Bond Dissociation Energies. Atomization Enthalpy

Bond Dissociation Energy D(HN-H) in kJ/mol. The bond dissociation energy Dg = 380.9 2.1 was calculated from AfHg(NH)-356.4 kJ/mol, AfHg(H)-216.1 kJ/mol, and AfHg(NH2) = 191.6 kJ/mol [1]. Three decades earlier, Do = 368 17 had been evaluated from thermochemical data [2]. De(HN-H) = 412.1 was determined from enthalpies of formation and spectroscopic data [3]. Ab initio calculations gave Do(HN-H) = 385 8 [1] and De(HN-H) = 378 [3]. Additional D values were obtained by ab initio [4, 5] and semiempirical [6, 7] calculations. 

12 Bond Dissociation Energy. Atomization Enthalpy Dissociation Energy D(FO-F) 

The dissociation energy (in kcal/mol, unless stated otherwise) is not very well known, as is shown by the large uncertainties in the data given in the table below. The three sources for D listed in the table are discussed separately. D = 39 (1.7 eV) has been estimated by a comparison with isoelectronic species [1], formerly D = 42 (1.8 eV) [2]. 

D(FO-F) from appearance potentials. Do = 159 15 kJ/mol has been derived from the measured difference 1.65 0.1s eV of the OF appearance potentials from OF and OF2 [3]. Do = 160.97 1.93 kJ/mol was likewise obtained [18] from the difference between the OF appearance potential from OF2 (14.44 eV) [19] and the ionization potential of the OF radical (12.77 eV) [20]. Previously, too large a value D = 64.6 (2.8 eV) was derived from the measured appearance potential 1.2 eV of F from OF2. An electron affinity of the F atom of 3.6 eV and an excess kinetic energy of 2.0 eV for the assumed products OF and F were taken into account. The latter was determined from the measured kinetic energy of the fluoride ion [6]. See also the 

For a comparison of Eb in compounds of N, O, and F, see [36]. For a calculation from covalent and ionic contributions, see [37]. AHat was derived from a Mulliken population analysis using Hiickel theory [38] and from the Mulliken [23] magic formula [24]. 

Vedeneyev, L. V. Gurvich, V. N. Kondrat yev, V. A. Medvedev, E. L. Frankevich (Bond Energies, Ionization Potentials, and Electron Affinities, Arnold, London 1966, p. 77). 

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Bond dissociation energy. The enthalpy change required to break a bond in a mole of gaseous molecules. (9.10) Bond length. The distance between the centers of two bonded atoms in a molecule. (9.8)... 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

Homolysis of a bond is an elementary reaction that is of profound influence on reactivity in many processes. The enthalpy of such a step, the bond dissociation energy (BDE), can be calculated from Eq. 1 with the products now being atoms or radicals. 

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

Photochemical processes and electronic slates of simple molecules with up to live atoms and radicals with up to four atoms in the gas phase are covered in Chapters V through VII. The absorption coellicients available for many molecules are shown in figures, as they are important in understanding the quantitative aspect of photochemistry. Bond dissociation energies given are calculated mostly from enthalpies of formation of atoms, radicals, and molecules tabulated in the Appendix. 

Values of A.H previously selected by Oolwell (5) and Mills (6) have been based primarily on the equilibrium data of Yost and Russell (4). The value selected by NB8 (]7) is -4.7 kcal mol. Our adopted results give an enthalpy of atomization and mean 8-01 bond dissociation energy of 126.8 0.9 kcal raol" and 63.4 kcal mol", respectively. 

The enthalpy change for the dissociation of a diatomic molecule such as H2 into its atoms in the gas phase can be termed the bond dissociation energy (or more strictly the bond dissociation enthalpy). If we consider the reaction... 

The standard enthalpy change for formation of KrF2(g) is +60.20 kJ/mol. Using atomization data from Appendix III, determine the bond dissociation energy for the Kr—F bond. 

The excellent linear plots confirm the Polanyi relation and lend considerable support to the values of the bond dissociation energies in the alkanes. It must be emphasized, however, that this type of relation is only applicable to reactions of radicals or atoms with a series of closely related compounds. It has been stressed by Benson and De More [385], from the example of the reactions of methyl radicals with a variety of substrates, that there is no way of relating the activation energies and enthalpy changes when the substrates are difficult classes of compounds. 

These terms combine to give the free energy of H-atom abstraction in Eq. (9.33) (BDE = bond dissociation energy), which can be used to calculate pseudoaqueous p Ka values by rearrangement, conversion to enthalpies instead of free energies,59 and use of an empirical constant ... 

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