TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹H NMR spectroscopy and all other proton shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - butan-1-ol here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of butan-1-ol represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the butan-1-ol molecule i.e. 3 : 2 : 2 : 2 :1.

Butan-1-ol C₄H₁₀O , .

Interpreting the H-1 NMR spectrum of butan-1-ol (1-butanol, n-butyl alcohol)

For relatively simple molecules, the low resolution H-1 NMR spectrum of butan-1-ol is a good starting point.

The hydrogen atoms (protons) of butan-1-ol occupy 5 different chemical environments so that the low resolution NMR spectra should show 5 peaks of different H-1 NMR chemical shifts (diagram above for butan-1-ol).

CH₃CH₂CH₂CH₂OH

Note the ratio 3:2:2:2:1 of the 5 colours of the protons in the 5 chemically different environments

Although there are 10 hydrogen atoms in the molecule, there only 5 possible chemical environments for the hydrogen atoms in butan-1-ol molecule.

The proton ratio 3:2:2:2:1 observed, corresponds with the structural formula of butan-1-ol.

The high resolution spectrum of butan-1-ol

The low and high resolution spectra of butanal show 5 groups of protons and in the ratio expected from the formula of butan-1-ol, namely 3:2:2:2:1, but the high resolution spectrum is very complex.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of butan-1-ol - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin filed splitting effects - see high resolution notes on butan-1-ol below.

CH₃CH₂CH₂CH₂OH (below I will refer to the 1st, 2nd and 3rd CH₂ groups from left to right)

So, using the chemical shifts and applying the n+1 rule to butan-1-ol

Chemical shift 0.94 ppm CH₃ protons

The end methyl group proton resonance is split into a 1:2:1 triplet by the first CH₂ group (2 protons, n+1 = 3 = triplet).

Evidence for the presence of a CH₂ group in the molecule of butan-1-ol.

Chemical shift 1.39 ppm 1st CH₂ protons

The first CH₂ proton resonance is split by the CH₃ protons AND the 2nd CH₂ protons into a 1:5:10:10:5:1 sextet (5 protons, n+1 = 6 = sextet).

Evidence for the presence of a CH₃CH₂CH₂ group in the molecule of butan-1-ol

Chemical shift 1.53 ppm 2nd CH₂ protons

The 2nd CH₂ proton resonance is split on either side by the 1st and 3rd CH₂ protons into a 1:4:6:4:1 quintet (4 protons, n+1 =5 = quintet).

Evidence for the presence of a CH₂CH₂CH₂ group in the molecule of butan-1-ol.

Chemical shift 2.24 ppm OH proton

The OH proton resonance is often seen and 'portrayed' as a singlet.

Normally the O-H proton resonance is not split by adjacent carbon atom protons and neither does it, in turn, split the resonance of the same adjacent protons (see extra note below).

Chemical shift 2.24 ppm 3rd CH₂ protons

You can consider that the 3rd CH₂ proton resonance is split by the 2nd CH₂ protons into a 1:2:1 triplet (2 protons, n+1 = 3 = triplet, as on diagram).

Note that no splitting due to OH proton is detected.

 

Note on the OH proton resonance

If the alcohol is impure, containing water or any source of labile protons, because water and the alcohol exchange protons e.g.

R-O-H  +  H-O-H    R-O-H  +  H-O-H

This means the CH₂ protons no longer experience a 'simple' local field from one singlet proton from two possible orientations, but, over a finite period, experience the averaging effect of exchanging protons.

This removes the spin - spin coupling effect and the OH proton resonance just shows up as a singlet if the butan-1-ol contains even a trace of water (or acid).

This sort of exchange cannot happen with the alkyl protons, but is common with molecules containing a hydroxylic (OH) hydrogen atom like alcohols and carboxylic acids.

Not only that, you also get proton transfer between the alcohol molecules i.e.

R-O-H  +  H-O-R    R-O-H  +  H-O-R

which gives the same effect as traces of water of acid.

So, in butan-1-ol, all you usually see in the H-1 NMR spectrum is the mutual splitting of the CH₂ and CH₃ proton resonances plus a singlet line for the OH proton resonance.

 

The chemistry of ALCOHOLS revision notes INDEX

H-1 proton NMR spectroscopy index

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