Aromatic Rule Research Articles

Connecting and combining rules of aromaticity. Towards a unified theory of aromaticity

Most of the archetypal aromatic compounds present high symmetry and have degenerate highest‐occupied molecular orbitals. These orbitals can be fully occupied resulting in a closed‐shell structure or can be same‐spin half‐filled. This closed‐shell or same‐spin half‐filled electronic structure provides an extra stabilization and it is the origin of several rules of aromaticity such as the Hückel 4N + 2 rule, the lowest‐lying triplet excited state 4N Baird rule, the 4N rule in Möbius‐type conformation, the 4N + 2 Wade–Mingos rule in closo boranes or the 2(N + 1)2 Hirsch rule in spherical aromatic species. Combinations of some of these rules of aromaticity can be found in some particular species. Examples of these combinations will be discussed and the validity of some of these rules will be assessed. Moreover, it is possible to establish connections with some of these rules of (anti)aromaticity and, therefore, they can be partially generalized, which represents a step forward to a future unified theory of aromaticity.This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Electronic Structure Theory > Density Functional Theory Software > Quantum Chemistry

Chemical Bonding in Transition Metal Nitride Os3N3 + Cluster: 6π Inorganic Benzene and δ2δ*1δ*1 Aromaticity.

Inorganic benzene-like clusters with a planar hexagonal ring are of interest in chemistry, as are new types of aromaticity, multifold aromaticity, and in particular δ aromaticity beyond carbon-based organic systems. Here we report on a computational study of chemical bonding in a binary Os3N3+D3h (7A2″) cluster. This transition metal nitride cluster assumes a perfectly planar, heteroatomic, hexagonal geometry. An array of quantum chemistry tools is exploited to elucidate the electronic, structural, and bonding properties of D3h Os3N3+ cluster, which include canonical molecular orbitals, adaptive natural density partitioning, natural bond orbital analysis, orbital composition calculations, and nucleus-independent chemical shifts. The computational data collectively support the bonding picture of 2-fold π/δ aromaticity: 6π electrons delocalized over all Os/N centers versus an Os-based 4δ framework in the unique δ2δ*1δ*1 configuration. The π sextet renders this heteroatomic cluster an inorganic analog of benzene. Transition metal-based inorganic benzenes are unknown in the literature, to our knowledge. The triplet 4δ electron-counting is a rare case of d-orbital aromaticity and δ-aromaticity, following the reversed 4n Hückel rule for aromaticity in a triplet system. This bonding picture is concrete, differing fundamentally from a recent study on the relevant system.

Electronic structure and stability of anionic AuGen (n = 1–20) clusters and assemblies: a density functional modeling

In the present study electronic structure and stabilities of cationic gold-doped germanium clusters, AuGen (n = 1 to 20), and their assemblies have been investigated by density functional theory (DFT) modeling. Computational results show a good relationship between the thermodynamic parameters, average binding energy, embedding energy, fragmentation energy, etc., with the percentage hybridization between different Ge 4s, Ge 4p, and Au 5d atomic orbitals, which plays a dominating role in the stabilization of anionic AuGe7, AuGe10, Au(Ge7)2, Au(Ge9)2, and Au(Ge10)2 clusters. Other thermodynamic and chemical parameters are also found consistent with the observed thermodynamic stabilities of the nanoclusters. In smaller size range (n < 11), Au atom always absorbs on the surface or vertex of pure Ge cluster. From n = 11, endohedral doping starts. In the assembled clusters, Au atom play the role as a bridging atom in Au(Ge7)2, Au(Ge9)2, and Au(Ge10)2 clusters. Stability of the AuGe7, AuGe10, Au(Ge7)2, Au(Ge9)2, and Au(Ge10)2 are explained using magic number in shell-filled model and mixed (π-σ) aromatic rule. As per the symmetry and structure of AuGe12 cluster, it is comparable to a nido-cluster, and hence, its stability is explained using Wade-Mingos rule. Calculated VDE, ADE, HOMO-LUMO gap, and VIP have very close agreement with the experimental results. IR and Raman frequencies show that the vibration nature of the clusters could produce electromagnetic radiation in the far infrared region which is useful for medical applications.

