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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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - pent-1-ene here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of pent-1-ene represent the peaks of the intensity of the chemical shifts of pent-1-ene (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 pent-1-ene molecule.

Pent-1-ene, , , (1-pentene)

Interpreting the H-1 NMR spectrum of Pent-1-ene

For relatively simple molecules, the low resolution H-1 NMR spectrum of ? is a good starting point.

The hydrogen atoms (protons) of pent-1-ene occupy 5 different chemical environments so that the H-1 proton low resolution NMR spectra should show 5 peaks, which it does (diagram above).

CH₃CH₂CH₂CH=CH₂  (note the five different colours of the protons in pent-1-ene)

Since all 5 hydrogen atoms in the molecule are in different chemical environments, pent-1-ene will show 5 different chemical shifts in its H-1 NMR spectrum.

The ratio of the area under the NMR 'peaks' should be 3 : 2 : 2 : 1 : 2 from the molecular structured of pent-1-ene (colour coded above) i.e. the proton ratios in the molecule.

As you can see, the high resolution spectrum of pent-1-ene is complex

The end CH₃ group is split into a triplet (1 : 2 : 1 lines at 0.91 ppm) by the adjacent CH₂ group.

The end =CH₂ group is split into a doublet (1 : 1 lines at 4.97 ppm) by the adjacent CH= group.

Both sets of CH₂ protons and the HC= proton are split on both sides by adjacent non-equivalent protons into multiple resonance lines.

CH₃CH₂CH₂CH=CH₂

The 1.43 ppm chemical shift:

From the n+1 rule, the 'left-hand' 1st CH₂ (H₂) protons are split by the CH₃ (H₃) protons and by the 'right-hand' 2nd CH₂ protons (H₂), (5 protons in total), into a 1:5:10:10:5:1 sextet of resonance lines.

This is pattern of resonances is a good indication of a propyl group (CH₃CH₂CH₂).

The 2.02 ppm chemical shift:

The 'middle' CH₂ protons (H₂) are split by the left CH₂ protons (H₂) and the CH proton on the right (H), (3 protons in total), in to a 1:3:3:1 quartet of resonance lines.

The 5.81 ppm chemical shift:

The CH proton (H) is split by the left CH₂ protons (H₂) and the end CH₂ protons (H), (4 protons in total), into a 1:4:6:4:1 quintet of resonance lines.

Non of these is clear on the H-1 NMR spectra of pent-1-ene shown above.

The chemistry of ALKENES revision notes INDEX

H-1 proton NMR spectroscopy index  (Please read 8 points at the top of the ¹H NMR index page)

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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