Methyl ions, decomposition

The o-substituted benzoic acid methyl ester of formula (CH3)2NC6H4C02CH3 has been found to lose both the ester methyl and the amine methyl in metastable ion decompositions. It has been proposed that the bond cleavage to eliminate the ester methyl is accompanied by a hydrogen transfer from one of the other methyl groups to the carbonyl oxygen in a 7-membered cyclic transition state [83]. The mechanistic proposal rests upon isotope effects observed on substituting deuterium in methyl groups. 

No metastable peak has been observed for loss of a methyl group following El of acetaldehyde, CH3CHO however, the metastable ion decomposition to lose CD3 has been seen with CD3CDO [695], This intermolecular isotope effect has been interpreted in terms of the isomerisation (CH3CHO)f - (CH2=CHOH)t occurring in the unlabelled molecule and precluding methyl loss [695]. 

The loss of a methyl radical from the 4- -butylpyridine ion has been shown to exhibit an isotope effect of 1.1 on decomposition in the El source and an isotope effect in the range 1.3—1.6 for metastable ion decomposition [642] [see discussion of t-butylbenzene in Sect. 7.5.4(a)]. 

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