N]Cyclo-para-biphenylmethine Polyradicaloids: [n]Annulene Analogs and Unusual Valence Tautomerization

Summary Annulenes and their charged analogs play an important role in validating Huckel's aromaticity rule. Whereas annulenes with [4n + 2] π electrons tend to delocalize π electron density, annulenes with [4n] π electrons prefer to form local double bonds (bond alternation) resulting in two interconvertible valence isomers. However, it is very challenging to experimentally detect such a valence tautomerization process as a result of the very fast conversion. In this work, we synthesized a series of [n]cyclo-para-biphenylmethine macrocycles ([n]CPBM-An, n = 3–8) as analogs of [n]annulenes and observed the unusual valence tautomerization process by variable-temperature NMR measurements for [4]CPBM-An, which showed a moderate interconversion barrier (11.22 kcal/mol at coalescence temperature) via a C4 tetraradicaloid transition state. These macrocycles showed [n]annulene-like geometries but with unique open-shell polyradical character. Their optical, electrochemical, and magnetic properties were systematically investigated and compared with those of the recently studied [n]cyclo-para-phenylenes.

The structure and superconductivity of ultrathin Ga films on GaN substrate: A first-principles calculations

A free-standing ultrathin Ga film with hexagonal symmetry is unstable due to its incompatibility with the chemical aromatic rule. Recently, two or three monolayers Ga films have been grown on GaN substrate and exhibit superconductivity [Zhang et al., Phys. Rev. Lett. 114, 107003 (2015) and Xing et al. Science 350, 542 (2015)]. To elucidate the role of the GaN substrate, the properties of slab consisting of ultrathin Ga films and GaN substrate are calculated from first-principles. The results show that the stable interface structure is ultrathin Ga films deposited on N-terminated GaN substrate. Covalent and metallic bonding coexist in the deposited ultrathin Ga films and this peculiar feature contributes to their stability. For two monolayers Ga film deposited on GaN substrate, the obtained electron-phonon coupling λ is about 1.18 and the estimated superconducting temperature (5.1 K) is in excellent agreement with the experimental value, suggesting that it is a conventional BCS-type two-dimensional superconductor. For the deposited three monolayer Ga film, our calculations of the phonon structures show that it is dynamically unstable. While for one monolayer Ga film deposited on GaN substrate, due to the deposited metallic Ga layer with partial covalent character and meanwhile having the behavior of phonon softening, the obtained electron-phonon coupling λ (2.31) and the estimated superconducting transition temperature (9.4 K) are much larger than the case of the deposited two monolayer Ga film.

Rational Synthesis of Antiaromatic 5,15‐Dioxaporphyrin and Oxidation into β,β‐Linked Dimers

Abstract5,15‐Dioxaporphyrin was synthesized for the first time by a nucleophilic aromatic substitution reaction of a nickel bis(α,α′‐dibromodipyrrin) complex with benzaldoxime, followed by an intramolecular annulation of the α‐hydroxy‐substituted intermediate. This unprecedented molecule is a 20π‐electron antiaromatic system, in terms of Hückel's rule of aromaticity, because lone pair electrons of oxygen atoms are incorporated into the 18π‐electron conjugated system of the porphyrin. A theoretical analysis based on the gauge‐including magnetically induced current method confirmed its antiaromaticity and a dominant inner ring pathway for the ring current. The unique reactivity of 5,15‐dioxaporphyrin forming a β,β‐linked dimer upon oxidation was also revealed.

Rational Synthesis of Antiaromatic 5,15-Dioxaporphyrin and Oxidation into β,β-Linked Dimers.

5,15-Dioxaporphyrin was synthesized for the first time by a nucleophilic aromatic substitution reaction of a nickel bis(α,α'-dibromodipyrrin) complex with benzaldoxime, followed by an intramolecular annulation of the α-hydroxy-substituted intermediate. This unprecedented molecule is a 20π-electron antiaromatic system, in terms of Hückel's rule of aromaticity, because lone pair electrons of oxygen atoms are incorporated into the 18π-electron conjugated system of the porphyrin. A theoretical analysis based on the gauge-including magnetically induced current method confirmed its antiaromaticity and a dominant inner ring pathway for the ring current. The unique reactivity of 5,15-dioxaporphyrin forming a β,β-linked dimer upon oxidation was also revealed.

Continuous group and electron-count rules in aromaticity

In the present article a group theoretical approach has been used to explain different electron-count rules for aromaticity. A general group theoretical method is presented to derive and unite the different electron count rules (Huckel, Baird, Mobius, and Spherical aromaticity). It is shown that continuous groups play important role in understanding of these electron count rules. SYNOPSIS Group theory has been used to investigate the origin of electron-count rules. Present work sucessfully classified the different types of aromaticity in terms of irrudicible representations of continuous groups.

Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π Conjugation Involving a Mo-Mo δ Bond.

We have synthesized and characterized the mixed-ligand dimolybdenum paddlewheel complex Na[(DAniF)3Mo2(C3S5)] (Na[1]; DAniF = N,N'-di-p-anisylformamidinate, dmit = 1,3-dithiole-2-thione-4,5-dithiolate), which has a six-membered chelating [Mo2S2C2] ring created by equatorial coordination of the dmit (C3S5) ligand to the Mo2 unit. One-electron oxidation of Na[1] using Cp2FePF6 yields the neutral complex [(DAniF)3Mo2(C3S5)] ([1]), and removal of two electrons from Na[1] using AgBPh4 gives [(DAniF)3Mo2(C3S5)]BPh4 ([1]BPh4). In the crystal structures, [1]- and [1] present dihedral angles of 118.9 and 142.3° between the plane defined by the Mo-Mo bond vector and the dmit ligand, respectively, while DFT calculations show that in [1]+ the Mo-Mo bond and the dmit ligand are coplanar. Complex [1] is paramagnetic with a g value of 1.961 in the EPR spectrum and has a Mo-Mo bond distance of 2.133(1) Å, increased from 2.0963(9) Å for [1]-. Consistently, a broad absorption band is observed for [1] in the near-IR region, which arises from charge transfer from the dmit ligand to the cationic Mo25+ centers. Interestingly, complex [1]+ has an aromatic [Mo2S2C2] core, as evidenced by a large diamagnetic anisotropy, in addition to the coplanarity of the core structure, which shifts downfield the 1H NMR signal of the horizontal methine proton (ArN-(CH)-NAr) but upfield those of the vertical protons, relative to the methine proton resonances for the precursor ([1]-). The magnetic anisotropy (Δχ = χ⊥ - χ∥) for the [Mo2S2C2] ring in [1]+ is -105.5 ppm cgs, calculated from the McConnell equation, which is about 2-fold larger than that for benzene. The aromaticity of the [Mo2S2C2] ring is supported by theoretical studies, including single-point calculations and gauge-including atomic orbital (GIAO) NMR spectroscopic calculations at the density functional theory (DFT) level. DFT calculations also show that the [Mo2S2C2] core in [1]+ possesses a set of three highest occupied and three lowest unoccupied molecular orbitals in π character, corresponding to those of benzene in symmetry, and six π electrons that conform to the Hückel 4n + 2 rule for aromaticity. Therefore, this study shows that an aromatic [Mo2S2C2] core is formed by coupling the δ orbital of the Mo≣Mo bond with the π orbital of the C═C bond through the bridging atoms (S), thus validating the equivalency in bonding functionality between δ and π orbitals.

Structural Monitoring of the Onset of Excited-State Aromaticity in a Liquid Crystal Phase.

Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird's rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials.

Electron affinity of modified benzene

AbstractThe electron affinities of organic molecules obeying Hückel's rule of aromaticity are vanishingly small, if not negative. For example, benzene, a classic example of an aromatic molecule, has an electron affinity of −1.15 eV. Using density functional theory, we have systematically calculated the electron affinities and vertical detachment energies of C6H6 by substituting H with halogen (F) and superhalogen (BO2) moieties, as well as replacing one of the C atoms with B. The ground state geometries were obtained by examining about 330 isomers. The electron affinities are found to steadily increase with these substitutions/replacements, even surpassing that of Cl, the element with the highest electron affinity in the periodic table, in the case of C5BH(BO2)5. In some special cases such as C6H5(BO2) the electron affinity and vertical detachment energy differ by as much as 5 eV, indicating substantial changes in the geometry as the electron is removed from the anion. We hope that the ability to change the negative electron affinity of C6H6 to large positive values by substituting H and/or replacing C atom will motivate experimental studies.

A Naphtho-p-quinodimethane Exhibiting Baird's (Anti)Aromaticity, Broken Symmetry, and Attractive Photoluminescence.

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T1 state with a phosphorescence 3(7a)* lifetime of τP = 395 μs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.

