Aromaticity Of Ring Research Articles

Assessment of the Performance of the Bond Resonance Energy, Circuit Resonance Energy, Magnetic Resonance Energy, Ring Currents, and Aromaticity of Anthracene, Biphenylene, Phenalenyl, and p-Terphenyl.

This paper contributes to the further understanding of bond resonance energy (BRE), circuit resonance energy (CRE), and ring currents (RC = I) as indices for the measure of local ring aromaticity and topological resonance energy (TRE) and magnetic resonance energy (MRE) as indices for the measure of global molecular aromaticity. Calculations of ring currents in the 3-ring polycyclic hydrocarbons of anthracene, biphenylene, phenalenyl, and p-terphenyl are demonstrated. This comparative study provides new molecular insights. A number of shortcut techniques are briefly illustrated. p-Terphenyl is a benzenoid total resonant sextet (TRS) and biphenylene is shown to be a nonbenzenoid TRS, i.e., both obey Clar's sextet principle. p-Terphenyl has only ring circuits, whereas anthracene, biphenylene, and phenalenyl have ring circuits and larger circuits engulfing the ring circuits. The perimeter circuits of anthracene, biphenylene, and phenalenyl engulf all other circuits, thus leading to the definition of MRE. By these several aromatic indices, the monocation, monoanion, and monoradical of phenalenyl are equally aromatic. The agreement in the prediction of local ring aromaticity between the three indices of m-BRE, t-BRE, and CRE is evaluated.

Synthesis and characterization of the layered insensitive pentazolate salts based on two triazolium isomers

The all-nitrogen cyclo-pentazolate (c-N5ˉ) anion has attracted interest because of its promising application in high-energy materials. Now, nonmetallic pentazolate salts were prepared by assembling cyclo-N5ˉ and cations but it was shown to be challenging to balance excellent detonation performance and good stability, which results in restrictions for practical applications. However, we find the layer-by-layer crystal stacking structure can enhance the sensitivity of pentazolate salts. Herein, we designed and synthesized two targeted cyclo-pentazolate salts with layered stacking mode in their crystal structures based on two isomeric cations: 3,4,5-triamino-1,2,4-triazolium and 3-hydrazino-4-amino-1,2,4-triazolium. The results demonstrate that the aromaticity of triazole and pentazole rings arouse the π−π interaction force, promoting the formation of a layer-by-layer crystal stacking structure. Two compounds show very low sensitivity characteristics (FS>40 and IS>360) and good detonation properties (detonation velocity: 3, Dv= 8796m⋅s−1; 4, Dv=8574m⋅s−1. detonation pressure: 3, P = 26.6 GPa; 4, P = 25.1 GPa). Computational analysis based on noncovalent interaction index, localized orbital locator-π and electron density difference provide a better understanding about the effects of layer-by-layer stacking structure on the stabilities. These findings contribute to preparation of both high-energy and low sensitivity pentazolate salts.

Electronic structures of zinc octaalkylthio- and octaphenylthio-tetrapyrazinoporphyrazines, and comparison with those of phthalocyanines and pyrazinoporphyrazines

Eight strongly electron-donating alkylthio- or arylthio-groups have been introduced to a strongly electron-withdrawing pyrazinoporphyrazine (PyZ) core, to form the Zn complexes of octadecylthiolated ZnPyZ, (SDc)[Formula: see text]ZnPyZ, and octaphenylthiolated ZnPyZ, (S[Formula: see text])[Formula: see text]ZnPyZ, and their electronic structures studied by 1H NMR and electronic absorption spectroscopy. The strength of the ring current, aromaticity, and flow of charge were discussed by calculating the ring current, nucleus-independent chemical shift (NICS), and anisotropy of the current-induced density (ACID), respectively. Good agreement was seen for the results of these three calculations, demonstrating that a strong ring current of the macrocycle is produced when the aromatic ring fused to the tetraazaporphyrin core has a weaker diamagnetic ring current, [Formula: see text]. weaker aromaticity (negatively smaller NICS values). TD-DFT molecular orbital (MO) calculations succeeded in explaining the differences of the absorption spectra between these octathiolated ZnPyZs and those without thiolated groups. The singlet oxygen quantum yield in pyridine (0.62) of (S[Formula: see text])[Formula: see text]ZnPyZ was similar to that (0.61) of normal ZnPc.

