Fragmentation behaviour and ring expansion of 1‐methylimidazole and 1‐methylpyrazole upon electron‐impact

The relative losses of unlabelled vs. labelled HCN from the [M]+˙ and [M – 1]+ ions of a number of specifically labelled 1‐methylimidazoles (I) and 1‐methylpyrazoles (II) have been determined. Hydrogen randomisation in the molecular ions prior to fragmentation is insignificant. Expulsion of HCN follows two distinct pathways: elimination involving positions 2 and 3 (predominant in I) and elimination involving the methyl group and the nitrogen atom at position 1 (predominant in II). The molecular ions eject H˙ from the methyl groups to a high degree of specificity. In both cases some contribution by position 5 is observed. The resultant [M – 1]+ ions exhibit extensive, but incomplete hydrogen randomisation. Loss of HCN from these ions is consistent with intermediacy of ring‐expanded ions, but notably in II a proportion of the HCN is generated from the group. A mechanism for this observation is presented.