Bond dissociation energies hybridization

Summary Ab initio calculated bond dissociation energies of silicon compounds will be discussed by means of atomic ionization energies and atomic orbital overlap. Ring strain energies of C- as well as Si-rings are estimated by homodesmotic reactions. The hybridization concept is critically examined in the case of silicon compounds. From the most important results a set of basic rules will be presented. 

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 ... 

Of this group only benzyl chloride is not an aryl halide its halogen is not attached to the aromatic ring but to an. v/r -hybridized carbon. Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 

The increase in bond strength going from id to 4d to 5d correlates with an increase in the bonding overlaps between the 2cr CH3 orbital and the (p, d) hybrid cr MLn orbital on the metal center (in [M(CH3)(CO)5]), as shown in Fig. 4. The M-C(methyl) bond dissociation energies in [M(CH3)(C0)5] (M = Mn, Re) have been measured and were found to be 117 kJ/mol for M = Mn and 223 kJ/mol for M = Re, indicating a much stronger Re-C(alkyl) bond (49). More recent calculations have been reported by McQuillan et al. (65) these results were discussed in Section III. 

It is known that the immediate molecular environment significantly affects the bond energy, as is illustrated by the data in Part B of Table 3.2. For hydrocarbons the C—H bond dissociation energy depends on the degree of substitution and hybridization... 

Thus, in these last three sections, we have seen that there are associated with covalent bonds not only characteristic bond lengths and bond dissociation energies but also characteristic bond angles. These bond angles can be conveniently related to the arrangement of atomic orbitals—including hybrid orbitals—involved in bond formation they ultimately go back to the Pauli exclusion principle and the tendency for unpaired electrons to get as far from each other as possible. 

Orbital theories relate an increase in the electronegativity of a carbon atom relative to that of a bonded hydrogen to an increase in the s character of its bonding hybrid orbital. Thus the decrease in bond length and pK and the increase in force constant and bond dissociation energy for C—H through the series cyclohexane, ethylene and... 

Luo, Y. R. (2003). Bond Dissociation Energies of Organic Compounds, CRC Press. Caveat one has to be careful not to attribute the BDE trends solely to the orbital size effects. Other factors (e.g. polarization, hybridization, conjugation, hyperconjugation, non-bonding electron repulsion) are in play as well as will be discussed in more detail in the subsequent parts of this hook. 

---