Chemical graph theory and n-center electron delocalization indices: a study on polycyclic aromatic hydrocarbons

Relations between aromaticity indices derived from chemical graph theory and those based on 6-center electron delocalization are investigated for a series of polybenzenoid hydrocarbons. Aromatic stabilization obtained by means of the effective scaled electron delocalization is highly correlated to the resonance energy, RE, obtained both from SCF MO calculations and conjugated ring circuits model. Local aromaticity of benzene rings is discussed using two different criteria, in one of them aromaticity is just given by the cyclic pi-electron conjugation of the ring, whereas terms involving more than one ring are also considered in the other one. Indices derived from chemical graph theory and those obtained from the 6-center electron delocalization give rise to the same local aromaticity. Moreover, 6-center electron delocalization provides more quantitative information.     

Designing organic spin filters in the coherent tunneling regime

Spin filters, that is, systems which preferentially transport electrons of a certain spin orientation, are an important element for spintronic schemes and in chemical and biological instances of spin-selective electronic communication. We study the relation between molecular structure and spin filtering functionality employing a theoretical analysis of both model and stable organic radicals based on substituted benzene, which are bound to gold electrodes, with a combination of density functional theory and the Landauer-Imry-Büttiker approach. We compare the spatial distribution of the spin density and of the frontier central subsystem molecular orbitals, and local contributions to the transmission. Our results suggest that the delocalization of the singly occupied molecular orbital and of the spin density onto the benzene ring connected to the electrodes, is a good, although not the sole indicator of spin filtering functionality. The stable radicals under study do not effectively act as spin filters, while the model phenoxy-based radicals are effective due to their much larger spin delocalization. These conclusions may also be of interest for electron transfer experiments in electron donor-bridge-acceptor complexes.     

Characterization of pericyclic reactions using multicenter electron delocalization analysis.

This work investigates the applicability of multicenter delocalization analysis to the characterization of pericyclic reactions. The results indicate that multicenter delocalization indices are a powerful tool for studying concerted processes, allowing the characterization of aromatic transition states with a significant increase in the electron delocalization. Moreover, an advantage over magnetic-based indices is that multicenter delocalization indices are not influenced by local electron currents but by the electron delocalization along the multiple (n) centers, and provide, in a quantitative sense, more reliable results. A thorough comparison with magnetic-based indices is carried out for the trimerization reaction of acetylene. Tracking the values of multicenter delocalization indices along the reaction path allows investigation of the nature of concerted mechanisms. Six-center electron delocalization displays a maximum at the transition state of the Diels-Alder reaction, whereas a similar maximum of four-center electron delocalization is slightly displaced to butadiene for the ring opening of cyclobutene. The profile of multicenter electron delocalization indices along the reaction path of [2+2] cycloaddition of ketene to ethene shows the presence of the two independent mechanisms that agree with the two HOMO/LUMO orbital interactions previously proposed to dominate this reaction.     

共轭效应和芳香性本质的争论和它们的历史发展

“共轭效应是稳定的”是有机化学的最最基本原理之一。但是,自30年代起,键长平均化,4N＋2芳香性理论,苯环D~6~h构架的起因,分子的构象和共轭效应的因果关系,π-电子离域的结构效应等已经受到了广泛的质疑。其中,最引人注目的是Vollhardt等合成了中心苯环具有环己三烯几何特征的亚苯类化合物,Stanger等合成了键长平均化,但长度在0.143～0.148nm的苯并类衍生物。最近(1999年),Stanger又获得了在苯环中具有单键键长的苯并类化合物。在理论计算领域,争论主要表现在计算方法上,集中在如何将作用能分解成π和σ两部分。随着论战的发展,作用能分解法在有机化学中的应用不断地发展和完善,Huckel理论在有机化学中的绝对权威也受到了挑战。为此,简要地介绍了能量分解法的发展史,对kollma法的合理性提出了质疑。此外特别介绍了我们新的能量分解法,及在共轭效应和芳香性的研究中的新观点和新的思维模式。     

