Chemistry

Editorial

3 pages, 610 KiB

Open AccessEditorial

Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday

by Andrea Peluso and Guglielmo Monaco

Chemistry 2022, 4(1), 118-120; https://doi.org/10.3390/chemistry4010010 - 23 Feb 2022

Viewed by 1991

Abstract

It is our great pleasure to introduce the Festschrift of Chemistry to honor Professor Riccardo Zanasi (Figure 1) on the occasion of his 70th birthday and to recognize his important contributions to quantum chemistry, particularly in the field of magnetic response and chiroptical [...] Read more.

It is our great pleasure to introduce the Festschrift of Chemistry to honor Professor Riccardo Zanasi (Figure 1) on the occasion of his 70th birthday and to recognize his important contributions to quantum chemistry, particularly in the field of magnetic response and chiroptical spectroscopies [...] Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

Research

Jump to: Editorial, Review

11 pages, 3895 KiB

Open AccessEditor’s ChoiceArticle

Photoelectron Circular Dichroism as a Probe of Chiral Hydrocarbons

by Piero Decleva

Chemistry 2022, 4(1), 31-41; https://doi.org/10.3390/chemistry4010003 - 13 Jan 2022

Cited by 3 | Viewed by 2995

Abstract

The sensitivity of Photoelectron Circular Dichroism (PECD) in the angular distribution of photoelectrons, a recent chiral technique, to detect chirality in pure hydrocarbons is investigated in a number of benchmark molecules. It is found that a very large chiral signal is expected, surpassing [...] Read more.

The sensitivity of Photoelectron Circular Dichroism (PECD) in the angular distribution of photoelectrons, a recent chiral technique, to detect chirality in pure hydrocarbons is investigated in a number of benchmark molecules. It is found that a very large chiral signal is expected, surpassing most current examples, giving a sure fingerprint of absolute configuration. On the other hand, the sensitivity to specific isomers or closely related molecules is relatively modest. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

11 pages, 5960 KiB

Open AccessArticle

The Magnetic Response of Starphenes

by Mesías Orozco-Ic and Gabriel Merino

Chemistry 2021, 3(4), 1381-1391; https://doi.org/10.3390/chemistry3040099 - 20 Nov 2021

Cited by 10 | Viewed by 3483

Abstract

The aromaticity of [n]starphenes (n = 1, 4, 7, 10, 13, 16), as well as starphene-based [19]dendriphene, is addressed by calculating the magnetically induced current density and the induced magnetic field, using the pseudo-π model. When an external magnetic field [...] Read more.

The aromaticity of [n]starphenes (n = 1, 4, 7, 10, 13, 16), as well as starphene-based [19]dendriphene, is addressed by calculating the magnetically induced current density and the induced magnetic field, using the pseudo-π model. When an external magnetic field is applied, these systems create diatropic currents that split into a global peripheral current surrounding the starphene skeleton and several local currents in the acene-based arms, resulting in large shielding cones above the arms. In particular, the arm currents are smaller than their linear analogs, and in general, the strengths of the ring currents tend to weaken as the starphene get larger. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

8 pages, 1840 KiB

Open AccessArticle

On the Aromaticity and ¹³C-NMR Pattern of Pentagonal-Pyramidal Hexamethylbenzene Dication [C₆(CH₃)₆]²⁺: A {C₅(CH₃)₅}⁻–{CCH₃}³⁺ Aggregate

by Desmond MacLeod-Carey and Alvaro Muñoz-Castro

Chemistry 2021, 3(4), 1363-1370; https://doi.org/10.3390/chemistry3040097 - 11 Nov 2021

Cited by 1 | Viewed by 2776

Abstract

The experimentally characterized hexamethylbenzene dication C₆(CH₃)₆²⁺ shows a pentagonal-pyramidal structure involving a carbon-capped five-membered ring. The structural characterization of this hypercoordination (or hypervalency) gives rise if the aromatic behavior remains in the resulting pentagon ring. Here, we [...] Read more.

