Antiaromatic compounds paramagnetic ring current

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... 

Aromatic compounds are defined as cyclic or polycyclic systems which sustain a diamagnetic ring current and consequently exhibit a total diatropic lowfield 1H NMR chemical shift relative to that of vinylic protons. Paramagnetic ring current is expected and was shown to be induced in antiaromatic species and to result in a total highfield H NMR band displacement, while non aromatic compounds give rise to characteristic vinylic XH patterns. 

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. 

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