Mass Spectrum Analysis - Fragmentation

Fragmentation

The fragmentation pattern of the spectra beside the determination of the molar weight of an unknown compound also suitable to give structural information, especially in combination with the calculation of the degree of unsaturation from the molecular formula (when available). Neutral fragments frequently lost are carbon monoxide, ethylene, water, ammonia, and hydrogen sulfide.

fragmentations arise from:

• homolysis processes. An example is the cleavage of carbon-carbon bonds next to a heteroatom

In this depiction single-electron movements are indicated by a single-headed arrow.

• Rearrangement reactions, for example a retro Diels-Alder reaction extruding neutral ethylene:

or the McLafferty rearrangement. As it is not always obvious where a lone electron resides in a radical cation a square bracket notation is often used.

• Ion-neutal complex formation. This pathway involves bond homolysis or bond heterolysis, in which the fragments do not have enough kinetic energy to separate and, instead, reaction with one another like an ion-molecule reaction.

Some general rules:

• A useful aid is the nitrogen rule: if the m/z ratio is an even number, the compound contains no nitrogen or an even number of nitrogens.
• Cleavage occurs at alkyl substituted carbons reflecting the order generally observed in carbocations.
• Double bonds and arene fragments tend to resist fragmentation.
• Allylic cations are stable and resist fragmentation.
• the even-electron rule stipulates that even-electron species (cations but not radical ions) will not fragment into two odd-electron species but rather to another cation and a neutral molecule.