Homolytic bond cleavage enthalpy

Some of the enthalpies (AH) for homolytic bond cleavage of the chemical bonds are given in Table 10.1 to show that there are no major differences in stability between the more xenobiotic bonds and the more biological ones. Homolytic cleavage enthalpy gives a rough measure of the stability of the compound ... 

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. 

A covalently bonded complex M-X can be envisioned to undergo M-X bond cleavage by the three modes depicted in Scheme 1. Here, Equation (4) represents the homolytic bond cleavage that defines the BDE, normally defined in enthalpy terms - as will be the case in all tabulated values here. Equations (5) and (6) in Scheme 1 are the two corresponding heterolytic bond cleavage processes. The processes are interrelated by the electron affinities (EAs) and ionization potentials (IPs) of the radicals M and X in the gas phase. In solution, EA and IP are replaced by the corresponding electrode potentials (multiplied by the Faraday constant). 

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. 

When a bond is broken or formed, the energy required is known as the bond dissociation enthalpy (D° or, more commonly, H° for a bond broken or formed in a reaction). It is also called bond dissociation energy for convenience, the values listed will be used for both heterolytic and homolytic bond cleavages. 

Bond dissociation enthalpies (BDEs) are the enthalpies of homolytic bond cleavage 8.33-8.34 for a given type of bond. 

M. Xian, X.-Q. Zhu, J. Lu, Z. Wen, J.-P Cheng. The First O-NO Bond Energy Scale in Solution Heterolytic and Homolytic Cleavage Enthalpies ofO-Nitrosyl Carboxylate Compounds. Org. Lett. 2000, 2, 265-268. 

Because most heteroatom-carbon single bonds are less stable than carbon-carbon bonds, traceless linkers can be synthesized on the basis of nearly all heteroatoms. The enthalpies of C-X bonds are, however, only relevant for homolytic bond scission. Many linkers are cleaved heterolytically, and kinetic stability toward hetero-lytic bond cleavage is decisive in these linkers. 

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... 

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. 

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

The breaking of a bond yielding radical products is expected to have a small reverse barrier. As a consequence, the reaction energy is similar to the reaction barrier and has been used as an input parameter in kinetic simulations of lignin pyrolysis [21]. For the homolytic cleavage, the reaction enthalpy is equal to the bond dissociation enthalpy (BDE). 

Homolytic bond dissociation energy, DFP (Section 10.2) The enthalpy change that accompanies the homolytic cleavage of a covalent bond. 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

The S-S bond dissociation energies of H2S2, H2S3 and H2S4 have been studied by Steudel et al. at the GCSD(T)//6-311 G(2df,p) level [42]. The calculated enthalpies AH° for the dissociation at the central bonds at 298 K are 247, 201 and 159 kJ mol respectively. The lower stability of the tri- and tetrasulfanes towards homolytic S-S cleavage is attributed to the stability of the generated HSS radical which is stabilized by the formation of a three-electron n bond. 

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

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