal and four heptagonal rings. Born-Oppenheimer Molecular Dynamics simulations show that continuous conversions in between six and seven membered rings take place. The activation energy barrier for such a transformation is found to be 14.3 kcal · mol(-1). The completely delocalized σ- and π-frameworks, as well as the conservation of the bonding pattern during rearrangement, facilitate the dynamical behavior of B40. B40 is predicted to act as a support-free spherical two-dimensional liquid at moderate temperature. In other words, B40 could be called as a nanobubble.

The potential energy surfaces of a series of clusters with the formula CAl3E (E = P, As, Sb, Bi) are systematically explored using density functional theory and high level ab initio calculations. The global minimum structure of these clusters contains a planar tetracoordinate carbon atom. The presence of a C=E double bond is supported by the Wiberg bond indices, the adaptive natural density partitioning analysis, and the magnetic response. Our results show that these planar tetracoordinate carbon clusters are both thermodynamically and kinetically viable species in the gas phase.

The potential energy surfaces of a series of clusters with formula CBe5Lin(n-4) (n = 1 to 5) have been systematically explored. Our computations show that the lithium cations preserve the CBe5(4-) pentagon, such that the global minimum structure for these series of clusters has a planar pentacoordinate carbon (ppC) atom. The systems are primarily connected via a network of multicenter σ-bonds, in which the C atom acts as σ-acceptor and this acceptance of charge is balanced by the donation of the 2pz electrons to the π-cloud. The induced magnetic field analysis suggests that the clusters with formula CBe5Lin(n-4) (n = 1 to 5) are fully delocalized. The fact that these ppC-containing clusters are the lowest-energy forms on the corresponding potential energy surfaces raises expectations that these species can be prepared experimentally in the gas phase.

The B19(-) anion and other boron species have been dubbed 'Wankel motors' for the almost barrierless rotation of inner and outer concentric rings relative to each other in these compounds. A single substitution in B19(-) is shown to shut down the well-established fluxionality in the anion. A carbon atom substituted in the structure to give a neutral CB18 species is shown computationally to enforce bond localization.

A quasi-planar member of the so-called 'Wankel motor' family, B18(2-), is found. This boron cluster is an electronically stable dianion and a concentric doubly σ- and π-aromatic system. The inner B6 unit in B18(2-) undergoes quasi-free rotation inside the perimeter of the B12 ring. The absence of any localized σ-bond between the inner ring and the peripheral boron atoms makes the system fluxional.

Inspired by their geometrical perfection, intrinsic beauty, and particular properties of polyhedranes, a series of carbo-cages is proposed in silico via density functional theory computations. The insertion of alkynyl units into the C-C bonds of polyhedranes results in a drastic lowering of the structural strain. The induced magnetic field shows a significant delocalization around the three-membered rings. For larger rings, the response is paratropic or close to zero, suggesting a nonaromatic behavior. In the carbo-counterparts, the values of the magnetic response are shifted with respect to their parent compounds, but the aromatic/nonaromatic character remains unaltered. Finally, Born-Oppenheimer molecular dynamics simulations at 900 K do not show any drastic structural changes up to 10 ps. In the particular case of a carbo-prismane, no structural change is perceived until 2400 K. Therefore, although carbo-cages have enthalpies of formation 1 order of magnitude higher than those of their parent compounds, their future preparation and isolation should not be discarded, because the systems are kinetically stable, explaining why the similar systems like carbo-cubane have already been synthesized.

In this article, we employed the induced magnetic field method to show that the Al2X6 (X = F, Cl, Br, I) clusters cannot be classified as aromatic systems. Interestingly, even nucleus independent chemical shift (NICS) reveals the same conclusion when analyzed in greater detail, showing that a superficial analysis of this index can easily lead to incorrect interpretations. In view of the fact that the NICS index is extensively used by computational and theoretically oriented experimental chemists, this is an important warning against superficial analyses, as it can lead to erroneous chemical interpretation.

