infrared spectroscopy

[purple_rose]

can some1 explain to me how infrared spectroscopy works and how it can be used as any qualitative and quantitative analysis tool? I'm using heinemann and i've read it like three times but i still don't get it. Thanks

[Collin Li]

Qualitative analysis
Can determine what functional groups are present in a molecule by exposing the molecule to infrared light. Particular wavelengths are absorbed, and at particular wavenumbers (just another way of saying wavelength), there will be absorbances - these are unique for the presence of a particular functional group, therefore you can determine the type of functional group that is present.

Quantitative analysis
In the same way as AAS and UV-visible spectroscopy, the absorbance can be correlated to concentration, which means you can construct a calibration curve, and then quantitatively find the concentration of something.

[Mao]

you'd most likely use the IR spectrum with wavelength and transmittence.

the basic theory is, particular bonds absorb particular energy. When shining IR at molecules, these bonds will absorb certain wavelength/frequency (because a variation in this results in variation in energy). The resulting graph from how much IR was shone through (transmittence) against the wavelength the IR was could give an indication of what bond it is. A broad spectrum of wavelength is used to measure the transmittence, allowing all sorts of bonds to absorb the IR light, and what you note on these graphs are the "peaks" (which are actually "drop", signifying that this wavelength was absorbed -> little was passed through the sample).

In the data booklet you have a table of common bonds and their relative wavelength at which they absorb the IR. Using this information, you can identify the molecular structure of a compound, its functional groups, etc etc. The information obtained from IR is not exhaustive, and (my teacher has said), it will most probably be presented on the exam in an extended response with NMR, mass spectroscopy (or other things) so you can deduct the exact shape and molecular structure of the compound.