A question about Infra-red Spectrum

[biochemist]

I'm a foreign student in England studying biochemistry, 1st year undergraduate. I did an experiment yesterday on infrared spectrum and was confused by the output diagram. I also don't know about how it works. Could anyone tell me how do I look and analyse the infrared spectrum diagram and what is the function of such technique, please. Waiting for your reply and many many thanks!

 

[eagleone]

Well it’s mainly used for identification of compounds (putting them into right classes, easy id of functional groups, part of microanalysis with nmr, different chromatography’s etc.). Diagram shows specific absorption spectra of compound you’re working with… How does it works,, huh take a look at any organic chem. Or Physical chem. or some other book, it won't take you a lot of time (+ you should find something about rotational and vibration levels and spectra) …

 

[thunderfvck]

http://www.chem.ucla.edu/~webspectra/irtable.html

That's a table of the important IR absorption patterns. You look at the spectrum you received and you look for certain peaks. How I do it, and probably a lot of other people, is that I start with looking at around 1700 cm^-1, and if there's a sharp peak then that means there's a carbonyl group present in the sample. Then I looked to see if there were two peaks in the 1500-1600 area (I THINK it was that anyway, there was usually a peak at 1500, and another at 1600) and that meant there was a benzene ring involved. THen there was the usual crap with the sp3 C-H stretches at about 2900-3000. O-H stretches are pretty massive, I remember something like 2500-3600 or something like that, all due to H-bonding in solution (your sample was put in a solution of some solvent). ANyway, I think you get the idea. It really helps to know what you're expecting to get out of the spectrum. Seeing as how you did a lab and received this spectrum I'm guessing there's some kind of reaction involved that you performed. So you should know what you're getting as a result and just make the information from the spectrum correspond to what you've expected. And don't be so overwhelmed by every little peak there is, just look for what you're expecting.

Oh and some peaks are IR inactive, like C=C (sometimes) for example. But only if the dipole moment on the carbons don't change. eg. in ethene there is not change in the dipole moment because each carbon is bonded to the same thing, it's symmetric.
And another thign, don't be so intent on finding every peak from everything in the expected compound, many of the peaks are most likely overshadowed by other stronger groups.

 

[biochemist]

Thank you very much!I'm clearer now!