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Infrared Spectroscopy… What is infrared radiation? Electromagnetic radiation with a wavelength of 700nm – 1mm (frequency of 430THz – 300GHz) that sits between visible light and microwaves on the spectrum Infrared Spectroscopy… Infrared Radiation (heat) makes everything hotter because it increases the kinetic energy of molecules. This motion is mainly within the bonds of the molecule. There are 2 main movement types: • Compression/Rarefaction • Bending Infrared Spectroscopy… In order to vibrate the bond must absorb a specific frequency of IR No bonds present as no radiation absorbed Infrared Spectroscopy… In order to vibrate the bond must absorb a specific frequency of IR What is the absorption of this carbonyl bond? 1700cm-1 3500 3000 2500 2000 1500 1000 500 Infrared Spectroscopy… Each functional group has a different absorption which allows us to identify whether or not it is present: An OH bond has an absorption range of 2500-3600cm-1 Which functional groups contain an OH bond? Infrared Spectroscopy… Alcohols cause an OH peak between 3300cm-1 and 3600cm-1 Infrared Spectroscopy… Carboxylic acids cause an OH peak between 2500cm-1 and 3300cm-1 The peak is described a broad because it is wide at the top and pointy at the bottom Infrared Spectroscopy… Carboxylic acids cause an OH peak between 2500cm-1 and 3300cm-1 What other bonds do you think will be obvious in the spectra of the carboxylic acid? Infrared Spectroscopy… Carboxy groups (C=O) cause a peak between 1630cm-1 and 1820cm-1 Infrared Spectroscopy… We can’t tell the difference between an aldehyde and ketone peak Infrared Spectroscopy… Isomers are often distinguished using IR spectra as their different functional groups give rise to unique absorption peaks. Let's have a look…