Inhibiting and activating effects of antimicrobial peptides on complement

The PAH family of organic compounds (polycyclic aromatic hydrocarbons), involved in several fields of chemistry, has received particular attention in astrochemistry, where their vibrational spectroscopy, thermodynamics, dynamics, and fragmentation properties are now abundantly documented. This survey aims at drawing trends for low spin-multiplicity surfaces of PAHs bearing internal energies in the range 1–10 eV. It addresses some typical alternatives to the ground-state regular structures of PAHs, making explicit possible intramolecular rearrangements leading to high-lying minima. These isomerisations should be taken into consideration when addressing PAH processing in astrophysical conditions. The first part of this double-entry study focuses on the hydrogen-shifted forms, which bear both a carbene center and a saturated carbon. It rests upon DFT calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono- and multi-cationic states. Systematically searched for in neutral species, these H-shifted minima are lying 4–5 eV above the regular all-conjugated forms, and are separated by barriers of about 1 eV. General hydrogen-shifting is found to be easier for cationic species as the relative energies of their H-shifted minima are 1–1.5 eV lower than those for neutral species. As much as possible, classical knowledge and concepts of organic chemistry such as aromaticity and Clar's rules are invoked for result interpretation.

Can Baird's and Clar's Rules Combined Explain Triplet State Energies of Polycyclic Conjugated Hydrocarbons with Fused 4nπ- and (4n + 2)π-Rings?

Compounds that can be labeled as "aromatic chameleons" are π-conjugated compounds that are able to adjust their π-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chemical calculations to explore how the fusion of benzene rings onto aromatic chameleonic units represented by biphenylene, dibenzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T1) of the compounds. Decreases in T1 energies are observed when going from isomers with linear connectivity of the fused benzene rings to those with cis- or trans-bent connectivities. The T1 energies decreased down to those of the parent (isolated) 4nπ-electron units. Simultaneously, we observe an increased influence of triplet state aromaticity of the central 4n ring as given by Baird's rule and evidenced by geometric, magnetic, and electron density based aromaticity indices (HOMA, NICS-XY, ACID, and FLU). Because of an influence of triplet state aromaticity in the central 4nπ-electron units, the most stabilized compounds retain the triplet excitation in Baird π-quartets or octets, enabling the outer benzene rings to adapt closed-shell singlet Clar π-sextet character. Interestingly, the T1 energies go down as the total number of aromatic cycles within a molecule in the T1 state increases.

NO2 sensing properties of a borazine doped nanographene: A DFT study

Nanographenes are definite segments of graphene whose end atoms are saturated with hydrogens and their properties are different from those of the graphene because of size confinement. Herein, we investigated the structural properties, reactivity and electronic sensitivity of a newly synthesized borazine-like ring doped nanographene (BNG) to NO2 gas by means of density functional theory. We found that the central and peripheral rings of BNG are aromatic, while the middle ones are non-aromatic, revealing that this pattern follows the Clar’s sextet rule of aromaticity. Different thermodynamically stable NO2/BNG complexes are predicted, but most of them cannot be formed at room temperature because of the high transition barrier energies. The most favorable complex is that in which one oxygen atom of NO2 molecule is located above the center of B3N3 doped ring with adsorption energy of about −5.1kcal/mol. It was found that the electronic properties of BNG are highly sensitive to the presence of NO2 molecules so that after the NO2 adsorption, the BNG is converted from a semiconductor to a semimetal compound. Thus, it was concluded that the BNG may be used in the NO2 sensor devices. Also, a short recovery time is predicted for the NO2 desorption process.

PAH chemistry at eV internal energies. 1. H-shifted isomers

The PAH family of organic compounds (polycyclic aromatic hydrocarbons), involved in several fields of chemistry, has received particular attention in astrochemistry, where their vibrational spectroscopy, thermodynamics, dynamics, and fragmentation properties are now abundantly documented. This survey aims at drawing trends for low spin-multiplicity surfaces of PAHs bearing internal energies in the range 1–10eV. It addresses some typical alternatives to the ground-state regular structures of PAHs, making explicit possible intramolecular rearrangements leading to high-lying minima. These isomerisations should be taken into consideration when addressing PAH processing in astrophysical conditions. The first part of this double-entry study focuses on the hydrogen-shifted forms, which bear both a carbene center and a saturated carbon. It rests upon DFT calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono- and multi-cationic states. Systematically searched for in neutral species, these H-shifted minima are lying 4–5eV above the regular all-conjugated forms, and are separated by barriers of about 1eV. General hydrogen-shifting is found to be easier for cationic species as the relative energies of their H-shifted minima are 1–1.5eV lower than those for neutral species. As much as possible, classical knowledge and concepts of organic chemistry such as aromaticity and Clar's rules are invoked for result interpretation.

