So, on to some real ¹³C NMR spectra. Our very first compound, hexanedioic acid, has the simple NMR spectrum shown here. The fi rst question is: why only three peaks for six carbon atoms? Because of the symmetry of the molecule, the two carboxylic acids are identical and give one peak at 174.2 ppm. By the same token C2 and C5 are identical, and C3 and C4 are identical. These are all in the saturated region 0–50 ppm but the carbons next to the electron withdrawing CO₂H group will be more deshielded than the others. So, we assign C2/C5 to the peak at 33.2 ppm and C3/C4 to 24.0 ppm.

Heptan-2-one is the bee pheromone mentioned on p. 48. It has no symmetry so all its seven carbon atoms are different. The carbonyl group is easy to identify (208.8 ppm) but the rest are more difficult. The two carbon atoms next to the carbonyl group come at lowest fi eld, while C7 is at highest fi eld (13.9 ppm). It is important that there is the right number of signals at about the right chemical shift. If that is so, we are not worried if we cannot assign each frequency to a precise carbon atom (such as atoms 4, 5, and 6, for example). As we said before, don’t be concerned with the intensities of the peaks.

¹³C NMR spectrum

You met BHT on p. 8: its formula is C15H24O and the fi rst surprise in its NMR spectrum is that there are only seven signals for the 15 carbon atoms. There is obviously a lot of symmetry; in fact, the molecule has a plane of symmetry vertically as it is drawn here, and the coloured blobs indicate pairs or groups of carbons related to each other by symmetry which therefore give only one signal. The very strong signal at δ = 30.4 ppm belongs to the six identical methyl groups on the t-butyl groups (coloured red) and the other two signals in the 0–50 ppm range are the methyl group at C4 and the brown central carbons of the t-butyl groups. In the aromatic region there are only four signals as the two halves of the molecule are the same. As with the last example, we are not concerned with exactly which is which— we just check that there are the right number of signals with the right chemical shifts.

Paracetamol is a familiar painkiller with a simple structure—it too is a phenol but in addition it carries an amide substituent on the benzene ring. Its NMR spectrum contains one saturated carbon atom at 24 ppm (the methyl group of the amide side chain), one carbonyl group at 168 ppm, and four other peaks at 115, 122, 132, and 153 ppm. These are the carbons of the benzene ring. Why four peaks? The two halves of the benzene ring must be the same (only one signal for each pair of carbons coloured red and green), which tells us that the NHCOCH3 group doesn’t really lie just to one side as shown here, but rotates rapidly, meaning that on average the two sides of the ring are indistinguishable, as in BHT. Why is one of these aromatic peaks in the C O region at 153 ppm? This must be C4 because it is bonded to oxygen, a reminder that carbonyl groups are not the only unsaturated carbon atoms bonded to oxygen (see the chart on p. 56), although it is not as deshielded as the true C O group at 168 ppm.

The ¹H NMR spectrum

1H NMR (or ‘proton NMR’) spectra are recorded in the same way as ¹³C NMR spectra: radio waves are used to study the energy level differences of nuclei, but this time they are ¹H and not ¹³C nuclei. Like ¹³C, ¹H nuclei have a nuclear spin of 1/2 and so have two energy levels: they can be aligned either with or against the applied magnetic field. Here, as an example, is the ¹H NMR spectrum of acetic (ethanoic) acid, MeCO₂H, and below it the ¹³C NMR spectrum.

¹H NMR spectra have many similarities with ¹³C NMR spectra: the scale runs from right to left and the zero point is given by the same reference compound, though it is the proton resonance of Me4Si rather than the carbon resonance that defines the zero point. However, as you immedi ately see in the spectrum above, the scale is much smaller, ranging over only about 10 ppm instead of the 200-ppm needed for carbon. This makes perfect sense: the variation in the chemical shift is a measure of the shielding of the nucleus by the electrons around it. There is inevitably less change possible in the distribution of two electrons around a hydrogen nucleus than in that of the eight valence electrons around a carbon nucleus. Nonetheless the acetic acid spectrum above shows you that, just as you would expect, the H atom of the carboxylic acid group, directly attached to an oxygen atom, is more deshielded than the H atoms of acetic acid’s methyl group. We can also divide up the 1H NMR spectrum into regions that parallel the regions of the 13C NMR spectrum. Hydrogen atoms bonded to saturated carbon atoms appear in the right-hand, more shielded (between 5 and 0 ppm) region of the spectrum, while those bonded to unsaturated carbon atoms (alkenes, arenes, or carbonyl groups primarily) appear in the left-hand, less shielded region between 10 and 5 ppm. As with the 13C spectrum, nearby oxygen atoms withdraw electron density and make the signals appear towards the left-hand end of each of these regions.