Extension and Quantification of the Fries Rule and Its Connection to Aromaticity: Large-Scale Validation by Wave-Function-Based Resonance Analysis.

The Fries rule is a simple, intuitive tool to predict the most dominant Kekulé structures of polycyclic aromatic hydrocarbons (PAHs), which is valuable for understanding the structure, stability, reactivity, and aromaticity of these conjugated compounds. However, it still remains an empirical hypothesis, with limited qualitative applications. Herein, we verify, generalize, and quantify the Fries rule based on the recently developed resonance analysis of the DFT wave functions of over 1500 PAH and fullerene molecules with over a billion Kekulé structures. The extended rules, counting the numbers of electrons within all rings (not just sextets), are able to rank the relative importance of all Kekulé structures for all considered systems. The statistically meaningful quantification also opens a way to evaluate ring aromaticity based on the resonance theory, which generally agrees well with conventional aromaticity descriptors. Furthermore, we propose a purely graph-based aromaticity indicator nicely applicable to PAHs and fullerenes, with no need of any quantum chemistry calculations, so that it can make valuable predictions for molecular properties that are related to local aromaticity.

A study on the aromaticity and ring currents of polycyclic neutral oxocarbon isomers

AbstractIn this study, topological resonance energy (TRE) methods and ring current (RC) methods were used to describe the global aromaticity of a series of polycyclic neutral oxocarbon isomers. The observed trends in aromaticity are explained. Our results obtained using the TRE method are compared with those obtained using the nucleus independent chemical shift (NICS) method as reported in the literature. The bond resonance energy (BRE) and circuit resonance energy (CRE) indices were applied to study the local aromaticity of oxocarbons. The aromaticity of the individual rings of each of the polycyclic neutral oxocarbon isomers studied was assessed using the Hückel–London ring current (RC) model. The results regarding the local aromatic and antiaromatic states of the individual rings as obtained using the BRE and CRE indices show direct correspondence with the RC results, which indicate the strength of the diatropic and paratropic currents within those rings.

Analytical pyrolysis of bagasse and groundnut shell briquettes: Kinetics and pyrolysate composition studies

Biomass briquettes are expanding its reach from domestic stoves to giant power plants. This study aims to evaluate the effect of compositional heterogeneity of bagasse and groundnut shell briquettes on slow pyrolysis kinetics, fast pyrolysis product distribution and time evolution of pyrolysates. The average apparent activation energy of pyrolysis was low for 0.71–1.4 mm particles of bagasse (175.5 kJ mol−1) and 1.4–2 mm particles of groundnut shell (169.3 kJ mol−1). The production of phenolics, furan derivatives, monoaromatics and condensed ring aromatics exhibited marked changes with respect to particle size of biomass and temperature. The phenolic selectivity was more from large biomass particles. The production of aromatic hydrocarbons is correlated with the particle size of biomass, and the extent of secondary reactions is shown to be coupled with heat and mass transport of pyrolysates within the confined pores of the sample. The evolution rate of pyrolysis vapors increased with increasing temperature.

Lewis or Brønsted? A Rectification of the Acidic and Aromatic Nature of Boranol-Containing Naphthoid Heterocycles.

Boron-containing heterocycles are important in a variety of applications from drug discovery to materials science; therefore a clear understanding of their structure and reactivity is desirable to optimize these functions. Although the boranol (B-OH) unit of boronic acids behaves as a Lewis acid to form a tetravalent trihydroxyborate conjugate base, it has been proposed that pseudoaromatic hemiboronic acids may possess sufficient aromatic character to act as Brønsted acids and form a boron oxy conjugate base, thereby avoiding the disruption of ring aromaticity that would occur with a tetravalent boronate anion. Until now no firm evidence existed to ascertain the structure of the conjugate base and the aromatic character of the boron-containing ring of hemiboronic "naphthoid" isosteres. Here, these questions are addressed with a combination of experimental, spectroscopic, X-ray crystallographic, and computational studies of a series of model benzoxazaborine and benzodiazaborine naphthoids. Although these hemiboronic heterocycles are unambiguously shown to behave as Lewis acids in aqueous solutions, boraza derivatives possess partial aromaticity provided their nitrogen lone electron pair is sufficiently available to participate in extended delocalization. As demonstrated by dynamic exchange and crossover experiments, these heterocycles are stable in neutral aqueous medium, and their measured pKa values are consistent with the ability of the endocyclic heteroatom substituent to stabilize a partial negative charge in the conjugate base. Altogether, this study corrects previous inaccuracies and provides conclusions regarding the properties of these compounds that are important toward the methodical application of hemiboronic and other boron heterocycles in catalysis, bioconjugation, and medicinal chemistry.

