Reaction from isomeric parent ions in the dissociation of dimethylpyrroles

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Abstract
The major dissociation pathways of the [M-H]+ (loss of NH3 or CH4) and the [M+H]+ (loss of NH3 or CH3) ions from dimethylpyrroles have been determined to occur from isomeric parent ions. For the [M-H]+ ion (formed by loss of a methyl hydrogen), loss of NH3 leads to the formation of the phenylium ion and is preceded by consecutive carbon ring expansions followed by a ring contraction to form protonated aniline. Loss of CH4 occurs after the first carbon ring expansion, which forms protonated picoline. The relative partitioning between the two dissociation paths depends upon the internal energy content of the parent ion; the highest point on the potential energy surface is the second ring expansion step. The [M+H]+ ion reacts through a similar pathway via dihydro analogs of picoline and aniline. The proposed reaction pathways are supported by results of semiempirical molecular orbital calculations.