Aromaticity and relative stabilities of azines

The most refined nucleus-independent chemical shift index (NICS(0)(πzz)) and the extra cyclic resonance energies (ECREs), based on the block localized wave function (BLW) method, show that the aromaticity of all azines is like that of benzene. The same is true for aza-naphthalenes relative to naphthalene. The lower relative energies of isomers with vicinal N's are due to the weakness of NN bonds rather than to reduced aromaticity.     

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.     

Theoretical study of structure, bonding, and aromaticity of borazyne and B-substituted borazynes

The density functional theory (DFT) is used to study the geometries, and electronic structures of triplet and singlet of borazyne and B-substituted of borazyne. The aromaticity of these systems is analyzed in the light of nucleus-independent chemical shift (NICS), average of two-center indices (ATI). These methods show increasing of aromaticity in deactivating groups. The relation between electron density in ring critical point (RCP) and NICS(1.0) is observed. The most important interaction in these molecules has been investigated by natural bonding orbital method (NBO).     

The delocalization index as an electronic aromaticity criterion: application to a series of planar polycyclic aromatic hydrocarbons

This work introduces a new local aromaticity measure, defined as the mean of Bader's electron delocalization index (DI) of para-related carbon atoms in six-membered rings. This new electronic criterion of aromaticity is based on the fact that aromaticity is related to the cyclic delocalized distribution of pi-electrons. We have found that this DI and the harmonic oscillator model of aromaticity (HOMA) index are strongly correlated for a series of six-membered rings in eleven planar polycyclic aromatic hydrocarbons. The correlation between the DI and the nucleus-independent chemical shift (NICS) values is less remarkable, although in general six-membered rings with larger DI values also have more negative NICS indices. We have shown that this index can also be applied, with some modifications, to study of the aromaticity in five-membered rings.     

过渡金属团簇Nbn、Con(n≤4)和NbxCoy(x+y≤8)的芳香性

采用杂化密度泛函(DFT)方法优化了过渡金属纯团簇Nbn, Con(n≤4)和二元铌钴团簇NbxCoy(x+y≤8)的结构, 并计算了较稳定结构的NICS(核独立化学位移)值, 分析这些过渡金属团簇的成键情况, 讨论不同轨道对各过渡金属团簇芳香性的贡献, 发现在过渡金属团簇中, 除了具有s、p轨道贡献的σ、π芳香性外, 很重要的地d轨道的参与而形成的δ芳香性.     

Aromaticity and curvature in heteroacepentalenes

Heteroatom-substituted acepentalene derivatives which are isoelectronic with the known acepentalenediide dianion are nonplanar, fused aromatic tricycles which are hemifullerenes of the corresponding C20 heteroanalogue. Depending on the number and position of heteroatoms, they may be anionic, neutral, or cationic. A nucleus-independent chemical shift study indicates that substitution of the central carbon of the acepentalenediide system with N or O results in a substantial increase in aromaticity. Peripheral aza substitution on the other hand tends to increase curvature and decrease aromaticity. Alkylation or protonation at the central position of asymmetrically substituted heteroacepentalenides leads to chiral, bowl-shaped, 10 pi aromatic species.     

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.     

The aromaticity/antiaromaticity continuum. 1. Comparison of the aromaticity of the dianion and the antiaromaticity of the dication of tetrabenzo[5.5]fulvalene via magnetic measures

