Proton NMR Spectroscopy (HL) (DP IB Chemistry): Revision Note

Proton NMR spectroscopy

• Nuclear magnetic resonance (NMR) spectroscopy is used for analysing organic compounds

• Only atoms with odd mass numbers show signals on NMR spectra and have the property of nuclear spin

• In ¹H NMR, the magnetic field strengths of protons in organic compounds are measured and recorded on a spectrum

• Samples are irradiated with radio frequency energy while subjected to a strong magnetic field

• The nuclei can align themselves with or against the magnetic field

• Protons on different parts of a molecule (in different molecular environments) absorb and emit (resonate) different radio frequencies

• All samples are measured against the reference compound tetramethylsilane (TMS)

  • TMS shows a single sharp peak on an NMR spectrum at 0 ppm

  • The protons in TMS are highly shielded

    • This means that they absorb at the lowest chemical shift and give a sharp upfield signal at 0 ppm

  • Sample peaks are then plotted as a ‘shift’ away from this reference peak

  • This gives rise to ‘chemical shift’ values for protons on the sample compound

  • Chemical shifts are measured in parts per million (ppm)

Features of an NMR spectrum

• An NMR spectrum shows the relative absorption at each chemical shift

• The area under each peak is proportional to the number of protons in a particular environment

• The height of each peak shows the intensity/absorption from protons

Low resolution proton NMR

Chemical environments

• Hydrogen atoms of an organic compound are said to reside in different chemical environments

• Eg. Methanol has the molecular formula CH₃OH

  • There are 2 environments:

    • -CH₃

    • -OH

  • The hydrogen atoms in these environments will appear at 2 different chemical shifts

• Different types of protons are given their own range of chemical shifts

¹H NMR Chemical Shifts

Type of proton	Chemical shift / ppm
-CH3	0.9 - 1.0
-CH2-R	1.3 - 1.4
-CHR2	1.5
	2.0 - 2.5
	2.2 - 2.7
	2.5 - 3.5
-CC-H	1.8 - 3.1
-CH2-Hal	3.5 - 4.4
-R -O-CH2	3.3 - 3.7
	3.7 - 4.8
	9.0 - 13.0
-R -O-H	1.0 - 6.0
-CH=CH2	4.5 - 6.0
	4.0 - 12.0
	6.9 - 9.0
	9.4 - 10.0

• Protons in the same environment are chemically equivalent

• Each peak on an NMR spectrum relates to protons in the same environment

• Peaks on a low resolution NMR spectrum refer to environments of an organic compound

• E.g. Ethanol has the molecular formula CH₃CH₂OH

  • This molecule has 3 separate environments:

    • -CH₃

    • -CH₂

    • -OH

  • So, 3 peaks would be seen on its spectrum at:

    • 1.2 ppm (-CH₃)

    • 3.7 ppm (-CH₂)

    • 5.4 ppm (-OH)

Low resolution proton NMR of ethanol

• The area under each peak is determined by computer and an integration trace overlaid on the spectrum

• The integration trace has stepped lines whose steps are in the same proportion as the peak areas

• This makes it easier to determine the relative abundance of the different proton environments