Ion Rearrangements

Rearrangement ion. An electrically charged dissociation product, of a molecular or parent ion, in which atoms or groups of atoms have transferred from one portion of the molecule or molecular moiety to another during the fragmentation process. 

Abundant ions are observed in the mass spectra of straight-chain carboxylic acids at m/z 60 and 73 from n-butanoic to n-octadecanoic acid. The formation of an abundant rearrangement ion at m/z 60 requires a hydrogen in position four of the carbon chain. Most mass spectra of acids are easy to identify with the exception of 2-methylpropanoic acid, which does not have a hydrogen at the C-4 position and cannot undergo the McLafferty... 

Subtract 58 Daltons from this rearrangement ion to find R2. 

Providing the Ri group has a y-hydrogen, [CH3C(0)NR2R3]" is a common fragment. Subtract 57 Daltons from this rearrangement ion to find R2 + R3. 

McLafferty rearrangement m/z 88 for ethyl esters If the a-carbon has a methyl substitution, the major rearrangement ion will be at m/z 102. 

Fluorine compounds (m/zs 19, 31, 50, and 69) Rearrangement ion characteristic of hydroxyl compounds... 

The mass spectra of pyrrole derivatives usually contain prominent ions at m/z 53, and 80, and sometimes the m/z 67 rearrangement ion. 

O- and N-containing heterocyclic compounds Characteristic rearrangement ion of butyrates except methyl Dinitrotoluenes Trimethylsilyl derivatives... 

Appendix 10 Structurally Significant McLafferty Rearrangement Ions... 

Amine Characteristic Fragments Rearrangement Ions Losses from the Molecular Ion... 

Characteristic fragment ions Aliphatic ketones also give abundant McLafferty rearrangement ions at m/z 58, 72, 86, and so forth. Methyl ketones produce an abundant ion at m/z 43. Low-intensity ions at m/z 31, 45, 59, 73, and so on reveal oxygen in the unknown ketone and are especially useful in distinguishing ketone spectra from isomeric paraffin spectra. Subtract 43 from the mass of the rearrangement ion to determine R. 

A characteristic rearrangement ion of monobasic carboxylic acids above C4 (also see m/z 73)... 

Alkyl indoles (characteristic rearrangement ion) Cinnamates (C6H5CH=CH—)... 

Pedersen and coworkers10 studied the El mass spectra of several alkyl 2-hydroxyphenyl sulfoxides (10) and found that, contrary to methyl phenyl sulfoxide2,11 and the corresponding sulfones10, they do not show any abundant skeletal rearrangement ions (see Section III). This is obviously due to an ortho effect as shown in structure 10. 

Efremov and coworkers investigated the mass spectra of 18 methyl-substituted diphenyl (63)40 and substituted phenyl mesityl sulfones (64)41. The mass spectra of practically all the compounds showed by the rearrangement ions [M — OH]+, [M —H20]+ and [M — (H20 + OH)]+, the relative abundances of which depend on the position of the substituent in the phenyl moiety (ortho effect). It was also evident that in 63 the introduction of the first methyl substituent clearly decreases the contribution of the sulfone sulfinate isomerization (equation 30) to their fragmentation whereas the further substitution had little or no effect on the isomerization process in both 6340 and 6441. 

Rearrangement ion An ion formed nnder certain ionization conditions in which the original molecnlar strncture of the analyte has undergone some modification, i.e. has not been prodnced by simple bond scission. 

The rearrangement ions [M —OH] , [M —H20] and [M — H2O- -OH] (equation 30) were not present if 63 had no ortho methyl substituent but they were present in all of 64. Accordingly, at least one of the eliminated hydrogens must be supplied by the methyl group and not by the aromatic rings. Some formation of [M — SO2] occurred also in both 63 and 64. 

The new (rearranged) ion-radical pair then collapses with extreme rapidity (as indicated by the high quantum yields) to a stereospecific cycloadduct, i.e.,... 

Dienes and polyenes show a pronounced molecular ion in the mass spectra and hence the molecular weight of polyenes can be determined by positive ion mass spectra. The easy removal of a 7r-electron from a diene is usually the reason for the distinct M +. The mass spectral investigation of conjugated polyenes is somewhat similar to that of aromatic structures, due to the high stability of the rearranged ions formed after the... 

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