Aromaticity antiaromatic compounds

Thus cyclobutadiene like cyclooctatetraene is not aromatic More than this cyclo butadiene is even less stable than its Lewis structure would suggest It belongs to a class of compounds called antiaromatic An antiaromatic compound is one that is destabi lized by cyclic conjugation... 

Aromaticity/antiaromaticity in cluster systems has certain peculiarities when compared with organic compounds. The striking feature of chemical bonding in cluster systems is the multifold nature of aromaticity, antiaromaticity, and conflicting aromaticity [3-10]. Double aromaticity (the simultaneous presence of [Pg.439]

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

Anti-aromatic 1,2-dithiins 179 display properties opposite to those of 1,4-dithiins 180, whose dications show aromatic stabilization. Unlike other antiaromatic compounds, the 1,2-dithiin derivatives, with eight jr-electrons (such as 181 and 182), appear in... 

A property associated with compounds that are destabilized by a closed loop of electrons. Antiaromatic compounds are typically planar and contain An electrons, where n is a positive integer, in overlapping parallel p orbitals. These compounds also have a paramagnetic ring current. Thus, protons on the outside of the ring will exhibit an upheld NMR chemical shift. See Aromatic... 

Estimation of the aromaticity (antiaromaticity) of various compounds from the values of the ISE, HSE, and HHSE will be discussed in their respective sections. For the present, we will merely observe that these values, some of which are given in Table II, correlate with those of the HSRE and TRE. 

The contributions of the first three types are practically local in character they are close in value for two protons with similar structural environment, such as the ethylenic- and aromatic-type protons. It is only the last term in Eq. (35) that defines the values of the chemical shifts characteristic of aromatic or antiaromatic compounds. 

Clearly, in view of a diversity in the types of heterocyclic compounds, one may hardly expect that all the manifestations of their aromaticity (antiaromaticity) could be rationalized in terms of some simple regularities. We shall therefore attempt to trace certain characteristic trends in the dependence of the aromaticity on the type of heteroatoms, their number and positions in the molecular structure. Our reasoning will be based on the nature of the aromaticity criteria and of the electron count rules. Then turning to individual compounds, we shall add details to the picture. 

The scheme for treating these subjects is as follows. We compare typical aromatic and antiaromatic compounds. Similar to the comparison between benzene and cyclobutadiene, we concern ourselves here with pyridine (34) and azete (57). Such an approach provides, apart from other advantages,... 

X and Y -CH=CH-). This suggests that the geometry distortion around the C-F bonds in [27](X and Y CH=CH-) when compared with the [27](X = Y = H) geometry, increases the corresponding As values between the fluorine lone pair orbitals and the (C-F) antibonding orbitals. It should be recalled that the d(F-F) distance in [27](X and Y -CH=CH-) is notably larger than in [27](X = Y = H). There is also about 2 Hz difference between the C-C contributions. Probably, this difference comes from the difference in aromaticity between these two compounds in fact while [27](X = Y = H) is an aromatic compound, [27](X and Y CH=CH-) is an antiaromatic compound. This seems to indicate that the outlier condition of [27]... 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (44).135 Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4n + 2. There is a long history of attempts to prepare this compound and its simple derivatives, and, as we shall see, the evidence fully bears out Hiickel s prediction— cyclobutadienes display none of the characteristics that would lead us to call them aromatic. More surprisingly, there is evidence that a closed loop of four electrons is actually ami-aromatic.1 If such compounds simply lacked aromaticity, we would expect them to be about as stable as similar nonaromatic compounds, but both theory and experiment show that they are much less stable.137 An antiaromatic compound may be defined as a compound that is destabilized by a closed loop of electrons. 

It is clear, as Katritzky et al. [7, 8] and ourselves [9] have pointed out, that aromaticity cannot be described with a single parameter. It is possible to select a parameter and classify aromatic compounds according to it and this approach is correct if one bears in mind that the aromaticity scale thus obtained is valid only for the chosen parameter. One of the most successful is Schleyer s NICS (nuclear independent chemical shifts) [10-12], a criterion we have used to separate aromatic and antiaromatic compounds [13], Cyranski et al. [14] as well as Sadlej-Sosnowska [15] have tried, with moderate success, to find an agreement between these different points of view. 

Explain whether each of these compounds is aromatic, antiaromatic, or nonaromatic ... 

Aromatic Antiaromatic Ions Aromatic Compounds (page 662 ... 

Oligounsaturated Five-Membered Carbocycles -Aromatic and Antiaromatic Compounds in the Same Family... 

I 2 Oiigounsaturated Five-Membered Carbocycies - Aromatic and Antiaromatic Compounds... 

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