PAH chemistry at eV internal energies. 2. Ring alteration and dissociation

Recognized as important interstellar constituents, polycyclic aromatic hydrocarbons (PAHs) have been intensively studied in astrochemistry and their spectroscopy, thermodynamics, dynamics, and fragmentations are now amply documented. There exists typical alternatives to the ground-state regular planar structures of PAHs, as long as they bear internal energies in the range 1–10eV. Resulting from intramolecular rearrangements, such high-lying minima on the potential-energy surfaces should be taken into consideration in the studies of PAH processing in astrophysical conditions. Resting upon DFT calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono- and multi-cationic states, this second survey addresses the following alternatives: (1) opened forms containing ethynyl or 2-butynyl groups, (2) vinylidene isomers, in which phenanthrene patterns are reorganized into dibenzofulvene ones, (3) “twisted” forms, where external CHCH bonds can be partly twisted, and (4) bicyclobutane forms, in which the latter are integrated in saturated bicyclic forms. A few scenarios for elimination of fragments H, H2, C2H2 and C2H4 are explored. As far as possible, familiar concepts of organic chemistry, such as aromaticity or Clar's rules, are invoked for interpretations.

Detection of NO2 by hexa-peri-hexabenzocoronene nanographene: A DFT study

It has been previously indicated that pristine graphene cannot detect NO 2 gas. Nanographene is a segment of graphene whose end atoms are saturated with hydrogen atoms and its properties are different from those of graphene. Herein, we investigated the reactivity, electronic sensitivity, and structural properties of hexa- peri -hexabenzocoronene (HBC) nanographene toward NO 2 gas using density functional theory calculations. It was found that the central and peripheral rings of HBC are aromatic but the middle rings are non-aromatic, following Clar's sextet rule of aromaticity. The NO 2 molecule prefers to be adsorbed on the central ring with a nitro configuration, releasing an energy of about 13.2 kJ/mol. The NO 2 molecule significantly stabilizes the LUMO level of the HBC, thereby reducing the HOMO–LUMO energy gap from 3.60 to 1.35 eV. This indicates that the HBC is converted from a semiconductor to a semimetal. It was shown that the adsorption of NO 2 gas by HBC can produce an electrical signal selectively in the presence of O 2 , H 2 , N 2 , CO 2 , and H 2 O gases. A short recovery time about 1.9 ns is predicted and the effect of density functional is investigated.

Rational design of super-alkalis and their role in CO2 activation.

Super-alkalis are clusters of atoms. With ionization potentials smaller than those of the alkali atoms, they are playing an increasing role in chemistry as highlighted by recent applications in solar cells as well as in Li-ion batteries. For the past 40 years superalkalis were designed using inorganic elements with the sp orbital character. Here, we show that a large class of superalkalis composed of only simple metal atoms, transition metal complexes as well as organic molecules can be designed by making use of electron counting rules beyond the octet rule. Examples include Al3+, Mn(B3N3H6)2+, B9C3H12+, and C5NH6+ which obey the jellium shell closure rule, the 18-electron rule, the Wade-Mingos rule, and Hückel's aromatic rule, respectively. We further show that the ability of superalkalis to transfer an electron easily can be used to activate a CO2 molecule by transforming it from a linear to a bent structure. These results, based on density functional theory with generalized gradient approximation for exchange-correlation potential, open the door to a new class of catalysts for CO2 activation.

Peculiar All‐Metal σ‐Aromaticity of the [Au2Sb16]4− Anion in the Solid State

AbstractGas‐phase clusters are deemed to be σ‐aromatic when they satisfy the 4n+2 rule of aromaticity for delocalized σ electrons and fulfill other requirements known for aromatic systems. While the range of n values was shown to be quite broad when applied to short‐lived clusters found in molecular‐beam experiments, stability of all‐metal cluster‐like fragments isolated in condensed phase was previously shown to be mainly ascribed to two electrons (n=0). In this work, the applicability of this concept is extended towards solid‐state compounds by demonstrating a unique example of a storable compound, which was isolated as a stable [K([2.2.2]crypt)]+ salt, featuring a [Au2Sb16]4− cluster core possessing two all‐metal aromatic AuSb4 fragments with six delocalized σ electrons each (n=1). This discovery pushes the boundaries of the original idea of Kekulé and firmly establishes the usefulness of the σ‐aromaticity concept as a general idea for both small clusters and solid‐state compounds.