A simple alkene has an area of low electron density in the plane of the molecule because the π orbital has a node there, and the carbons and hydrogen nuclei lying in the plane gain no shielding from the π electrons. The benzene ring looks similar at first sight, and the plane of the molecule is indeed a node for all the π orbitals. However, as we discussed in Chapter 7, benzene is aromatic—it has extra stability because the six π electrons fit into three very stable orbitals and are delocalized round the whole ring. The applied field sets up a ring current in these delocalized electrons that produces a local field rather like the field produced by the electrons around a nucleus. Inside the benzene ring the induced field opposes the applied field, but outside the ring it reinforces the applied field. The carbon atoms are in the ring itself and experience neither effect, but the hydrogens are outside the ring, feel a stronger applied field, and appear less shielded (i.e. more deshielded; larger chemical shift).

Cyclophanes and annulenes , You may think that it is rather pointless imagining what goes on inside an aromatic ring as we cannot have hydrogen atoms literally inside a benzene ring. However, we can get close. Compounds called cyclophanes have loops of saturated carbon atoms attached at both ends to the same benzene rings. You see here a structure for [7]-para-cyclophane, which has a string of seven CH₂ groups attached to the para positions of the same benzene ring. The four H atoms on the benzene ring itself appear as one signal at 7.07 ppm—a typical ring-current deshielded value for a benzene ring. The two CH₂ groups joined to the benzene ring (C1) are also deshielded by the ring current at 2.64 ppm. The next two sets of CH₂ groups on C₂ and C₃ are neither shielded nor deshielded at 1.0 ppm. But the middle CH₂ group in the chain (C₄) must be pointing towards the ring in the middle of the π system and is heavily shielded by the ring current at –0.6 ppm).

With a larger aromatic ring it is possible actually to have hydrogen atoms inside the ring. Compounds are aromatic if they have 4n + 2 delocalized electrons and this ring with nine double bonds, that is, 18 π electrons, is an example. The hydrogens outside the ring resonate in the aromatic region at rather low field (9.28 ppm) but the hydrogen atoms inside the ring resonate at an amazing –2.9 ppm, showing the strong shielding by the ring current. Such extended aromatic rings are called annulenes: you met them in Chapter 7.

مواضيع ذات صلة

How electron donation and withdrawal change chemical shifts

Uneven electron distribution in aromatic rings

Summary chart of proton NMR shifts

Chemical shifts of CH groups

CH and CH2 groups have higher chemical shift than CH3 groups

Methyl groups give us information about the structure of molecules

Proton chemical shifts tell us about chemistry

Protons on saturated carbon atoms Chemical shifts are related to the electronegativity of substituents

Regions of the proton NMR spectrum

Integration tells us the number of hydrogen atoms in each peak

The differences between carbon and proton NMR

Kinetic versus thermodynamic products