Paramagnetic rings

Annulene is a very imstable compound that undergoes cyclization to bicyclic isomers and can be kept only at very low temperature. The NMR spectrum has been studied at low temperature." Besides indicating the double-bond geometry shown in the stmcture below, the spectrum reveals a paramagnetic ring current, the opposite to what is observed for aromatic systems. This feature is quite characteristic of the [4 ]annulenes and has been useful in characterizing the aromaticity or lack of it in annulenes." ... 

As we have seen (p. 59), these systems are expected to be not only nonaromatic but actually antiaromatic. The chief criterion for antiaromaticity in annulenes is the presence of a paramagnetic ring current, which causes protons on the outside of the ring to be shifted upfield while any inner protons are shifted downfield, in sharp... 

We can therefore conclude that in 4n systems antiaromaticity will be at a maximum where a molecule is constrained to be planar (as in 59 or the dianion of 83) but, where possible, the molecule will distort itself from planarity and avoid equal bond distances in order to reduce antiaromaticity. In some cases, such as cyclooctatraene, the distortion and bond alternation are great enough for antiaromaticity to be completely avoided. In other cases (e.g., 96 or 103), it is apparently not possible for the molecules to avoid at least some p-orbital overlap. Such molecules show paramagnetic ring currents and other evidence of antiaromaticity, although the degree of is not as great as in molecules such as 59 or the dianion of 83. 

Characteristic shifts due to paramagnetic ring currents are also met with in the singly annellated [2]paracyclo[2](l,4)naphthalenophane (35) 16>-... 

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... 

By contrast, in the case of antiaromatic molecules with N = An the contribution is positive (paramagnetic ring current). 

For example, the question of a paramagnetic ring current in pyracylene 1C. A. Coulson and R. B. Mallion, J. Am. Chem. Soc. 98, 592 (1976)1, which is closely related to hydrazino-bridged [ 12]annulenes, treated in Section VIII,A. 

Fig. 9.6 The net paramagnetic ring current in 1,2-C60H2 derived from a London methodology (Meier et al. 2002a)...

For example, a pure symmetry argument based on the angular-momentum characteristics of 77 molecular orbitals implies diamagnetic ring currents for 4/1 + 277 monocycles, and paramagnetic ring currents for An it monocycles, and furthermore ascribes essentially all the ring current in each case to the activity of 4 HOMO electrons for the aromatic (diatropic) and 2 HOMO electrons for the antiaromatic (paratropic) systems [7a,b]. 

Fig. la,b Effect of induced paramagnetic ring currents in low-valent transition-metal hydrides. See also Table 4. (Reprinted with permission from Ruiz-Morales et al. [81], copyright 1996 American Chemical Society)... 

Anti-aromaticity was predicted by the Hiickel approach for conjugated cyclic planar structures with 4n 7i electrons due to the presence of two electrons in antibonding orbitals, such as in the cydopropenyl anion, cydobutadiene, and the cydopentadienyl cation (n = 1), and in the cydoheptatrienyl anion and cydooctatetraene (n = 2). It has been argued that a simple definition of an anti-aromatic molecule is one for which the 1H NMR shifts reveal a paramagnetic ring current, but the subject is controversial. The power of the Hiickel theory indeed resides not only in the aromatic stabilization of cydic 4n + 2 electron systems, but also in the destabilization of those with An electrons [22, 27, 42]. 

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