Charge site initiation mechanism

For a localized charge site, the initiation of the cleavage takes place by the attraction of the electron pair forming the bond charge site initiation mechanism). 

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. 

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... 

Figure 2.8. Proposed mechanism of action of BPI (a) The external polysaccharide chain of lipopolysaccharide (LPS) is a hydrophilic barrier stabilised by Ca2+ and Mg2+, which cross-link the LPS chains, (b, facing) BPI is highly basic and initially interacts by displacing the cations that bind to charged sites on the surface. See text for details.

The sites of formation of hot spots may be provided in both liquid and solid expls not only by bubbles of gas but also by bubbles of vapor, if the expl is more or less volatile. The importance of some heterogeneous initiation mechanism is shown by the failure of deton to propagate, in at least some expls, when the density of material exceeds a certain value [See under "Detonation (and Explosion), Resistance to ]. For the type of expls referred to by D. Price as Group 2 (Ref 33), see under "Detonation (and Explosion), Critical or Limiting Charge Densities. Price noted that in other expls, however, the homogeneous process plays the major part in the transmission, if not the initiation, of detonation (Ref 34)... 

Fujii, T., Hiramoto, Y., Trao, J., and Fukuzawa, K. 1991 Site-specific mechanism of initiation by chelated iron and inhibition by a-tocopherol of lipid peroxide-dependent lipid peroxidation in charged micelles. Arch. Biochem. Biophys. 284 120-126. 

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. 

Presumably the radical sites generated do not propagate but are destroyed in some chain-transfer reaction or radical-radical combination. The charge-transfer complex has not been included in the initiation mechanism for reasons to be discussed. 

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