Chemistry, Third edition

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

There are three signals because there are protons in three different environments,

those present in CH 3 , in CH 2 and the single Hattached to the oxygen.

The resonance frequencies of the different protons are expressed as chemical shifts

() relative to a standard. Tetramethylsilane, (TMS), or (CH 3 ) 4 Si, is widely used as a

standard because it is inert and has a spectrum with a single absorption (there is only

one type of hydrogen in the molecule). Different NMR spectrometers have magnets

that generate different fields so, in order that scientists can compare data from differ-

ent instruments, the chemical shift of a proton () is calculated as

shift from TMS for a particular proton in Hz

= ——————————————————

spectrometer frequency in MHz

On this scale the resonance of TMS is exactly 0.00. As a result of using this system val-

ues of for a given proton will always be the same, no matter what instrument is used,

and data is easily compared.

If the spectrum of ethanol is run under conditions of high resolution, the original

peaks are seen to be split into multiplets as shown in Fig. 20.28(a). This is called

spin–spin splitting.Each type of proton experiences the protons onneighbouring atoms

and its resonance peak is split into n+ 1 components, where n is the number of pro-

tons on the nearest neighbouring atom(s).

Look again at Fig. 20.28(a). The resonance at 0.00 is due to the protons from TMS

and the resonances centered at 4.35,3.50 and 1.10 are due to the protons of

ethanol. The ethanol resonances are split; at 1.10 the resonance is split into three (a

triplet); this is because the CH 3 protons experience two protons on the next carbon

atom (the CH 2 group and n= 2) and so their resonance splits into n+ 1 or 2 + 1 = 3

peaks. At 3.50 the resonance is split into five (a quintet): the CH 2 protons experience

four neighbouring protons (three from CH 3 and one from OH) so the resonance is

split into n+ 1, or five, peaks.

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Fig. 20.28(a) High-resolution NMR spectrum of pure ethanol, CH 3 CH 2 OH. (b) The same
spectrum with integration.

NMR signals

How many different signals

will there be in the low

resolution 1 H-NMR

spectrum of:

(i)CH 3 CH 2 CHO

CH 3

(ii) CH—Br

CH 3

Exercise 20L