Ionization homolytic

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... 

Here, and in the following thermochemical schemes, AHs denotes solvation energy, D homolytic bond dissociation energy, I ionization potential, A electron affinity. 

In the gas phase, homolytic bond dissociation enthalpies (D//) relate the thermochemical properties of molecules to those of radicals while ionization potentials (IP) and electron affinities (EA) tie the thermochemistry of neutral species to those of their corresponding ions. For example, Scheme 2.1 represents the relationships between RsSiH and its related radicals, ions, and radical ions. This representation does not define thermodynamic cycles (the H fragment is not explicitly considered) but it is rather a thermochemical mnemonic that affords a simple way of establishing the experimental data required to obtain a chosen thermochemical property. 

A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

Fio. 1. Schematic potential energy diagrams for the homolytic (I) and heterolytic (II) splitting of hydrogen by Ag+. All processes and energy terms are in solution. D = dissociation energy I = ionization potential. [Webster, A. H., and Halpern, J., J. Phys. Chem. 61, 1239 (1967).]... 

The most widely studied reference acid is the proton. Proton affinity, PA(B), is defined for a base B as the heterolytic bond dissociation energy for removing a proton from the conjugated acid BH+ (equation 20). The homolytic bond dissociation energy D(B+—H) defined by equation 21 is related to PA(B) and the adiabatic ionization potentials IP(H) and IP(B) (equation 22) are derived from the thermochemical cycle shown in Scheme 6. 

TABLE 19. Nitrile proton affinities, adiabatic ionization potential and homolytic bond dissociation energies0... 

The prediction of the homolytic bond dissociation enthalpy (BDE) and adiabatic ionization potential (IP) of 4-hydroxy-2,2,3,5,6-pentamethylbenzoselenete and benzotelluretes has been calculated <20060BC846>. 

Platinacyclobutane complex 118 undergoes equilibrium heterolytic scission of the exocyclic carbon-carbon bond to form a cationic allyl complex and the organic enolate ion (Equation 35) <1993OM3019>. Similar dissociative ionization was previously reported for rearrangements of iridium and rhodium metallacyclobutane complexes formed by nucleophilic alkylation < 1990JA6420>. This carbon-carbon bond activation is generally associated with reversible central carbon alkylation of Jt-allyl complexes (Section 2.12.9.3.3), but the homolytic equivalent has recently been... 

Bernhard and co-workers have performed a series of experiments to determine the mechanisms of DNA strand breakage by direct ionization of plasmid DNA. A big surprise in this work was the discovery that the total yield of single strand breaks exceeds the yield of trapped sugar radicals. Even at very low hydration levels (2.5 waters per nucleotide residue) nearly 2/3 of the strand breaks are derived from precursors other than deoxyribose radicals [74], The authors conclude that a majority of the strand breaks observed do not result from dissociative electron capture, homolytic bond cleavage from excited states, or from hydroxyl radical attack. Rather, the authors conclude that doubly oxidized deoxyribose is responsible for the high yield of strand breaks. 

Because of their extreme reactivity, the existence of free radicals in biological systems was generally not considered possible until Gerschman and associates (G3) hypothesized that oxygen poisoning and X-irradiation have a common basis of action. Here, they reported the homolytic dissociation of water by ionizing radiation. 

Ultraviolet light and Ionizing radiation homolytically split water to form the hydroxyl radical and hydrogen. 

In solution, ions are produced by the heterolysis of covalent bonds in ionogens. This ionization reaction is favored by solvents due to their cooperative EPD and EPA properties (c/ Section 2.6). In the gas phase, however, ionization of neutral molecules to form free ions is rarely observed because this reaction is very endothermic. For example, in order to ionize gaseous H—Cl into H and Cl , an energy of 1393 kJ/mol (333 kcal/ mol) must be provided. This considerably exceeds the 428 kJ/mol (102 kcal/mol) needed to homolytically cleave H—Cl into hydrogen and chlorine atoms. Thus, for the creation of isolated ions in the gas phase, energy must be supplied by some means other than solvation with EPD/EPA solvents. The most widely used method is ionization by elec-... 

Free radicals can be formed by thermolytic cleavage, photolysis (nltravi-olet light photolysis of hydrogen peroxide to form hydroxyl radicals), radiolysis (ionizing radiation of water to form hydroxyl radicals), or by homolytic cleavage with the participation of another molecule (i.e., Fenton reaction). Perkins, J., Radical Chemistry The Eundamentals, Oxford University Press, Oxford, UK, 2000. 

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