Search

The potential energy surfaces of the [Si.sub.5][Li.sub.n.sup.5-6] (n = 5?7) systems were explored in detail to find feasible way to design three-dimensional star-like silicon structures using the appropriate ligands. The calculation results revealed that the important contribution of ionic character of the Si-Li interactions to induce the formation of a bridged structure and the contribution of reduction of the Pauli repulsion and the maximization of the orbital towards formation of star-like structure.

The [C.sub.5.sup.2-] skeleton in combination with saturated hydrocarbon fragments is used to obtain a series of planar tetracoordinate carbon molecules in cyclic semisaturated hydrocarbons. The system is the first example of semisaturated cycles containing a planar tetracoordinate carbon stabilized only by electronic factors.

The article discusses the designing of different sets of boron rings enclosing the planar hypercoordinate group 14 elements on the basis of the geometrical and electronic fit principles. The set is designed in a way that the boron ring can accommodate the central atom easily.

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]

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.)

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.