Charge site-initiated fragmentation

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... 

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. 

As mentioned earlier [see reaction (6.43)], cyclic ketones undergo three distinct fragmentation pathways elimination of an alkene via cleavage of two 0-bonds in the ring, charge site-initiated reactions, and H-transfer reactions. 

Another common one-bond cleavage is charge-site-initiated or inductive cleavage, often indicated in a fragmentation mechanism by the symbol i. Inductive cleavage involves the attraction of an electron pair by an electronegative heteroatom that ends up as a radical or as a closed-shell neutral... 

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. 

One particularly intriguing aspect of Cl for molecular weight determination for polymers is the appearance of the quasimolecular ion (MH+). Cl leads to less fragmentation compared to EL Proton attachment preferentially occurs on heteroatoms, and the initial fragmentation is at the charge site normally by heterolytic cleavage (inductive effect). 

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