Mass spectral fragmentation alkynes

Example The oxidative addition of dimethyl disulfide (DMDS) transforms the double bond to its 1,2-bis-thiomethyl derivative (a). Induced by charge localization at either sulfur atom, the molecular ions of DMDS adducts are prone to a-cleavage at the former double bond position (b). This gives rise to sulfonium ions that are readily identified from the mass spectrum (Chap. 6.2.5). The method can be extended to dienes, trienes, and alkynes. [70,71] (For the mass spectral fragmentation of thioethers cf. Chap. 6.12.4). 

Mass spectral fragmentation patterns in the spectra of these compounds are in accord with the formation of alkynes. The first step in the fragmentation of 1,2,3-selenadiazoles is the loss of N2 followed by extrusion of selenium and formation of the corresponding alkyne. The abundance of the alkynic ion in the mass spectrum appears to be dependent on the nature of the substituent group present in the selenadiazole. When the alkynic ion cannot be stabilized by the formation of a cation on the adjacent carbon atoms, the abundance of the alkynic ion decreases (10% in the parent compound and zero for 4-f-butyl-l,2,3-selenadiazole). On the basis of the mass spectral pattern it is possible to predict the yield of the alkynic compound formed through pyrolysis or photolysis of a given... 

Mass spectral fragmentation of alkynes gives resonance-stabilized cations... 

Regrettably, alkyne-substituted cluster complexes seem particularly prone to fragmentation and very few accurate mass spectroscopic studies have been reported. A recent exception has been the field-desorption and electron-impact mass spectral investigation of mono-and oligo-nuclear ferracyclic ring systems of the from Fei(CO)J,(C2R2)2 (x = 1, 2, 3 y = 6, 8) (384). These species show intense molecular ion peaks, which enable ready recognition of the molecular composition. 

The extensive mass spectral studies of the triafulvenes have been summarized The radical cation of 85 is observed in the mass spectrum of methylenecyclopropene and is the most stable C4H4 species . By analogy with cyclopropenones fragment ions derived from cycloreversion to alkynes are often observed. 

Alkoxyalkynes have characteristic spectral properties . The triple bond stretching in the IR of alkoxyacetylenes varies from 2290 cm in 1-ethoxyhexyne to 2130-2160 cm in the parent" " and silylated" alkoxyethynes. The signals of both acetylenic carbons in the NMR are located in the normal alkyne region of 80-90 ppm f , which is distinctly different from siloxyacetylenes (see Section II.D.l) and ynol esters (Section IV.B). The mass spectra of alkoxyacetylenes almost always have a peak for the molecular ion and easily identifiable, characteristic fragmentation" " . ... 

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