The experimentally characterized hexamethylbenzene dication C₆(CH₃)₆²⁺ shows a pentagonal-pyramidal structure involving a carbon-capped five-membered ring. The structural characterization of this hypercoordination (or hypervalency) gives rise if the aromatic behavior remains in the resulting pentagon ring. Here, we investigated the induced magnetic field of C₆(CH₃)₆²⁺ to gain a deeper understanding of the resulting non-classical structural situation in a representative pentagonal-pyramidal structure. Our results support the view of a C₅(CH₃)₅⁻/CCH₃³⁺ structure, depicting a π-aromatic pentamethylcyclopentadienyl anion with a 6π-electron kernel, with a capped carbon which does not decrease the characteristic shielding cone property of the aromatic ring. Hence, carbon-capped rings are suggested to retain the aromatic behavior from the former aromatic ring. We expect that the analysis of both the overall magnetic response and NMR chemical shifts may be informative to unravel the characteristic patterns in the formation of hypervalent carbon atoms involving non-classical chemical environments. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

12 pages, 2073 KiB

Open AccessArticle

Are Metallacyclopentadienes Always Non-Aromatic?

by Ricardo Casiano-González and José Enrique Barquera-Lozada

Chemistry 2021, 3(4), 1302-1313; https://doi.org/10.3390/chemistry3040094 - 4 Nov 2021

Cited by 2 | Viewed by 3393

Abstract

Even though metallacyclopentadienes (MCPs) are among the most common metallacycles, their electron delocalization (aromaticity) has received far less attention than other metallacycles, such as metallabenzenes. We systematically studied the aromaticity of MCPs with energetic (isomerization stabilization energy), density (delocalization index) and magnetic (current [...] Read more.

Even though metallacyclopentadienes (MCPs) are among the most common metallacycles, their electron delocalization (aromaticity) has received far less attention than other metallacycles, such as metallabenzenes. We systematically studied the aromaticity of MCPs with energetic (isomerization stabilization energy), density (delocalization index) and magnetic (current density) aromaticity indices. The indices agree that metallacyclopentadienes are, in general, weakly aromatic at most. The 18

e^{-}

complexes showed the expected weak aromaticity, and only the

d^{8}

molecules are somewhat anti-aromatic. However, the theoretical account of the aromaticity of the 16

e^{-}

MCPs is more convoluted. We find that the aromatic criteria for a 16

e^{-}

d^{4}

ruthenacyclopentadiene disagree. The lack of agreement shows that significant electron delocalization is not always related to great stability or to strong diatropic currents. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

19 pages, 7817 KiB

Open AccessArticle

Partitioning Hückel–London Currents into Cycle Contributions

by Wendy Myrvold, Patrick W. Fowler and Joseph Clarke

Chemistry 2021, 3(4), 1138-1156; https://doi.org/10.3390/chemistry3040083 - 8 Oct 2021

Cited by 5 | Viewed by 1944

Abstract

Ring-current maps give a direct pictorial representation of molecular aromaticity. They can be computed at levels ranging from empirical to full ab initio and DFT. For benzenoid hydrocarbons, Hückel–London (HL) theory gives a remarkably good qualitative picture of overall current patterns, and a [...] Read more.

Ring-current maps give a direct pictorial representation of molecular aromaticity. They can be computed at levels ranging from empirical to full ab initio and DFT. For benzenoid hydrocarbons, Hückel–London (HL) theory gives a remarkably good qualitative picture of overall current patterns, and a useful basis for their interpretation. This paper describes an implemention of Aihara’s algorithm for computing HL currents for a benzenoid (for example) by partitioning total current into its constituent cycle currents. The Aihara approach can be used as an alternative way of calculating Hückel–London current maps, but more significantly as a tool for analysing other empirical models of induced current based on conjugated circuits. We outline an application where examination of cycle contributions to HL total current led to a simple graph-theoretical approach for cycle currents, which gives a better approximation to the HL currents for Kekulean benzenoids than any of the existing conjugated-circuit models, and unlike these models it also gives predictions of the HL currents in non-Kekulean benzenoids that are of similar quality. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

