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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 - ethylamine here.

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

Interpreting the H-1 NMR spectrum of ethylamine

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

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

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

CH₃CH₂NH₂

Note the proton ratio 3:2:2 of the 3 colours of the 8 protons in the 3 chemically different proton environments

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

Although there are 7 hydrogen atoms in the molecule, there are only 3 possible different chemical environments for the hydrogen atoms in ethylamine molecule.

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

The high resolution 1H NMR spectrum of ethylamine

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

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

(a) ¹H Chemical shift 1.10 ppm, methyl protons: CH₃CH₂NH₂

This resonance is split 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 ethylamine

(b) ¹H Chemical shift 2.61 ppm, CH₂ protons: CH₃CH₂NH₂

This resonance is split into a 1:3:3:1 quartet by the adjacent CH₃ protons (n+1 = 4), but not by the N-H protons.

I have assumed the N-H protons do NOT cause splitting (see (c)).

Evidence for the presence of a CH₃ group in the molecule of ethylamine.

Resonances (a) and (b) provide evidence for the ethyl group in ethylamine.

(c) ¹H Chemical shift 1.04 ppm, amine group protons: CH₃CH₂NH₂

This resonance appears as a singlet chemical shift for ethylamine.

I have assumed the adjacent CH₂ group protons do NOT cause splitting of the N-H proton resonance, therefore the amine group proton resonance is not split by the adjacent CH₂ protons, so appears a singlet.

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