Unexpectedly strong anion–π interactions on the graphene flakes

Interactions of anions with simple aromatic compounds have received growing attention due to their relevancy in various fields. Yet, the anion–π interactions are generally very weak, for example, there is no favorable anion–π interaction for the halide anion F^? on the simplest benzene surface unless the H‐atoms are substituted by the highly negatively charged F. In this article, we report a type of particularly strong anion–π interactions by investigating the adsorptions of three halide anions, that is, F^?, Cl^?, and Br^?, on the hydrogenated‐graphene flake using the density functional theory. The anion–π interactions on the graphene flake are shown to be unexpectedly strong compared to those on simple aromatic compounds, for example, the F^?‐adsorption energy is as large as 17.5 kcal/mol on a graphene flake (C₈₄H₂₄) and 23.5 kcal/mol in the periodic boundary condition model calculations on a graphene flake C₁₁₃ (the supercell containing a F^? ion and 113 carbon atoms). The unexpectedly large adsorption energies of the halide anions on the graphene flake are ascribed to the effective donor–acceptor interactions between the halide anions and the graphene flake. These findings on the presence of very strong anion–π interactions between halide ions and the graphene flake, which are disclosed for the first time, are hoped to strengthen scientific understanding of the chemical and physical characteristics of the graphene in an electrolyte solution. These favorable interactions of anions with electron‐deficient graphene flakes may be applicable to the design of a new family of neutral anion receptors and detectors. © 2012 Wiley Periodicals, Inc.     

Magic Clusters PtMg2,3H5− Facilitated by Local σ-Aromaticity

The concept of local aromaticity has been successfully utilized in understanding the stability of certain atomic clusters. However, all the skeleton atoms in these clusters are covered by at least one local aromatic feature, collectively making the multiple local aromaticities coexist globally. Herein we show the robustness of local aromaticity as a tool for the discovery of novel magic clusters: not all of the skeleton atoms need to be covered by an aromatic feature to make the cluster magic. In this study, the PtMg_2,3H₅⁻ cluster anions are generated by a unique high-current pulsed discharge ion source and found to be magic numbers using mass spectrometry. Photoelectron spectroscopy and calculations confirm that only the PtH₄²⁻ kernels in these clusters are locally aromatic. Based on these results, we propose that local aromaticity can be gainfully utilized as a new potential magic rule in the search for magic numbers.     

Criteria for aromaticity of mesoionic heterocycles

The quantum chemical calculations of the basic criteria for aromaticity (nucleus-independent chemical shift (NICS), aromatic stabilization energy (ASE), and parameters of harmonic oscillator model of aromaticity (HOMA), and geometric indices (I ₅)) of 54 mesoionic heterocycles in the 6–31G* split-valence basis set were performed in terms of the density functional theory (DFT) with the B3LYP exchange-correlation hybrid functional. The aromatic nature of the mesoionic heterocycles containing the pyridinium N atom was shown.     

Effects on the aromatic character of DNA/RNA nucleobases due to its adsorption onto graphene

In the last years, experimental/theoretical studies have shown that graphene has a strong affinity toward nucleobases, serving as a promising nanomaterial for self‐assembly, sensing, and/or sequencing of DNA/RNA constituents. Then, a complete picture of the properties of the nucleobase–graphene systems is required for its use in technological applications. This work describes a detailed quantum chemical analysis of the aromaticity of adsorbed nucleobases on graphene, comparing between aromaticity indexes based on magnetic, geometry, electron density, and electron delocalization properties of graphene–nucleobase systems. Contrary to the stated by magnetic‐based aromaticity criteria (such as nucleus‐independent chemical shifts), it is proved that the aromatic character of nucleobases is not increased/decreased upon binding on graphene. Therefore, magnetic aromaticity criteria are not recommended to analyze aromaticity in related systems, unless a fragmented scheme be adopted. Finally, these results are expected to expand the knowledge about the understanding of biomolecules‐graphene interactions.     

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

It has been previously indicated that pristine graphene cannot detect NO₂ 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 NO₂ 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 NO₂ molecule prefers to be adsorbed on the central ring with a nitro configuration, releasing an energy of about 13.2 kJ/mol. The NO₂ 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 NO₂ gas by HBC can produce an electrical signal selectively in the presence of O₂, H₂, N₂, CO₂, and H₂O gases. A short recovery time about 1.9 ns is predicted and the effect of density functional is investigated.     