Searches for planar hexacoordinate carbon (phC) species comprised of only seven atoms uncovered good CX(3)M(3) prototypes, D(3h) CN(3)Be(3)(+) and CO(3)Li(3)(+). The latter is the global minimum. It might also be possible to detect the deep-lying kinetically-viable D(3h) CN(3)Be(3)(+) local minimum, based on its robustness toward molecular dynamic simulations and its very high isomerization barrier.

Al(4)(2-) was the first discovered σ + π aromatic all-metal cluster. In the present work we analyze the molecular structure, relative stability, and aromaticity of lowest-lying isomers of related M(2)N(2)(2-) (M and N = B, Al, and Ga) clusters, with special emphasis devoted to the cis (C(2v)) and trans (D(2h)) isomers of the M(2)N(2)(2-) clusters. For such purpose, we start by performing the search of the global minimum for each cluster through the Gradient Embedded Genetic Algorithm (GEGA). Energy decomposition analyses and the calculated magnetic- and electronic-based aromaticity criteria of the lowest-lying isomers help to understand the nature of the bonding and the origin of the stability of the global minima. Such methodology should allow guiding future molecular design strategies.

Aromaticity is indispensable for explaining a variety of chemical behaviors, including reactivity, structural features, relative energetic stabilities, and spectroscopic properties. When interpreted as the spatial delocalization of π-electrons, it represents the driving force for the stabilization of many planar molecular structures. A delocalized electron system is sensitive to an external magnetic field; it responds with an induced magnetic field having a particularly long range. The shape of the induced magnetic field reflects the size and strength of the system of delocalized electrons and can have a large influence on neighboring molecules. In 2004, we proposed using the induced magnetic field as a means of estimating the degree of electron delocalization and aromaticity in planar as well as in nonplanar molecules. We have since tested the method on aromatic, antiaromatic, and nonaromatic compounds, and a refinement now allows the individual treatment of core-, σ-, and π-electrons. In this Account, we describe the use of the induced magnetic field as an analytical probe for electron delocalization and its application to a large series of uncommon molecules. The compounds include borazine; all-metal aromatic systems Al(4)(n-); molecular stars Si(5)Li(n)(6-n); electronically stabilized planar tetracoordinate carbon; planar hypercoordinate atoms inside boron wheels; and planar boron wheels with fluxional internal boron cluster moieties. In all cases, we have observed that planar structures show a high degree of electron delocalization in the π-electrons and, in some examples, also in the σ-framework. Quantitatively, the induced magnetic field has contributions from the entire electronic system of a molecule, but at long range the contributions arising from the delocalized electronic π-system dominate. The induced magnetic field can only indirectly be confirmed by experiment, for example, through intermolecular contributions to NMR chemical shifts. We show that calculating the induced field is a useful method for understanding any planar organic or inorganic system, as it corresponds to the intuitive Pople model for explaining the anomalous proton chemical shifts in aromatic molecules. Indeed, aromatic, antiaromatic, and nonaromatic molecules show differing responses to an external field; that is, they reduce, augment, or do not affect the external field at long range. The induced field can be dissected into different orbital contributions, in the same way that the nucleus-independent chemical shift or the shielding function can be separated into component contributions. The result is a versatile tool that is particularly useful in the analysis of planar, densely packed systems with strong orbital contributions directly atop individual atoms.

Is it possible to achieve molecules with starlike structures by replacing the H atoms in (CH)(n)(q) aromatic hydrocarbons with aluminum atoms in bridging positions? Although D(4h) C(4)Al(4)(2-) and D(2) C(6)Al(6) are not good prospects for experimental realization, a very extensive computational survey of fifty C(5)Al(5)(-) isomers identified the starlike D(5h) global minimum with five planar tetracoordinate carbon atoms to be a promising candidate for detection by photoelectron detachment spectroscopy. BOMD (Born-Oppenheimer molecular dynamics) simulations and high-level theoretical computations verified this conclusion. The combination of favorable electronic and geometric structural features (including aromaticity and optimum C-Al-C bridge bonding) stabilizes the C(5)Al(5)(-) star preferentially., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Theoretical evidence for the first neutral and anionic global-minimum structures possessing a planar pentacoordinate carbon is reported.

