L5 - H NMR

HSC Chem Homepage

Previous Lesson

Next Lesson

objectives

WHAT IS NMR AND WHAT DOES AN NMR SPECTRUM TELL YOU?

Proton nuclear magnetic resonance spectroscopy provides...
information about the hydrogen atoms in molecules

How does it work?

involves the interaction of materials with the LOW ENERGY RADIO WAVES

It provides the information by...

spinning a sample of the compound in a magnetic field
hydrogen atoms in different environments respond differently to the field
each different environment of hydrogen produces a signal in a different position
the area under each peak / signal is proportional to the number of hydrogens
signal can be split according to how many H’s are on adjacent atoms

Origins of NMR Spectra

All nuclei possess charge and mass. Those with either an odd mass number or an odd atomic number also possess spin. This means they have angular momentum.

A spinning nucleus such as ¹H behaves as a spinning charge and generates a magnetic field.
It can be likened to a bar magnet. When it is placed in an externally applied field it can align with, or against, the field.
The energy difference between the two states (DE) depends on the applied field.

Information on Spectra

NMR spectra provide information about the structure of organic molecules from the ...
number of different signals in the spectrum
position of the signals (chemical shift)
intensity of the signals
splitting pattern of the signals

OBTAINING SPECTRA

a liquid sample is placed in a tube which spins in a magnetic field
solids are dissolved in deuterated solvents (CDCl₃) or solvents without H’s (CCl₄ )
[solvents with hydrogen atoms in them will produce peaks in the spectrum]
spectrometers are now linked to computers to analyse data and store information

Low Resolution - High Resolution

low resolution nmr gives 1 peak for each environmentally different group of protons
high resolution gives more complex signals - doublets, triplets, quartets, multiplets
the signal produced indicates the number of protons on adjacent carbon atoms

The splitting pattern depends on the number of hydrogen atoms on adjacent atoms

Integration

the area under a signal is proportional to the number of hydrogen atoms present
an integration device scans the area under the peaks
the ratio of relative areas is displayed on the spectrum
historically, lines on the spectrum showed the relative abundance of each H type

By measuring the distances between the integration lines one can

work out the simple ratio between the various types of hydrogen.