PHỔ IR

Chapter 13: Spectroscopy

Infra-Red (IR) Spectroscopy

Basic priciples of IR
Hookes' Law model
Important absorptions
Sample spectra
Basics:
Infra red (IR) spectroscopy deals with the interaction between a molecule and radiation from the IR region of the EM spectrum (IR region = 4000 - 400 cm-1). The cm-1 unit is the wave number scale and is given by 1 / (wavelength in cm).
IR radiation causes the excitation of the vibrations of covalent bonds within that molecule. These vibrations include the stretching and bending modes.

An IR spectrum show the energy absorptions as one 'scans' the IR region of the EM spectrum. As an example, the IR spectrum of butanal is shown below.

In general terms it is convienient to split an IR spectrum into two approximate regions:

4000-1000 cm-1 known as the functional group region, and
< 1000 cm-1 known as the fingerprint region




Most of the information that is used to interpret an IR spectrum is obtained from the functional group region.
In practice, it is the polar covalent bonds than are IR "active" and whose excitation can be observed in an IR spectrum.
In organic molecules these polar covalent bonds represent the functional groups.
Hence, the most useful information obtained from an IR spectrum is what functional groups are present within the molecule (NMR spectroscopy typically gives the hydrocarbon fragments).
Remember that some functional groups can be "viewed" as combinations of different bond types. For example, an ester, CO2R contains both C=O and C-O bonds, and both are typically seen in an IR spectrum of an ester.
In the fingerprint region, the spectra tend to be more complex and much harder to assign.

MOST IMPORTANT THING TO REMEMBER.....

When analysing an IR spectrum avoid the temptation to try to assign every peak.
The fingerprint region, however, can be useful for helping to confirm a structure by direct comparison with a known spectra.