Standard bond dissociation energies

De Vos Burchart et al. have recently developed a force field for modeling zeolites.21 The model originally was intended for all-silica zeolites but was quickly extended to aluminum-containing zeolites. The parameters were derived from several sources. Standard bond dissociation energies were used, and the force constants were refined to fit the structure of ZSM-5, the structure and frequencies of a-quartz, in addition to unit cell dimensions of other zeolites. With the all-silica model, the authors were able to calculate heats of formation... 

If the fragments and the compound are gases in their standard states, at 1 bar and 25°C, then we can define the standard bond dissociation energy, DH° for the bond X—Y as ... 

The standard bond dissociation energy (BDE) in each case is therefore DH°(HO— H)=494 kJ, and DH°(0—H)=431 kJ. Methane provides a second, more extreme example. The successive removal of hydrogen atoms may be summarized ... 

This reaction involves the breaking of all four C—H bonds, and so the enthalpy change will be a sum of all four standard bond dissociation energies. However, we can write, from equation 2.10 ... 

A standard bond dissociation energy is different from an average bond dissociation energy. The latter is just the value obtained by calculating the heat of atomization of a compound (the enthalpy change on converting the molecule to individual atoms) divided by the number of bonds from one atom to another in the molecule. For more details on this distinction, see reference 67. Blanksby, S. J. Ellison, G. B. Acc. Chem. Res., 2003, 36, 255. This reference provides the uncertainties for the values in Tables 1.10 and 1.11. 

BOND ENERGIES OF H2" AND H2 Another useful experimental quantity that reflects electronic structure is bond-dissociation energy. The standard bond-dissociation energy... 

Accordingly, the energy required to separate Li from F at a bond distance of 2.88 au is 8.38 eV. This is called the coordinate-bond energy. However, we want to calculate the standard bond-dissociation energy, which refers to the process... 

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. 

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

The standard heats of formation AH of gaseous HX diminish rapidly with increase in molecular weight and HI is endothermic. The very small (and positive) value for the standard free energy of formation AGj of HI indicates that (under equilibrium conditions) this species is substantially dissociated at room temperature and pressure. However, dissociation is slow in the absence of a catalyst. The bond dissociation energies of HX show a similar trend from the very large value of 574kJmol for HF to little more than half this (295kJmol ) for HI. 

Table 4.56. Statistical means and standard deviations (SD) for bond dissociation energies (BDEs) o/ M—H and M—Me bonds of saturated MH X (X = H, Me) transition-metal complexes from the first three series of the d block...

The bond-dissociation energy is defined as the standard enthalpy change of the reaction in which the bond is broken R X - R —X. 

There is a clear antiperiplanar preference for the reaction (Scheme 4.2) due to the stabilization of the radical by coupling of the unpaired electron with bromine (ESR) in the first case. The weaker bond dissociation energy leads to a more favorable standard potential and a weaker intrinsic barrier. When the two conformers are present and can convert one into the other, the reduction follows a CE mechanism (Section 2.2.2), which goes through the more reducible of the two.1 2... 

No symbol has been approved by the IUPAC for dissociation energy in the chemical thermodynamics section [13]. Under Atoms and Molecules, either El or D is indicated. The latter is more common, and IUPAC recommends Do and De for the dissociation energy from the ground state and from the potential minimum, respectively. Because the bond energy concept will be omnipresent in this book and can be explored in a variety of ways, some extra names and symbols are required. This matter will be handled whenever needed, but for now we agree to use DUP for a standard bond dissociation internal energy and DHj for a standard bond dissociation enthalpy, both at a temperature T. In cases where it is clear that the temperature refers to 298.15 K, a subscript T will be omitted. 

The A-B bond dissociation energy, on the other hand, is the standard internal energy of reaction 5.1. It is abbreviated by DUf(A-B) and its relation with DHj A-B) is given by... 

The types of values reported in the database standard enthalpies of formation at 298.15 K and 0 K, bond dissociation energies or enthalpies (D) at any temperature, standard enthalpy of phase transition—fusion, vaporization, or sublimation—at 298.15 K, standard entropy at 298.15 K, standard heat capacity at 298.15 K, standard enthalpy differences between T and 298.15 K, proton affinity, ionization energy, appearance energy, and electron affinity. The absence of a check mark indicates that the data are not provided. However, that does not necessarily mean that they cannot be calculated from other quantities tabulated in the database. 

B. D. Darwent. Bond Dissociation Energies in Simple Molecules. National Standard Reference Data Series, National Bureau of Standards 31 U.S. Department of Commerce Washington, D.C., 1970. 

As will be discussed in Chapter 13, calculated energies of one particular class of isodesmic reactions, so-called bond separation reactions, may be combined with experimental or high-quality calculated thermochemical data in order to lead directly to accurate heats of formation. These in turn can be used in whatever types of thermochemical comparisons are of interest. We start our assessment of isodesmic processes with bond separation reactions. Following this, we consider description of bond dissociation energies, hydrogenation energies and acid and base strengths in terms of isodesmic processes, that is, not as absolute quantities but expressed relative to standard compounds. 

It is clear that proper description of the energetics of homolytic bond dissociation requires models that account for electron correlation. Are correlated models also needed for accurate descriptions of relative homolytic bond dissociation energies where the relevant reactions are expressed as isodesmic processes A single example suggests that they may not be. Table 6-15 compares calculated and measured CH bond dissociation energies in hydrocarbons, R-H, relative to the CH bond energy in methane as a standard ... 

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