Question

What is anisotropy in nmr?

Answer 1

Well this is a feature of NMR spectroscopy that profs use to batten down and confuse students with, and it implies the directional dependence of a species under a given analysis.

Explanation:

In NMR spectroscopy, possibly the best example of anisotropy occurs with the benzene molecule, in which the `6-pi` electrons are delocalized and free to move around the aromatic ring:

Now in the diagram consider the behaviour of the `pi-"electrons"` in the applied magnetic field. You know from physics when an electron encounters a magnetic field, it sticks it little right hand out, and moves in a direction so as to generate a magnetic field OPPOSED to the applied field (cf. the `"right hand rule"`, counterclockwise movement with respect to the diagram). Of course, the benzene ring will not be in this flat, opposed orientation all the time, but constants can be factored into the equation to account for the time it is perpendicular to `B_0`, the applied field.

So a current, i.e. movement of electrons, is induced around the benzene ring. This OPPOSES the applied field with respect to the interior of the ring, but with respect to the exterior of the ring, it REINFORCES the applied magnetic field. The result? Well, protons on an aryl ring are said to be deshielded with respect to conventional protons, and their resonance occurs significantly downfield. The physical manifestation of this deshielding is the appearance of the protons of benzene at `7.15*"ppm"`, way DOWNFIELD with respect to non-aromatic protons.

And now consider the performance of another aromatic species in the NMR experiment, i.e. 18-annulene, which is another aromatic molecule with 18 delocalized `pi` electrons.

The 12 protons EXTERIOR to the ring are deshielded like the aryl protons. However, the protons INTERIOR to the ring are STRONGLY SHIELDED, in that the induced magnetic field OPPOSES the applied field, with the result that a signal is observed at `delta=-1.9*"ppm"`, i.e. way UPFIELD. The exocyclic protons are deshielded from the applied magnetic field, and these occur at `delta=8.2*"ppm"`. And so we would see TWO signals in a 2:1 ratio in the `""^1H` `"NMR spectrum"`.

I acknowledge that there is a whole lot of info here to digest.....