12 pages, 3598 KiB

Open AccessArticle

Analysis of Local and Global Aromaticity in Si₃C₅ and Si₄C₈ Clusters. Aromatic Species Containing Planar Tetracoordinate Carbon

by Juan J. Torres-Vega, Diego R. Alcoba, Ofelia B. Oña, Alejandro Vásquez-Espinal, Rodrigo Báez-Grez, Luis Lain, Alicia Torre, Víctor García and William Tiznado

Chemistry 2021, 3(4), 1101-1112; https://doi.org/10.3390/chemistry3040080 - 25 Sep 2021

Cited by 7 | Viewed by 2884

Abstract

The minimum energy structures of the Si₃C₅ and Si₄C₈ clusters are planar and contain planar tetracoordinate carbons (ptCs). These species have been classified, qualitatively, as global (π) and local (σ) aromatics according to the adaptive natural density [...] Read more.

The minimum energy structures of the Si₃C₅ and Si₄C₈ clusters are planar and contain planar tetracoordinate carbons (ptCs). These species have been classified, qualitatively, as global (π) and local (σ) aromatics according to the adaptive natural density partitioning (AdNDP) method, which is an orbital localization method. This work evaluates these species’ aromaticity, focusing on confirming and quantifying their global and local aromatic character. For this purpose, we use an orbital localization method based on the partitioning of the molecular space according to the topology of the electronic localization function (LOC-ELF). In addition, the magnetically induced current density is analyzed. The LOC-ELF-based analysis coincides with the AdNDP study (double aromaticity, global, and local). Moreover, the current density analysis detects global and local ring currents. The strength of the global and local current circuit is significant, involving 4n + 2 π- and σ-electrons, respectively. The latter implicates the Si-ptC-Si fragment, which would be related to the 3c-2e σ-bond detected by the orbital localization methods in this fragment. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

12 pages, 2274 KiB

Open AccessCommunication

Chirality Transfer in a Calixarene-Based Directional Pseudorotaxane Complex

by Gerardo Concilio, Carmine Gaeta, Paolo Della Sala, Veronica Iuliano, Carmen Talotta, Guglielmo Monaco, Stefano Superchi, Sandra Belviso and Placido Neri

Chemistry 2021, 3(3), 1089-1100; https://doi.org/10.3390/chemistry3030079 - 18 Sep 2021

Cited by 2 | Viewed by 2423

Abstract

Hexamethoxycalix[6]arene 3 forms a directional pseudorotaxane complex with the chiral axle (S)-(α-methyl-benzyl)benzylammonium 2⁺. Between the two (endo-chiral)-2⁺@3 and (exo-chiral)-2⁺@3 pseudorotaxane stereoisomers, the former is preferentially formed. This result confirms [...] Read more.

Hexamethoxycalix[6]arene 3 forms a directional pseudorotaxane complex with the chiral axle (S)-(α-methyl-benzyl)benzylammonium 2⁺. Between the two (endo-chiral)-2⁺@3 and (exo-chiral)-2⁺@3 pseudorotaxane stereoisomers, the former is preferentially formed. This result confirms the validity of the “endo-α-methyl-benzyl rule”, previously reported by us. DFT calculations suggest that C-H … π interactions between the methyl group of 2⁺ and the calixarene aromatic rings, determine the stereoselectivity of the threading process toward the “endo-α-methyl-benzyl preference”. An amplification of optical rotation is observed upon formation of the pseudorotaxane complex (endo-chiral)-2⁺@3 with respect to free axle 2⁺. Thus, the specifical rotation of the 1:1 mixture of chiral 2⁺·B(ArF)₄⁻ salt and achiral 3 was augmented upon formation of the pseudorotaxane and DFT calculations were used to rationalize this result. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

