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 - cyclohexene here.

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

Interpreting the H-1 NMR spectrum of cyclohexene

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

The 10 hydrogen atoms (protons) of cyclohexene occupy 3 different chemical environments so that the low resolution NMR spectra should show 3 principal resonance peaks of different H-1 NMR chemical shifts (diagram above for cyclohexene).

Note the proton ratio 4:4 2 in the molecule of the 3 chemically different proton environments

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

Although there are 10 hydrogen atoms in the molecule, the proton NMR spectrum shows there are only 3 possible different chemical environments for the hydrogen atoms in cyclohexene molecule - due to the symmetry of the hexagonal ring on either side of the C=C double bond.

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

The high resolution 1H NMR spectrum of cyclohexene

In terms of spin-spin coupling from the possible proton magnetic orientations, for cyclohexene I have only considered the interactions of non-equivalent protons on adjacent carbon atoms

e.g. -CH₂-CH₂, -CH-CH₂-, protons etc.

The high resolution spectra of cyclohexene would show 3 groups of proton resonances and in the 4:4:2 (2:2:1) ratio expected from the structural formula of cyclohexene, but we can now consider the splitting of resonance lines from the spin-spin coupling in the molecule of cyclohexene.

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

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

¹H NMR resonance (a) ¹H Chemical shift δ 1.61 ppm:

The resonance for the 2 x CH₂ protons furthest from the C=C bond.

This will be split into a triplet by the CH₂ protons nearer the C=C bond (n+2 = 3).

This resonance is not split by the other CH₂ protons furthest from the C=C bond because they are adjacent and equivalent to each other (the cyclohexene molecule is symmetrical about the -CH₂-CH₂- grouping furthest from the C=C bond)..

¹H NMR resonance (b) ¹H Chemical shift δ 1.98 ppm:

The resonance for the 2 x CH₂ protons nearest to the C=C bond.

This resonance will be split into a quartet by the CH₂ protons furthest from the C=C bond and the CH proton of the C=C bond (n+1 = 4).

¹H NMR resonance (c) ¹H Chemical shift δ 5.66 ppm:

The resonance for the 2 x CH protons of the C=C bond.

Here the presence of the pi orbitals of the double bond results in a much greater shift than is observed for the other four CH₂ protons.

This resonance will be split into a triplet by the CH₂ protons nearest the C=C bond (n+1 = 3).

This resonance is not split by the other CH proton of the C=C bond because they are adjacent and equivalent to each other (the cyclohexene molecule is symmetrical about the -CH=CH-).