Doc Brown's Chemistry Advanced Level Pre-University Chemistry

H-1 proton NMR spectroscopy - spectra index

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 - ethanol here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of ethanol 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 ethanol molecule.

Ethanol C₂H₆O, , , ,

Interpreting the H-1 NMR spectrum of ethanol

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

The hydrogen atoms (protons) of ethanol occupy 3 different chemical environments so that the H-1 proton low resolution NMR spectra should show 3 peaks (diagram above).

CH₃CH₂OH

Note the ratio of the 3 colours for the 3 proton chemical environments in ethanol.

In terms of the H-1 chemical shifts for ethanol (a) to (c) and applying the n+1 rule:

(a) Centred at 1.22 ppm, the CH₃ protons are split by the CH₂ protons into a 1 : 2 : 1 triplet (n+1 = 3).

(b) Centred at 3.69 ppm, the CH₂ protons are split by the CH₃ protons into a 1 : 3 : 3 : 1 quartet (n+3 = 4).

(c) The hydroxy proton O-H gives a chemical shift of 2.61 ppm and shows no significant splitting.

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

Very high resolution 1H NMR spectrum of ultra-pure ethanol under the right condition!

With ultra-pure anhydrous ethanol it is possible to observe the splitting effects by, and of, the hydroxyl proton OH - but its a bit tricky in place!

(a) The CH₃ protons give a triplet from the CH₂ protons- as above (n+1 = 3) - no change.

(b) You might think the CH₂ proton resonance might seem to be split into a quintet (n+1 = 5) by the CH₃ and OH protons on either side, and not a quartet. Some diagrams I've seen look like this, but this not actually what happens.

In fact the CH₂ protons are split by the CH₃ protons into a 1:3:3:1 quartet, but this quartet is split into doublets by the single OH proton (n+1 = 2), so a series of 1:3:3:1 doublets, or 1:1:3:3:3:3:1:1 to be a purist, but this is going beyond pre-university level!

(c) As above, the OH resonance would be split into a triplet (n+1 = 3) by the CH₂ protons.

(This is more university level NMR spectroscopy, so don't worry, concentrate on the basic proton ratio of 3:2:1 to match the structure of ethanol).

The integrated NMR proton ratio observed of 3 : 2 : 1, corresponds with the structural formula of ethanol.

Extra note on the hydroxy (O-H) resonance in the H-1 NMR of alcohols

(a) The first important point to make is, that despite the variety of different chemical shifts for the OH proton resonance quoted in data books, textbooks and internet sources, the integration of the NMR resonances always gives the correct proton ratio in the molecule, in this case 3 : 2 : 1 for ethanol CH₃CH₂OH

(b) Apart from the O-H group resonance the NMR spectra of most alcohols conform to what would be expected e.g. the triplet and quartet of the spin-spin splitting effects for the alkyl part resonances of the ethanol molecule.

(c) However, the OH proton NMR resonance for ethanol is typically quoted as ~2.6 ppm (here) and ~5.3 ppm, and some in between too!

BUT, the main difference you find from various sources is not the OH proton chemical shift, but is it a singlet or a triplet?

(d) The problem arises if the alcohol is impure e.g. containing water or any source of labile protons, because water and the alcohol, ethanol, exchange protons e.g.

CH₃CH₂OH  +  H-O-H    CH₃CH₂OH  +  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 ethanol 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.

CH₃CH₂OH  +  H-O-CH₂CH₃    CH₃CH₂OH  +  H-O-CH₂CH₃

which can give the same effect as traces of water of acid.

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

(e) So, in ethanol, 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.

Note the actual chemical shift for the OH proton resonance of alcohols can depend on the solvent and concentration used in the NMR machine, often CDCl₃ - deuterated trichloromethane solvent.

Number of protons 1H causing splitting	Splitting pattern produced from the n+1 rule and the theoretical ratio of line intensities
0 means no splitting							1
1 creates a doublet						1		1
2 creates a triplet					1		2		1
3 creates a quartet				1		3		3		1
4 creates a quintet			1		4		6		4		1
5 creates a sextet		1		5		10		10		5		1
6 creates a septet	1		6		15		20		15		6		1

---

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H6O NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanol (ethyl alcohol) and methoxymethane (dimethyl ether) image sizes.
INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking difference is the broad O-H stretching band ~3400 cm-1, found in the infrared spectrum of alcohols, but absent in the infrared spectrum of ethers.
MASS SPECTRA: Both ethanol and methoxymethane show some similarities in their mass spectra, but their base ion peaks are quite different - for ethanol it is m/z 31 and for methoxymethane it is m/z 45.
1H NMR SPECTRA: The 1H NMR spectra of ethanol and methoxymethane are quite significantly different. Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different chemical environments), whereas methoxymethane only gives one 1H chemical shift peak (all 6 protons in the same chemical environment). Ethanol does not have the symmetry of methoxymethane and has the hydroxyl group, thus producing 3 different chemical environments
13C NMR SPECTRA: The 13C NMR spectra of ethanol and methoxymethane are different. Ethanol gives two 13C resonances, but methoxymethane only one (2 different 13C chemical environments and a 13C single chemical environment).

H-1 proton NMR spectroscopy index

(Please read the 8 points at the top of the ¹H NMR index page)

The infrared spectrum of Ethanol (ethyl alcohol)

The mass spectrum of Ethanol (ethyl alcohol)

The C-13 NMR spectrum Ethanol (ethyl alcohol)

The chemistry of ALCOHOLS revision notes INDEX

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box