Search

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 Al42−, which is the first all‐metal molecule catalogued as aromatic and its non‐metallic isoelectronic analog, B42− were also analyzed. The antiaromatic clusters Al44− and B44− were studied in form of lithium salts. And the end, the non‐planar B12 boron cluster and its isoelectronic analogs (B11Be− and B11C+) 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. [ABSTRACT FROM AUTHOR]

Inspired by carbo-benzene and its inorganic analogues, in the current work, the viability of extended systems (called carbomers) formed from aromatic small rings was studied. The aluminum aromatic cluster, Al42−, and its isoelectronic carbon analogue, C42+, were employed as starting point. The insertion of alkynyl units into the Al-Al and C-C bonds results in the extended molecules named carbomers. These molecules were compared with the global minima structures, which were searched employing the genetic algorithm program, GEGA. The electronic delocalization (aromaticity) of the isomers was studied with the induced magnetic field (Bind). The results showed that global minimum of C122+ (formed from C42+) was an unexpected diatropic structure which presented a similar magnetic response to the C42+ cluster. Also, optical properties of C122+ were computed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this article, we employed theinduced magnetic field methodto show that the Al2X6(X = F, Cl, Br, I) clusterscannot be classified as aromatic systems. Interestingly, even nucleusindependent chemical shift (NICS) reveals the same conclusion whenanalyzed in greater detail, showing that a superficial analysis ofthis index can easily lead to incorrect interpretations. In view ofthe fact that the NICS index is extensively used by computationaland theoretically oriented experimental chemists, this is an importantwarning against superficial analyses, as it can lead to erroneouschemical interpretation. [ABSTRACT FROM AUTHOR]

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 Al4n–; molecular stars Si5Lin6–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. [ABSTRACT FROM AUTHOR]

Objective: To identify acute complications that occur during tracheal intubation and if the physician's experience is crucial in presenting them. Material and methods: An observational, transversal, descriptive, which included patients over 16 years of both sexes admitted to the Emergency Department from 1 September 2007 to January 31, 2008 with pathology requiring advanced management air. Results: A total of 150 intubation was realized, esophageal intubation main complication was 13.24%, where 52% of intubations were performed by residents. Conclusions: Esophageal intubation is a major acute complication in intubation, infl uencing the experience of the medical staff for their presentation. [ABSTRACT FROM AUTHOR]

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 π 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. [ABSTRACT FROM AUTHOR]

The authors discuss the role of the σ and π 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 σ electrons is short-ranged, the π 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 π electrons, they are shielded by the total induced magnetic field. The induced magnetic field of the π electrons support Pople's model on the basis of first-principles calculations. © 2006 Wiley Periodicals, Inc. J Comput Chem 2007 [ABSTRACT FROM AUTHOR]

In this work, the induced magnetic field is analyzed for a series of substituted benzenes dications with formula C6R62+ (R=I, At, SeH, SeCH3, TeH, TeCH3), 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. [ABSTRACT FROM AUTHOR]

Metal influence over triple-decker, sandwich-like and pyramidal structured benzenes was studied by means of Energy Decomposition Analysis (Morokuma–Ziegler), combined with Extended Transition State Natural Orbitals for Chemical Valence, finding that metal–ring bonding was a covalent contribution of about 60% due to the bonding interaction between dxz and dyz, dx2−y2 and dxy orbitals with pz orbitals, respectively, adapted by symmetry in the ring, to form π and δ bonding interactions. Finally, an important amount of electron density between the ring and the metal was found. This has a key role in the electron delocalization in this zone. This electronic delocalization was analysed via Induced Magnetic Field and Nucleus-Independent Chemical Shift calculations, finding a pattern between metal atomic radii and shielding tensor. Furthermore, similar behaviour for Mo and W, in the enhancement of the diatropic magnetic response, was displayed while Cr had a slightly lower diatropic character. [ABSTRACT FROM AUTHOR]

We have explored in detail the potential energy surfaces of the Si5Lin5-6 (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 Si5Li7+ has a perfect seven-peak star-like structure. The title compounds comprise, essentially, the Si56- 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 Si5 unit. This is confirmed by the magnetic properties, which show that electron delocalization enhances the stability of the star-like structures proposed here. [ABSTRACT FROM AUTHOR]