15 pages, 4081 KiB

Open AccessArticle

Absorption and Isomerization of Azobenzene Guest Molecules in Polymeric Nanoporous Crystalline Phases

by Nicola Coscia, Antonietta Cozzolino, Manohar Golla and Paola Rizzo

Chemistry 2021, 3(3), 1074-1088; https://doi.org/10.3390/chemistry3030078 - 16 Sep 2021

Cited by 5 | Viewed by 2974

Abstract

PPO co-crystalline (CC) films including azobenzene guest molecules have been prepared and characterized by WAXD, FTIR and UV-Visible measurements. Isomerization reactions of azobenzene (photo-induced trans to cis and spontaneous cis to trans) included in α and β nanoporous-crystalline (NC) phases leading to CC [...] Read more.

PPO co-crystalline (CC) films including azobenzene guest molecules have been prepared and characterized by WAXD, FTIR and UV-Visible measurements. Isomerization reactions of azobenzene (photo-induced trans to cis and spontaneous cis to trans) included in α and β nanoporous-crystalline (NC) phases leading to CC phases, or simply absorbed in amorphous phase have been studied on thick and thin films. Spectroscopic analysis shows that photo-isomerization of azobenzene occurs without expulsion of azobenzene guest molecules from crystalline phases. Sorption studies of α and β NC films immersed into photo-isomerized azobenzene solution reveal a higher selectivity of the β NC phase toward cis azobenzene isomer than the α NC phase, inducing us to propose the β NC phase as particularly suitable for absorbing spherically bulky guest molecules. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

12 pages, 4443 KiB

Open AccessArticle

The B₂ Structural Motif as a Tool for Modulating Ring Currents in Monocyclic Li Clusters

by Slađana Đorđević and Slavko Radenković

Chemistry 2021, 3(3), 1063-1073; https://doi.org/10.3390/chemistry3030077 - 14 Sep 2021

Cited by 1 | Viewed by 1705

Abstract

Magnetically induced current densities, calculated at the M06-2X/def2-TZVP level using the diamagnetic-zero version of the continuous transformation of origin of current density (CTOCD-DZ) method, were employed to study the aromaticity in

{Li}_{3} B_{2}^{-}

and

{Li}_{4} B_{2}

. It [...] Read more.

Magnetically induced current densities, calculated at the M06-2X/def2-TZVP level using the diamagnetic-zero version of the continuous transformation of origin of current density (CTOCD-DZ) method, were employed to study the aromaticity in

{Li}_{3} B_{2}^{-}

and

{Li}_{4} B_{2}

. It was found that the

{Li}_{3} / {Li}_{4}

rings in

{Li}_{3} B_{2}^{-}

and

{Li}_{4} B_{2}

remarkably resemble the monocyclic

{Li}_{3}^{+}

and

{Li}_{4}^{2 +}

clusters. Unlike the parent

{Li}_{3}^{+}

and

{Li}_{4}^{2 +}

systems that sustain negligibly weak global current density circulation, the

{Li}_{3} B_{2}^{-}

and

{Li}_{4} B_{2}

clusters exhibit a strong diatropic current density. The present work demonstrates how structural modifications introduced by the

B_{2}

unit can be used for modulating the current density in cyclic Li-based clusters. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

15 pages, 385 KiB

Open AccessFeature PaperArticle

Electronic Currents and Anapolar Response Induced in Molecules by Monochromatic Light

by Francesco Ferdinando Summa and Paolo Lazzeretti

Chemistry 2021, 3(3), 1022-1036; https://doi.org/10.3390/chemistry3030073 - 5 Sep 2021

Cited by 4 | Viewed by 1870

Abstract

It is shown that the electric dipole- and electric quadrupole–anapole polarizabilities, denoted respectively by

f_{α β}^{'}

and

g_{α, β γ}^{'}

, and the anapole magnetizability

a_{α β}

, are intrinsic properties of the electron cloud of molecules [...] Read more.

