Electron ionization fragmentation pathways

The chemical potentials and free energies of the 2-isoxazolines have also been studied and the electron impact and chemical ionization mass spectra determined (77MI41614). Fragmentation pathways and retrocycloadditions of various derivatives were discussed in these reports. 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

Electron ionization (El) mass spectra of 1,3,4-oxadiazole itself and its 2-mono- and 2,5-disubstituted derivatives, including the proposed main fragmentation pathways have already been discussed in CHEC(1984) and CHEC-11(1996) <1984CHEC(6)427, 1996CHEC-II(4)268>. Molecular ions of the compounds are usually of high intensity and the most important fragmentation pathways of the molecular ions involve loss of respective HCN, RCN molecules, or RCO cations. Loss of HNCO is significant in the spectra of 2-amino derivatives. 

Electron transfer dissociation (ETD) is an ECD-like method with most of the same characteristics [21]. Like ECD, ETD yields abundant peptide backbone c- and z-type ions while often retaining such labile groups as peptide O/TV-glycosylation and phosphorylation [22]. Unlike ECD, ETD can be performed in the presence of an RF field. Here, anions created in a chemical ionization (Cl) source (see Section 2.1.7) are used as electron donors but the fragmentation pathways are essentially the same as for ECD. Commercial linear QIT instruments have recently become available with the ETD option. 

During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms. 

This classic text describes fragmentation pathways and mechanisms for ions formed using electron impact (El) ionization. In addition, this edition contains additional information regarding desorption ionization and the corresponding related fragmentation mechanisms. 

The first process involves electron ionization to form radical M-1"1 molecular ions. This process has been observed primarily for nonpolar molecules. The proposed mechanisms are charge-exchange transitions between sputtered ions and the neutral organic molecules or electron attachment of low-energy secondary electrons to neutral molecules. The fragmentation reactions of the M ions usually follow the dissociation pathways for odd-electron gas-phase ions. 

No molecular ions appear in the electron ionization (El) spectra of mesoionic 1,2,3,4-oxatriazoles 4 and 6 or their derivatives. Instead, [M+ —30] peaks are observed due to loss of nitric oxide <1979J(P1)747, 1999CHE363>. The typical fragmentation pathway for such compounds and their analogues is summarized in Scheme 1 <1984CHEC(4)579, 1996CHEC-II(4)679>. In their fast atom bombardment (FAB) spectra, intact molecular ion peaks are observed for the A2-l,2,3,4-oxatriazolines 23 114 [M+H] and 24 142 [M+H] <2002HAC307>. 

Audier and Das [48] published a specific report on the MS fragmentation of cucurbitacins which exemplified how the use of modem MS techniques leads to a better understanding of molecule fragmentation. In a similar vein, Johnson et al. [49] reported the diagnostic fragmentation pathways that occur in cucurbitacins isolated from Fevillea cordifolia after using electron impact (El)-, chemical ionization (Cl)-, and fast-atom bombardment (FAB)-MS. 

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