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Advanced Organic Chemistry: 1H NMR spectrum of methanol

The H-1 hydrogen-1 (proton) NMR spectrum of methanol

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing H-1 NMR spectra of methanol

CH3OH low and high resolution 1H proton nmr spectrum of methanol analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr methyl alcohol  explaining spin-spin coupling causing line splitting doc brown's advanced organic chemistry revision notes

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 resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - methanol here.

In terms of spin-spin coupling from the possible proton magnetic orientations, for methanol I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-OH.

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

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Interpreting the H-1 NMR spectrum of methanol

For relatively simple molecules, the low resolution H-1 NMR spectrum of methanol is a good starting point (low resolution diagram above).

The hydrogen atoms (protons) of methanol occupy 2 different chemical environments so that the low resolution NMR spectra should show 2 principal peaks of different H-1 NMR chemical shifts (diagram above for methanol).

CH3OH

Note the proton ratio 3:1 of the two colours of the protons in the two chemically different environments

Chemical shifts (a) to (b) on the H-1 NMR spectrum diagram for methanol.

Although there are 4 hydrogen atoms in the molecule, there are only 2 possible different chemical environments for the hydrogen atoms in methanol molecule.

The integrated signal proton ratio 3:1 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of methanol.

The high resolution 1H NMR spectrum of methanol

All low and high resolution spectra of methanol show 2 groups of proton resonances and in the 3:1 ratio expected from the formula of methanol.

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

So, using the chemical shifts and applying the n+1 rule to methanol and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 3.43 ppm, CH3 protons: CH3OH

This resonance is split into a 1:1 doublet by the adjacent OH proton (n+1 = 2)

Evidence for the presence of a methyl group in the molecule of methanol

(b) 1H Chemical shift 3.66 ppm. OH proton: CH3OH

This resonance is split into a 1:3:3:1 quartet by the adjacent CH3 protons (n+1 = 4)

Evidence for the presence of an OH group in the molecule of methanol


Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling 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

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