It is shown that the electric dipole- and electric quadrupole–anapole polarizabilities, denoted respectively by

f_{α β}^{'}

and

g_{α, β γ}^{'}

, and the anapole magnetizability

a_{α β}

, are intrinsic properties of the electron cloud of molecules responding to optical fields.

f_{α β}^{'}

is a nonvanishing property of chiral and achiral compounds, whereas

a_{α β}

is suitable for enantiomer discrimination of chiral species. They can conveniently be evaluated by numerical integration, employing a formulation complementary to that provided by perturbation theory and relying on the preliminary computation of electronic current density tensors all over the molecular domain. The origin dependence of the dynamic anapolar response is rationalized via related computational techniques employing numerical integration, as well as definitions of molecular property tensors, for example, electric dipole and electric quadrupole polarizabilties and magnetizability. A preliminary application of the theory is reported for the

R_{a}

enantiomer of the hydrogen peroxide molecule, evaluating tensor components of electric dipole-anapole polarizability and anapole magnetizability as functions of the dihedral angle

ϕ \equiv ∠

H-O-O-H in the range

0^{\circ} \leq ϕ \leq 180^{\circ}

. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

17 pages, 26215 KiB

Open AccessArticle

Spatial Contributions to ¹H NMR Chemical Shifts of Free-Base Porphyrinoids

by Heike Fliegl, Maria Dimitrova, Raphael J. F. Berger and Dage Sundholm

Chemistry 2021, 3(3), 1005-1021; https://doi.org/10.3390/chemistry3030072 - 5 Sep 2021

Cited by 7 | Viewed by 2941

Abstract

A recently developed methodology for calculating, analyzing, and visualizing nuclear magnetic shielding densities is used for studying spatial contributions including ring-current contributions to

^{1}

H nuclear magnetic resonance (NMR) chemical shifts of aromatic and anti-aromatic free-base porphyrinoids. Our approach allows a visual inspection [...] Read more.

A recently developed methodology for calculating, analyzing, and visualizing nuclear magnetic shielding densities is used for studying spatial contributions including ring-current contributions to

^{1}

H nuclear magnetic resonance (NMR) chemical shifts of aromatic and anti-aromatic free-base porphyrinoids. Our approach allows a visual inspection of the spatial origin of the positive (shielding) and negative (deshielding) contributions to the nuclear magnetic shielding constants. Diatropic and paratropic current-density fluxes yield both shielding and deshielding contributions implying that not merely the tropicity of the current density determines whether the contribution has a shielding or deshielding character. Instead the shielding or deshielding contribution is determined by the direction of the current-density flux with respect to the studied nucleus. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

14 pages, 2318 KiB

Open AccessArticle

Magnetic Aromaticity of Cycloporphyrin Nanorings

by Alessandro Landi, Francesco Ferdinando Summa and Guglielmo Monaco

Chemistry 2021, 3(3), 991-1004; https://doi.org/10.3390/chemistry3030071 - 2 Sep 2021

Cited by 7 | Viewed by 2244

Abstract

The ascertainment of magnetic aromaticity is not necessarily straightforward, especially for large and bent systems, such as the cycloporphyrin nanorings recently synthesized by the group of Anderson. Six of these cycloporphyrin nanorings were studied here computationally. Indirect methods, based on nuclear shielding and [...] Read more.

The ascertainment of magnetic aromaticity is not necessarily straightforward, especially for large and bent systems, such as the cycloporphyrin nanorings recently synthesized by the group of Anderson. Six of these cycloporphyrin nanorings were studied here computationally. Indirect methods, based on nuclear shielding and magnetizabilities, and direct methods, based on standard quantum mechanics, were both used effectively to determine their magnetically induced current strength, which mostly confirmed Anderson’s classification. However, in the case of hexanions, and in particular for cyclohexaporphyrin hexacations, a significant cancellation of delocalized diatropic and paratropic flow occurred, showing that the resultant faint aromatic character was a result of competing aromatic and antiaromatic contributions, as also evidenced by the ipsocentric method. A warning is renewed on the use of isotropic shielding to determine the tropicity of the magnetically induced current. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

