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3.15.1Organic Chemistry

Nuclear Magnetic Resonance Spectroscopy

5 min read 5 parts Lesson 50 of 55 Virtual lab

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Section 1 of 5Principles of NMR

3.15.1 Nuclear Magnetic Resonance Spectroscopy
1
Nuclei such as ¹H and ¹³C have a property called spin, so they behave like tiny magnets.
2
In a strong external magnetic field the nuclei line up with the field or against it, and the two alignments have slightly different energies.
3
Radio-frequency radiation of the right frequency is absorbed as nuclei flip to the higher energy alignment; this is resonance.
4
The frequency needed depends on the electron density around the nucleus, so nuclei in different environments absorb at different positions.
5
The spectrum plots absorption against chemical shift δ in ppm, measured relative to the reference compound TMS at 0 ppm.

Section 2 of 5TMS and Solvents

TMS
  • Tetramethylsilane, Si(CH₃)₄, is the reference standard, and its signal is assigned a chemical shift of exactly 0 ppm.
Single sharp peak
  • All 12 hydrogen atoms (and all 4 carbon atoms) are in the same environment, so TMS gives one strong sharp peak.
Other advantages
  • TMS is inert, non-toxic and volatile, so it is easy to remove from the sample after the spectrum is run.
Chemical shift
  • A measure of how far a signal is from the TMS signal, in parts per million, which is independent of the spectrometer.
Solvents
  • The sample is dissolved in a solvent with no ¹H, such as CCl₄ or deuterated CDCl₃, so the solvent does not produce peaks.
Deuterium
  • Deuterium (²H) is an isotope of hydrogen, and it does not absorb in the region used for ¹H NMR.

Section 3 of 5What the Spectrum Shows

What the Spectrum Shows
Number of peaks
  • Each peak is a different environment, so the number of peaks equals the number of different environments of the nucleus.
Chemical shift
  • The position of a peak tells you what type of group the nucleus is in, using data tables.
Equivalent nuclei
  • Atoms in identical environments, for example three hydrogens on one CH₃ group, give a single peak.
Combined techniques
  • Elemental analysis, mass spectrometry, infrared and NMR together are used to find the formula and structure.

Section 4 of 5Worked Example

Question How many peaks would 2-methylpropan-1-ol, (CH₃)₂CHCH₂OH, show in its ¹H NMR spectrum and in its ¹³C NMR spectrum?

  1. Hydrogen environmentsthe two CH₃ groups are equivalent (1), then CH (2), CH₂ (3) and OH (4), so there are four.
  2. Carbon environmentsthe two CH₃ carbons are equivalent (1), the CH carbon (2) and the CH₂ carbon (3), so there are three.

Answer Four ¹H peaks and three ¹³C peaks.

Section 5 of 5Exam Checklist & Tips

3.15.1 – Key Points for the Exam

  • Chemical shift The position of an NMR signal relative to TMS, measured in ppm.
  • TMS Tetramethylsilane, the standard reference compound at 0 ppm.
  • Deuterated solvent A solvent in which hydrogen is replaced by deuterium so it gives no ¹H signal.
  • Molecular environment The surroundings of an atom within the molecule, determined by what it is bonded to.

Common Mistakes

  • Counting peaks by the number of hydrogen atoms rather than the number of different environments.
  • Saying TMS is chosen because it has a high chemical shift; its shift is 0 ppm.
  • Using a solvent containing ¹H, which would give its own peak.
  • Thinking that NMR gives the molecular mass; that comes from mass spectrometry.
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