Bonds homolytic cleavage

Such reactions are commonly found as a result of the decomposition of charge transfer excited states. For example, while excitation of the MC bands of Co(NH3)5X (X = Cl, Br, I) leads to photosolvation and the formation of Co(NH3)5(0112) and Co(NH3)4(OH2)X, shorter wavelengths yield the LMCT state which decomposes into Cc II) ions and halogen atoms. The quantum yield for the reaction is found to depend on the excitation energy, indicating a role for the initially formed radical pair (Eq. 5). This may reform the starting complex (Eq. 6) or decompose to the redox products stabilised by the solvent or some other species (Eq. 7). The Co(II) complexes eventually decomposes (Eq. 8). 

Bands associated with the formation of ion pairs are observed. Excitation within these Charge Transfer to Ion (CTTI) bands causes reduction of metal centre (e.g. Eq, 9). Similarly short wavelength U.V. excitation of pentamino-Co(III) complexes in organic solvents is believed to cause solvent oxidation (Eq. 10). 

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Configuration correlation diagram for homonuclear case in which homolytic bond cleavage is energetically favored. 

The energy required for homolytic bond cleavage is called the bond dissociation energy (BDE) A list of some bond dissociation energies is given m Table 4 3... 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

A photochemical variant, the so-called photo-Fries rearrangement, proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ort/zo-derivative is formed in an analogous way ... 

Furthermore, photochemically induced homolytical bond cleavage can also be applied when the prepolymer itself does not contain suitable chromophoric groups [113-115]. Upon thermolysis of ACPA in the presence of styrene, a carboxyl-terminated polystyrene is formed. This styrene-based prepolymer was reacted with lead tetraacetate and irradiated with UV light yielding free radicals capable of initiating the polymerization of a second monomer (Scheme 33) [113]. 

Excited states can also decay by means of chemical reaction via heterolytic bond cleavage, leading to ions, or by homolytic bond cleavage generating free radicals. 

Homolytic bond cleavage from excited states in irradiated polymers [30] can lead to a pair of free radicals via bond scission, involving main chain or side-chain substituents. 

Processes accompanied by a decrease in volume, such as C—C bond formation, in which the distance between two carbon atoms decreases from the van der Waals distance of ca 3.6 A to the bonding distance of ca 1.5 A, are accelerated by raising the pressure and equilibria are shifted toward the side of products (AV < 0, AV < 0). The reverse reaction, a homolytic bond cleavage, leads to an increase in volume (AV / > 0, AV > 0). Pressure induces a deceleration of such a process and a shift in equilibrium toward the side of reactants. However, in an ionization, such as an ionic dissociation, the attractive interaction between the ions generated and the solvent molecules leads to a contraction... 

The corresponding hyperconjugative interactions in an open-shell singlet description of radical dissociation would similarly promote homolytic bond cleavage. 

DR. JAMES ESPENSON (Iowa State University) I should like to raise a slightly different aspect of substitution reactions, an aspect which relates to a different kind of mechanism. This involves a change from the conventional heterolytic process of metal ligand bond cleavage to one dealing with homolytic bond cleavage. 

The oxidation pathways for alkylated heteroaromatics start with the formation of a radical species, via hydrogen atom loss or alkyl group homolytic bond cleavage. We calculated these BDEs for methyl- and ethyl-substituted derivatives of several key heteroaromatics (Tables 1-3). Few of these experimental values exist therefore. 

A. The H2 Case in Which Homolytic Bond Cleavage is Favored... 

Figure 3.9b portrays homolytic bond formation by the recombination of radicals and is accompanied by charge transfer from A to B. The radicals must be singlet coupled. The interaction of triplet-coupled electron pairs is repulsive and does not lead to bond formation. The reverse process describes homolytic bond cleavage and results in singlet-coupled free radicals. 

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