15.9 : 质谱法：胺碎裂

Amines undergo ⍺-cleavage in the mass spectrometer to yield a resonance-stabilized nitrogen-containing cation and an alkyl radical.

Interestingly for amines, the number of nitrogen atoms in the compound affects the molecular weight of the molecular ion, also called the nitrogen rule of mass spectrometry.

As per the rule, a neutral organic compound containing an odd number of nitrogen atoms will produce a molecular ion with an odd-numbered molecular weight.

For instance, consider the mass spectrum of triethylamine, which has one nitrogen atom.

Here, the molecular ion has an odd mass-to-charge ratio of 101.

Fragmentation of the molecular ion upon ⍺-cleavage frees a methyl radical and generates a base peak at a mass-to-charge ratio of 86.