Role of heteroatoms and substituents on the structure, reactivity, aromaticity, and absorption spectra of pyrene: a density functional theory study

Abstract Effect of heteroatoms viz. BN and substituents viz. –Me (methyl), –OH (hydroxyl), –NH2 (amine), –COOH (carboxyl), and –CN (cyano) on the structural parameters, global reactivity, aromaticity, and UV-visible spectra of pyrene are studied with the help of density functional theory (DFT). Global reactivity parameters such as global hardness (η) and electrophilicity (ω) are calculated using density functional reactivity theory (DFRT). Time dependent density functional theory (TD-DFT) is explored for interpreting the UV-visible absorption spectra. Aromaticity of the pyrene rings are predicted from the nucleus independent chemical shift (NICS) values. Presence of BN unit and substituent induces reasonable impact on the studied parameters. The observed absorption spectra lie predominantly within the UV-region (both blue and red shifts are observed in presence of BN and substituent). HOMO energy and absorption spectra are affected nominally in solvent phase.

Synthesis, Spectroscopic, and Theoretical Study of Copper and Cobalt Complexes with Dacarbazine.

Dacarbazine (DAC) 5-(3,3-dimethyl-1-triazenyl)imidazole-4-carboxamide is an imidazole-carboxamide derivative that is structurally related to purines. DAC belongs to the triazene compounds, which are a group of alkylating agents with antitumor and mutagenic properties. DAC is a non-cell cycle specific drug, active in all phases of the cellular cycle. In the frame of this work the 3d metal complexes (cobalt and copper) with dacarbazine were synthesized. Their spectroscopic properties by the use of FT-IR, FT-Raman, and 1HNMR were studied. The structures of dacarbazine and its complexes with copper(II) and cobalt(II) were calculated using DFT methods. The effect of metals on the electronic charge distribution of dacarbazine was discussed on the basis of calculated NBO atomic charges. The reactivity of metal complexes in relation to ligand alone was estimated on the basis of calculated energy of HOMO and LUMO orbitals. The aromaticity of the imidazole ring in dacarbazine and the complexes were compared (on the basis of calculated geometric indices of aromaticity). Thermal stability of the investigated 3d-metal complexes with dacarbazine and the products of their thermal decomposition were analyzed.

Effects of Multiple OH/SH Substitution on the H‐Bonding/Stability versus Aromaticity of Benzene Rings: From Computational Insights

AbstractProperties of intramolecular H‐bond (IHB) for a chemical system can affect the changes in geometrical/electronic features in the vicinity of the bridge. Substitution of an atom (H) in benzene with OH/SH groups forming IHB has little effect on its aromaticity. This report is devoted to the depiction of the effect of OH/SH substituent on stability/aromaticity of multi‐substituted benzene ringsviaIHB formation. The computational studies on the stability/aromaticity in the homo (OH/SH) and hetero (OH and SH) substituted form of the benzene systems have been investigated using B3LYP/aug‐cc‐pVDZ and MP2/aug‐cc‐pVDZ approaches with the aid of aromaticity indices (AIs) descriptors (NICS, HOMA, AIBIC, and PDI) and IHB/stability diagnostics; relative energy, QTAIM based interaction energy acquired from the potential energy density and electron density, HBSBIC, and HOMO‐LUMO gap including NCI plots. The IHBs formed by different OH/SH substituents have a vital role as an aromaticity modulator and deploying the above‐said tools, it turns out that the cyclic 4n+2 π‐electrons delocalization is weakened/strengthened depending on the number and types of OH/SH substituents forming the IHBs. Here, all the AIs vary in a small amount and indicative of the modulator of the π‐electron structure to the substituent effectviaIHB formation. As in practical applications, the diagnostics based on different criteria do not speak with the same voice, hence interestingly, in many cases, the changes in AIs notably correlate with their stability.

