Ionic fragmentation process

Thermolysis and photolysis reactions are both more selective than ionic fragmentation processes. This is because there is genrally more energy available to ions in a mass spectrometer and the activation energies for the... 

Fraas, R. E. Ionic Fragmentation Processes of Beta-Diketonate Complexes of the Group III Metals, Doctoral Dissertation, University of Kentucky, Lexington, Kentucky, 1972. 221 pp. (Order no. 73- 7,343, University Microfilms, Ann Arbor, Mich.)... 

Collision-induced dissociation (CID) An ionic/neutral process in which the projectile ion is dissociated as a result of interaction with a target neutral species. Part of the translational energy of the ion is converted to internal energy causing subsequent fragmentation. 

The experimental approaches described above are examples of relative methods, wherein a thermochemical property is measured with respect to that of a standard, or an anchor. The quality of these measurements ultimately depends on the quality of the anchor. Alternatively, there are methods of determining thermochemical properties, in which the energy for a chemical process is measured on an absolute basis. Among the more common of these are the appearance energy measurements, in which the threshold energy for formation of an ionic fragment from an activated precursor is measured. There are mauy differeut methods of activation that can be used. Some of these are discussed here. 

Somewhat surprisingly, the pressure dependence of quantum yields from the photolysis of NO at 1236 A, where the primary process is almost entirely photoionization199, is very similar to that observed for 1470-1650 A198. Clearly, recombination of the ionic fragments must lead to an excited state of NO which predissociates upon collision. However, the neutralization reaction... 

The sole gas-phase study on a cationic magnesium cluster examined the photodissociation spectrum of the Mg2(CH4)+" complex . Mg2 is only a minor product (equation 66) while Mg is the main ionic fragment and may arise via either of the processes shown in equations 67 and 68. The latter reaction is predicted to only be slightly more endothermic. 

Studies of this nature are just at their inception, with solvent variation and different chromophoric species to be explored. This form of ionic fragmentation chemistry is quite interesting and is highly dependent on solvation structure dependence of these fragmentations can also be investigated in small clusters (n < 6). The opportunity for more experiments and theory here is quite clear. These are perhaps the most solvent rich and dependent processes thus far characterized in clusters. 

G. The Generation of Ionic and Neutral Silylenes via Fragmentation Processes 483... 

Table III also shows that hydrogen and the chlorinated butanes are reduced substantially when ethyl chloride is irradiated in the presence of benzene. The other products are essentially unaffected by this additive. In the radiolysis of certain alkanes (4), benzene, added in small amounts, does not interfere with the fast ion-molecule reactions of primary ionic fragments or with free radical processes, but it will efficiently condense unreactive or long-lived ions in the system. It is reasonable to assume that this is also true for alkyl halide systems and that the reduction in product yields compared with the pure compound upon adding benzene may be attributed to the interception of unreactive ions. Since the rate constants for reactions of the expected primary ions with ethyl chloride are very large (see Table II), the concentration of benzene used in our experiments should not interfere with the initial fast ion-molecule reactions. For ethyl chloride ion-molecule reactions, C4Hi0C1+ is the only unreactive ion of appreciable abundance which is expected in this system at the elevated pressures used in the radiolysis experiments. Thus, the reduced product yields in the presence of benzene additive can be identified tentatively with the removal of this stable ion and the elimination of its resultant neutralization products.

The radiolysis product yields in the presence of ion scavenger (Table III) also show that ethane is not formed from neutralization of stable ions. Therefore, the remainder of the ethane product (above that indicated to result from neutral decomposition) must be produced by an ion-molecule process—i.e., a yield of G = 1.47. The ion-molecule reactions previously listed show that ethylene ions react with ethyl chloride to form ethane. From the relative rates indicated for Reactions 3a-3d and the ethane yield just derived, a relative yield of 2.46 may be deduced for the ionic fragmentation to ethylene ion in the radiolysis. 

Allyltributyltin (5) is the most commonly used reagent for carrying out allylation reactions via a free radical fragmentation process [5]. Keck reported the first practical use of allyltributyltin for free radical allylation reactions in 1982 in the context of a synthesis of perhydrohistrionicotoxin [6]. Heating bromide 4 with allyltributyltin in the presence of AIBN as a radical initiator gave the allylated derivative 6 (Scheme 3) in high yield with complete control of stereochemistry. Similar transformations had proven to be very difficult by standard ionic reactions. 

As in the case of EED processes, nonresonant dipolar dissociation (DD) involves a dissociative electronic excited state of the molecule AB (Figure 16.3), which brings the spontaneous scission of the molecule into ionic fragments (Abdoul-Carime et al. 2000, Herve du Penhoat et al. 2001, Sanche 2003) ... 

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