The behaviour of a number of deuterated imidazoles upon electron-impact has been studied. Loss of H' from the molecular ion occurs specifically at the 4(5)-position. Loss of HCN is less specific, the order being 2 > 4(5) > 1.

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The relative losses of unlabelled us. labelled HCN from the [MI+. and [M -1]+ ions of a number of specifically labelled 1-methylimidazoles (I) and 1-methylpyrazoles (11) 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 11). 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 LI a proportion of the HCN is generated from the N=CH, group. A mechanism for this observation is presented.

The mass spectra of ten pyrazole compounds have been determined. Fragmentation schemes have been derived by means of the metastable defocusing method. The predominant process is cleavage of the nitrogen-nitrogen bond resulting in expulsion of HCN. The process second in prominence is the loss of a nitrogen molecule after initial removal of a hydrogen radical or a substituent, giving the species [C3HzRl+, probably a cyclopropenyl ion. In general, the fragmentation pattern is strongly influenced by the substituent.

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The mass spectra of deuterated pyrazoles show that loss of H' and of HCN from the molecular ion occurs with a very high specificity from the 3(5)-position. For the two processes isotope effects and preference factors have been determined. Metastable ion decompositions involving the loss of HCN from the [M -HI b-fragment indicate that the identity of the hydrogen atoms in this fragment is lost to a large extent.

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