Isomerization of hydrocarbon ions. VIII-The electron impact induced decomposition of n-dodecane

The literature on the mass spectrometry of 2H and 13C labelled higher alkanes is reviewed and the decomposition behaviour of both the molecular and the fragment ions of n‐dodecane, n‐dodecane‐1, 12‐[13C2] and n‐dodecane‐1,1,1,12,12,12‐[2H6] studied with special emphasis on metastable decompositions. It is shown that the elimination of alkane molecules and alkyl radicals from the n‐dodecane molecular ion occurs primarily by simple splitting of the CC bond. In addition, both small alkane molecule and alkyl radicals are eliminated with low probability from centreal parts of the molecular ion. The alkane elimination is less specific than the alkyl elimination. The methyl elimination shows an exceptionally high non‐specificity, but is of negligible abundance in the 70 e V electron impact spectrum. The metastable ion spectra suggest, but do not prove unambiguously, that those small alkyl ions (with up to four carbon atoms) originating directly from the molecular ion, may be formed both by direct cleavage of the terminal groups and from central parts of the molecular ion. However, the majority of the small alkyl fragment ions in the 70 eV spectrum are formed by secondary decomposition explaining their apparent non‐specific formation. The strikingly different fragmentation behaviour of even electron, [CnH2n+1]+, and odd electron fragment ions, results from differences in the product stabilities. Using collisional activation and metastable ion spectra it is shown that the odd electron fragments have the structure of the linear alkene (most probably the 1‐alkene) molecular ion. In contrast to the molecular ions, alkyl fragment ions decompose with complicated skeletal rearrangements, which lead to substantial, but not complete, carbon randomization. The terminal hydrogen atoms, however, show little scrambling.