Borazine: to be or not to be aromatic

Aromaticity of borazine, which has been subject of controversial discussions, is addressed. Beside a short review on aromaticity of borazine we report a detailed analysis of two molecular fields, the induced magnetic field (B ^ind) and the electron localization function (ELF). The induced magnetic field of borazine shows a long-range shielding cone perpendicular to the molecular plane, as in benzene, but lower in magnitude. Contrary to benzene, borazine shows two weakly paratropic regions, one of them inside the ring, and the second one enveloping the boron atoms. It is necessary to separate σ and π contributions to identify whether borazine exhibits π-aromatic character comparable to benzene. Nucleus-independent chemical shift (NICS) isolines show that the σ electrons are much stronger localized than π electrons, their local paramagnetic contributions generate a short-range response and a paratropic (deshielding) region in the ring center (similar to an anti-aromatic response). Three regions can be identified as chemically meaningful domains exhibiting an internally strong electron delocalization (ELF = 0.823). Borazine may be described as a π aromatic compound, but it is not a globally aromatic species, as the electronic system is not as delocalized as in benzene. Dedicated to the 70th birthday of Prof. Tadeusz Marek Krygowski.     

Revisiting Aromaticity and Chemical Bonding of Fluorinated Benzene Derivatives

The electron delocalization of benzene (C₆H₆) and hexafluorobenzene (C₆F₆) was analyzed in terms of the induced magnetic field, nucleus-independent chemical shift (NICS), and ring current strength (RCS). The computed out-of-plane component of the induced magnetic field at a distance (r) greater than or equal to 1.0 Å above the ring center correlates well (R²>0.99) with the RCS value. According to these criteria, fluorination has two effects on the C₆ skeleton; concomitantly, the resonant effects diminish the π electron delocalization and the inductive effects decrease the charge density at the ring center and therefore reduce the magnitude of the paratropic current generated in this region. The equilibrium between both effects decreases aromaticity in the fluorinated benzene derivatives. These results can be extrapolated to determine the aromaticity of any derivative within the series of fluorinated benzene derivatives (C₆H_(6−n)F_n, where n=1–5).     

Oxatriphyrins(2.1.1) Incorporating an ortho‐Phenylene Motif

An understanding of fundamental aspects of archetypal organic structural motifs remains a key issue faced by the experimental and theoretical chemists. Two possible bonding modes for a disubstituted benzene ring, that is a meta and para, determines the π delocalization for oligomeric structures. When the less abundant ortho‐substituted variant is introduced into a triphyrin(2.1.1) skeleton an aromatic molecule is obtained and the carbocyclic ring participates in the conjugation of the macrocycle. The two‐electron reduction and introduction of boron(III) changes the aromatic character and results in an anti‐aromatic structure which has been confirmed by single‐crystal analysis and supported by theoretical calculations.     

Researches on aromatic heterocyclic compounds III. The aromatic character of piazthiol

The PMR spectra of piazthiol, which is benzo[1,2-c]-[1,2,5]-thiodiazole, and of itsα- andβ-nitro, chloro, methyl and amino derivatives are investigated. The effects of these substituents on the chemical shift is in qualitative agreement with the like effects in the benzene series. The parameters of the corresponding PMR spectra are greatly dependent on the electrophilic action of the heterocyclic ring. It is shown that there is conjugation in the thiadiazole ring, although, however, it is much weakened compared with that existing in the benzene ring.     

Exploration of the electronic structure of dendrimerlike acetylene-bridged oligothiophenes by correlating Raman spectroscopy, electrochemistry, and theory

A series of radial thiophene-based structures consisting of a central benzene or thiophene ring surrounded by acetylene-bridged terthienyl arms has been investigated by physical and theoretical methods. Fourier transform Raman spectroscopy of the neutral solids shows that the terthiophene arms are weakly coupled across the core (benzene plus acetylene groups) likely due to cross-conjugation or meta-conjugation effects that may prevent full delocalization. By increasing the number of arms around the central ring, the electronic structure of the molecules seems to be affected only at the core, whereas the outer terthiophene arms remain almost unaltered. Raman spectroelectrochemistry and quantum chemical calculations provide further insight into the charge delocalization of the oxidized species. There is no evidence to suggest that these oxidized forms, obtained upon electrochemical doping of the molecules, show charge delocalization across the core.     

