Bond dissociation energy The

The same conclusion is reached using bond dissociation energies. The following equation shows the bond dissociation energies of the reactants and products taken from Table 4.3 ... 

Figure S-1. Form of a potential energy curve for diatomic molecule AB. VfrAa) is the potential energy, Tab is the intemuclear distance, is the equilibrium intemuclear distance, and D is the bond dissociation energy. (The zero point energy is neglected in the figure.)...

The important elementary reactions of coal liquefaction are the decomposition of coal structure with low bond dissociation energy, the stabilization of fragments by the solvent and the dissolution of coal units into the solution. 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

For 1, X=F, however, A ° is independent of the bond dissociation energies. The greater carbene-stabilizing ability of F28 (vs. Cl or Br) drives the partition of excited alkylfluorodiazirines mainly to carbene, thus minimizing the RIES pathway. In the case of the cyclobutylhalodiazirine 21-X photolyses (Scheme 4), RIES accounted for 68% of reaction when X=C1, but only 12% when X=F the balance of reaction was due to carbene in each case.28... 

The three parameters in the Morse function D, B, re are positive and are usually chosen to fit the bond dissociation energy, the harmonic vibrational frequency and the equilibrium bond length. At r = re, the Morse function V = 0. As r — D, V approaches D. For r re, V is large and positive, corresponding to short range repulsion. Although the Morse function has been used extensively, its representation of the potential away from re is not satisfactory. Several modifications have been proposed in Morse function. 

Local density models also provide a poor account of homolytic bond dissociation energies. The direction of the errors is the opposite as noted for Hartree-Fock models (reaction energies are too large), but the magnitudes of the errors are comparable. In fact, the average of Hartree-Fock and local density homolytic bond dissociation energies is typically quite close to the experimental energy. ... 

In aprotic solvents, the radical anion, RX , for aryl halides has been detected as intermediate. In cyclic voltammetry of aryl halides, though an irreversible two-electron reduction occurs at low scan rate, a reversible one-electron reduction occurs at high scan rate. Thus, it is possible to get the values of the standard potential ( °) for the RX/RX couple and the rate constant (k) for RX -> R (therefore, the lifetime of RX ). In Fig. 8.18, the relation between ° and log k for aryl bromides in DMF is linear with a slope of 0.5 [5If], It is apparent that the lifetime of RX , obtained by 1/k, increases with the positive shift of E0. In contrast, the existence of RX for alkyl monohalides has never been confirmed. With these compounds, it is difficult to say whether the two processes, i.e. electron transfer and bond cleavage, are step-wise or concerted (RX+e -> R +X ). According to Sa-veant [5le], the smaller the bond dissociation energy, the larger the tendency for the concerted mechanism to prevail over the step-wise mechanism. 

Hydrogen bonds are relatively weak. Those in liquid water have a bond dissociation energy (the energy required to break a bond) of about 23 kJ/mol, compared with 470 kJ/mol for the covalent O—H bond in... 

During coal conversion, the coal structure influences both thermal and catalytic reactions. Thermal reactions of solid coals initiate the breakage of weak bonds at rates proportional to their bond dissociation energies. The radicals thus produced require stabilization by hydrogenation or addition of small molecules otherwise the radicals couple to produce much more thermally stable bonds, which eventually leads finally to the formation of infusible and insoluble coke. 

Even though the energy of activation may be theoretically related to bond dissociation energy, the comparative rates and correlation, adequate parameters were found for the interpretation of the substituent effect. An isomerization of equation 38 and an equilibrium process of the primary products formed from 4-bromobutyronitrile are shown in a... 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

Bond dissociation energy The energy required to break a bond. The usual units are kilojoules mol-1. 

The potential curve for a diatomic molecule (blue) where / e represents the equilibrium bond distance and De is the bond dissociation energy. The parabolic curve (red) represents the behavior of a true harmonic oscillator. 

These different mechanisms and prodnct distribntions, to some extent, are related to the bond dissociation energies, the chain defects of the polymers, and the degree of aromaticity, as well as the presence of halogen and other heteroatoms in the polymer chains. Large amount of styrene monomers can be obtained by pyrolysis of PS, while a wide range of hydrocarbons are prodnced by random degradation of PE and PP [3, 59, 60]. 

In Section 1.3 three measures of bond strength were suggested, the bond dissociation energy, the bond energy term, and the bond stretching force constant. We now examine the definition and status of the third of these quantities. 

The higher the bond dissociation energy, the stronger the bond (6.4). 

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