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Advanced Organic Chemistry: 1H NMR spectrum of N-methylethanamine

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of N-methylethanamine (ethylmethylamine)

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 N-methylethanamine (ethylmethylamine)

1H proton nmr spectrum of N-methylethanamine (ethylmethylamine) low/high resolution diagrams C3H9N CH3NHCH2CH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for N-methylethylamine 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 - N-methylethanamine (ethylmethylamine) here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of N-methylethanamine (ethylmethylamine) 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 N-methylethanamine (ethylmethylamine) molecule.

N-methylethanamine, N-methylethylamine, ethylmethylamine, (c) doc b, (c) doc b

Interpreting the H-1 NMR spectrum of N-methylethanamine (ethylmethylamine)

In terms of spin-spin coupling from the possible proton magnetic orientations, for N-methylethanamine (ethylmethylamine) I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3 protons (but see note on problems with N-H proton resonance).

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

The 9 hydrogen atoms (protons) of N-methylethanamine (ethylmethylamine) occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts (diagram above for N-methylethanamine (ethylmethylamine)).

CH3NHCH2CH3

Note the proton ratio 3:1:2:3 of the 4 colours of the 9 protons of N-methylethanamine (ethylmethylamine) in the 4 chemically different proton environments

Chemical shifts (a) to (d) on the H-1 NMR spectrum diagram for N-methylethanamine (ethylmethylamine).

Although there are 9 hydrogen atoms in the molecule, there are only 9 possible different chemical environments for the hydrogen atoms in N-methylethanamine (ethylmethylamine) molecule.

The integrated signal proton ratio 3:1:2:3 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of N-methylethanamine (ethylmethylamine).

The high resolution 1H NMR spectrum of N-methylethanamine (ethylmethylamine)

The high resolution spectra of N-methylethanamine (ethylmethylamine) shows 4 groups of proton resonances and in the 3:1:2:3 ratio expected from the structural formula of N-methylethanamine (ethylmethylamine).

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of N-methylethanamine (ethylmethylamine) - 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 N-methylethanamine (ethylmethylamine) below.

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

(a) 1H Chemical shift 2.30 ppm, methyl group protons: CH3NHCH2CH3

This resonance appears as a singlet chemical shift, the adjacent amine protons do not normally cause splitting.

(b) 1H Chemical shift 1.20 ppm, amine group proton: CH3NHCH2CH3

This resonance appears as a singlet chemical shift, the amine proton resonance is not normally split by adjacent C-H protons.

(c) 1H Chemical shift 2.65 ppm, CH2 group protons: CH3NHCH2CH3

This resonance is spilt into a 1:3:3:1 quartet by the adjacent CH3 protons (n+1 = 3), the adjacent amine protons do not normally cause splitting.

Evidence for the presence of a CH3 group in the molecule of N-methylethanamine (ethylmethylamine)

(d) 1H Chemical shift 1.10 ppm, methyl group protons: CH3NHCH2CH3

This resonance is spilt into a 1:2:1 triplet by the adjacent CH2 protons (n+1 = 3)

Evidence for the presence of a CH2 group in the molecule of N-methylethanamine (ethylmethylamine)


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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