Polarography of some 2,1,3-benzothiadiazoles,benzofurazans, 2,1,3-benzoselenadiazoles,and 3,4-disubstituted and fused 1,2,5-thiadiazoles

The redox behavior has been determined in acetonitrile solutions at a mercury and platinum electrode for 2,1,3-benzo(group VI)diazoles, and 3,4-disubstituted and fused 1,2,5-thiadiazoles. The derivatives studied contained alkyl, phenyl, bromo, chloro, cyano, nitro, methylsulfonyl, and trifluoromethylsulfonyl groups. All ring systems and their derivatives are reversibly reduced initially in a one-electron step, to their respective radical anion, but the nitro and bromo derivatives are reduced preferentially at the substituent group. The potential at which the production of the radical anion occurred became more anodic as the electron withdrawing ability of the substituent and the number of substituents increased.     

Structure and chemistry of 1-silafluorenyl dianion, its derivatives, and an organosilicon diradical dianion.

1-Silafluorene dianion was synthesized by potassium reduction of 1,1-dichloro-1-silafluorene in refluxing THF. The X-ray structure of 1,1-dipotassio-1-silafluorene (3b) shows C-C bond length equalization in the five-membered silole ring and C-C bond length alternation in the six-membered benzene rings, indicating aromatic delocalization of electrons in the silole ring. The downfield (29)Si chemical shift at 29.0 ppm and theoretical calculations also support electron delocalization in the silole ring of 3b. Dianion salt 3b underwent nucleophilic reactions with Me(3)SiCl and EtBr to form the corresponding disubstituted products. Benzaldehyde underwent reductive coupling in the presence of 3b. Slow oxidation of 3b yielded 1,1'-dipotassio-1,1'-bis(silafluorene) (16). The X-ray structure and (29)Si chemical shift (-38.0 ppm) of 16 indicate localized negative charges at the silicon atoms and no aromatic character. Heating a DME/hexane solution of 3b in the presence of 18-crown-6 led to a novel diradical dianion salt.     

Quantum delocalization of benzene in the ring puckering coordinates

The effect of quantum mechanical delocalization of atomic nuclei on the conformation of the six‐membered ring structure in two hydrocarbons, cyclohexane and benzene, is investigated using ab initio path integral approach. A striking feature of benzene species is revealed using ring puckering coordinate representation, which demonstrates that the zero point motion of the heavy atom skeleton dominates over the out‐of‐plane thermal motions of the ring. Even more unexpected is the fact, that this is true not only at low temperature of 150 K, at which such behavior would not be surprising, but also at room temperature, where the nuclear quantum effects are usually of lesser importance, especially in the case of such heavy nuclei as carbon. In view of this finding the planar conformation of benzene, whose equilibrium (T = 0 K) geometry results from the well‐known properties of the electronic structure, can be elucidated also at nonzero temperature. According to our simulations, it appears as a consequence of quantum delocalization of the carbon nuclei rather than a trivial time average over the classical configurations of the puckered ring. This interesting behavior is contrasted with the clearly nonplanar structure of cyclohexane, whose ring puckering states can be unequivocally assigned even if the nuclear delocalization is taken into account. © 2014 Wiley Periodicals, Inc.     