The effect of ring aromaticity on ESIPT behavior and photophysical properties of 2-(2′-hydroxyphenyl)- 4-chloromethylthiazole derivatives: A TD-DFT study

By using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, the effect of ring aromaticity on the excited state intramolecular proton transfer (ESIPT) process and photophysical properties of HCO, HCSe, HPDCT and HPZCT compounds were explored in detail. The strengthened excited state intramolecular hydrogen bonds (Hbonds) of the studied compounds were confirmed by analysis of the structure, infrared vibration frequency, electron density and reduced density gradient. The stronger Hbond is favorable to ESIPT process. Frontier molecular orbitals evidently showed that the proton transfer was induced by the rearrangement of electronic density. The HOMO-LUMO energy gap (Egap) correlated with the Stokes shift in the enol form. A smaller Egap would produce a larger Stokes shift. The heteroatom substitution in thiazole ring and nitrogen substitution in benzene ring of HCT molecule affected the ring aromaticity, and then had an obvious effect on the intramolecular Hbond. The stronger the ring aromaticity, the stronger the intramolecular Hbond, and the easier the ESIPT process to occur.

General approach to substituted naphtho[1,2-b]benzofurans via photochemical 6π-electrocyclization of benzofuranyl containing cinnamonitriles

At the first time, we elaborated the method of synthesis of naphtho[1,2- b ]benzofurans based on photoinduced reaction of acrylonitriles containing 2-arylbenzofuran fragment as part of 1,3,5-hexatriene system. The considered reaction proceeds via initial 6π-electrocyclization followed by [1,9]- H sigmatropic shift and final oxidative aromatization of central benzene ring. It was shown that 5,6-dihydronaphtho[1,2- b ]benzofurans are key intermediates of studied process. The structures of obtained 5,6-dihydronaphtho[1,2- b ]benzofuran and one of the target naphtho[1,2- b ]benzofurans were confirmed by X-ray diffraction. • Photocyclization of benzofuranyl containing cinnamonitriles was studied. • The photochemical method of the synthesis of naphtho[1,2- b ]benzofurans was elaborated. • The key intermediate 5,6-dihydronaphtho[1,2- b ]benzofuran was isolated. • Typical structures were confirmed by X-ray crystallographic analysis.

Probing the origin of ambiphilic reactivity in osmapentalyne complexes: Interplay of ring strain, aromaticity, and phosphonium substituent

Ambiphilic reactivity is a fascinating topic in chemical reactions, attracting considerable interest because ambiphilic reagents can display properties of both nucleophilicity and electrophilicity. However, most of the previous attentions have been focused on the characterization of the ambiphilic reactivity, whereas the origin is less understood. Here we carry out thorough density functional theory (DFT) calculations to probe the origin of the ambiphilic reactivity of the carbyne atom in osmapentalynes, observed previously in experiment. A series of osmium carbyne complexes ranging from acyclic to cyclic, non-conjugated to conjugated, non-aromatic to aromatic, unsubstituted to substituted were designed for comparable studies. Our results indicate that the origin of ambiphilic reactivity in osmapentalyne complexes should be attributed to an interplay of ring strain, aromaticity, and phosphonium substituent.

Aromaticity of ortho and meta 8-Cycloparaphenylene and Their Dications: Induced Magnetic Field Analysis with Localized and Delocalized Orbitals in Strained Nanohoops.

Dications of cycloparaphenyles ([n]CPPs) are known to exhibit in-plane global aromaticity, contained in a nanobelt structure. Recently synthesized ortho and meta isomers of [n]CPPs break the radial symmetry of π structure incorporating perpendicular oriented π orbitals. Herein we set to explore the aromaticity of neutral and dicationic ortho and meta isomers of [8]CPP by dissecting the induced magnetic field to contributions of the twofold radial/perpendicular π system using delocalized canonical molecular orbitals (CMO), and introducing the natural localized molecular orbitals (NLMO) analysis with DFT methods. The dications sustain a reduced global aromatic character of the radial π system under a perpendicular orientation of the external field which declines from ortho to meta isomer and reinforces local aromaticity of ortho ring while it destroys aromaticity of meta ring. Aromaticity variations are determined by symmetry governed rotational excitations of frontier π orbitals. The parallel orientation reveals a substantial reduction of local aromaticity verified with NICSπ analysis and electron delocalization indices.

