1H proton nmr spectrum of dimethylamine C2H7N CH3NHCH3 low/high resolution analysis interpretation of chemical shifts ppm spin spin line splitting H-1 N-methylmethanamine 1-H nmr explaining spin-spin coupling for line splitting doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: 1H NMR spectrum of dimethylamine

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Interpreting the H-1 NMR spectrum of dimethylamine (N-methylmethanamine)

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 dimethylamine

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H-1 proton NMR spectroscopy - spectra index

1H proton nmr spectrum of dimethylamine low/high resolution diagrams C2H7N CH3NHCH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for N-methylmethanamine 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 - dimethylamine here.

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

Dimethylamine, N-methylmethanamine, C₂H₇N, (c) doc b , ,

Interpreting the H-1 NMR spectrum of dimethylamine

In terms of spin-spin coupling from the possible proton magnetic orientations, for dimethylamine I have only considered the interactions of non-equivalent protons on adjacent carbon atoms which doesn't apply here, and the N-H proton resonance is not normally split by adjacent C-H protons and neither is the adjacent C-H resonance split by the N-H protons, see note on resonance (b) below.

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

The hydrogen atoms (protons) of dimethylamine 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 dimethylamine).

CH₃NHCH₃ or (CH₃)₂NH

Note the proton ratio 6:1 of the 2 colours of the ? protons of dimethylamine in the 2 chemically different proton environments

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

Although there are 7 hydrogen atoms in the molecule, there are only 7 possible different chemical environments for the hydrogen atoms in dimethylamine molecule.

The integrated signal proton ratio 6:1 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of dimethylamine.

The high resolution 1H NMR spectrum of dimethylamine

The high resolution spectra of dimethylamine shows 2 groups of proton resonances and in the 6:1 ratio expected from the structural formula of dimethylamine.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of dimethylamine - 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 dimethylamine below.

However, applying the n+1 rule to dimethylamine isn't necessary, both proton resonances

(a) ¹H Chemical shift 2.13 ppm, methyl protons: CH₃NHCH₃

There are no adjacent C-H protons, and the N-H protons do not cause a splitting, so this 1H NMR resonance shows up as a singlet in the dimethylamine spectrum.

(b) ¹H Chemical shift ?: CH₃NHCH₃

The adjacent C-H protons do not cause a splitting, so this 1H NMR resonance shows up as a singlet in the dimethylamine spectrum.

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 (and vice versa) - even a trace of water catalyses this effect.

e.g. for aliphatic primary/secondary aliphatic amines, if R = H or alkyl

R₂N-H + H-O-H H-R₂N-H⁺ + OH^- R₂N-H + H-O-H

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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Links associated with dimethylamine

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

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