Annulene computational studies

The concept of Mobius aromaticity has been extended from transition structures to discussions of the stability of organic molecules. Extensive reviews of Mobius aromaticity and of Mobius structures were provided by Rzepa and by Herges. A computational study of the (CH)i2, (CH)i6, and (CH)2o annulenes found twisted Mobius structures to be energy minima, but in each case steric strain caused such structures to be higher in energy than their nontwisted Hiickel coimterparts. Similar results from another study are shown in Table 11.7. In order to test the stabilizing effect of Mobius... 

The conventional procedure would be to regard the molecule as a derivative of norcaradiene, in which two H atoms have been replaced by a butadiene moiety with a frozen pair of conjugated double bonds. The reaction would then be analysed in the local symmetry of norcaradiene. Such an assumption implies that these two tt bonds remain isolated from the other three in the product isomer, but methano[10]annulene is a nearly planar molecule with AN + 2 mobile electrons that has been established by spectroscopic methods to be aromatic. [17] A computational study [18] supports this conclusion, indicating that the global symmetry of the annulene, like that of bisnorcaradiene -from which it is separated by a very low barrier - is C2v... 

A theoretical study of the acidity (proton loss) of the porphyrin 266 and some related systems was carried out by means of the B3LYP/6-31+G(d,p) computational level. These include, on one hand, porphyrin fragments 267 and pyrrole 11 (X = H) and the monoanion and dianion of 266 and, on the other hand, chalcogen-related porphyrins 268,269, and 270 and their anions, as well as cyclic nitrogen derivatives 271, 272, 273, and 274, simplified models of porphyrin, dibenzo[14]annulene, phthalocyanine, and porphycene. 

In addition to the classical [6]annulene, benzene, neutral in-plane trishomoaromatic "benzene" frameworks (such as that depicted in Fig. 5) were also studied.Although not yet synthesized, these compounds were collectively studied using a variety of computational techniques. Unique from benzene, the six over-... 

Mislow proposed in 1952 that the internal hydrogens in the [18]annulene ring system would interfere with one another (from van der Waals interactions) to such an extent, that the molecule would probably not be planar (as is clearly the case with [lOJannu-lene). The compound was synthesized in 1962 by Sondheimer and co-workers and has been subjected to extensive smdies subsequently. In spite of numerous studies by a variety of physical methods, and extensive computations as well, it can only be said at this writing that the structure is still not completely agreed upon. A brief exposition of the situation here up to the present time follows. 

Other species that are potentially aromatic such as the annulenes and radialenes have also been discussed, based on DFT computations and several experimentally prepared structures [112, 113]. The key conclusion of these studies is that aromaticity is an important consideration for these types of structures but the considerable strain often found in these molecules can override some of the aromatic stabilization. Owing to their cyclic delocalization virtually all carbomers are excellent electron acceptors, as evident from their high electron affinities [113]. 

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