Aromaticity of Polycyclic Conjugated Hydrocarbons

Randić, Milan

doi:10.1021/cr9903656

Cited by 704 publications

(619 citation statements)

References 714 publications

(1,419 reference statements)

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“…Still, the structural similarity is sufficiently manifest to use the analogous linear polyacenes well-known as one of the most studied aromatic organic compounds [36][37][38] , as a natural reference for the polycyclic Na-systems. NICS values calculated at the HF level for Na 4n+2 and C 4n+2 H 2n+4 (n = 1-5) are given in Table 1 and In the case of Na-clusters NICS-values monotonically become less negative as one moves from the molecular plane, whereas in the case of acenes NICS-values are the most negative at about 1 Å above the molecular plane ( Figure S1).…”

Section: Resultsmentioning

confidence: 99%

Ring Currents in Polycyclic Sodium Clusters

Radenković

Bultinck

2011

J. Phys. Chem. A

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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 multicentre 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. 1,2 the concept of aromaticity was extended from its usual organic realm to the field of all-metal inorganic clusters. Compared to the aromaticity encountered in organic molecules, primarily being of the traditional π-type, all-metal systems can exhibit a multifold aromaticity [3][4][5] and conflicting aromaticity, 2,3,[6][7][8][9] arising from their σ-, π-, δ- The problem with claims that a certain species is aromatic is that this conclusion may depend rather significantly on the property used to characterize aromaticity. As an example: a ring that exhibits bond length equalization as in benzene but does not sustain a ring current could be classified by some as aromatic but by others as not aromatic depending on the relative importance they attach to the property of bond lengths equalization or ring currents respectively. The same is true for e.g., electron delocalization and ring currents. Although one of us has previously shown that electron delocalization and ring currents in carbohydrates go hand in hand, 20-23 the more general and subtle interrelationship is that a delocalized system is a necessary but not sufficient requirement for a ring current. In previous work on Na 6 systems, electron delocalization was gauged by the multicentre index and information on a ring current was obtained indirectly from NICS values. The problem with NICS, however, is that although usually a negative (or aromatic) NICS value reflects the 3 existence of a ring current, there is no direct way to extract and thus beyond doubt prove the existence of an underlying ring current. INTRODUCTION COMPUTATIONAL METHODSThe structures of the Na 4n+2 (n = 1-5) entities (Figure1) were taken from the experimental structural data given by Khatua et al. 13,14 The molecular structures of the linear polyacenes C 4n+2 H 2n+4 (n = 1-5) were optimized at the B3LYP/6-311+G* level.Computed Hessian matrices showed the optimized structures to correspond to minima on the potential energy surface.The current density maps presented in this pap...

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Section: Resultsmentioning

confidence: 99%

Ring Currents in Polycyclic Sodium Clusters

Radenković

Bultinck

2011

J. Phys. Chem. A

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“…Although it is one of the most important concepts in chemistry, aromaticity still has no unified definition [1][2][3]. It was postulated in many reports [4][5][6][7][8][9][10][11][12][13][14] and summarized in Tetrahedron report 520 [3] that aromaticity is a collective phenomenon and hence a variety of criteria should be used in the discussion of this term in any particular subject of investigation [15].…”

Section: Introductionmentioning

confidence: 99%

EL: the new aromaticity measure based on one-electron density function

Dominikowska

Palusiak

2012

Struct Chem

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Ellipticity of the bond, being the quantity which numerically reflects how far the given chemical bond has elliptic cross-section, may be used to estimate p-electron contribution in bonding. We make use of that fact and develop a new measure of aromaticity-EL index. Since ellipticity is available from calculations on oneelectron density function, EL can be used for both theoretical and experimental data. The EL measure is normalized to make interpretation of this parameter as easy and comfortable as possible. We compare EL values with the values of other commonly used aromaticity measures, such as HOMA, PDI, FLU, and NICS. It appears that the indications of EL are in agreement with indications of other indices and general expectations.

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“…These currents determine the typical magnetic response of PAHs, for example, enhanced diamagnetism, pronounced anisotropy of magnetizability and proton shielding tensors, and strong current susceptibilities, which can be rationalized by simple classical models. [26][27][28][29][30][31][32][33] However, the extension of theoretical procedures useful to rationalize the magnetic response of diamagnetic PAHs, [34][35][36][37][38][39] soon appears quite problematic. Magnetic-field induced current densities in a large graphene fragment are hard to visualize and its magnetic properties become more and more difficult to calculate ab initio on increasing its size.…”

Section: Introductionmentioning

confidence: 99%

Polygonal current models for polycyclic aromatic hydrocarbons and graphene sheets of various shapes

Pelloni

Lazzeretti

2017

J Comput Chem

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Assuming that graphene is an "infinite alternant" polycyclic aromatic hydrocarbon resulting from tessellation of a surface by only six-membered carbon rings, planar fragments of various size and shape (hexagon, triangle, rectangle, and rhombus) have been considered to investigate their response to a magnetic field applied perpendicularly. Allowing for simple polygonal current models, the diatropicity of a series of polycyclic textures has been reliably determined by comparing quantitative indicators, the p-electron contribution to I B , the magnetic field-induced current susceptibility of the peripheral circuit, to n k and to r k ðCMÞ52NICS k ðCMÞ, respectively the out-of-plane components of the magnetizability tensor and of the magnetic shielding tensor at the center of mass. Extended numerical tests and the analysis based on the polygonal model demonstrate that (i) n k and r k ðCMÞ yield inadequate and sometimes erroneous measures of diatropicity, as they are heavily flawed by spurious geometrical factors, (ii) I B values computed by simple polygonal models are valid quantitative indicators of aromaticity on the magnetic criterion, preferable to others presently available, whenever current susceptibility cannot be calculated ab initio as a flux integral, (iii) the hexagonal shape is the most effective to maximize the strength of p-electron currents over the molecular perimeter, (iv) the edge current strength of triangular and rhombic graphene fragments is usually much smaller than that of hexagonal ones, (v) doping by boron and nitrogen nuclei can regulate and even inhibit peripheral ring currents, (vi) only for very large rectangular fragments can substantial current strengths be expected.

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Aromaticity of Polycyclic Conjugated Hydrocarbons

Abstract: For Abstract see ChemInform Abstract in Full Text.

Cited by 704 publications

References 714 publications

Ring Currents in Polycyclic Sodium Clusters

Ring Currents in Polycyclic Sodium Clusters

EL: the new aromaticity measure based on one-electron density function

Polygonal current models for polycyclic aromatic hydrocarbons and graphene sheets of various shapes

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