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Advanced Organic Chemistry: H-1 NMR spectrum of propane

The H-1 hydrogen-1 (proton) NMR spectrum of propane

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 propane

low and high resolution H-1 proton nmr spectrum of propane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for propane 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - propane here.

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

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Interpreting the H-1 NMR spectrum of propane

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

The hydrogen atoms (protons) of propane occupy 2 different chemical environments so that the low resolution NMR spectra should show 2 peaks of different H-1 NMR chemical shifts (diagram above for propane).

CH3CH2CH3 

Note the ratio 6:2 = 3:1 of the two colours of the protons in the two chemically different environments

Although there are 8 hydrogen atoms in the molecule, there only 2 possible chemical environments for the hydrogen atoms in propane molecule.

The overall proton ratio 3:1 observed, corresponds with the structural formula of propane.

The high resolution H-1 NMR spectrum of propane

All low and high resolution spectra of propane show two groups of protons and in the ratio 3:1 expected from the formula of propane.

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

So, using the chemical shifts and applying the n+1 rule to propane

Chemical shift 0.90 ppm, the two groups of CH3 protons

This proton resonance split by the CH2 protons into a 1:2:1 triplet (n+1 = 3)

Evidence for the presence of a CH2 group in the molecule of propane

Chemical shift 1.34 ppm, the one group of CH2 protons

The central CH2 group protons are split on either side by a group of CH3 protons (6 protons in total) into a 1:6:15:20:15:6:1 septet (n+1 = 7)

Evidence for the presence of a two CH3 groups separated by a carbon atom with at least on proton on it in the molecule of propane


Number of protons 1H causing splitting Splitting pattern produced from the n+1 rule 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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