Fragmentation routes

Metastable ions are useful for determining the paths by which molecular ions of an unknown substance have decomposed to give fragment ions. By retracing these fragmentation routes, it is often possible to deduce some or all of the molecular structure of the unknown. 

The loss of an oxygen atom from the molecular ions of 10 occurs also to some extent and is most pronounced when R = CH3 (route I in equation 2). The predominant fragmentation route of methyl and ethyl 2-hydroxyphenyl sulfoxides is II (equation 2) which, however, is not important when R > Et. Ethyl 2-hydroxyphenyl sulfoxide undergoes also fragmentation via route III, which is the main path of fragmentation when R > Et10. 

The mechanism of the polymerization contains ionic intermediate steps. The free H+ goes to a carbenium ion and, as shown in route B, rearranges to form tetrapropylene. It is highly likely that this actual tetrapropylene exists only in very small concentrations. The product variety is explained by the rearrangement of the carbenium ion to dodecene isomers according to route C. In addition, short-chain olefins formed by fragmentation (route D). Polymerization proceeds at almost 100% to mono olefins. Aromatics, paraffins, and diolefins exist only in trace amounts. The propylene tetramer is best characterized by its distillation range. 

In case of alkanols, the methylene oxonium ion, CH2=OH, m/z 31, deserves special attention. Resulting from a-cleavage, it undoubtedly marks spectra of primary alkanols, where it either represents the base peak or at least is the by far most abundant of the oxonium ion series (Fig. 6.8). [32] The second important fragmentation route of aliphatic alcohols, loss of H2O, is discussed in Chap. 6.10. 

SCHEME 8. The fragmentation routes of Rha-Rha-Cio-Cio (Rha = Rhamnosyl) [M + Na]+ and [M + Li]+ precursor ions during high-energy CID ... 

The mass spectrum of 2-butenal shows a peak at m/z 69 that is 28.9% as intense as the base peak. Propose at least one fragmentation route to account for this peak, and explain why this fragment would be reasonably stable. 

The fragmentation route of hydroxy-1,4-biradicals could be effectively removed as a viable possibility by replacing the cyclohexyl rings of 82 with a 1-adamantyl group the 1-adamantene which would be formed violates... 

Although collision induced dissociation (CID) is a well-known method for investigating the structures of cations in the gas phase (McLafferty, 1983), it has been applied much less to anions (Bowie, 1986). Actually, in some cases CID has been used to study the fragmentation mechanisms of anions, such as the elimination of molecular hydrogen from alkoxide ions (Hayes el al., 1984) or the primary fragmentation routes of ester enolate ions (Froelicher et al., 1985). 

The molecular ion peaks of all the compounds studied are the most intense ones, while the m/z 147 and 148 peak intensities are negligible. It has been shown [1337] that metastable ion peaks (m/z 133, [M-NO]+) can be of help in distinguishing the five nitroindazole isomers, but in general their spectra are much alike. The fragmentation route to [M-NO]+ for the 4-nitro isomer is as follows (Scheme 3.70) ... 

The knowledge of the key fragmentation routes of eburnamine and some related alkaloids confirmed or elucidated the structures of the alkaloids 11-methoxy-vincamine,100 vincaminine,102 and vinci-nine,102 of the same skeletal type as eburnamine. Alkaloids of the mavacurine-pleiocarpamine group (212)103 are characterized by a key fragment corresponding to the loss of the ester group and... 

Figure 5 Fragmentation route of melatonin isomers leading to the intense peak at m/z 160,...

A second fragmentation route is observed starting from the molecular ion, peaks are observed at m/z 328 (15.7%), 292 (15.7%), 257 (19.1%) and 221 (10.1%) which correspond to the successive loss of one to four chlorine atoms (again associated with H-transfers) from the molecular ion. Each of these four fragments gives a related peak corresponding to the loss of one CH2 group from the SPIRO loop at m/z 311 (23.5%), 278(66.2%), 243 (17.9%) and 207 (8.9%) respectively. 

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