Bond dissociation enthalpies homolytic

F. G. Bordwell, X.-M. Zhang. Acidities and Homolytic Bond Dissociation Enthalpies of 4-Substituted-2,6-di-tert-Butylphenols. J. Phys. Org. Chem. 1995, 8, 529-535. 

Table 2.2. Homolytic bond dissociation enthalpies, AHohom. and heats of formation, AHf(x.), of some selected bonds and radicals [kJ mol ] ...

The value of 169.9 kJ mol" (1.75 eV) corresponds to AE(ch3+) = 14.35 eV which is in good agreement with published values of about 14.3 eV. [28,39] In addition, this is only 40 % of the homolytic C-H bond dissociation enthalpy of the neutral methane molecule, thereby indicating the weaker bonding in the molecular ion. 

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. 

In this chapter, we have collected and discussed the available data in both gas and liquid phases related to Scheme 2.1. Emphasis will be given to homolytic bond dissociation enthalpies of silanes. Generally, Df/values are extrapolated from the gas phase to solution without concerning solvent effects (particularly in the... 

Due to the importance of homolytic bond dissociation enthalpies for understanding radical chemistry, a set of Me3Si—bond dissociation enthalpies was derived via the relationship... 

Bordwell, F. G. Zhang, S. Zhang, X.-M. Liu, W.-Z. Homolytic bond dissociation enthalpies of the acidic H-A bonds caused by proximate substituents in sets of methyl ketones, carboxylic esters, and carboxamides related to changes in ground state energies./. Am. 

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>. 

Peroxides are often added to free-radical reactions as initiators because the oxygen-oxygen bond cleaves homolytically rather easily. For example, the bond-dissociation enthalpy of the O —O bond in hydrogen peroxide (H —O —O —H) is only 213 kJ/mol (51 kcal/mol). Give a mechanism for the hydrogen peroxide-initiated reaction of cyclopentane with chlorine. The BDE for HO — Cl is 210 kJ/mol (50 kcal/mol). 

Tab. 3.5 Relationship of homolytic bond dissociation enthalpy D298, threshold wavelength A.thw and absorption onset of selected molecules in the gas phase...

The bond dissociation enthalpy (BDE) for a diatomic molecule AB is the amount of energy necessary to bring about homolytic cleavage, that is, the enthalpy of the dissociation reaction... 

Reactions. - The equilibrium acidities (pKha) of six p-substituted benzyltriphenylphosphonium salts, and also those of related allylphosphonium salts, have been determined, together with the homolytic bond dissociation enthalpies of the acidic C-H bonds. A study of the data available in the Cambridge Structural Database reveals that tetraphenylphosphonium cations in crystals associate through phenyl-phenyl non-bonded interactions which are attractive, concerted, and widespread in nature. An attractive force of 60-85 kJmoP has been calculated. ... 

For any species containing -XH groups there is a relationship between the redox and acidity properties and the homolytic X-H bond dissociation enthalpy as illustrated by the thermochemical cycle in scheme 1. 

Consequently, the homolytic gas-phase X-H bond dissociation enthalpy, Dx h, for the corresp>onding neutreil molecule can be calculated using Hess law and the one-electron reduction potential of tiie radical cation (X-H ) in combination with the pl< of the radical cation or the one-electron reduction potential of the radical (X ) in combination with the pK, of the neutral molecule (X-H). This results in equation 5 where C is a constant that depends on the family of compounds and the solvent. [25]... 

The hydrogenolysis reaction proceeds at a near collisional rate, but the activation of methane, the reverse reaction, does not proceed at a measurable rate in the FTICR. Thus the equilibrium constant (=kf/kj.) for Equation 1 is > 1, and the product Zr-H homolytic bond dissociation enthalpy, D(Cp2Zr -H), is therefore greater than D(Cp2Zr -CH3) since D(H-H) = D(H3C-H) =... 

An interesting point concerning results in Table I is that for M = V, Ru, and Ni both oxidized and reduced forms of a metallocene have the same values of homolytic bond dissociation enthalpies within the estimated error... 

This paper is concerned with certain aspects of the thermodynamics and kinetics of transition metal-alkyl homolytic bond dissociation processes, notably of stable, ligated complexes in solution (L M-R, where L is a ligand and R = alkyl, benzyl, etc.)(l ). The metal-alkyl bond dissociation energy of such a complex (BDE, strictly bond dissociation enthalpy) is defined as the enthalpy of the process represented by Equation 1. 

The bond dissociation enthalpy (BDH or 2)77298.15(A-B)) for a species A-B is the enthalpy required for homolytic bond cleavage at T = 298.15 K, reaction (17), and depends exclusively on the relative enthalpies of formation of the reactant and product states. 

The bond dissociation enthalpy is a valuable quantity for deriving enthalpies of formation of organic radicals, provided that all the enthalpies of formation of the neutral species present in the homolytic cleavage reaction are known. 

The energy required for homolytic bond cleavage is called the bond dissociation enthalpy (D). A list of some bond dissociation enthalpies is given in Table 4.3. 

The bond-dissociation enthalpy (BDE, also called bond-dissociation energy) is the amount of enthalpy required to break a particular bond homolytically, that is, in such a way that each bonded atom retains one of the bond s two electrons. In contrast, when a bond is broken heterolytically, one of the atoms retains both electrons. 

TABLE 4-2 Bond-Dissociation Enthalpies for Homolytic Cleavages ... 

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