In search of the smallest boroxol-type heterocyclic ring system: Planar hexagonal B3S3+ cluster with double 6π/2σ aromaticity

Boroxol (B₃O₃) rings and relevant hexagonal B₃S₃ structural blocks are ubiquitous in boron oxide/sulfide glasses, crystals, and high-temperature liquids. However, the isolation of an ultimate heterocyclic B₃O₃ or B₃S₃ cluster in the free-standing form, with as few as six atoms, has been unsuccessful so far. We report on computational design of the simplest case of such a system: the highly symmetric D_3h B₃S₃⁺ (¹A₁′) cluster. It is the well-defined global minimum on the potential energy surface, following global searches and electronic structure calculations at the B3LYP and single-point CCSD(T) levels. Chemical bonding analysis reveals an ideal system with skeleton Lewis B S σ single bonds and unique double 6π/2σ aromaticity, which underlies its stability. The cluster turns out to be an inorganic analog of the 3,5-dehydrophenyl cation, a typical double π/σ aromatic system. It offers an example for chemical analogy between boron-based heterocyclic clusters and aromatic hydrocarbons. Double π/σ aromaticity is also a new concept in heterocyclic boron clusters. Previous systems such as borazine, boroxine, and boronyl boroxine only deal with π aromaticity as in benzene.     

Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird‐Aromatic Triplet Ground State Compounds

Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed‐shell singlet state (normally ground state, S₀) and lowest ππ* triplet state (T₁ or T₀), as given by Hückel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T₁ and S₀ states. This effect is caused by a reversal in the dipole moment when going from S₀ to T₁ as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S₀ reduces (enhances) aromaticity in T₁, allowing for rationalizations of the triplet state energies (E_T) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted E_T. As a means to increase the π‐electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6‐dicyanopentafulvenes can be a route to on‐demand formation of a fulvenium dication with a triplet ground state (T₀). The five‐membered ring of this species is markedly Baird‐aromatic, although less than the cyclopentadienyl cation known to have a Baird‐aromatic T₀ state.     

Theoretical study of hydrogenated tetrahedral aluminum clusters

We report on the structures of aluminum hydrides derived from a tetrahedral aluminum (Al₄) cluster using ab initio quantum chemical calculation. Our calculation of binding energies of the aluminum hydrides reveals that stability of these hydrides increases as more hydrogen atoms are adsorbed, while stability of Al – H bonds decreases. We also analyze and discuss the chemical bonds of those clusters by using recently developed method based on the electronic stress tensor. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Z/E-photoisomerizations of olefins with 4npi- or (4n + 2)pi-electron substituents: zigzag variations in olefin properties along the T(1) state energy surfaces

[Figure: see text]. A quantum chemical study has been performed to assess changes in aromaticity along the T1 state Z/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1 energy surface with highest substituent aromaticity corresponds to the minimum. According to Baird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged when going from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partially antiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twist of the C=C bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1 should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins. This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bond lengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independent chemical shifts whose differences along the T1 PES display zigzag dependencies on the number of -electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in the profiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in set A have minima at 3p* whereas those in set B have maxima at such structures. The proper combination (fusion) of the substituents of set A and B olefins could allow for design of novel optical switch compounds that isomerize adiabatically with high isomerization quantum yields.     

Performance of 3D‐space‐based atoms‐in‐molecules methods for electronic delocalization aromaticity indices

Several definitions of an atom in a molecule (AIM) in three‐dimensional (3D) space, including both fuzzy and disjoint domains, are used to calculate electron sharing indices (ESI) and related electronic aromaticity measures, namely, I_ring and multicenter indices (MCI), for a wide set of cyclic planar aromatic and nonaromatic molecules of different ring size. The results obtained using the recent iterative Hirshfeld scheme are compared with those derived from the classical Hirshfeld method and from Bader's quantum theory of atoms in molecules. For bonded atoms, all methods yield ESI values in very good agreement, especially for C–C interactions. In the case of nonbonded interactions, there are relevant deviations, particularly between fuzzy and QTAIM schemes. These discrepancies directly translate into significant differences in the values and the trends of the aromaticity indices. In particular, the chemically expected trends are more consistently found when using disjoint domains. Careful examination of the underlying effects reveals the different reasons why the aromaticity indices investigated give the expected results for binary divisions of 3D space. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011.     

