Structures and ¹H‐chemical shifts of conjugation deficient hydrocarbons

Abstract

The geometrical and electtonic structures and the 'H-chemical shifts of some conjugation deficient hydrocarbons and their benzenoid isomers 1-3 which consist of two concentric perimeters have been studied by means of semiempirical procedures. Conclusions on the importance of macrocyclic annulenoid conjugation are drawn from the bond lengths pattern and the proton shifts in these compounds.

Analytical Study of Superaromaticity in Cycloarenes and Related Coronoid Hydrocarbons

“…TREs for 1, 2, and their molecular ions are graphically summarized in Figure 5 as sequential line plots (i.e., sequential line graphs) of TRE against the number of π-electrons (N π ). 36,49 Each plot represents the N π dependence of global aromaticity, where N π corresponds to the neutral or charged (7) 102.486088 0.000357 1.6 C 90 H 30 (8) 132.513299 0.048368 −11.6 C 96 H 48 (9) 136.248163 0.000060 1.0 C 126 H 42 (10) 185.085176 0.028038 −11.9 C 144 H 36 (11) 215.463454 0.007936 4.4 C 120 H 60 (12) 169.959117 0.000014 0.7 C 162 H 54 (13) 237.558707 0.018135 −11.1 C 192 H 48 (14) 286.872173 0.001440 3.1 C 210 H 42 (15) 317.142129 0.031855 −10.1…”

“…Cycloarenes are defined as comprising many annelated benzene rings that form a macrocycle with inward-pointing C–H bonds. Since before the synthesis of kekulene ( 1 ), − the prototypical cycloarene, electronic structure of cycloarenes has been a target of many theoretical and computational studies. − Kekulene had been viewed as a closed cycle of angularly annelated benzene rings and also as a combination of two interacting [4 n +2]annulenes. The synthesis and characterization of kekulene ( 1 ) − in 1978 answered a fundamental question about the nature of macrocyclic conjugation.…”

Combinatorial Enumeration of Isomers of Superaromatic Polysubstituted Cycloarenes and Coronoid Hydrocarbons with Applications to NMR

“…Topological and graph theoretical techniques − have found extensive applications and use over the decades in enhancing our understanding of polycyclic aromatic compounds, fullerenes, large organic polymers, toxicity − ,, and other properties of heterocyclic aromatic compounds, periodic systems, and combinatorial chemistry of these species. Although fully ab initio quantum chemical studies are feasible, they are formidably difficult and consequently, graph-theoretical techniques have provided alternative platforms to gain new insight into especially larger circumscribed superaromatic compounds.…”

“…Ever since the synthesis of kekulene, − a circumcised coronoidal polycyclic aromatic compound exhibiting macrocyclic conjugation and a close nonalternant relative septulene, the topic of aromaticity and superaromaticity has been revived with considerable interest. − The enhanced thermodynamic stability of these polycyclic aromatic compounds is attributed to the phenomenon of superaromaticity. Kekulene, the first structure with the smallest cavity comprising a closed structure of angularly annulated benzene rings exhibiting D 6 h symmetry can be envisaged as a combined unit of two interacting [4 n + 2]­annulenes.…”

“…Superaromaticity is often used synonymously with macrocyclic aromaticity, and it is enhanced thermodynamic stabilization arising from macrocyclic conjugation. Consequently, combinatorial enumeration of various macrocyclic conjugated circuits − has played a critical role in explaining the relative stability of these species. − A number of combinatorial and graph theoretical tools such as the enumeration of conjugated circuits, Clar’s aromatic sextets, enumeration of polycyclic aromatics, spectral polynomials, matching polynomials, distance polynomials, and enumeration of polysubstituted compounds have been considered over the years for not only polycyclic aromatics but also related fullerene cages. − …”