PAHs controlling soot nucleation in 0.101—0.811MPa ethylene counterflow diffusion flames

On the heels of a recent study in an atmospheric pressure ethylene diffusion flame in which the transition from parent fuel molecule to Polycyclic Aromatic Hydrocarbons (PAHs) and, eventually, soot was studied by spatially resolved measurements of PAH concentrations and soot quantities, we extended the focus to diffusion flames with self- similar structure in the 0.101–0.811 MPa pressure range. To that end, we complemented pyrometry based measurements of soot volume fraction with light scattering measurements that, once corrected for beam steering, yielded soot particle size and number concentration profiles. A chemistry model, that had been validated for all species up to 3 ring aromatics in one of the flames investigated at each pressure and up to 4-rings for the flame at atmospheric pressure, was used to compare profiles of chemical species to those of soot quantities. Further analysis yielded the assessment of number nucleation rates of soot and their comparison to the dimerization rates of PAHs. Soot nucleation rate is consistent only with the dimerization of one- and two-ring PAHs, an observation that confirms findings in the atmospheric pressure flame. Changes in pressure and temperature have a progressively larger impact on the concentration of aromatics of increasingly larger molecular weight and, even more so, on soot volume fraction and nucleation rate. A four-fold increase in pressure from 0.101 MPa to 0.405 MPa increases the soot nucleation rate and PAH dimerization rate in flames with constant peak temperature, which is primarily a concentration effect on bimolecular collision rates; a similar but less pronounced effect is observed in the higher (0.405–0.811 MPa) pressure range. Changes in pressure and temperature tend to be progressively more consequential on aromatics of increasing molecular weight and soot.

Hole Transport Materials by Direct C‐H Arylation for Organic Solar Cells: Effect of Structure and Conjugation on Electrical, Optical and Computational Properties

AbstractSignificant efforts have been devoted to the synthesis of conjugated molecules due to their interesting properties for organic electronic applications, it is surprising that very few studies have been reported on the development of small molecules for hole transport material (HTM) in organic solar cells through direct C−H arylation. In this work, we have studied the reaction conditions used for the facile synthesis of a series of π conjugated small molecules through direct C−H arylation. The reaction condition was optimized by using different solvents, bases and ligands to achieve the maximum possible yield. Ten phenyl(s)‐flanked conjugated small molecules were prepared using the optimized reaction condition and were characterized. Density functional theory was studied on these molecules to understand the structure‐property relationship and predicting the reliable geometry, electronic structure and properties of conjugated systems. Electrical and optical properties were measured by cyclic voltammetry and UV‐vis absorption spectroscopy. Finally, the organic solar cells were fabricated by solution‐processable deposition of these HTM with device geometry of ITO/HTM/PCDTBT:PC70BM/Al and achieved a power conversion efficiency of 2.40 % under ambient conditions. This method has the advantages of using different aromaticity of heterocyclic rings combined with conjugation length and electronegativity of heteroatom(s) to allow fine‐tuning of HOMO level and band gap engineering.

Synthesis, structural characterization, and bonding analysis of two-coordinate copper(I) and silver(I) complexes of pyrrole-based bis(phosphinimine): new metal-pyrrole ring π-interactions.

