Radical site-initiated fragmentation

Other examples are shown in Figure 7.18. f-Butylamineand n-butylamine yield only one intense fragment, the ion corresponding to the radical-site-initiated fragmentation. These... 

DBT, 2-DBT (2-methyldibenzo[3]thiophene), and 4,6-DBT (4,6-dimethyldibenzo[3]thiophene) in ESI tandem mass spectra have a a consistent 32 Da neutral loss, which is believed to be sulfur. Based on the tandem mass spectrum of the PASH compounds, the mechanism of fragmentation is considered to be a charge site-initiated reaction followed by a radical site-initiated fragmentation. Taking the example of DBT, Scheme 4 shows a possible mechanism. 

The alpha bond to the radical cation site can be broken by a radical-site-initiated reaction, that is by a transfer of the unpaired electron to form a new bond to an adjacent atom (a atom) with concomitant cleavage of another bond of this atom. The new bond compensates energetically for the cleaved bond. In this case we speak about a radical-site-initiated a fragmentation ... 

The presence of heteroatoms with non-bonding n electrons favors the localization of charges. When the radical and/or the charge are localized, this influences the fragmentation and the reaction mechanisms can be classified as sigma electron ionizations, localized charge initiations, radical site initiations, and rearrangements. 

Aliphatic Ethers The abundance of the molecular ion from aliphatic ethers is usually low but higher than that of the corresponding alcohols. These compounds exhibit radical site- and charge site-initiated fragmentation reactions. Radical site-initiated reactions prodnce resonance-stabilized oxyninm ions (e.g., of m/z 45,59,73,. ..). As exemplified in the reaction... 

The tendency for the fragmentation initiation with the radical site is parallel to the donor properties of this site. The most spectacular examples involve the processes triggered by the removal of a nitrogen n-electron. Halogens are the least active in these reactions. 

As shown In Table IV, the total incorporation values are greater than 2 at both initiator concentrations. Since MA terminates predominately by combination (16), the presence of more than 2 Initiator fragments/polymer is suggestive of an additional mode of Incorporation. A possible mechanism Is H-abstractlon from polymer by Initiator radicals which provides radical sites for chain branching. However, further discussion of these findings must await confirmation of the molecular weight data by more direct means. 

H2l-thioglycolate suggest that the dependence on thiol structure may be due to internal quenching of one radical site of the activated chromophore by the hydrogen atoms of the thiol side chain. A comparison of DNA damage produced by activation of the neocarzinostatin chromophore with several thiol activators revealed that the fragmentation pathway (i.e. the initial site... 

Alkenes Ionization occurs at the site of the double bond, and the relative abundance of M+ ion is higher than that formed in the corresponding saturated alkanes. The double bond is, however, mobile, and leads to structurally insensitive spectra for various isomeric alkenes. The allylic cleavage initiated by the radical site is a favored fragmentation of alkene radical cations [reaction (6.19)] to produce a dominant C H2n-i+ ion series (i.e., ions of m/z 27, 41, 55, 69,...). The McLafferty rearrangement can occur prior to double-bond migration for certain alkenes and is structurally diagnostic. 

Initiation. Free-radical sites for polymerization are formed by reaction between primary initiator free-radical fragments and vinyl molecules, as shown in Fig. 2.8. 

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