Ion Parity

Ion Parity

Almost all molecules have an even number of electrons. If you remove an electron to form an ion in the source of a mass spectrometer, then a molecular ion with an odd number of electrons will result. For example, the ionization of a molecule in the source gives:
M + e^- -> M⁺• + 2e^-
Molecules or ions with an odd number of electrons are called radicals. Molecular ions usually fragment by the loss of a neutral radical:
M⁺• -> F⁺ + R•
Therefore, fragment ions are usually even electron species. However, the loss of a stable neutral molecule is also a common feature of fragmentation of ions. Therefore, fragments resulting from the loss of stable neutrals like H₂, H₂O, CO, CO₂, HCN, N₂, HF, HCl, HBr will leave the ion produced by primary fragmentation of the molecular ion an odd electron radical. For example, alcohols often loose H₂O to form odd electron fragments:
R-OH⁺• -> [R-H]⁺• + H₂O

Even electron ions rarely fragment to form odd electron ions. Therefore, once an even electron fragment forms, subsequent fragmentations produce even electron fragments. The following rules then result(1).
In the absence of nitrogen atoms or for an even number of nitrogen atoms:

fragments occurring at odd numbered m/z values are even-electron species resulting mainly from simple bond fission
fragments occuring at even numbered m/z values are odd electron species produced by mulitple bond cleavage, suggesting that rearrangement may have occurred.

For an odd number of nitrogens the rules are reversed.

Common stable molecule losses include

CO from aldehydes, ketones, carboxylic acids, esters, amides, and phenols
CO₂ from carboxylic acids and esters
H₂O from alcohols
HCN from aromatic nitriles
H₂C=CH₂ from ethyl esters and > C₃ aldehydes and ketones.

In summary the following rules are normally observed:

The molecular ion should be an odd electron species
Odd electron fragments should correspond to the loss of a stable neutral molecule from an odd electron ion

Reference: 1. Robert A. W. Johnstone, Malcolm E. Rose, Mass spectrometry for chemists and biochemists, 2nd. Ed., Cambridge University Press, Cambridge, Great Britain, 1996.

Colby College Chemistry, 1999