General Note on Interpretation of  Mass Spectral Data

 

Below is a rather simplified way of  mass spectral data interpretation approach:

1.  Identify the molecular ion peak

2.  Making sense of the isotope peaks

3.  The "double bond equivalent"  rule - total number of rings and/or double bonds in the molecule

This is one of by far the most interesting and useful rule to follow in structural interpretation exercises. Fortunately, mastering this rule does not require you to know much about mass spectrometry. Carbon is tetravalent and hence  forms four bonds with other atoms in order to fill its valence shell. This can be fulfilled in one of the following ways:

In the above examples the degree of unsaturation and/or ring systems can easily be calculated since one double bond or a ring is equivalent to a loss of two protons from the saturated system. Similarly, a triple bond in a molecule would have a  2 double bond equivalent.

Thus 'double bond equivalent' for the molecular formula of CxHyOn is: [(2x + 2)-y]/2

4.  Having all the above accounted for, try to postulate the molecular structure consistent with the abundance and m/z of fragments. Remember that we use data coming from other spectroscopic methods, such as NMR, too.

 

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