Mass and Infra-Red Spectroscopy
Mass spectrometry is a method for finding the molecular formula of a particular compound, and is one method that we can use to determine the structure of a molecule. But it is not perfect since many different molecules have the same mass (C6H12 and C4H8N2).
A key feature of mass spectrometry is fragmentation and this occurs because the ionising beam causes the molecules to break apart, in the following reaction.
M+° ® X+ + Y°
The parent ion, fragements to a positively charged ions and a radical; the radical is not detected by the mass spectrometer. For example, the fragmentation of pentan-3-one is written as follows.
[CH3CH2COCH2CH3]+° ® [CH3CH2CO]+ + [CH2CH3]°
Fragmentation can give you clues as to the bonding in a substance, since weaker bonds are broken first so more stable species such as methyl and ethyl groups are likely to have taller peaks.
Infra-red radiation is a form of electromagnetic radiation. It is used in identifying molecules because different bonds will absorb IR at different frequencies. Therefore allowing us to identify the bonding in a particular molecule.
At a particular frequency (measured in cm-1) of infra-red radiation the bonds in a molecule will vibrate as the energy is absorbed. Therefore, it is possible to compare the absorptions with the known characteristic absorptions and deduce the functional groups present.
Fingerprinting is a method of identifying a compound. Since if two molecules have the same spectrum they must be the same molecule. In particular the area of the spectrum above 1500cm-1 is known as the fingerprint region. Chemists can compare this region with a database of reference regions to easily identify a molecule, in much the same way as the police use fingerprints to identify criminals.
Applying Infra-red Spectroscopy
Below is an example of an infra-red spectrum, next to it is a table of some absorption values and the peaks (which are actually troughs) are labelled with what they are to help you.
An infra-red spectrum has obvious dips where particular absorptions have taken place. Given a spectrum you would use a table and identify what the major peaks are.
In the example above, the molecule has C-H bonds (most organic compounds do). But crucially the compound has C=O and C-O, and as you should know, this is the functional group for esters. By itself this is not enough to identify the compound so you must use this information along with other techniques.
Using mass spectrometry you can find the exact molecule. So if you knew the molecular mass to be 102 you can deduce the molecule to have the formula CH3COOCH2CH2CH3 and so it is propyl ethanoate.