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Advanced Organic Chemistry: 1H NMR spectrum of methylamine

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of methylamine

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 methylamine

1H proton nmr spectrum of methylamine low/high resolution diagrams CH5N CH3NH2 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for methylamine explaining spin-spin coupling for 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 - methylamine here.

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

Methylamine  CH5N   (c) doc b   (c) doc b   (c) doc b   (c) doc b

Interpreting the H-1 NMR spectrum of methylamine

In terms of spin-spin coupling from the possible proton magnetic orientations, for methylamine I have only considered the interactions of non-equivalent protons on adjacent carbon atoms, BUT this doesn't apply to methylamine.

For relatively simple molecules, the low resolution H-1 NMR spectrum of methylamine is not a good starting point (low resolution diagram above) because both proton resonances have a similar chemical shift of ~2.4 ppm.

CH3NH2

Note the proton ratio 3:2 of the 2 colours of the protons in the 2 theoretically chemically different proton environments

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

Although there are 5 hydrogen atoms in the molecule, there are only 2 possible different chemical environments for the hydrogen atoms in methylamine molecule.

The high resolution 1H NMR spectrum of methylamine

In the case of some amines applying the n+1 rule is sometimes not applicable because unless the amine is ultra-pure, the N-H proton resonance is not split by adjacent alkyl group protons and neither are the adjacent alkyl proton resonance split by the N-H protons.

(a) 1H Chemical shift ~2.4 ppm, methyl group protons: CH3NH2

A singlet, assuming no splitting of the resonance due to the amine group protons.

(b) 1H Chemical shift ~2.4 ppm, amine group protons: CH3NH2

A singlet, assuming no splitting of the resonance due to the methyl group protons.

So what you observe is a singlet, which is actually two singlet resonances close together.

The lack of resonance splitting is due to exchange of protons between the amine group of the amine molecules which inhibits the coupling between amine group protons and any adjacent alky group protons - even trace of water catalyses this effect.


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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