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Principles of NMR

Dr. Davinder Bhachu

Teacher

Dr. Davinder Bhachu

Contents

Principles of NMR Spectroscopy

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The Basic Principle

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The NMR Spectrum

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Practice Questions

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Principles of NMR Spectroscopy

What is NMR?

Nuclear Magnetic Resonance (NMR) Spectroscopy is an analytical technique used to determine the structure of unknown molecules by studying the environment of certain atomic nuclei, typically hydrogen-1   and carbon-13  .

The Basic Principle

NMR is based on the phenomenon that some atomic nuclei have spin (they act like tiny magnets). When placed in a strong magnetic field, these nuclei can absorb radio wave energy and change their spin state.

  • Nuclei with an odd mass number (like  and ) have nuclear spin and can be detected using NMR.

  •  When exposed to radio frequency radiation in a magnetic field, these nuclei resonate - they absorb energy and flip their spin.

  • The amount of energy needed depends on the chemical environment of the nucleus.

Diagram showing the NMR resonance process where nuclei absorb radio frequency energy during excitation and release energy during relaxation.

Chemical environments

By measuring the strength of the magnetic field that must be applied, NMR spectroscopy gives information about the local environment of specific atoms in a molecule.

A “different environment” means that an atom is bonded to different atoms or groups of atoms.

e.g. for ethanol.

Diagram comparing ¹³C NMR and ¹H NMR spectra by showing two carbon environments and three hydrogen environments in an alcohol molecule.

The NMR Spectrum

The NMR machine generates a spectrum where:

  • Each peak corresponds to nuclei in a unique chemical environment.

  • The position of the peak is the chemical shift (δ) measured in ppm and depends on the electronic surroundings of the nucleus.

  • Chemical shifts are provided in the data booklet.

Diagram showing the ¹³C NMR spectrum of ethanol with two peaks representing the two different carbon environments in the molecule.

Table showing characteristic ¹³C NMR chemical shift ranges in ppm for different carbon environments such as alkanes, alcohols, alkenes, aromatics, esters, aldehydes, and ketones.

Two carbon environments so two peaks in spectrum.


Reference Peak

All chemical shifts are measured relative to the reference peak at produced by a standard called TMS (tetramethylsilane) with formula .

This peak is often removed from the final spectrum.

Structural formula of tetramethylsilane (TMS) showing a silicon atom bonded to four methyl groups in a tetrahedral arrangement.

TMS is used as a standard as:

  • It produces a strong single peak ( in the same environment).

  • Its peak is found at the far right of an NMR spectrum (away from other signals).

  • It is volatile (easy to remove).

  • It is inert (so will not react with samples being analysed).

NMR spectrum showing tetramethylsilane (TMS) as a reference peak at 0 ppm on the chemical shift scale.

Solvents

Samples are dissolved in a solvent before entering the spectrometer.

Solvents should be unreactive and not contain any atoms as this would mask the spectrum of the sample.

Deuterium (D) is an isotope of hydrogen with a mass number of 2. This even mass number makes it NMR inactive.

Like dissolves like and so polar solvents are used to dissolve polar samples and vice versa.


Typical solvents:

Table comparing deuterated solvents used in NMR spectroscopy, including tetrachloromethane, deuterated trichloromethane, and deuterated water for dissolving non-polar and polar compounds.

Practice Questions

Question 1

Determine the number of signals expected (excluding TMS) in the NMR spectra of the following compounds.

a)Structural formula of butanone showing a ketone functional group with carbon and hydrogen atoms bonded in a four-carbon chain.

b) Structural formula of butane showing four carbon atoms single-bonded in a straight chain with attached hydrogen atoms.

c) Structural formula of nitrophenol showing a benzene ring with hydroxyl and nitro functional groups attached.

Answer

a) 4Structural formula of butanone with coloured highlights showing the different carbon environments used in carbon-13 NMR spectroscopy.

b) 2 (look out for symmetry!)Structural formula of butane with coloured highlights showing two different carbon environments identified in carbon-13 NMR spectroscopy.

c) 6Structural formula of nitrophenol with coloured highlights showing different carbon environments around the benzene ring for carbon-13 NMR analysis.

Question 2

Suggest a suitable solvent to use in the analysis of a non-polar organic compound.

Answer

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