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

40 items
Editorial

Celebrating the 150th anniversary 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’.

Graphical abstract: Celebrating the 150th anniversary of the Kekulé benzene structure
Perspective

Beyond organic chemistry: aromaticity in atomic clusters

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.

Graphical abstract: Beyond organic chemistry: aromaticity in atomic clusters
From the themed collection: PCCP Perspectives
Communication

Aromatic cages B0/+42: unprecedented existence of octagonal holes in boron clusters

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

Graphical abstract: Aromatic cages B0/+42: unprecedented existence of octagonal holes in boron clusters
Communication

Bis(triisopropylsilylethynyl)-substituted pyrene-fused tetraazaheptacene: synthesis and properties

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

Graphical abstract: Bis(triisopropylsilylethynyl)-substituted pyrene-fused tetraazaheptacene: synthesis and properties
Communication

Endohedral Ca@B38: stabilization of a B382− borospherene dianion by metal encapsulation

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

Graphical abstract: Endohedral Ca@B38: stabilization of a B382− borospherene dianion by metal encapsulation
Communication

How does tetraphenylethylene relax from its excited states?

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

Graphical abstract: How does tetraphenylethylene relax from its excited states?
Paper

The degree of π electron delocalization and the formation of 3D-extensible sandwich structures

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

Graphical abstract: The degree of π electron delocalization and the formation of 3D-extensible sandwich structures
Paper

Aromatic character of planar boron-based clusters revisited by ring current calculations

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

Graphical abstract: Aromatic character of planar boron-based clusters revisited by ring current calculations
Open Access Paper

New insights into aromatic pathways of carbachlorins and carbaporphyrins based on calculations of magnetically induced current densities

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

Graphical abstract: New insights into aromatic pathways of carbachlorins and carbaporphyrins based on calculations of magnetically induced current densities
Paper

The stability of biradicaloid versus closed-shell [E(μ-XR)]2 (E = P, As; X = N, P, As) rings. Does aromaticity play a role?

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.

Graphical abstract: The stability of biradicaloid versus closed-shell [E(μ-XR)]2 (E = P, As; X = N, P, As) rings. Does aromaticity play a role?
Paper

10-π-Electron arenes à la carte: structure and bonding of the [E–(CnHn)–E]n−6 (E = Ca, Sr, Ba; n = 6–8) complexes

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

Graphical abstract: 10-π-Electron arenes à la carte: structure and bonding of the [E–(CnHn)–E]n−6 (E = Ca, Sr, Ba; n = 6–8) complexes
Open Access Paper

Edge chlorination of hexa-peri-hexabenzocoronene investigated by density functional theory and vibrational spectroscopy

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

Graphical abstract: Edge chlorination of hexa-peri-hexabenzocoronene investigated by density functional theory and vibrational spectroscopy
Paper

Heteroleptic Ru(II)-bipyridine complexes based on hexylthioether-, hexyloxy- and hexyl-substituted thienylenevinylenes and their application in dye-sensitized solar cells

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

Graphical abstract: Heteroleptic Ru(ii)-bipyridine complexes based on hexylthioether-, hexyloxy- and hexyl-substituted thienylenevinylenes and their application in dye-sensitized solar cells
Paper

An electronic aromaticity index for large rings

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.

Graphical abstract: An electronic aromaticity index for large rings
Paper

Understanding the molecular switching properties of octaphyrins

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

Graphical abstract: Understanding the molecular switching properties of octaphyrins
Paper

Magnetic resonance energy and topological resonance energy

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

Graphical abstract: Magnetic resonance energy and topological resonance energy
Paper

Electron conjugation versus π–π repulsion in substituted benzenes: why the carbon–nitrogen bond in nitrobenzene is longer than in aniline

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).

Graphical abstract: Electron conjugation versus π–π repulsion in substituted benzenes: why the carbon–nitrogen bond in nitrobenzene is longer than in aniline
Paper

Cyclic π-electron delocalization in non-planar linear acenes

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.

Graphical abstract: Cyclic π-electron delocalization in non-planar linear acenes
Paper

Computational study on donor–acceptor optical markers for Alzheimer's disease: a game of charge transfer and electron delocalization

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.

