A comparative study on the behaviour of 1,3‐diheterocyclopentanes (X = O, O; S, S; O, S) under electron impact. The formation of thioacetyl and thiiranyl cations

The electron‐impact‐induced mass spectra of 1,3‐dioxolane (la), 1,3‐dithiolane (2a) and 1,3‐oxatbiolane (3a) and their 2‐methyl (1b–3b) and 2,2‐dimethyl [(CH3)2: 1c–3c or (CD3)2: 1d–3d] derivatives have been studied in detail to gain further insight into their ion structures and competing reaction pathways with low‐resolution, high‐resolution, metastable and collision‐induced dissociation (CID) techniques. For compounds 1a–1d the most significant reaction is loss of H˙ and CH3˙ by α‐cleavage and a subsequent formation of CHO+ and C2H3O+ ions. The [M  H]+ ions from 1a and 1b give a C2H3O+ ion which does not have the acyl cation structure as shown by their CID spectra. In compounds 3a–3d the sulphur‐containing ions predominate, the C2H3O+ now having the acyl cation structure. 1,3‐Dithiolanes (2a–2d) exhibit the most complicated fragmentation patterns. Furthermore the [M  H]+ ion from 2a and [M  CH3]+ ion from 2b have different structures as well as the [M − H]+ ion from 2b and [M  CH3]+ ion from 2c, as shown by their CID spectra. This can be utilized to explain why 3a–3c and 2a give principally a thiiranyl cation, whereas 2b gives a mixture of this and the thioacyl cation and 2c practically only the open‐chain thioacetyl cation.