C7H16 2-methylhexane low high resolution 1H proton nmr spectrum of analysis interpretation of chemical shifts ppm spin spin line splitting H-1 2-methylhexane 1-H nmr doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: 1H NMR spectrum of 2-methylhexane

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The H-1 hydrogen-1 (proton) NMR spectrum of 2-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 2-methylhexane

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H-1 proton NMR spectroscopy - spectra index

C7H16 low and high resolution 1H proton nmr spectrum of 2-methylhexane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for 2-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 - 2-methylhexane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 2-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 2-methylhexane molecule.

2-methylhexane C₇H₁₆

Interpreting the H-1 NMR spectrum of 2-methylhexane

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-methylhexane is may or may not be a good starting point (low resolution diagram above).

You can observe three main peaks, but two include resonances that overlap each other.

Theoretically, the hydrogen atoms (protons) of 2-methylhexane occupy 6 different chemical environments so that a very high resolution NMR spectra should show 6 peaks of different H-1 NMR chemical shifts (diagram above for 2-methylhexane).

(CH₃)₂CHCH₂CH₂CH₂CH₃

Note that from the structure of 2-methylhexane and theory, the proton ratio in the spectrum should be 6:1:2:2:2:3 of the 6 colours of the protons in the 6 chemically different environments

Chemical shifts (a) to (f) on the H-1 NMR spectrum diagram for 2-methylhexane.

Although there are 7 hydrogen atoms in the molecule, there are only 6 possible different chemical environments for the hydrogen atoms in 2-methylhexane molecule.

The two 'left-hand' methyl groups include six equivalent protons.

However, because several resonances are close together you seem to observe ...

a ratio of 1:6:9 applied to the proton ratio of the groups 1 x CH : 3 x CH₂: 3 x CH₃

The high resolution 1H NMR spectrum of 2-methylhexane

This is problematical because of the close proximity of several of the chemical shifts.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 2-methylhexane - 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 2-methylhexane below.

So, using the chemical shifts and applying the n+1 rule to 2-methylhexane and make some predictions using some colour coding! (In problem solving you work the other way round!)

However, whatever the theory, you need very high resolution to fully sort out the 1H NMR resonances for 2-methylhexane.

(a) ¹H Chemical shift 0.87, resonance for the C(CH₃)₂ groups of protons

(CH₃)₂CHCH₂CH₂CH₂CH₃

Theoretically this resonance would be split into a 1:1 doublet by the CH proton (n+1 = 2).

Evidence for the presence of a -CH- group in the molecule of 2-methylhexane

(b) ¹H Chemical shift 1.52 ppm, resonance for the CH proton

(CH₃)₂CHCH₂CH₂CH₂CH₃

Theoretically this resonance would be split into a nonet by the 2 x CH₃ and CH₂ protons on either side (n+8 = 9).

(c) ¹H Chemical shift 1.17 ppm, resonance for the 1st CH₂ group (left to right)

(CH₃)₂CHCH₂CH₂CH₂CH₃

Theoretically this resonance would be split into a 1:3:3:1 quartet by the CH and CH₂ protons on either side (n+3 = 4).

(d) ¹H Chemical shift 1.27 ppm, resonance for the 2nd CH₂ group (left to right)

(CH₃)₂CHCH₂CH₂CH₂CH₃

Theoretically this resonance would also be split into a quintet by the CH₂ protons on either side (n+4 = 5).

(e) ¹H Chemical shift 1.27 ppm, resonance for the 3rd CH₂ group (left to right)

(CH₃)₂CHCH₂CH₂CH₂CH₃

Theoretically this resonance would also be split into a sextet by the CH₂ and CH₃ protons on either side (n+5 = 6).

(f) ¹H Chemical shift 0.89 ppm, resonance for the right-hand end methyl group protons

(CH₃)₂CHCH₂CH₂CH₂CH₃

Theoretically this resonance would be split into a 1:2:1 triplet by the CH₂ protons on left (n+2 = 3).

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

Comparing the 1H NMR and 13C NMR spectra of the nine alkane structural isomers of C₇H₁₆ You can distinguish all 9 isomers from a data combination of their number of ¹H NMR chemical shifts, and their resulting integrated ¹H proton ratios, plus, their number of ¹³C chemical shifts.
Name of the alkane structural isomer of molecular formula C₇H₁₆	Abbreviated structural formulae of the nine isomers of molecular formula C₇H₁₆ (interpretation complications with 3-methylhexane and 2,3-dimethylpentane because they exhibit R/S isomerism due to a chiral carbon)	Skeletal formula of the nine alkane isomers of molecular formula C₇H₁₆	Number of ¹H NMR chemical shifts (δ) and proton ratio (links to spectrum)	Number of ¹³C chemical shifts (δ) (links to spectrum)
heptane			4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule)	4 δ shifts
2-methylhexane			6 δ: proton ratio : 6:3:2:2:2:1	6 δ shifts
3-methylhexane			7 δ: proton ratio: 3:3:3:2:2:2:1 (simplification) !!!	7 δ shifts
3-ethylpentane			3 δ: proton ratio: 9:6:1	3 δ shifts
2,2-dimethylpentane			4 δ: proton ratio: 9:3:2:2	5 δ shifts
2,3-dimethylpentane			6 δ: proton ratio: 6:3:3:2:1:1 (simplification) !!!	6 δ shifts (simplification) !!!
2,4-dimethylpentane			3 δ: proton ratio: 12:2:2	3 δ shifts
3,3-dimethylpentane			3 δ: proton ratio: 3:3:2 (6:4:4 in the molecule)	4 δ shifts
2,2,3-trimethylbutane			3 δ: proton ratio: 9:6:1	4 δ shifts

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