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 |
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Authors: | Kalevi Pihlaja Pentti Kuosmanen Pirjo Vainiotalo |
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Abstract: | 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. |
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