Reaction from isomeric parent ions in the dissociation of dimethylpyrroles

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.