The aromaticity of the dianion (2) and the antiaromaticity of the dication (3) of tetrabenzo[5.5]fulvalene have been evaluated through magnetic criteria, (1)H NMR shifts, nucleus-independent chemical shifts, NICS, and magnetic susceptibility exaltation, Lambda. The sum of the NICS values, using the GIAO (gauge-independent atomic orbital) method, for 2 is -35.2; that of 3 is +38.2, indicating the aromaticity of 2 and the antiaromaticity of 3. Calculation of magnetic susceptibility exaltation using the CSGT (continuous set of gauge transformations) method gives a similar result, with Lambda of -81.8 ppm cgs for 2 and 95.8 ppm cgs for 3. The general validity of these values is supported by excellent agreement between the NMR shifts calculated by the GIAO and CSGT methods with experimental shifts. Comparison of 1H NMR shifts with those of model compounds allows evaluation of the magnitude of the diatropic shift in 2 and paratropic shift in 3 and supports their assignment as aromatic/antiaromatic, respectively. The agreement between calculated and experimental 1H NMR shifts is excellent for 3 in the absence of counterions but much better for 2 when counterions are included. Inclusion of counterions in the evaluation of diatropic shift for 2 gave a smaller shift than in the absence of counterions, suggesting a decreased aromaticity. When counterions were included in the calculation of Lambda, the value was also decreased, suggesting a decreased aromaticity. This observation has important consequences in the use of experimental data for the evaluation of aromaticity, and presumably antiaromaticity, of anions since, in most cases, there will be close interaction with counterions.     

Are chelate rings aromatic? Calculations of magnetic properties of acetylacetonato and o-benzoquinonediimine chelate rings

The aromaticity of the chelate rings of acetylacetonato (acac) and o-benzoquinonediimine (bqdi) ligands was investigated theoretically by calculating nucleus-independent chemical shifts (NICS). The calculations were done for the complexes with various metals and various other ligands. The results show that acac chelate rings in none of the complexes satisfy this magnetic criterion for aromaticity. According to the results for bqdi chelate rings, there is only the Ru2+-bqdi chelate ring with large negative NICS values, indicating possible aromaticity by magnetic criterion.     

NIR-Absorbing π-Extended Azulene: Non-Alternant Isomer of Terrylene Bisimide

The first planar π-extended azulene that retains aromaticity of odd-membered rings was synthesized by [3+3] peri-annulation of two naphthalene imides at both long-edge sides of azulene. Using bromination and subsequent nucleophilic substitution by methoxide and morpholine, selective functionalization of the π-extended azulene was achieved. Whilst these new azulenes can be regarded as isomers of terrylene bisimide they exhibit entirely different properties, which include very narrow optical and electrochemical gaps. DFT, TD-DFT, as well as nucleus-independent chemical shift calculations were applied to explain the structural and functional properties of these new π scaffolds. Furthermore, X-ray crystallography confirmed the planarity of the reported π-scaffolds and aromaticity of their azulene moiety.     

Quantification of the aromaticity of 2-alkylidenethiazolines subjected to push-pull activity

Through-space NMR shieldings (TSNMRSs) of a series of 2-alkylidenethiazolines subjected to push-pull activity have been calculated by the GIAO method employing the nucleus-independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs). The ICSSs were applied to quantify and visualize the degree of aromaticity of the studied compounds, which has been shown to be in excellent correlation with the push-pull behavior, quantified by the quotient (π*/π) method. Dissection of the absolute magnetic shielding values into individual contributions of bonds and lone pairs by the natural chemical shielding-natural bond orbital (NCS-NBO) analysis has revealed unexpected details.     

An Additivity Scheme for Aromaticity: The Heteroatom Case

The nucleus-independent chemical shift (NICS)-XY-Scan is a simple and easy tool for the quantitative measurement of the aromaticity of polycyclic aromatic hydrocarbons and identification of the existence of local and global ring currents. We recently introduced an additivity scheme that uses the NICS-XY-Scans of smaller building blocks to predict the aromatic profiles of larger polycyclic aromatic hydrocarbon systems. We now report on an expansion of the methodology to include systems of varying aromatic natures containing the heteroatoms B, N, O, and S. The additivity approach allows for rapid and resource-efficient generation of NICS-XY-Scans of large, complex systems. Moreover, it reveals that the magnetic criterion of aromaticity behaves in an additive manner, and that the ring currents of multi-ring systems appear to be mostly localized within subunits of up to three rings.     