19 pages, 14101 KiB

Open AccessArticle

Topological Analysis of Magnetically Induced Current Densities in Strong Magnetic Fields Using Stagnation Graphs

by Tom J. P. Irons, Adam Garner and Andrew M. Teale

Chemistry 2021, 3(3), 916-934; https://doi.org/10.3390/chemistry3030067 - 26 Aug 2021

Cited by 9 | Viewed by 2176

Abstract

Stagnation graphs provide a useful tool to analyze the main topological features of the often complicated vector field associated with magnetically induced currents. Previously, these graphs have been constructed using response quantities appropriate for modest applied magnetic fields. We present an implementation capable [...] Read more.

Stagnation graphs provide a useful tool to analyze the main topological features of the often complicated vector field associated with magnetically induced currents. Previously, these graphs have been constructed using response quantities appropriate for modest applied magnetic fields. We present an implementation capable of producing these graphs in arbitrarily strong magnetic fields, using current-density-functional theory. This enables us to study how the topology of the current vector field changes with the strength and orientation of the applied magnetic field. Applications to CH

_{4}

, C

_{2}

H

_{2}

and C

_{2}

H

_{4}

are presented. In each case, we consider molecular geometries optimized in the presence of the magnetic field. The stagnation graphs reveal subtle changes to this vector field where the symmetry of the molecule remains constant. However, when the electronic state and symmetry of the corresponding equilibrium geometry changes with increasing field strength, the changes to the stagnation graph are extensive. We expect that the approach presented here will be helpful in interpreting changes in molecular structure and bonding in the strong-field regime. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

12 pages, 4537 KiB

Open AccessArticle

Magnetic Shielding Study of Bonding and Aromaticity in Corannulene and Coronene

by Peter B. Karadakov

Chemistry 2021, 3(3), 861-872; https://doi.org/10.3390/chemistry3030063 - 12 Aug 2021

Cited by 15 | Viewed by 3175

Abstract

Bonding and aromaticity in the bowl-shaped C_5v and planar D_5h geometries of corannulene and the planar D_6h geometry of coronene are investigated using 3D isosurfaces and 2D contour plots of the isotropic magnetic shielding σ_iso(r) and, [...] Read more.

Bonding and aromaticity in the bowl-shaped C_5v and planar D_5h geometries of corannulene and the planar D_6h geometry of coronene are investigated using 3D isosurfaces and 2D contour plots of the isotropic magnetic shielding σ_iso(r) and, for planar geometries, of the out-of-plane component of the shielding tensor σ_zz(r). Corannulene and coronene both feature conjoined shielded “doughnuts” around a peripheral six-membered carbon ring, suggesting strong bonding interactions and aromatic stability; a deshielded region inside the hub ring of corannulene indicates that this ring is antiaromatic, more so in planar corannulene. The switch from the planar to the bowl-shaped geometry of corannulene is shown to enhance both bonding and the local aromaticities of the five- and six-membered rings; these factors, in addition to ring strain reduction, favour the bowl-shaped geometry. The most and least shielded bonds in both corannulene and coronene turn out to be the spoke and hub bonds, respectively. The higher π electron activity over spoke bonds in planar corannulene and coronene is supported by σ_zz(r) contour plots in planes 1 Å above the respective molecular planes; these findings about spoke bonds are somewhat unexpected, given that ring current studies indicate next to no currents over spoke bonds. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

18 pages, 2875 KiB

Open AccessArticle

Singlet/Triplet State Anti/Aromaticity of CyclopentadienylCation: Sensitivity to Substituent Effect

by Milovan Stojanović, Jovana Aleksić and Marija Baranac-Stojanović

Chemistry 2021, 3(3), 765-782; https://doi.org/10.3390/chemistry3030055 - 21 Jul 2021

Cited by 7 | Viewed by 3866

Abstract

It is well known that singlet state aromaticity is quite insensitive to substituent effects, in the case of monosubstitution. In this work, we use density functional theory (DFT) calculations to examine the sensitivity of triplet state aromaticity to substituent effects. For this purpose, [...] Read more.