The reaction between 2,5-bis(diphenylphosphinomethyl)pyrrole and Me3SiN3 gave the new pyrrole-based bis(phosphinimine) L1H in an excellent yield. L1H reacts with [CuCl(COD)]2, AgBF4, or AgOTf to give the corresponding two-coordinate mononuclear ionic complex formulated as [M{(L1H)-κ2N,N}]+[X]- where M = Cu and Ag; X = [CuCl2], BF4 or OTf. Their single crystal X-ray diffraction studies confirmed the two-coordinate geometry formed by the chelate bonding mode of L1H. These 10-membered metalacycles exhibit planar chirality and were also characterized by spectroscopic methods. In addition, in all three structures, there exists a hitherto unknown π-interaction between the pyrrole ring atoms and metal, represented as η2-(Cα-N) in the copper(i) complex, and η3-(Cα-N-Cα') in the silver(i) complexes. These weak interactions were supported by DFT calculations in terms of their electron densities, non-covalent interaction plots and the decrease in the aromaticity of the pyrrole ring.

Photochemical synthesis of novel naphtho[1,2-b]benzofuran derivatives from 2,3-disubstituted benzofurans

A novel approach to the synthesis of naphtho[1,2-b]benzofuran derivatives based on the photochemical reaction of 2,3-disubstituted benzofurans was developed. The studied process includes the photocyclization of the hexatriene system and subsequent aromatization of benzene ring via elimination of water molecule. The starting terarylenes were prepared via a new three-component condensation of phenols, arylglyoxals, and cyclic 1,3-diketones.

New Insight into the Aromaticity of cyclo-N5– by Constructing 3D Arrays in Crystal Structures

Pentazole (HN5) has been actively pursued due to its intriguing aesthetic structure and potential applications as ultrahigh energetic materials. It is generally accepted that cyclo-N5– is a highly symmetric ring with good aromaticity. In this work, we designed two innovative pentazolate salts with 3D arrays in their crystal structures and found that the aromaticity of the cyclo-N5– anion could be damaged by specific weak interactions. In such fascinating crystal structures, multiple cyclo-N5– anions are regularly stacked with two modes (“antiparallel” and “vertical”) in the parallelepiped spaces constructed by chain cations, forming 3D arrays of cations and anions, respectively. Notably, the aromaticity of “vertical” cyclo-N5– anions is damaged due to the geometric changes caused by strong local hydrogen bonds at room temperature; meanwhile, the unstable “vertical” cyclo-N5– can be stabilized by sufficiently strong π–π interactions. This is the first time that the aromaticity of a pentazole ring has been damaged by weak interactions. Such findings not only widen the understanding of aromaticity but also deepen the fundamental understanding of cyclo-N5– and contribute to the emerging pentazole chemistry.

Planar Tetracoordinated Silicon (ptSi): Room-Temperature Stable Compounds Containing Anti-van't Hoff/Le Bel Silicon.

While a variety of compounds containing planar tetracoordinated carbon (ptC), the so-called anti-van't Hoff/Le Bel carbon, are known experimentally, stable systems containing planar tetracoordinated silicon (ptSi) are barely known. As part of our studies on the application of stereoelectronically well-defined transition-metal fragments to stabilize silicon in unprecedented bonding modes, we report herein the synthesis and full characterization of a series of thermally stable complexes of the general formula [Tp'(CO)2MSiC(R1)C(R2)M(CO)2Tp'] (M = Mo, W; R1 = R2 = Me or R1 = H, R2 = SiMe3, Ph; Tp' = κ3-N,N',N″-hydridotris(3,5-dimethylpyrazolyl)borate), which incorporate a ptSi atom in addition to two ptC atoms. The complexes were obtained by reacting the metallasilylidyne complexes [Tp'(CO)2M≡Si-M(CO)2(PMe3)Tp'] with alkynes R1C≡CR2 and were comprehensively analyzed by experimental studies and quantum chemical calculations. The analyses revealed that the ptSi atom is embedded in a tricyclic trapezoidal core featuring one internal SiC2 and two outer M-Si-C three-membered rings, which are fused via two Si-C bonds. The structural peculiarities evoked by the presence of an anti-van't Hoff/Le Bel ptSi center, such as the short M-Si bonds, a nearly linear M-Si-M spine, long M-C bonds, and the presence of two planar tetracoordinated carbon atoms were elucidated by a detailed analysis of the electronic structure, suggesting that one factor for the stabilization of the ptSi geometry is the aromaticity of the central SiC2 ring having two delocalized π electrons. Remarkably, the results further indicate the existence of both anti-van't Hoff/Le Bel carbon and silicon centers next to each other in the isolated complexes.