NMR 6 - Magnetic anisotropy

Protons attached to simple vinyl groups resonate from 4.5 to 7 ppm. Part of the explanation is that sp² carbons are more electronegative, since the bond contains more s character (33% compared to 25%). The carbon s orbital pulls bonds in tighter, away from the attached protons.

By itself this does not explain such a large shift. A greater effect is magnetic anisotropy. Anisotropy also explains why acetylenic protons are found from 3 to 2 ppm, since we would expect them to be higher than vinyl protons based on hybridisation alone. It also explains why the chemical shift of benzylic protons is so high.

An anisotropic field is a field that is not isotropic. An isotropic field either has a uniform density, or a spherically symmetric density distribution.

When aromatic molecules are placed in a magnetic field, mobile pi electrons in it are induced to circulate around, which itself produces a new magnetic field - an anisotropic one.

The benzylic protons are in the region deshielded by the induced anisotropic field. Protons above the ring would be shielded by the induced anisotropic field, as shown empirically in these molecules:

A similar effect is found in pi bonds, with the current moving around the internuclear axis:

For double bonds, this deshields the end protons. For triple bonds it shields them:

So an aromatic molecule in an NMR machine experiences three different magnetic fields. The external field of the machine, the diamagnetic shielding from valence electrons (an isotropic field that counteracts the external field) and any anisotropic fields induced by currents induced by the external field. The chemical shift of a proton environment depends on the net effect of these.

These opens a lot of questions such as:

Why do magnetic fields induce current in some systems but not others?

Why does the moving current itself create another magnetic field?

Why are pi electrons more mobile?

Why do pi electrons in triple bonds induce in the opposite direction to double bonds?

And so on. The answers, if any, are found in physics textbooks. Spectroscopy textbooks will just summarize what happens and what the effects are.

NMR 5

NMR does not only show relative deshielding. You can use the chemical shift value itself to predict what type of enviroment it is in. Correlation charts can be used for this:

While it difficult to memorize a lot of number ranges, it is easy to get a "feel" for what regions the peaks are in, especially for the less-cluttered left half of the chart.

Notice that all protons attached to sp³ carbons are at 2-1 ppm, assuming the carbon is not attached to a heteroatom. Protons on primary carbons have a higher shift than on tertiary carbons:

This can be explained with the same reason for tertiary carbocations are more stable - alkyl group donate electrons inductively, shielding the central CH.

Sulfonates infrared spectra

Methyl p-toluene sulfonate

S=O stretch: Strong asymmetric at 1350, strong symmetric at 1175.

S-O stretch: Several strong bands at 1000-750.

Sufonyl chloride infrared spectra

Benzenesulfonyl chloride

S=O stretch: Strong asymmetric at 1375, and strong symmetric at 1185.

Thiol infrared spectra

Benzenethiol

S-H stretch: One weak peak near 2550

IR of amino acids

Leucine

Amino acids exist as zwitterions, so they can be considered both carboxylate salts and amine salts.

COO^- stretch: Frequency is lowered compared to parent acid, since resonance gives the carbonyl much more single-bond character. Strong asymmetric at 1600 and strong symmetric at 1400.

N-H⁺ stretch: Broad stretch at 3300-2600. Ammonium ions absorb to the left of this range, while tertiary amine salts absorb to the right. A broad peak often appears near 2100.

N-H⁺ bend: Occurs at 1610 to 1500. Tertiary amines absorb only weakly.

Nitro compound infrared spectra

1-nitrohexane

Nitrobenzene

Aliphatic NO₂ stretch: Strong asymmetric at 1600-1530 and medium symmetric at 1390-1300.

Conjugated NO₂ stretch: Strong asymmetric at 1550-1490 and strong symmetric at 1355-1315.

These bands may partially overlap the C=C region, but are usually easy to distinguish.