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Annulenes paramagnetic ring current

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." ... [Pg.519]

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... [Pg.67]

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. [Pg.365]

We therefore conclude that n.m.r. will differentiate between the [4 ]- and [4n + 2]-annulenes, for it is quite apparent that proton chemical shifts are sensitive to the magnetic properties of the tr-electrons therein. Insofar as this property parallels the Hiickel predictions regarding aromatic and anti-aromatic character (which we now identify with diamagnetic and paramagnetic ring currents) then n.m.r. is a qualitative8) criterion of aromatic character in the annulenes. [Pg.30]

Hexadehydro-[ 18]annulene 80>, 15, (see Table 4) possesses no inner protons and the six equivalent outer protons resonate at t 2.98. This represents a downfield shift of over 1 ppm from a model compound, cis-hex-3-ene-l,5-diyne, 9, (t 4.11). This model compound also served to clarify the interpretation of the singlet at t 5.55 in the case of l,5,9-tridehydro-[12]annulene, 1, a molecule for which aromaticity is not predicted. The downfield shift of 15 and the upfield shift of 1 from the acyclic model can then be attributed to a diamagnetic ring current in 15 and a paramagnetic ring current in 1. [Pg.52]

Of the various [20]annulenes prepared 115>116> only one compound has as yet been investigated by n.m.r. This is l,ll-didehydro-[20]-annulene, 24, whose n.m.r.66) indicates the presence of a paramagnetic ring current. The room temperature spectrum is of the mobile type arising from the rotation of two pairs of trans double bonds, which leads to an averaged "symmetrical molecule (a similar case was seen for l,9-didehydro-[16]annulene, 12). When the bond rotation is "frozen out at — 80 °C, one sees the deshielded inner protons (t —1.60 to —0.45) and shielded outer protons (t4.40 to 4.93) characteristic of an antiaromatic compound. [Pg.55]

Thus, by the mere addition of two electrons to [16]annulene the inner protons are shifted from r —0.61 (—140 °C) to t 18.17, whilst the outer protons t 4.70 (—140 °C) are shifted downfield to t 1.17 and t 2.55, such noticeable shifts paralleling the change from a system possessing a paramagnetic ring current to one with a diamagnetic current. [Pg.101]

A potentially anti-homoaromatic species (79, see Table 16) has been generated by protonation of l,6-methano-[10]annulene at the 2-position 224>. As expected, 1,3 overlap is not significant and the compound can be formulated as a classical carbonium ion. In fact the abnormally large H(2a) to H(2a) coupling constant of —24.9 Hz indicates that the 1,3 distance has been maximized in order to avoid the expected destabilization, and thus there is no evidence for a paramagnetic ring current in this system. [Pg.106]

The l,6-dithia[10]annulene (126) shows no paramagnetic ring-current and is probably non-planar. The synthesis failed with the salt (125 X == O). [Pg.199]

In anti-aromatic systems, on the other hand, a paramagnetic ring current is produced, in which the magnetic field strengthens the outer magnetic field inside, above and below the loop, and weakens it on the periphery. [16]Annulene, an example of an antiaromatic substance, therefore displays absorption signals at 5.33 ppm for the outer protons, and 9.44 ppm for the inner protons. [Pg.12]


See other pages where Annulenes paramagnetic ring current is mentioned: [Pg.61]    [Pg.138]    [Pg.327]    [Pg.64]    [Pg.666]    [Pg.501]    [Pg.85]    [Pg.37]    [Pg.223]    [Pg.224]    [Pg.154]    [Pg.42]    [Pg.23]    [Pg.30]    [Pg.34]    [Pg.49]    [Pg.57]    [Pg.88]    [Pg.5]    [Pg.734]    [Pg.14]    [Pg.157]    [Pg.211]    [Pg.391]    [Pg.369]    [Pg.28]    [Pg.20]    [Pg.27]    [Pg.31]    [Pg.46]    [Pg.49]    [Pg.85]    [Pg.14]    [Pg.28]   
See also in sourсe #XX -- [ Pg.37 ]

See also in sourсe #XX -- [ Pg.42 ]




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Annulenes currents

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Paramagnetic ring currents

Paramagnetic rings

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