Cleavage of a Sigma-Bond

Electron ionization mainly creates singly charged positive ions by ejection of one electron out of the neutral. If the precursor was a molecule, M, it will have an even number of electrons, i.e., an even-electron or closed-shell species. The molecular ion formed upon EI must then be a positive radical ion, M , odd-electron or open-shell) ion. 

Definition The molecular ion has the same empirical formula as the corresponding neutral molecule. The neutral and its molecular ion only differ by one (or more) electron(s). A singly charged molecular ion can either be a positive radical ion, M , or a negative radical ion, M (not in case of El). The mass of this ion corresponds to the sum of the masses of the most abundant isotopes of the various atoms that make up the molecule (with a correction for the elec-tron(s) lost or gained. Chap. 3.1.4). [8,9] 

Note The symbolism M does not mean one added electron. The radical symbol is added to the molecular ion only to indicate a remaining unpaired electron after ionization. Addition of one electron to a neutral would make it a negative radical ion, M (electron capture). 

When initially formulating the ionization process, we did not consider where the charge would reside (Chap. 2.2.1), e.g., for methane we wrote  

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Consistent with these results, as shown in Fig. 7.33, the DFT calculations of the CH activation mechanism show that the lowest energy pathway involves pre-equilibrium, dissociative loss of pyridine to generate a trans-5rate-determining trans-cis isomerization to generate the cis-benzene complex and fast CH bond cleavage by a sigma-bond metathesis transition state. 

There are much fewer examples in the literature of the heterolytic cleavage of Si H bonds compared to H-H and even fewer other examples in which a cr-silane complex was observed as an intermediate in the activation. We have synthesized alkyl-substituted diimine ligands with sidearms containing labile functional groups that reserve a vacant site for binding and activation of sigma bonds as shown in Eq. (32) (108,109). 

The coupling of two radicals to form a sigma bond is the reverse of the homolytic cleavage reaction and occurs without any activation barrier. Therefore, we might expect this reaction to be exceedingly fast. However, the rate of a reaction depends not only on the barrier height but also on the concentrations of the reacting species. Since the radical concentration in a usual reaction is low, radical-radical collisions are infrequent. 

The results can be rationalized in terms of an oxidative addition-reductive elimination mechanism as illustrated with XXII. A similar mechanism has been proposed for the acid cleavage of Pt(II)-C sigma bond (57, 58). 

Photochemical Cleavage Reactions of Benzyl-Heteroatom Sigma Bonds Steven A. Fleming and James A. Pincock... 

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