It sounds rather unlikely that the humble methyl group could tell us much that is important about molecular structure—but just you wait. We shall look at four simple compounds and their NMR spectra—just the methyl groups, that is.

The first compound, the acid chloride in the margin, shows just one methyl signal containing nine Hs at δH 1.10. This tells us two things. All the protons in each methyl group are identical, and all three methyl groups in the tertiary butyl (t-butyl, or Me3C–) group are identical. This is because rotation about C–C single bonds, both about the CH3–C bond and about the (CH3)3C–C bond, is fast. Although at any one instant the hydrogen atoms in one methyl group, or the methyl groups in the t-butyl group, may differ, on average they are the same. The time-averaging process is fast rotation about a σ bond. The second compound shows two 3H signals, one at 1.99 and one at 2.17 ppm. Unlike a C–C bond, the C=C double bond does not rotate at all and so the two methyl groups are different. One is on the same side of the alkene as (or ‘cis to’) the –COCl group while the other is on the opposite side (or ‘trans’). The next pair of compounds contain the CHO group. One is a simple aldehyde, the other an amide of formic acid: it is DMF, dimethylformamide. The first has two sorts of methyl group: a 3H signal at δH 1.81 for the SMe group and a 6H signal at δH 1.35 for the CMe2 group. The two methyl groups in the 6H signal are the same, again because of fast rotation about a C–C σ bond. The second compound also has two methyl signals, at 2.89 and 2.98 ppm, each 3H, and these are the two methyl groups on nitrogen. Restricted rotation about the N–CO bond must be making the two Me groups different. You will remember from Chapter 7 (p. 155) that the N–CO amide bond has considerable double-bond character because of conjugation: the lone pair electrons on nitrogen are delocalized into the car bonyl group.

Like double bonds, cage structures prevent bond rotation and can make the two protons of a CH₂ group appear different. There are many flavouring compounds (terpenoids) from herbs that have structures like this. In the example here—myrtenal, from the myrtle bush—there is a four-membered ring bridged across a six-membered ring. The methyl groups on the other bridge are different because one is over the alkene while one is over the CH₂. No rotation of any bonds within the cage is possible, so these methyl groups resonate at different frequencies (0.74 and 1.33 ppm). The same is true for the two H atoms of the CH₂ group.

مواضيع ذات صلة

How electron donation and withdrawal change chemical shifts

Uneven electron distribution in aromatic rings

The benzene ring current causes large shifts for aromatic protons

Summary chart of proton NMR shifts

Chemical shifts of CH groups

CH and CH2 groups have higher chemical shift than CH3 groups

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