It is well known that singlet state aromaticity is quite insensitive to substituent effects, in the case of monosubstitution. In this work, we use density functional theory (DFT) calculations to examine the sensitivity of triplet state aromaticity to substituent effects. For this purpose, we chose the singlet state antiaromatic cyclopentadienyl cation, antiaromaticity of which reverses to triplet state aromaticity, conforming to Baird’s rule. The extent of (anti)aromaticity was evaluated by using structural (HOMA), magnetic (NICS), energetic (ISE), and electronic (EDDB_p) criteria. We find that the extent of triplet state aromaticity of monosubstituted cyclopentadienyl cations is weaker than the singlet state aromaticity of benzene and is, thus, slightly more sensitive to substituent effects. As an addition to the existing literature data, we also discuss substituent effects on singlet state antiaromaticity of cyclopentadienyl cation. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Graphical abstract

Review

Jump to: Editorial, Research

26 pages, 9313 KiB

Open AccessReview

Aromaticity of Heterocirculenes

by Nataliya N. Karaush-Karmazin, Glib V. Baryshnikov and Boris F. Minaev

Chemistry 2021, 3(4), 1411-1436; https://doi.org/10.3390/chemistry3040102 - 3 Dec 2021

Cited by 11 | Viewed by 3922

Abstract

This review summarizes the results on the aromaticity of a series of synthesized and hypothetical neutral heterocirculene molecules and their double charged ions. The aromaticity of heterocirculenes is a direct reflection of their electronic structure responsible for the specific optoelectronic and photophysical properties. [...] Read more.

This review summarizes the results on the aromaticity of a series of synthesized and hypothetical neutral heterocirculene molecules and their double charged ions. The aromaticity of heterocirculenes is a direct reflection of their electronic structure responsible for the specific optoelectronic and photophysical properties. We show how the presence of a heteroatom in the outer macrocycle affects the aromaticity of hetero[8]circulenes. In addition, we also describe the change in aromaticity and strain energy for a series of the “lower” (n < 8) and “higher” (n > 8) hetero[n]circulenes. It was demonstrated that the loss of planarity with increased strain leads to an increased antiaromaticity of the lower hetero[n]circulenes, whereas higher hetero[n]circulenes demonstrate a more pronounced aromatic nature because of the small departure from planarity of each heteroarene ring in hetero[n]circulene molecule. Finally, we discuss the aromatic nature of the first examples of π-extended hetero[8]circulenes. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

16 pages, 816 KiB

Open AccessReview

Is Aromatic Nitration Spin Density Driven?

by Amedeo Capobianco, Alessandro Landi and Andrea Peluso

Chemistry 2021, 3(4), 1286-1301; https://doi.org/10.3390/chemistry3040093 - 4 Nov 2021

Cited by 5 | Viewed by 2957

Abstract

The mechanism of aromatic nitration is critically reviewed with particular emphasis on the paradox of the high positional selectivity of substitution in spite of low substrate selectivity. Early quantum chemical computations in the gas phase have suggested that the retention of positional selectivity [...] Read more.

The mechanism of aromatic nitration is critically reviewed with particular emphasis on the paradox of the high positional selectivity of substitution in spite of low substrate selectivity. Early quantum chemical computations in the gas phase have suggested that the retention of positional selectivity at encounter-limited rates could be ascribed to the formation of a radical pair via an electron transfer step occurring before the formation of the Wheland intermediate, but calculations which account for the effects of solvent polarization and the presence of counterion do not support that point of view. Here we report a brief survey of the available experimental and theoretical data, adding a few more computations for better clarifying the role of electron transfer for regioselectivity. Full article

(This article belongs to the Special Issue Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday

Share This Special Issue

Special Issue Editors

Special Issue Information

Keywords

Benefits of Publishing in a Special Issue

Published Papers (18 papers)

Editorial

Research

Review

Further Information

Guidelines

MDPI Initiatives

Follow MDPI