Various isomers of the trigonal dianion metal clusters, X(3)(2-), X = Be, Mg, Ca, and their mono- and disodium complexes are optimized at the B3LYP/6-311+G(d) level. Different conceptual density functional theory based reactivity descriptors as well as the induced magnetic field values are calculated to understand the stability and aromaticity of these systems. Possibility of bond stretch isomerism is explored. Genetic algorithm results lend additional insights into the structures of these isomers., (Copyright 2009 Wiley Periodicals, Inc.)

We have explored in detail the potential energy surfaces of the Si(5)Li(n)(5-6) (n = 5-7) systems. We found that it is feasible to design three-dimensional star-like silicon structures using the appropriate ligands. The global minimum structure for Si(5)Li(7)(+) has a perfect seven-peak star-like structure. The title compounds comprise, essentially, the Si(5)(6-) ring interacting with lithium cations. The ionic character of the Si-Li interactions induces the formation of a bridged structure. Concomitantly, our calculations show that the reduction of the Pauli repulsion and the maximization of the orbital contribution are also significant for the star-like structure formation. Additionally, the MO analysis of the systems suggests that the role of the lithium atoms is to provide the precise number of electrons to the central Si(5) unit. This is confirmed by the magnetic properties, which show that electron delocalization enhances the stability of the star-like structures proposed here.

A series of planar tetracoordinate carbon molecules in cyclic semisaturated hydrocarbons resulting from the combination of the C5(2-) skeleton with saturated hydrocarbon fragments is reported. The electronic stabilization and the bonding situation are studied through the analyses of molecular orbitals and the electron localization function. The magnetic properties are also revised, giving particular attention to the induced magnetic field. These systems are the first semisaturated cycles containing a planar tetracoordinate carbon stabilized only by electronic factors.

Sets of boron rings enclosing planar hypercoordinate group 14 elements (ABn(n-8); A = group 14 element; n = 6-10) are designed systematically based on geometrical and electronic fit principles: the size of a boron ring must accommodate the central atom comfortably. The electronic structures of the planar minima with hypercoordinate group 14 elements are doubly aromatic with six pi and six in-plane radial MO systems (radial MOs are comprised of boron p orbitals pointing toward the ring center). This is confirmed by induced magnetic field and nucleus-independent chemical shift (NICS) computations. The weakness of the "partial" A-B bonds is compensated by their unusually large number. Although a C7v pyramidal SiB8 structure is more stable than the D8h isomer, Born-Oppenheimer molecular dynamics simulations show the resistance of the D8h local minimum against deformation and isomerization. Such evidence of the viability of the boron ring minima with group 14 elements encourages experimental realization.

Structure, dynamics, and electron delocalization of Al4(2-) and Al4(4-) based clusters are investigated. Gradient-corrected Density-Functional Born-Oppenheimer Molecular Dynamics simulations indicate that Al4(2-) based clusters have a rigid planar Al framework, while the Al4(4-) based moieties show large distortions from planarity. The induced magnetic field analysis of these species indicates that both systems have diatropic σ-systems, while the π-system is diatropic for Al4(2-) and paratropic for Al4(4-). The total magnetic response is diatropic for Al4(2-), while Al4(4-) is "bitropic":  it has typical antiaromatic long-range cones, while the magnetic field in the Al4(4-) ring plane is similar to that of aromatic annulenes.

The authors discuss the role of the sigma and pi contributions to the induced magnetic field for simple hydrocarbons containing a double or a triple bond, as well as for benzene and cyclobutadiene. While the magnetic field induced by the sigma electrons is short-ranged, the pi system is responsible for the formation of long-range cones. These cones influence the chemical shift of atoms by additional shielding (for aromatic) or deshielding (for antiaromatic molecules) contributions. While the hydrogen atoms of benzene are found to lie within the deshielded region of the magnetic field induced by the pi electrons, they are shielded by the total induced magnetic field. The induced magnetic field of the pi electrons support Pople's model on the basis of first-principles calculations., (Copyright (c) 2006 Wiley Periodicals, Inc.)