Coronene, structure

Copper(ll) chloride, aromatic iodination and, 551 Coproslanol, structure of, 304 Coral, organohalides from, 352 Corn oil, composition of. 1062 Cornforlh. John Warcup. 1085 Coronene, structure of, 532 Cortisone, structure of. 107 Couper, Archibald Scott, 7 Coupled reactions. 1128-1129 ATP and, 1128-1129 Coupling (NMU), 460... 

To the organic chemists of the early 1900s and before, an aromatic compound was one that underwent substitution reactions, as opposed to the addition reactions of ordinary alkenes and polyenes. They also noted that if one had fused benzene rings in different arrangements, such compounds were by and large aromatic. Smaller members of this group included naphthalene, anthracene, phenanthrene, and coronene (Structure 4) ... 

The only reported X-ray structure of a it-bonded diiodine exists in the 12/coronene associate [75], which shows the I2 to be located symmetrically between the aromatic planes and to form infinite donor/acceptor chains. -Coordination of diiodine over the outer ring in this associate is similar to that observed in the bromine/arene complexes (vide supra), and the I - C separation of 3.20 A is also significantly contracted relative to the stun of their van der Waals radii [75]. For the highly reactive dichlorine, only X-ray structures of its associates are observed with the n-type coordination to oxygen of 1,4-dioxane [76], and to the chlorinated fullerene [77]. 

Fujimoto et al. [77] synthesized a novel phase by coupling a dodecylamino-substituted P-cyclodextrin (P-CD) to 3-glycidoxypropyl-derivatized silica gel. The surface coverage of this phase was reported as 0.37 xmol/m, which amounts to a surface coverage of 2.6 xmol/m for C12 chains (seven chains per fi-CD). An increase in shape selectivity was observed when compared with a conventional Cis monomeric phase as determined by selectivity differences between j9/m-terphenyl, j9/o-terphenyl, and coronene/phenanthro[3,4-c]phenanthrene solute pairs and was attributed to the localized high ligand density as constrained by the fi-CD platform structure. 

Polynuclear aromatic hydrocarbons formed by the angular fusion of benzene rings may adopt either planar or nOnplanar minimum energy conformations, as demonstrated by the circulenes 8-10. [6]Circulene (coronene 9), the earliest known member of the family, prefers a planar structure of symmetry since the presence... 

In a third type of crystal structure (Type B2, Fig. 13) exhibited by the larger pericondensed hydrocarbons (e.g., coronene and ovalene), the characteristic excimer fluorescence band may originate from a small concentration of... 

Fig. 231. Coronene. (a) Form of the molecule. (6) hOl section of the weighted reciprocal lattice (areas of spots proportional to structure amplitudes), (c) Derivation of foreshortening of tilted molecule from peaks in reciprocal space. Alt A9, etc., are the positions of the benzene peaks derived from (b) perpendiculars to the A vectors are drawn at Blt B%, etc., at distances from the origin equal to reciprocals of A vectors these lines give the projected shape of the benzene ring.

Fig. 54. Molecular structure of [K(TMEDA)(THF)2][C24H12]", 84 (C24H 2 = coronene radical ion). (Reprinted with permission from C. Janiak and H. Hemling, Chem. Ber. 1994, 127, 1251. Copyright 1994 Wiley VCH.)...

Absorption and fluorescence spectra of aromatic hydrocarbons are not greatly affected by change of solvent, except for small solvent shifts. At low temperatures the vibrational structure of the bands sharpens up, and some peculiar solvent effects have been noted. When frozen in solution of normal paraffins coronene shows doubling of some of its vibrational bands, and the separation of the components varies with the number of carbon atoms in the solvent molecule chain. The most probable cause is some size-relationship factor between solvent and solute molecules (7). 

In addition to naphthalene, the triplet states of coronene, tri-phenylene, and 1,3,5-triphenylbenzene have been studied by this technique (van der Waals and de Groot, 1959, 1960). Hyperfine structure has been observed, and is presumably governed by the same principles as for aromatic cations and anions. The spin-spin splittings have been discussed theoretically (Gouterman, 1959 Gouterman and Moffitt, 1959 Hameka, 1959). 

Although the basic principles behind this Intact ejection mechanism can be Illustrated with carbon monoxide, the extrapolation to large bloorganlc molecules Is not necessarily obvious. Calculations have been performed for a series of organic molecules adsorbed on a Ni(OOl) surface to understand the mechanisms of molecular ejection (8-12). The first molecules which have more than just a few atoms examined are benzene which it-bonds on a metal surface and pyridine which can either ir-bond or o-bond on a metal surface. Larger structures, whose sizes approach the diameter of bloorganlc molecules, are naphthalene, biphenyl and coronene whose adsorption structures are unknown. All the molecules except pyridine are assumed to ir-bond on the surface. 

It should be noted, however, that planarity had not been excluded for corannulene until the X-ray analysis was performed. The molecule is a non-alternant hydrocarbon because it contains an odd-membered ring. Hence, the it-electron density distribution cannot be uniform in the ground state. One of such polar structures is the double Hiickel aromatic structure 66, c consisting of peripheral 14n and central 6ir systems. The contribution of resonance as shown in 6 c will be at its maximum when the whole molecule is planar like coronene (8). Notwithstanding, the planar corannulene will have huge angle strain. If we assume that all C-C bonds are 1.40 A in length and the... 

Fig. 4. Directed rooted trees of the Kekule patterns of coronene. Note their isomorphic structures and the similarity in their local structures...

The same comment can be made with respect to aromatic hydrocarbons. The relatively large amount of aromatic hydrocarbons is associated with a limited variety of structures cat,37,39) from benzene to coronene. 

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