Mass spectrometry of pyridine compounds-IV: The formation and decomposition of the [M - methyl]+ ion [C8H10N]+ from 4-t-butylpyridine in comparison with that of the [M - methyl]+ ion [C9H11]+ from t-butylbenzene, as studied by D- and 13C-labelling

A strong secondary isotope effect is observed in the preferred loss of methyl vs. trideutero‐methyl from the molecular ions of appropriately labelled 4‐t‐butylpyridine and t‐butylbenzene decomposing in the first and second field free regions of a double focusing mass spectrometer. This has been rationalised by invoking the theory of radiationless transitions2, which can account for the higher population of activated states responsible for loss of methyl vs. that for trideuteromethyl. 13C‐Labelling at the central carbon atom of the t‐butyl group indicates that the [M – methyl]+ ions, decomposing further by elimination of ethylene, cannot be represented exclusively by a pyridylated (or phenylated) cyclopropane ion if present at all. It is concluded that ions with structures generated by 1,2‐hydrogen‐, 1,2‐pyridyl‐ (or 1,2‐phenyl‐) and 1,2‐methyl shifts must also play a role. D‐labelling further shows an extensive randomisation of side‐chain hydrogen atoms in the [M‐methyl]+ ions of 4‐t‐butylbenzene; in this case, however, the expelled ethylene also contains ring hydrogen atoms (≤2). Presumably this is caused by exchange between the side‐chain and ortho‐hydrogen atoms in the initially generated phenyldimethylcarbinyl carbenium ion.