Bond Dissociation Energies for Some Common Bonds

Bond Dissociation Energies for Some Common Bonds [A-B A- + B]... 

There is at present no convenient, self-consistent source of all bond energies. The standard work is Cottrell. T. L. The Strengths of Chemical Bonds, 2nd ed. Butter-worths London, 1958, but it suffers from a lack of recent data. Darwent (National Bureau of Standards publication NSRDS-NBS 31, 1970) has summarized recent data on dissociation energies but did not include some earlier work or values known only for total energies of atomization rather than for stepwise dissociation. Three useful references of the latter type are Brewer, L. Brackett, E. Chem. Rev. 1961,61,425 Brewer, L. et al. Chem. Rev. 1963, 63, 111 Feber, R. C. Los Alamos Report LA-3164, 1965. The book by Darwent mentioned above also lists bond energy values for some common bonds. 

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. 

Based on bond dissociation energies (DDEs, Table 2.2), the keto form will dominate keto-enol equilibria for common carbonyl-containing structures. However, in many cases, enols can be spectroscopically observed or isolated, and in some cases they even dominate the equilibrium. 

The neutral bis-w-arene complexes form well defined crystals, moderately soluble in common organic solvents. They may be sublimed in vacuum at about 100°C. Thermally they are reasonably stable, frequently up to 300°C. Some mean bond dissociation energies of the metal-ring bonds are listed in Table XVI together with some equivalent data for bis-ir-cyclopentadienyl complexes. 

In conclusion, vitamin E and 2,6-di-tert-butyl-phenols (AOl, A02 and A04 for example) displays some commonality because of comparable O-H bond dissociation energy in the phenol group. Then, they have the same reactivity at least for the POO° -I- AH POOH -I- A° reaction. However, vitamin E can also trap P° radical which is not the case... 

The flame is a chemical reaction which takes place in the gas phase. The ideal flame for atomic absorption would generate the correct amount of thermal energy to dissociate the atoms from their chemical bonds. The most commonly used flames are aii -acetylene and nitrous oxide—acetylene. The choice of oxidant depends upon the flame temperature and composition required for the production of free atoms. These temperatures vary the molecular or chemical form of the element. Air and acetylene produce flame temperatures of about 2300°C and permit the analysis by atomic absorption of some thirty or so elements. The nitrous oxide—acetylene flame is some 650°C hotter and extends the atomic absorption technique to around 66 elements. It also permits the successful analysis of most elements by flame atomic emission, in many cases at fractional parts per million levels, providing adequate spectral resolution is available. 

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