Stable Molecule Loss

Stable Molecule Loss

Fragments resulting from the loss of stable neutral molecules like H₂, H₂O, CO, CO₂, HCN, N₂, HF, HCl, and HBr are common. Loss of a stable neutral molecule from the molecular ion results in an odd electron radical. For example, alcohols often loose H₂O to form odd electron fragments:
R-OH⁺• -> [R-H]⁺• + H₂O
Loss of a stable neutral molecule requires a rearrangment; more than one bond needs to be broken.

Common Neutral Molecule Losses

m/z	Species	Functional Group
2	H₂
16	CH₄
18	H₂O	alcohol
20	HF	R-F
26	HCCH	aromatic
27	HCN	aromatic nitrile
28	CO	aldehyde, ketone, carboxylic acid, ester, amide, phenol
28	H₂C=CH₂	ethyl ester, > C₃ aldehyde or ketone
30	H₂C=O	aromatic methyl ether
30	C₂H₆
30	NO	Ar-NO₂*
32	CH₃OH	methyl ester
34	H₂S	thiol
36	H³⁵Cl	R-Cl
38	H³⁷Cl	R-Cl
42	CH₂C=O	methyl ketone, aromatic acetate, ArNHCOCH₃
42	CH₃CH=CH₂	n- or iso- butyl ketone, aromatic propyl ether, Ar-n-C₄H₉
44	CO₂	carboxylic acid, ester, anhydride
46	C₂H₅OH	ethyl ester
46	NO₂	Ar-NO₂*
48	SO	aromatic sulphoxide
56	C₄H₈	Ar-n-C₅H₁₁, ArO-n-C₄H₉, Ar-iso-C₅H₁₁, ArO-iso-C₄H₉, pentyl ketone
58	C₄H₁₀
60	CH₃COOH	acetate
80	H⁷⁹Br	R-Br
82	H⁸¹Br	R-Br
128	HI	R-I

* NO and NO₂ are odd-electron molecules so that molecular ion fragments resulting from their loss will be even-electron species. Therefore, Formula Finder doesn't include m/z=46 in its molecule loss list.

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

2. Dudley H. Williams, Ian Fleming, Spectroscopic methods in organic chemistry, McGraw Hill(UK), London, Great Britain, 1987. Table 4.12.

Colby College Chemistry, 1999