Anisotropy Of Current-induced Density Research Articles

Four-membered heterocyclic molecules featuring boron and heavy group 14 elements that exhibit both σ-aromatic and π-aromatic properties: a new synthetic target.

In this study, we present a series of theoretically designed B2G14G14' molecules, featuring four-membered-ring heterocycles containing boron and heavy group 14 elements (G14 and G14' = Si, Ge, Sn, and Pb). Through the use of density functional theory (DFT), natural bond orbital (NBO) analysis, quantum theory of atoms in molecules (QTAIM), and electron localization function (ELF), our studies demonstrate a strong π single bond between the bridgehead G14 and G14' atoms, with minimal participation from a very weak G14-G14' σ bond. Additionally, the nucleus independent chemical shift (NICS), anisotropy of current-induced density (ACID), and adaptive natural density partitioning (AdNDP) analyses definitively establish the presence of both σ-aromaticity and π-aromaticity in these inorganic four-membered heterocyclic neutral molecules.

Planar Cyclopenten-4-yl Cations: Highly Delocalized π Aromatics Stabilized by Hyperconjugation.

Theoretical studies predicted the planar cyclopenten‐4‐yl cation to be a classical carbocation, and the highest‐energy isomer of C5H7 +. Hence, its existence has not been verified experimentally so far. We were now able to isolate two stable derivatives of the cyclopenten‐4‐yl cation by reaction of bulky alanes CpRAlBr2 with AlBr3. Elucidation of their (electronic) structures by X‐ray diffraction and quantum chemistry studies revealed planar geometries and strong hyperconjugation interactions primarily from the C−Al σ bonds to the empty p orbital of the cationic sp2 carbon center. A close inspection of the molecular orbitals (MOs) and of the anisotropy of current (induced) density (ACID), as well as the evaluation of various aromaticity descriptors indicated distinct aromaticity for these cyclopenten‐4‐yl derivatives, which strongly contrasts the classical description of this system. Here, strong delocalization of π electrons spanning the whole carbocycle has been verified, thus providing rare examples of π aromaticity involving saturated sp3 carbon atoms.

Dithieno-Fused Polycyclic Aromatic Hydrocarbon with a Pyracylene Moiety: Strong Antiaromatic Contribution to the Electronic Structure.

A synthetic route to dithieno-fused CP-PAHs with a pyracylene segment is reported. A combination of experimental and theoretical studies revealed a strong contribution of antiaromatic character to the electronic structure of this dithieno-fused CP-PAH. Anisotropy of current-induced density (ACID) calculations indicated a significantly increased paramagnetic ring current on the two pentagonal rings, which is more prominent than that of the dibenzo-fused analogue. Furthermore, enhanced electron affinity and a consequently decreased HOMO-LUMO gap were observed for this dithieno-fused CP-PAH.

Aromaticity in Pericyclic Transition State Structures? A Critical Rationalisation Based on the Topological Analysis of Electron Density

AbstractThe nature of the electron delocalisation pattern within a cyclic structure, i. e. the aromatic character, is examined for six‐membered pseudocyclic transition state structures (TSs) involved in five representative examples of so‐called pericyclic reactions. Results of the electron localisation function (ELF) and the quantum theory of atoms in molecules (QTAIM) analyses of the electron density evidence that in four of the cases, at least one pair of atoms are not bound at the TS configuration, thus precluding a possible cyclic conjugation. These findings make it possible to rule out the aromatic character of these TSs. High values of the synchronicity Sy index at the TSs contrast with the bonding changes evidenced by ELF topological analysis. The magnitude of the nucleus independent chemical shift (NICS) computed for the five TSs becomes much more negative than that of the reference system, benzene, with no obvious relation to the strong evidence of the pattern of delocalisation revealed by the topological analysis of ELF and QTAIM for each TS. The topology of the anisotropy of current induced density (ACID) at each TS reveals the actual existence of strong non‐symmetrical patterns of electron delocalisation of the five TSs.