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Advanced Organic Chemistry: 1H NMR spectrum of 3-methylhexane

The H-1 hydrogen-1 (proton) NMR spectrum of 3-methylhexane

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 3-methylhexane

C7H16 low and high resolution 1H proton nmr spectrum of 3-methylhexane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for 3-methylhexane 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 - 3-methylhexane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 3-methylhexane 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 3-methylhexane molecule.

 3-methylhexane   C7H16   alkanes structure and naming (c) doc b

 alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b

Interpreting the H-1 NMR spectrum of 3-methylhexane

For relatively simple molecules, the low resolution H-1 NMR spectrum of 3-methylhexane is NOT a good starting point (relatively low resolution diagram above).

The hydrogen atoms (protons) of 3-methylhexane occupy 7 different chemical environments so that the low resolution NMR spectra should show 7 principal peaks of different H-1 NMR chemical shifts (diagram above for 3-methylhexane).

CH3CH2CH(CH3)CH2CH2CH3

Note the proton ratio 3:2:1:3:2:2:3  of the 7 colours of the protons in the 7 chemically different environments (but see last comment in the notes below!).

Chemical shifts (a) to (g) on the H-1 NMR spectrum diagram for 3-methylhexane.

Theoretically, although there are 16 hydrogen atoms in the molecule, there are 7 possible different chemical environments for the hydrogen atoms in 3-methylhexane molecule (but see last comment in the notes below!).

The high resolution 1H NMR spectrum of 3-methylhexane

A very high resolution spectrum of 3-methylhexane will show 7 different resonances, though many of the chemical shifts are close together.

I've quoted the chemical shift data, but made no attempt to show the resonance splitting in terms of major peaks and subsequent spin-spin coupling splitting effects.

The full interpretation of the 1H NMR spectrum of 3-methylhexane is beyond the scope of my pre-university chemistry website.

There is also a further complications with resonances (b) and (d).

Due to the asymmetry of the molecule, which has a chiral carbon atom (*), the CH2 protons either side of the chiral carbon are NOT equivalent - two more chemical environments.

This means that technically, there are 9 different chemical environments for the protons!


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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