Electron correlations in molecules. III. Strength of electron correlations in localized and aromatic bonds of main-group atoms

We investigate electron correlations within a minimal basis of various chemical bonds formed by second- and third-row atoms and hydrogen. The effects of correlations are characterized by two quantities: (i) intrabond correlation energy ϵ_corr and (ii) the correlation strength parameter Σ. The latter describes the relative suppression of the charge fluctuations within a bond. The quantities ϵ_corr and Σ can be associated with chemical bonds due to the local nature of the correlation phenomenon. It is found that one may distinguish between different classes of chemical bonds which are characterized by similar values of Σ parameter within each class. For instance, all considered bonds between hydrogen and main-group atoms fall into one class. The homopolar bonds formed by the atoms of the third row are characterized by a weaker σ but stronger π correlation strength than the respective bonds of the second row. The strongest correlations are found in homopolar single π bonds of third-row atoms. A particular detailed discussion is given of heteropolar bonds. The electron correlations of aromatic π electrons are found to be considerably weaker than those within the corresponding localized bonds. For the SCF input of the correlation calculation a semi-empirical method is used. The bond orbital approximation allows for a transparent interpretation of the computational results.     

(Li6Si5)2–5: The Smallest Cluster-Assembled Materials Based on Aromatic Si56− Rings

Extensive explorations of their potential energy surfaces, combined with high-level quantum chemical computations, strikingly show that the lowest energy structures of the (Li₆Si₅)_2–5 systems consist of 2–5 Si₅⁶⁻ aromatic units, surrounded by Li⁺ counterions, respectively. These viable gas-phase compounds are the pioneering reported cases of oligomers made by planar aromatic silicon rings. Based on the key evidence that these oligomers are energetically favored, and that their silicon rings aromaticity is thoroughly preserved, the Li₆Si₅ cluster is proposed as a potential assembly unit to build silicon-lithium nanostructures, thus opening new paths to design Zintl compounds at the nanoscale level.     

DFT study of “all-metal” aromatic compounds

An overview of recent quantum chemical studies on all-metal aromatic compounds is presented. Novel classes of inorganic molecules containing bonds that are characterized by a common ring-shaped electron density are reviewed. The mechanistic insight gained for the aromatic character of all-metal aromatic molecules is discussed and the predictive nature of the electronic structure calculation methods particularly those based on density functional theory (DFT) is highlighted. The indicators of aromaticity (aromaticity criteria) - structural, magnetic, energetic and reactivity-based measures - many of which are accessible through quantum chemical calculations are also outlined herein.     

Molecular structure and bonding character of mono and divalent metal cations (Li+, Na+, K+, Be2+, Mg2+, and Ca2+) with substituted benzene derivatives: AIM,NBO, and NMR analyses

Cation–π complexes between several cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, and Ca²⁺) and different π-systems such as para-substituted (F, Cl, OH, SH, CH₃, and NH₂) benzene derivatives have been investigated by UB3LYP method using 6-311++G** basis set in the gas phase and the water solution. The ions have shown cation–π interaction with the aromatic motifs. Vibrational frequencies and physical properties such as dipole moment, chemical potential, and chemical hardness of these compounds have been systematically explored. The natural bond orbital analysis and the Bader’s quantum theory of atoms in molecules are also used to elucidate the interaction characteristics of the investigated complexes. The aromaticity is measured using several well-established indices of aromaticity such as NICS, HOMA, PDI, FLU, and FLUπ. The MEP is given the visual representation of the chemically active sites and comparative reactivity of atoms. Furthermore, the effects of interactions on NMR data have been used to more investigation of the studied compounds.     

Theoretical study of nitrogen‐doped graphene nanoflakes: Stability and spectroscopy depending on dopant types and flake sizes

As nitrogen‐doped graphene has been widely applied in optoelectronic devices and catalytic reactions, in this work we have investigated where the nitrogen atoms tend to reside in the material and how they affect the electron density and spectroscopic properties from a theoretical point of view. DFT calculations on N‐doped hexagonal and rectangular graphene nanoflakes (GNFs) showed that nitrogen atoms locating on zigzag edges are obviously more stable than those on armchair edges or inside flakes, and interestingly, the N‐hydrogenated pyridine moiety could be preferable to pure pyridine moiety in large models. The UV–vis absorption spectra of these nitrogen‐doped GNFs display strong dependence on flake sizes, where the larger flakes have their major peaks in lower energy ranges. Moreover, the spectra exhibit different connections to various dopant types and positions: the graphitic‐type dopant species present large variety in absorption profiles, while the pyridinic‐type ones show extraordinary uniform stability and spectra independent of dopant positions/numbers and hence are hardly distinguishable from each other. © 2018 Wiley Periodicals, Inc.     

