Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons

Balaban, Alexandrù T.; Schleyer, Paul V. R.; Rzepa, Henry

doi:10.1021/cr0300946

Cited by 123 publications

(74 citation statements)

References 48 publications

(53 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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“…Aromaticity is a chemical property associated with such cyclic and planar compounds and is attributed to these π-electrons which are free to cycle around the circular arrangements of atoms found in the aromatic moieties. It can be considered as a manifestation of cyclic delocalization and resonance which is found in planar ring systems such as benzene [1, 2]. The flat face of an aromatic ring has a partial negative charge owing to these π electrons.…”

Section: Introductionmentioning

confidence: 99%

Aromatic-aromatic interactions in structures of proteins and protein-DNA complexes: a study based on orientation and distance

Anjana

Vaishnavi²,

Sherlin³

et al. 2012

Bioinformation

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Interactions between the aromatic amino acid residues have a significant influence on the protein structures and protein-DNA complexes. These interactions individually provide little stability to the structure; however, together they contribute significantly to the conformational stability of the protein structure. In this study, we focus on the four aromatic amino acid residues and their interactions with one another and their individual interactions with the four nucleotide bases. These are analyzed in order to determine the extent to which their orientation and the number of interactions contribute to the protein and protein-DNA complex structures.

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“…6 The significance of the "rule of six" by Bamberger and by Thiele in the late 1890s followed by the recognition of the π-electron sextet by Crocker and its symbolization by Armit and Robinson's circle 7 was generalized in Hückel's "4n + 2 π-electron rule". 8 It was Clar's insight that gave a more profound significance to the sextet circle.…”

Section: Introductionmentioning

confidence: 99%

Claromatic Carbon Nanostructures

Balaban

Klein

2009

J. Phys. Chem. C

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Eric Clar’s qualitative ideas for benzenoids are described in application to various novel nanostructures: graphene, edges in graphene, carbon nanotubes, carbon nanocones, and carbon nanotori. The specially singled out most highly aromatic species with Clar structures consisting entirely of aromatic sextets are proposed to be termed “claromatic”. Several such claromatic nanostructures are identified as manifesting special properties. A molecular structural “(h,k)” characterization for benzo/grapheneic nanostructures is shown to link up neatly with Clar’s ideas.

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Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons

Cited by 123 publications

References 48 publications

Ring Currents in Polycyclic Sodium Clusters

Ring Currents in Polycyclic Sodium Clusters

Aromatic-aromatic interactions in structures of proteins and protein-DNA complexes: a study based on orientation and distance

Claromatic Carbon Nanostructures

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