Themed collection Electron delocalization and aromaticity: 150 years of the Kekulé benzene structure

This themed issue presents a collection of articles on ‘Electron delocalization and aromaticity: Celebrating the 150th Anniversary of the Kekulé Benzene Structure’.

We describe joint experimental and theoretical studies carried out collaboratively in the authors' labs for understanding the structures and chemical bonding of novel atomic clusters, which exhibit aromaticity.

The cluster B₄₂⁺ exhibits an aromatic cage-like structure containing remarkable octagonal holes.

The synthesis and characterisation of pyrene-fused tetraazaheptacene that is constituted of two terminal pyrene units and a central tetraazaanthracene core are reported.

We present the viability of an endohedral C_s Ca@B₃₈ which contains a C_s B₃₈²⁻ borospherene cage composed of interwoven boron double chains with a σ + π double delocalization bonding pattern, extending the B_n^q (q = n − 40) borospherene family from n = 39–42 to n = 38.

Photocyclization play a key role in the deactivation mechanism of tetraphenylethylene.

DFT calculations suggest that the feasible ligands of a 3D sandwich complex should have locally delocalized π electrons.

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability.

The aromatic pathways of carbaporphyrins and carbachlorins that are based on magnetically induced current density DFT-GIMIC calculations are presented and discussed.

High-level CASSCF calculations, supplemented with MCQDPT ab initio calculations, demonstrate that the [E(μ-XH)]₂ (E = P, As; X = N, P, As) compounds possess one planar and one butterfly-like isomers.

Alkaline earth metals (Ca, Sr, and Ba) can form inverted sandwich compounds with C₆H₆, C₇H₇⁺, and C₈H₈²⁺ of D_nh symmetry holding planar 10-π-electron aromatic cores.

The molecular structure and vibrational properties of perchlorinated HBC and the parent HBC have been investigated by density functional theory calculations and vibrational spectroscopy.

A series of eight Ru(II) sensitizers incorporating thienylenevinylene-functionalised bipyridines as ancillary ligand is reported.

We introduce a new electronic aromaticity index, AV1245, consisting of an average of the 4-center indices along the ring that keep a positional relationship of 1, 2, 4, 5.

Triggering Hückel–Möbius topological and aromaticity switches in octaphyrins by protonation and redox reactions.

Magnetic resonance energy (MRE) derived from the ring-current diamagnetic susceptibility was examined in conjunction with topological resonance energy (TRE).

Computations show that the enhanced π–π repulsion in nitrobenzene, contributed by both the electrostatic interaction and the Pauli exchange, is responsible for the stretched C–N bond in nitrobenzene (1.486 Å) compared with the C–N bond in aniline (1.407 Å).

Based on the experimental data of newly determined structures of per-substituted naphthalenes by halogen atoms (F, Cl and Br) and perchloroanthracene, as well as on the molecular modeling we have shown that deviation from planarity leads to relatively small changes in the cyclic π-electron delocalization of acenes.

The length of the conjugated double bond chain in DANIR dyes modulates the charge transfer character, non-radiative deactivation pathways and affinity for amyloid-β fibril.

In recent years, expanded porphyrins have emerged as a promising class of π-conjugated switches whose conductance is studied from the electron density.

Molecular orbitals of aniline explain electrophilic substitution, whereas for nitrobenzene charge rearrangements are needed.

Electron delocalization allows us to study the similarity and aromaticity of PAHs in excited states, and can be correlated with the excitation energies.

The π-electron diatropic ring current emerging throughout the trimerization of acetylene to benzene.

Herein we report a series of porphyrin and methano[60]fullerene that undergo self-assembly.

We examine in this contribution the possible relation between the spatial decay rate of real space delocalization measures and the insulating- or metallic-like character of molecular and extended systems.

Conservation of aromaticity of 6-membered rings along with vanishing anti-aromatic character of central 5-membered ring was found to be the main reason for exceptional stability of hub-isomer. Functionalization of corannulene moiety at rim- or spoke-site resulted in dramatic elimination of aromaticity of 6-membered rings thus resulting to dramatic reduce of their stability.

Benzenoid perimeter currents within the Randić conjugated-circuit model follow directly from a simple calculation of Pauling bond orders and Kekulé count.

A definition of ring currents in a velocity vector field is proposed according to topological criteria: ring currents are axial vortices confined in, or rotating beyond, a separatrix, i.e., the boundary which marks the limits of the vortex.

It is shown that no unique current density topology can be obtained from (sets of) NICS values. Therefore, the use of NICS indices as aromaticity indices without prior analysis of the current density map is strongly discouraged.

The structure, stability, bonding and π-aromaticity in novel cyclic all-pnictogen heterocyclic anions, P₂N₃⁻ and P₃N₂⁻, and in their heavier analogues are studied using quantum mechanical computations.

The CpM units in the biicosahedral CpNiB₂₀H₁₇ and CpCoCB₁₉H₁₇ structures energetically prefer to be located at a meta vertex of the biicosahedron. Similar biicosahedral CpFeB₂₀H₁₇ structures are found with an agostic hydrogen atom bridging an Fe–B edge.

The solid-state structure of centrohexaindane, 1, and the sixfold nitration of this three-dimensional polycyclic and topologically nonplanar hydrocarbon is reported.

Energy, bond length and electron delocalization are connected within the context of quantum chemical topology theories.

Quantum chemical modeling was used to investigate the electron-donating properties of the amino group in a series of meta- and para-X-substituted anilines (X = NMe₂, NH₂, OH, OMe, CH₃, H, F, Cl, CF₃, CN, CHO, COMe, CONH₂, COOH, NO₂, and NO).

Octahedral aromaticity was found in most clusters of formula X₆^q (X = Li–C and Be–Si) with q = −2 to +4 and spin states ranging from the singlet to the septet that have electronic configurations of closed-shells or open shells half-filled with the same spin electrons.

The structure, stability, bonding and σ-aromaticity in dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gas and carbon monoxide bound M₃⁺ (M = Cu, Ag, Au) complexes are analyzed.

The gold(I)-catalysed divinylcyclopropane–cycloheptadiene rearrangement has been studied computationally within the Density Functional Theory framework.

Linear versus cyclic π-electron conjugation is discussed in oligoparaphenylenes.

Of the three lowest isomers of C₅₀ fullerene, the minimal energy D₃ isomer comes closest to a spherical aromat or superaromat.

Transition-metal hydrides are essential for catalysis, organic synthesis, and hydrogen storage. In this work we study IrH₅(PPh₃)₂, (RuH₅(PⁱPr₃)₂)⁻, (OsH₅(PⁱPr₃)₂)⁻, and OsH₄(PPhMe₂)₃ polyhydride complexes, where the metal is five-fold coordinated in-plane. The unusual coordination of these compounds can be explained by σ-aromaticity.