In-Situ Electronegativity and the Bridging of Chemical Bonding Concepts

One challenge in chemistry is the plethora of often disparate models for rationalizing the electronic structure of molecules. Chemical concepts abound, but their connections are often frail. This work describes a quantum-mechanical framework that enables a combination of ideas from three approaches common for the analysis of chemical bonds: energy decomposition analysis (EDA), quantum chemical topology, and molecular orbital (MO) theory. The glue to our theory is the electron energy density, interpretable as one part electrons and one part electronegativity. We present a three-dimensional analysis of the electron energy density and use it to redefine what constitutes an atom in a molecule. Definitions of atomic partial charge and electronegativity follow in a way that connects these concepts to the total energy of a molecule. The formation of polar bonds is predicted to cause inversion of electronegativity, and a new perspective of bonding in diborane and guanine−cytosine base-pairing is presented. The electronegativity of atoms inside molecules is shown to be predictive of pK_a.     

An ab initio valence bond (VB) calculation of the π delocalization energy in borazine,B3N3H6

Valence bond (VB) calculations using a double‐zeta D95 basis set have been performed for borazine, B₃N₃H₆ and for benzene, C₆H₆ in order to determine the relative weights of individual standard Lewis structures. In the delocalized resonance scheme of borazine, the structure ( I ) with no double bonds and three lone pairs of electrons at the three nitrogen atoms is the major contributor with a structural weight of 0.17, followed by six equivalent Lewis structures with one double bond and two lone pairs at two nitrogen atoms ( II ) with weights of 0.08 each. In the case of benzene, the two Kekulé structures ( III ) contribute with structural weights of 0.15 each, followed by 12 equivalent ionic structures ( IV ) with weights of 0.03 each, followed by the three equivalent Dewar‐type structures ( V ) with structural weights of 0.02 each. The values of 54.1 and 45.8 kcal mol⁻¹ for the delocalization energies of borazine and benzene were estimated. Therefore, B₃N₃H₆ is calculated to have substantial aromatic character, similar to benzene, when we assume that the resonance energy can provide a criterion for aromaticity. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:311–315, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20095     

Aromatic gold and silver "rings": hydrosilver(I) and hydrogold(I) analogues of aromatic hydrocarbons

Quantum chemical calculations suggest that a series of molecules with the general formula cyclo-Mn(mu-H)n (M = Ag, Au; n = 3-6) are stable. All cyclo-MnHn species, except cyclo-Au(3)H(3), have the same symmetry with the respective aromatic hydrocarbons but differ in that the hydrogen atoms are in bridging positions between the metal atoms and not in terminal positions. The aromaticity of the hydrosilver(I) and hydrogold(I) analogues of aromatic hydrocarbons was verified by a number of established criteria of aromaticity, such as structural, energetic, magnetic, and chemical criteria. In particular, the nucleus-independent chemical shift, the relative hardness, Deltaeta, the electrophilicity index, omega, and the chemical reactivity toward electrophiles are indicative for the aromaticity of the hydrosilvers(I) and hydrogolds(I). A comprehensive study of the structural, energetic, spectroscopic (IR, NMR, electronic, and photoelectron spectra), and bonding properties of the novel classes of inorganic compounds containing bonds that are characterized by a common ring-shaped electron density, more commonly seen in organic molecules, is presented.     

Simulation of structure and stability of carbon nanoribbons

Results of carbon nanoribbons and nanotubes simulation by means of hybrid density functional method and using empirical potentials have been compared. Energy of the nanoribbons formation and their citting from graphene sheet as well as energy of the nanotubes folding from graphene and nanoribbons have been determined. The REBO force field satisfactorily reflects the result of quantum chemical simulations; however, it cannot reproduce the formation of triple bonds between the edge atoms of the nanoribbons in the armchair conformation and thus leads to underestimated stability of the latter. Energy of the nanotubes folding from the nanoribbons is linear with the nanotube diameter.     

Aromaticity Reversal in the Lowest Excited Triplet State of Archetypical Möbius Heteroannulenic Systems

The aromaticity reversal in the lowest triplet state (T₁) of a comparable set of Hückel/Möbius aromatic metalated expanded porphyrins was explored by optical spectroscopy and quantum calculations. In the absorption spectra, the T₁ states of the Möbius aromatic species showed broad, weak, and ill‐defined spectral features with small extinction coefficients, which is in line with typical antiaromatic expanded porphyrins. In combination with quantum calculations, these results indicate that the Möbius aromatic nature of the S₀ state is reversed to Möbius antiaromaticity in the T₁ state. This is the first experimental observation of aromaticity reversal in the T₁ state of Möbius aromatic molecules.