Annulene paramagnetic shifts

However, the most common appearance of paramagnetic shifts in NMR spectra is less spectacular for spectra of molecules that have a low-lying, electronic state with at least one unpaired electron. The applied magnetic field encourages mixing of the closed-shell wavefunction with the open-shell (paramagnetic), excited state wavefunction. This happens dramatically in the case of antiaromatic compounds, such as the [12]annulene drawn in Fig. 5.29, where the lone interior proton has a chemical shift of 16.4 ppm. 

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 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. 

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. 

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. 

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