Abstract:Cyclic conjugation in benzenoid hydrocarbons is studied in the series of peropyrene and its 42 mono-, di-, tri-, tetra-, penta-and hexabenzo derivatives. Some general regularities in the conjugation pattern of these molecules are pointed out.
“…Although the central ring of the Clar picture (Figure b) has an inscribed circle, none of its six bonds are in the 1.39 Å range . The results are inconsistent with previous Hückel-type (ef-model) calculations, which concluded that the Clar structure was not sufficient to describe the π-electron structure of peropyrenes. Peropyrene ( 1 ) is more stable than its isomers rubicene ( 14 ) and benz[ e ]indeno[1,2,3,4- hi ]acephenanthrylene ( 15 ), potential peri − peri coupling products of fluorenone (an isomer of 1 ), by 115.0 and 90.3 kJ/mol, respectively.…”
Reactions of phenalenone (2), benzanthrone (3), and naphthanthrone (4) with a low-valent titanium reagent (TiCl4/LiAlH4/THF) gave peropyrene
(1), tetrabenzo[a,cd,j,lm]perylene (6), and dibenzo[jk,uv]dinaphtho[2,1,8,7-defg;2‘,1‘,8‘,7‘-opqr]pentacene (10), respectively. The syntheses of
the LPAHs 6 and 10 were regioselective. An unsymmetrical pathway of reductive peri−peri “dimerization” of the phenalenones leading to
peropyrene-type LPAHs was proposed. Ab initio DFT B3LYP/6-311G** calculations indicated that D
2
h
-1, the most stable conformation, resembles
the Clar picture of 1.
“…Although the central ring of the Clar picture (Figure b) has an inscribed circle, none of its six bonds are in the 1.39 Å range . The results are inconsistent with previous Hückel-type (ef-model) calculations, which concluded that the Clar structure was not sufficient to describe the π-electron structure of peropyrenes. Peropyrene ( 1 ) is more stable than its isomers rubicene ( 14 ) and benz[ e ]indeno[1,2,3,4- hi ]acephenanthrylene ( 15 ), potential peri − peri coupling products of fluorenone (an isomer of 1 ), by 115.0 and 90.3 kJ/mol, respectively.…”
Reactions of phenalenone (2), benzanthrone (3), and naphthanthrone (4) with a low-valent titanium reagent (TiCl4/LiAlH4/THF) gave peropyrene
(1), tetrabenzo[a,cd,j,lm]perylene (6), and dibenzo[jk,uv]dinaphtho[2,1,8,7-defg;2‘,1‘,8‘,7‘-opqr]pentacene (10), respectively. The syntheses of
the LPAHs 6 and 10 were regioselective. An unsymmetrical pathway of reductive peri−peri “dimerization” of the phenalenones leading to
peropyrene-type LPAHs was proposed. Ab initio DFT B3LYP/6-311G** calculations indicated that D
2
h
-1, the most stable conformation, resembles
the Clar picture of 1.
“…However, as we have seen, according to our modified approach, among migrating sextets the central rings are more aromatic that the terminal migrating sextets. This finding can be contrasted to findings of Polansky and Derflinger, who used HMO, which have also late been found also later by Bultinck et al who used more advanced MO calculations. According to both MO calculations the terminal migrating rings have been found to have higher local aromaticity than the central migrating rings.…”
Section: Discussioncontrasting
confidence: 76%
“…In 2005, Bultinck and coworkers generalized the Polansky index as an aromaticity measure in polycyclic aromatic hydrocarbons by considering more advanced molecular orbitals instead of the simple HMO used by Polansky and Derflinger. Below we have reproduced the Abstract of the article of Bultinck et al, published approximately 40 years after Polansky and Derflinger: In this work, the ideas of molecular quantum similarity are used to generalize the Polansky similarity index.…”
Recently assigned numerical expressions to qualitative benzenoid formulas of Clar have transformed Clar's theory into a quantitative model that allowed a structural characterization of aromaticity in benzenoid polycyclic hydrocarbons and construction of aromaticity maps for benzenoid polycyclic hydrocarbons. The key structural elements of the novel quantitative aromatic sextet theory of Clar are (1) Kekul e valence structures, (2) Pauling bond orders, and (3) Clar's aromatic sextet benzenoid rings. In the present contribution we go beyond Clar and consider local aromaticities of benzenoid rings in benzenoid polycyclic hydrocarbons. We have characterized each benzenoid ring by two descriptors: (1) their content of C 5 C bonds in the set of Kekul e valence structures, which is reflected in the ring bond orders, and by (2) the variations of CC bond orders within rings, which are zero for benzene. This allows us to introduce for any set of benzenoid hydrocarbons local aromaticity map, which allows one to graphically view similarities and dissimilarities of local aromaticity of different benzenoid rings. For such map, one can calculate the "distance" of each ring from the benzene, with coordinates (3.00, 0) for benzene RBO and variations in CC bonds, respectively. We show that the central ring in peropyrene is visibly less aromatic that the terminal sextets, as has been claimed by Gutman and Agranat, and similarly we show that the central ring of anthracene is unexpectedly more aromatic than the terminal rings, in having greater similarity in bond lengths to benzene ring than the terminal rings. This suggests limitations of the assumption of the Clar model that migrating sextets have equal local aromaticities.Finally, we examined a pair of non benzenoid hydrocarbons related to peropyrene, built from terminal benzenoid rings and naphthalene fragments. Surprisingly, we have found the terminal benzene rings in the two non benzenoid compounds to have unusually high ring bond orders, approaching that of benzene.
“…Another model relating local aromaticity of polycyclic aromatic systems include harmonic oscillator model of aromaticity, and the para delocalization index (PDI) of Solà and coworkers . In addition, numerous references on aromaticity, which included among the references at the end of this article also some graph theoretical contributions, have been included. Among the letter, recent numerical representation of Clar formulas stand out as one of most important contribution, which has led to quantitative structural characterization of global and local aromaticity in benzenoid hydrocarbons …”
Section: Part 3 the Rre In Benzenoid Hydrocarbonssupporting
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