3.4.11 Structure Determination - Mass spectrometry
Students should:
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X+ + Y. gives rise to a characteristic relative abundance spectrum that
may give information about the structure of the molecule (rearrangement
processes not required)Mass spectrometry
Often the first stage in analysis is to find out is the relative molecular mass of the substance in question. Nowadays, this may be done accurately using a mass spectrometer.
In the past it was necessary to find the percentage composition of each component element to find the empirical formula and then to find the molecular mass by one of several physical techniques.
Once the relative molecular mass and chemical formula are known, structural information may be obtained by both electronic and 'wet' chemical means.
The molecular ion
The highest m/e peak in the mass spectrum always consists of the molecular ion; i.e. the ion that has been formed by dislodging one electron from the molecule under investigation.
Mass spectrometry m/e values can be read to many decimal places. As all atomic isotopes have slightly different masses, this allows the molecular formula to be obtained directly from the m/e value of the molecular ion.
Fragmentation
Once the molecular ion has been formed in the high energy beam of electrons, it is likely that this ion wlll break apart into fragments. Each fragment is either an ion itself, or a neutral species. The ions formed by fragmentation can also be detected in the mass spectrometer trace, but neutral fragments are neither accelerated, deflected nor detected.
The likelihood of a specific ion forming depends on the bond energy of the bond that must be broken and the stability of the fragment formed.
Typically, alkyl groups, acyl groups and allyl groups are most easily formed and often appear in mass spectra.
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Alkyl fragments
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Acyl fragments
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Allyl fragments
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| [CH3]+ | [CH3CO]+ | [CH2=CH2]+ |
| [C2H5]+ | [C2H5CO]+ | [CH3CH=CH2]+ |
| [C3H7]+ | [C3H7CO]+ |
As each of these fragments has a specific m/e value, an experienced analyst recognises the fragments as they are formed and uses this information to help build a structure of the fragmenting molecule. It's like being given pieces of a jigsaw puzzle and fitting them together to get the final picture.
The spectrum, showing several different peaks due to fragmentation, gives rise to a fragmentation pattern, which is the suggested way that a specific molecule has broken apart.