Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

You should now be able to work out why the resonance for the OH proton is a

triplet. Notice that the intensity of the peaks of the split resonances are not all the same

• these approximately follow a set of numbers known as Pascal’s triangle in which
  each number is the sum of the left and right numbers above it (Table 20.1 gives the first
  five lines of the triangle).

The area under each resonance (whether split or not) is proportional to the num-

ber of protons responsible for that resonance. The NMR instrument will integrate

each resonance and produce a vertical line over it, the height of which is propor-tional

to its area. We are now in a position to consider the complete NMR spectrum of

ethanol, with integration, in Fig. 20.28(b). Notice that the heights of the vertical lines

over the OH, CH 2 and CH 3 resonances are in a ratio of 1:2:3, i.e. in accordance with

the number of protons on each group.

Table 20.2 shows typical values of chemical shifts for protons in organic com-

pounds. Notice that protons attached to a carbon atom also bonded to a halogen tend

to have relatively high chemical shift () values; the electronegative atoms reduce the

electron density around the proton so that it experiences more of the external mag-

netic field (the proton is deshielded) and absorbs radiation at a higher frequency.

Similarly, if a group or atom pushes electron density on to a proton (shieldsit), the

proton absorbs radiation at a lower frequency (its chemical shift is lower).

Figures 20.29 to 20.33 show some typical NMR spectra.

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Table 20.1Pascal’s triangle

Number of peaks Intensities as shown by Pascal’s triangle

1 (singlet) 1

2 (doublet) 1 1

3 (triplet) 1 2 1

4 (quartet) 1 3 3 1

5 (quintet) 1 4 6 4 1

Table 20.2Typical values of chemical shifts for protons in organic

compounds

Group Type of compound Approximate chemical

shift / 

CH 3 –C alkane 0.9

C–CH 2 –C alkane 1.3

CH 2 =C alkene 2.6

C 6 H 5 – arene 7.3

CH 3 –N amine 2.3

CH 3 COO– ester, acid 2.0

CH 3 CO– ketone 2.1

CH 3 Br bromoalkane 2.6

CH 3 Cl chloroalkane 3.1

CH 3 F fluoroalkane 4.3