Abstract Here, we discuss the formation of several heterocyclic benzopyrylium compounds via joint experimental-theoretical efforts, in order to obtaining a better understanding of the diverse properties of these interesting organic species. Our results suggest these benzopyrylium dyes as interesting sensitizers in the novel design of new solar cells from the basis of their optical properties and pH-dependent characteristics. The role of aromatic stability of the pyrylium compounds in the obtained properties is discussed in terms of both NBO and analysis of the induced magnetic field toward the isotropic respond. Furthermore, optical properties were analyzed in the framework of TD-DFT methodologies. Noteworthy changes in red-shift and absorptions λ max are observed that contribute toward the interesting DSSCs applications. Additionally, the emission profile of 2a derivative was obtained in agreement to both theoretically and experimental strategy. We expect that our results can be informative for further development of heterocyclic benzopyrylium compounds and their application in DSSCs devices. Graphical abstract Image 1 Highlights • Formation of benzopyrylium compounds via joint experimental-theoretical efforts. • Benzopyrylium dyes as interesting sensitizers in the novel design of new solar cells. • Optical properties were analyzed in the framework of TD-DFT methodologies. • Aromatic stability in terms of NBO and induced magnetic field to isotropic respond. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, we report a simple experimental strategy for the one‐step electrochemical synthesis of nanogranular Cu2O films by copper anodization in fluoride‐containing ethylene glycol media. Microscopic exploration using field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM), shows the formation of spherical shape grains with sizes ranging from 20 to 40 nm. Raman and X‐Ray Photoemission Spectroscopy (XPS) results indicate that only CuI oxide is obtained. A band gap energy Eg=2.01 eV is estimated from UV–vis reflectance spectroscopy indicating that an indirect transition mechanism between semiconductor bands takes place. These evidences indicate that the present synthesis of nanogranular Cu2O films is a promising method for obtaining improved properties of materials for the design of photoelectronic devices. [ABSTRACT FROM AUTHOR]

The objective of the current work is to evaluate the effectiveness of the isoelectronic substitution (IS) principle on a series of complexes with the general formula OsCl2(SX3H3)(PH3)2, where X3 represents the moieties CCC, CCB, CCN, CBN, CNB or NCB, formed by substitution of the carbon atoms in CCC by either the isoelectronic B− or N+ separately, or by both. The SX3H3 moiety forms, together with Os, an aromatic five-membered ring (5-MR) called osmathiophene. The preservation of stability and aromaticity in the resulting systems is used to indicate the effectiveness of the IS principle. The aromaticity of the proposed molecules is analyzed according to the magnetic (induced magnetic field (Bind)) and electronic (through the multicenter index (MCI)) criteria. In addition a chemical bonding analysis on selected species is performed by the adaptive natural density partitioning (AdNDP) method. [ABSTRACT FROM AUTHOR]

The stability of the LinBnH2n (n = 3–6) series was analyzed using quantum chemical calculations, and it was found that cyclic isomers are not energetically favored. This is different to what happens in their organic counterparts (CnH2n), where cyclopentane (C5H10) and cyclohexane (C6H12) are the low-lying isomers. Apparently, aromaticity is a key-stabilizing factor that needs to be considered for designing stable lithium-boron hydride analogues of cyclic organic compounds. This is verified in the Li3B3H3+ system, which has been designed as the smallest aromatic carbocation (C3H3+) analogue. The global minimum structure of Li3B3H3+ contains a triangular B3H32− moiety, which has structural and chemical bonding features similar to its organic counterpart. Besides, this new cluster is classified as aromatic according to both the 4n + 2 Hückel rule and the analysis of the induced magnetic field. This theoretical evidence leads us to propose this cluster as a viable target for experimental detection in the gas phase. [ABSTRACT FROM AUTHOR]

A quasi-planar member of the so-called 'Wankel motor' family, B182-, is found. This boron cluster is an electronically stable dianion and a concentric doubly σ- and π-aromatic system. The inner B6 unit in B182- 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. [ABSTRACT FROM AUTHOR]

Abstract: Theoretical investigation on planar and rings has been performed. The aromaticity of the rings has been analyzed through their nucleus independent chemical shift (NICS) values as well as the σ- and π-contribution of the Z-component of the induced magnetic field. Counter to a previously reported result [Chem. Phys. Lett. 432 (2006) 331], the ring is found to be a local minimum and aromatic with planar D6h symmetry. The ring exhibits conflicting aromaticity like . Both the and rings can be stabilized in presence of suitable counter cations and systems like N6Ca2 and N4Li2 can trap molecular hydrogen. [Copyright &y& Elsevier]

Various isomers of the trigonal dianion metal clusters, X32-, 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. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [ABSTRACT FROM AUTHOR]

A series of planar tetracoordinate carbon molecules in cyclic semisaturated hydrocarbons resulting from the combination of the C52- 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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

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, CF3) and traveling (X = S, S = O, and CH2) 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Bowl-like structured B182- as the fifth member of the so-called 'Wankel motor' family is found. [ABSTRACT FROM AUTHOR]