Graphical abstract: Computational study on donor–acceptor optical markers for Alzheimer's disease: a game of charge transfer and electron delocalization
Paper

Understanding conductivity in molecular switches: a real space approach in octaphyrins

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

Graphical abstract: Understanding conductivity in molecular switches: a real space approach in octaphyrins
Open Access Paper

How amino and nitro substituents direct electrophilic aromatic substitution in benzene: an explanation with Kohn–Sham molecular orbital theory and Voronoi deformation density analysis

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

Graphical abstract: How amino and nitro substituents direct electrophilic aromatic substitution in benzene: an explanation with Kohn–Sham molecular orbital theory and Voronoi deformation density analysis
From the themed collection: 2016 most accessed PCCP articles
Paper

Electron delocalization and electron density of small polycyclic aromatic hydrocarbons in singlet excited states

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

Graphical abstract: Electron delocalization and electron density of small polycyclic aromatic hydrocarbons in singlet excited states
Paper

The making of ring currents

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

Graphical abstract: The making of ring currents
Paper

[60]Fullerene–porphyrin [n]pseudorotaxanes: self-assembly, photophysics and third-order NLO response

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

Graphical abstract: [60]Fullerene–porphyrin [n]pseudorotaxanes: self-assembly, photophysics and third-order NLO response
Paper

Decay rate of real space delocalization measures: a comparison between analytical and test systems

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.

Graphical abstract: Decay rate of real space delocalization measures: a comparison between analytical and test systems
Paper

Aromatic stabilization of functionalized corannulene cations

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.

Graphical abstract: Aromatic stabilization of functionalized corannulene cations
Paper

Perimeter ring currents in benzenoids from Pauling bond orders

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

Graphical abstract: Perimeter ring currents in benzenoids from Pauling bond orders
Paper

Topological definition of ring currents

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.

Graphical abstract: Topological definition of ring currents
Paper

Can the current density map topology be extracted from the nucleus independent chemical shifts?

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.

Graphical abstract: Can the current density map topology be extracted from the nucleus independent chemical shifts?
Paper

Viability of aromatic all-pnictogen anions

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.

Graphical abstract: Viability of aromatic all-pnictogen anions
Paper

Biicosahedral metallaboranes: aromaticity in metal derivatives of three-dimensional analogues of naphthalene

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.

Graphical abstract: Biicosahedral metallaboranes: aromaticity in metal derivatives of three-dimensional analogues of naphthalene
Paper

Centrohexaindane: six benzene rings mutually fixed in three dimensions – solid-state structure and six-fold nitration

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

Graphical abstract: Centrohexaindane: six benzene rings mutually fixed in three dimensions – solid-state structure and six-fold nitration
Paper

Unification of ground-state aromaticity criteria – structure, electron delocalization, and energy – in light of the quantum chemical topology

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

Graphical abstract: Unification of ground-state aromaticity criteria – structure, electron delocalization, and energy – in light of the quantum chemical topology
Paper

Towards physical interpretation of substituent effects: the case of meta- and para-substituted anilines

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).

Graphical abstract: Towards physical interpretation of substituent effects: the case of meta- and para-substituted anilines
Paper

Octahedral aromaticity in 2S+1A1g X6q clusters (X = Li–C and Be–Si, S = 0–3, and q = −2 to +4)

Octahedral aromaticity was found in most clusters of formula X6q (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.

Graphical abstract: Octahedral aromaticity in 2S+1A1g X6q clusters (X = Li–C and Be–Si, S = 0–3, and q = −2 to +4)
Paper

σ-Aromatic cyclic M3+ (M = Cu, Ag, Au) clusters and their complexation with dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gases and carbon monoxide

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.

Graphical abstract: σ-Aromatic cyclic M3+ (M = Cu, Ag, Au) clusters and their complexation with dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gases and carbon monoxide
Paper

The effect of the metal fragment on the aromaticity and synchronicity of the gold(I)-catalysed divinylcyclopropane–cycloheptadiene rearrangement

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

Graphical abstract: The effect of the metal fragment on the aromaticity and synchronicity of the gold(i)-catalysed divinylcyclopropane–cycloheptadiene rearrangement
Paper

From linear to cyclic oligoparaphenylenes: electronic and molecular changes traced in the vibrational Raman spectra and reformulation of the bond length alternation pattern

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

Graphical abstract: From linear to cyclic oligoparaphenylenes: electronic and molecular changes traced in the vibrational Raman spectra and reformulation of the bond length alternation pattern
Open Access Paper

Is C50 a superaromat? Evidence from electronic structure and ring current calculations

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

Graphical abstract: Is C50 a superaromat? Evidence from electronic structure and ring current calculations
Paper

σ-Aromaticity in polyhydride complexes of Ru, Ir, Os, and Pt

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.

Graphical abstract: σ-Aromaticity in polyhydride complexes of Ru, Ir, Os, and Pt
40 items

About this collection

Guest Edited by Gabriel Merino and Miquel Solà this collection of articles celebrates the 150th anniversary of the seminal paper ‘‘Sur la constitution des substances aromatiques’’ by August Kekulé and presents a snapshot of present-day research in aromaticity.

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