Ring currents in polycyclic sodium clusters

In the recent work by Khatua et al. (Khatua, S.; Roy, D. R.; Bultinck, P.; Bhattacharjee, M.; Chattaraj, P. K. Phys. Chem. Chem. Phys.2008, 10, 2461-2474) the synthesis and structure of a fac-trioxo molybdenum metalloligand and its sodium complex containing 1D hexagonal chains of sodium ions was reported. In the same paper, the aromaticity of hexagonal Na clusters was quantified by means of the nucleus-independent chemical shift and electronic multicenter indices. It was shown that the aromaticity of hexagonal Na-clusters is of the same order as the aromaticity of analogous benzenoid hydrocarbons. In the present study current density maps are used to rationalize the aromaticity of polycyclic Na clusters. It is shown that although polycyclic Na systems sustain a diatropic ring current, the induced current density is several times weaker than in analogous benzenoid hydrocarbons. A detailed analysis indicates that the current density in hexagonal Na systems is almost completely determined by four HOMO σ electrons.     

Non-conventional Hydrogen Bonding and Aromaticity: A Systematic Study on Model Nucleobases and Their Solvated Clusters

The conceptual development of aromaticity is essential to rationalize and understand the structure and behavior of aromatic heterocycles. This work addresses for the first time, the interconnection between aromaticity and sulfur/selenium centered hydrogen bonds (S/SeCHBs) involved in representative heterocycle models of canonical nucleobases (2-Pyridone; 2PY) and its sulfur (2-Thiopyridone; 2TPY) and selenium (2-Selenopyridone; 2SePY) analogs. The nucleus-independent chemical shift (NICS) and gauge induced magnetic current density (GIMIC) values suggested significant reduction of aromaticity upon replacement of exocyclic carbonyl oxygen with sulfur and selenium. However, we observed two-fold (57 %) and three-fold (80 %) enhancement in the aromaticity for 2TPY dimer, and 2SePY dimer, respectively which are connected through S/SeCHBs. Aromaticity enhancement was also noticed in 1 : 1 H-bonded complexes (heterodimers), micro hydrated clusters and for bulk hydration. It is expected that exocyclic S and Se incorporation into heterocycles without compromising aromatic loss would definitely reinforce to design new supramolecular building blocks via S/SeCH-bonded complexes.     

The 9-Borataphenanthrene Anion

The 9-borataphenanthrene anion is easily accessed by deprotonation of a 9,10-dihydro-9-boraphenanthrene and its diverse reactivity is investigated. Alkylation occurs at the carbon atom adjacent to boron, and room temperature hydroboration occurs across the B=C bond. The π-manifold of the central BC₅ ring coordinates to chromium in an η⁶ fashion while only the B=C unit binds η² to gold, indicating versatility of the 9-borataphenanthrene anion as a ligand. Supporting calculations rationalize the reactivity and aromaticity is corroborated by nucleus-independent chemical shift (NICS) indices.     

Electronic structures, intramolecular interactions, and aromaticity of substituted 1-(2-iminoethylidene) silan amine: a density functional study

The intramolecular hydrogen bond, molecular structure, π electrons delocalization, and vibrational frequencies in 1-(2-iminoethylidene) silan amine and its derivatives have been investigated by means of density functional method with 6-311++G** basis set, in gas phase, water, and carbon tetrachloride solutions. The obtained results showed that the hydrogen bond strength is mainly governed by resonance variations inside the chelate ring induced by the substituent groups. Furthermore, the topological properties of the electron density distributions for N–H···N intramolecular hydrogen bond were analyzed in terms of the Bader's theory of atoms in molecules. On the other hand, the aromaticity of the ring formed is measured using several well-established indices of aromaticity such as nucleus-independent chemical shift, harmonic oscillator models of the aromaticity, para-delocalization index, average two-center indices, aromatic fluctuation index, and π-fluctuation aromatic index. Natural population analysis data, the electron density and Laplacian properties, as well as γ(NH) and ν(NH) were further used for estimation of the hydrogen bonding interactions and the forces driving their formation.