3.1.2.2 Influencing factors on chemical shifts --- anisotropic在线视频

Last time we went through one of the influencing factors

on chemical shift --- inductive effect.

Today we are talking about the second one --- the magnetic anisotropic effect.

The word anisotropic basically means different property in various directions.

NMR is anisotropic because the following reason.

In the basic concepts of NMR, we assumed a spherical shape.

However, that is not the real shape for most of organic molecules.

A real molecule usually has a 3-dimensional unsymmetrical shape

because chemical bonds are generally anisotropic.

Thus when magnetic field is applied,

different magnetic moments are induced in different direction.

So we say that magnetic moment is orientation dependent.

The most representative result of magnetic anisotropic effect is

the ring currents of aromatic rings.

Here we use benzene as example.

The  electrons in the aromatic ring induce a circulation

and consequently a micro magnetic field.

In this region in the plane of aromatic ring,

the induced magnetic field reinforces the external magnetic field.

Protons residing here are exerted higher magnetic field and deshielded.

Thus aromatic protons have characteristic large chemical shifts.

Theoretically, if a proton appears here above or below the plane of the aromatic ring,

it will be highly shielded and have a very small chemical shift.

Even when compared with protons on isolated double bonds,

aromatic protons are deshielded, as shown in the examples here.

As we just mentioned, when protons are

above or below the plane of the aromatic ring, or in the middle of it,

upfield shift effects are observed.

Those kind of structure are usually obtained in bridged rings or large annulenes.

If a cyclic conjugated system is planar and antiaromatic,

chemical shift effects are in the opposite direction:

downfield over the ring, and upfield in the ring plane

due to the paramagnetic ring currents.

Those two ions are perfect examples.

They have similar skeleton but different charge.

The one on the left is positively charged,

and has 10  electrons, so it is aromatic.

We can see the protons in the plane have large chemical shifts

and those two protons on the bridging methylene are highly shielded,

with negative chemical shifts.

On the contrary, the one on the right is negatively charged.

It has 12  electrons so it is antiaromatic.

This time the protons in the plane have small chemical shifts

and those two protons on the bridging methylene are markedly deshielded,

with very large chemical shifts.

These are some more examples for the comparison

between aromatic and antiaromatic rings.

Ring currents is typical but not exclusive case for magnetic anisotropic effect.

Actually since chemical bonds are generally anisotropic,

chemical shift changes depending on the spatial relationship

between a proton and nearby functional groups.

This is called neighbouring group anisotropy.

A double bond is highly anisotropic and the result is similar to that of ring currents.

Olefinic protons are residing in the deshielding region

thus have large chemical shifts.

Here are some examples,

you will find those olefinic protons at five point something or six point something ppm.

Again, if a proton appears here in the shielding region, the chemical shift is small.

Triple bonds have an opposite spatial relationship with protons.

Protons on alkynes are in the shielding region and

have relatively small chemical shifts, usually around two ppm.

The deshielding region for a triple bond is perpendicular to the bond.

If a nucleus is pushed into this region by some rigid structural moiety,

like the proton here in this particular case,

anisotropy may also increase its chemical shift.

Anisotropy exists not only in multiple bonds,

but also in single bonds, but with just slight effect.

In acyclic compounds, this effect is diminished by the free rotation of single bonds.

While in cyclic compounds such as substituted cyclohexanes,

the effect contributes to the chemical shift difference between equatorial and axial protons.

Depending on the properties of substituents,

there could be a difference ranging from less than 0.1 ppm to over 1 ppm,

while the equatorial proton always has larger chemical shift

than the axial proton on the same carbon.

Ok, that is all for today and let’s do a little exercise.

Please point out the shielding and deshielding regions of an aromatic ring, double bonds and triple bonds.

If you are not sure about the answer, please go over today class.

In next section, we’ll talk about a third influencing factor --- resonance structures.