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 (original) (raw)

1988, Organic Mass Spectrometry

The electron-impact-induced mass spectra of 1,3-dioxolane (la), 1,3-dithiolane (2s) and 1,3oxatbiolane (3a) and their %methyl (lb-3b) and 2,2-dimethyl [(CH&: lc-3c or (CD,),: ld-3dI derivatives have been studied in detail to gain further insight into their ion structures and competing reaction pathways with low-resolution, highresolution, metastable and collision-induced dissociation (CID) techniques. For compounds la-ld the most significant reaction is loss of H and CH,' by a-cleavage and a subsequent formation of CHO' and C,H,O' ions. The [M-m' ions from l a and l b give a C,H,O' 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 C,H,O+ now having the acyl cation structure. 1,3-Dithiolanes (2a-2d) exhibit the most complicated fragmentation patterns. Furthermore the [M-HI+ ion from 2a and-CHJ+ ion from 2b have different structures as well as the [M-a' ion from 2b and [M-CH,]' ion from 2c, as shown by their CID spectra. This can be utilized to explain why 3a-3c and 2s 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.