Application of AIM parameters at ring critical points for estimation of pi-electron delocalization in six-membered aromatic and quasi-aromatic rings

The AIM parameters at the ring critical point (the electron density and its Laplacian, the total electron energy density and both its components, potential and kinetic electron energy densities), have been intercorrelated with aromaticity indices: the geometry-based HOMA and the magnetism-based NICS, NICS(1), and NICS(1)(zz). A set of 33 phenylic rings having possibly a diversified aromatic character, and a set of 20 quasi-rings formed by intramolecular hydrogen and lithium bonds, have been taken into consideration. It has been found that the density of total electron energy, H, may serve as a new quantitative characteristic of pi-electron delocalization. The dependences between H values and aromaticity indices are correlated (cc(H/HOMA)=0.99, cc(H/NICS(1)zz)=0.95).     

Rational Design of Star-shaped Molecules with Benzene Core and Naphthalimide Derivatives End Groups as Organic Light-emitting Materials

A series of star-shaped molecules with benzene core and naphthalimides derivatives end groups have been designed to explore their optical,electronic,and charge transport properties as charge transport and/or luminescent materials for organic light-emitting diodes(OLEDs). The frontier molecular orbitals(FMOs) analysis has turned out that the vertical electronic transitions of absorption and emission are characterized as intramolecular charge transfer(ICT). The calculated results show that the optical and electronic properties of star-shaped molecules are affected by the substituent groups in N-position of 1,8-naphthalimide ring. Our results suggest that star-shaped molecules with n-butyl(1),benzene(2),thiophene(3),thiophene S?,S?-dioxide(4),benzo[c][1,2,5]thiadiazole(5),and 2,7a-dihydrobenzo[d]thiazole(6) fragments are expected to be promising candidates for luminescent and electron transport materials for OLEDs. This study should be helpful in further theoretical investigations on such kind of systems and also to the experimental study for charge transport and/or luminescent materials for OLEDs.     

Local aromaticity study of heterocycles using n-center delocalization indices: the role of aromaticity on the relative stability of position isomers

A quantitative study on local aromaticity based on n-center electron delocalization indices, n being the number of atoms in the ring, is performed on a series of heterocycles containing N, O or S. The results indicate that the order of stability within a series of position isomers is not controlled by aromaticity but by other structural factors. Thus, for a certain series of monocycles position isomers (diazoles, triazoles, tetrazoles, diazines, triazines, and tetrazines) the most stable compound is the least aromatic one and vice versa. However, aromaticity controls the stability for series of isomers where these structural factors are similar. For the case of isocompounds, like isobenzopyrrole, isobenzofuran or isobenzothiophene, the large decrease in the aromaticity of the benzene ring with regard to their isomers makes them less stable.     

Carbon networks based on dehydrobenzoannulenes. 4. Synthesis of "star" and "trefoil" graphdiyne substructures via sixfold cross-coupling of hexaiodobenzene

The synthesis and characterization of star- and trefoil-shaped polyethynyl aromatic structures, which represent model substructures of the all-carbon network graphdiyne, are described. Assembly of these macrocycles is accomplished via 6-fold Sonogashira cross-coupling of hexaiodobenzene using Pd[P(o-Tol)(3)](2) and CuI as the catalytic system. The development of these modified Sonogashira conditions is detailed. This work has led to the synthesis of a new family of hexakis(phenylbutadiynyl)benzene derivatives (4a-c), the largest of which is the D(3)(h)()-symmetric "trefoil" 2 and is composed of three [18]annulenes fused at a common benzene ring. Attempts at the synthesis of "wheel" 3 are also described. Compound 2 represents the largest fragment of the graphdiyne network to date. UV-vis spectroscopic studies indicate enhanced electron delocalization throughout the extended pi-system.     

Bidimensionality of 8-atom clusters of Au: first principles study and comparison with Ag clusters

We have calculated the lowest energy structures of 8-atom neutral gold clusters using the density functional theory approach. In contrast with current literature that finds kinetic energy to be the determinant component, we have found that the 2D structure is energetically favored due to a higher electron delocalization that stems from the relativistic contraction of Au atom size which cause 3D clusters to deform. This higher delocalization lowers the total energy of the 2D structures against the 3D ones. Silver clusters do not suffer this size contraction, hence there is no higher delocalization in the 2D clusters, and their fundamental structure will be 3D.