Bond Dissociation Energy BDE

A quantity that represents the contribution of a particular n-bond in a molecule to the topological resonance energy. A TT-bond with large negative BRE is presumed to be antiaromatic in nature. 

Molecular equilibrium structures of the same spin state which - still hypothetically - differ only in the length of one or several bonds. 

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The energy required for homolytic bond cleavage is called the bond dissociation energy (BDE) A list of some bond dissociation energies is given m Table 4 3... 

Bond dissociation energies (BDEs) for the oxygen—oxygen and oxygen— hydrogen bonds are 167—184 kj/mol (40.0—44.0 kcal/mol) and 375 kj/mol (89.6 kcal/mol), respectively (10,45). Heats of formation, entropies, andheat capacities of hydroperoxides have been summarized (9). Hydroperoxides exist as hydrogen-bonded dimers in nonpolar solvents and readily form hydrogen-bonded associations with ethers, alcohols, amines, ketones, sulfoxides, and carboxyhc acids (46). Other physical properties of hydroperoxides have been reported (46). 

These data can be combined with ionization potential (IP) data according to the scheme below to determine bond dissociation energies (BDE). 

Table 2. Singlet-triplet energy separations (AEsx) in M-H monomers, M—M bond dissociation energies (BDEs), SCF calculated bond lengths for the hypothetical, linear dimers HMMH (M = B, Al, Ga, In, or Tl) ...

The reactivity shown in Scheme 3 results from the low bond dissociation energy (BDE) of the P-H bond [11] k=l.2 10 M s for the H-transfer from R02P(0)H to a primary C-centered radical) and the fast halogen-atom transfer from a C-halogen bond to a phosphonyl radical [9,12] (fc=4 10 M s for f-Bu-Br and k=83 10 M s for Cl3C-Br). Piettre et al. [13] pointed out that these chain reactions were even more efficient when dialkylthiophosphites and the corresponding dialkylphosphinothioyl radicals were involved. 

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. 

As with the Marcus-Hush model of outer-sphere electron transfers, the activation free energy, AG, is a quadratic function of the free energy of the reaction, AG°, as depicted by equation (7), where the intrinsic barrier free energy (equation 8) is the sum of two contributions. One involves the solvent reorganization free energy, 2q, as in the Marcus-Hush model of outer-sphere electron transfer. The other, which represents the contribution of bond breaking, is one-fourth of the bond dissociation energy (BDE). This approach is... 

Boc-protection, oxime reduction, 128 Bond dissociation energy (BDE), hydrosilanes, 6... 

C-H and N-H bond dissociation energies (BDEs) of various five- and six-membered ring aromatic compounds (including 1,2,5-oxadiazole) were calculated using composite ab initio CBS-Q, G3, and G3B3 methods. It was found that all these composite ab initio methods provided very similar BDEs, despite the fact that different geometries and different procedures in the extrapolation to complete incorporation of electron correlation and complete basis set limit were used. A good quantitive structure-activity relationship (QSAR) model for the C-H BDEs of aromatic compounds... 

The situation with polyarylmethanes is very similar. Due to the stabilization of free valence in arylmethyl radicals, the bond dissociation energy (BDE) of the bond C—02 for example, in triphenylmethyl radical is sufficiently lower than in alkylperoxyl radicals. This radical is decomposed under oxidation conditions (room temperature), and the reaction of Ph3C with dioxygen is reversible ... 

The hydroxyl group of alcohol weakens the a-C—H bond. Therefore, free radicals attack preferentially the a-C—H bonds of the secondary and primary alcohols. The values of bond dissociation energy (BDE) of C—H bonds in alcohols are presented in Table 7.1. The BDE values of C—H bonds of the parent hydrocarbons are also presented. It is seen from comparison that the hydroxyl group weakens BDE of the C—H bond by 23.4 kJ mol 1 for aliphatic alcohols and by 8.0 kJ mol 1 for allyl and benzyl alcohols. 

TABLE 2. Values of A//(4 5 eq. 6) for the reaction between di-tert-butoxy radicals and organogermanes and bond dissociation energies, BDE(Ge—H)... 

The bond dissociation energies (BDE) of several alkyl- and aryl-substituted germanium hydrides were measured by the laser-induced photoacoustic effect22. The alkyl-substituted compounds exhibited similar BDEs to that of GeEU (81.6-82.6 kcal mol 1) while aryl substitution results in a slight weakening of the Ge—H bond (BDEs 79.2-80.2 kcal mol 1). 

Figure 3.88 Successive bond-dissociation energies (BDEs) for S—Fbonds of SF , showing the alternation associated with breaking of strongly coupled cu-bond pairs.

Table 3.34. The Ar—F bond lengths R, average Ar—F bond-dissociation energies (BDE), and vibrational frequencies of ArF species...

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...

Figure 4.97 Periodic trends in bond dissociation energies (BDE) for M—CH3 bonds (left) and M—H bonds (right) of saturated MH X (X = CH3, H) compounds of row 1 (circles, solid line), row 2 (squares, dashed line), and row 3 (triangles, dotted line) of the d block. (For these comparisons [only], all calculations were carried out at lower B3LYP/LANL2DZ level.)...

Local HSAB principle can also be used to calculate the relative homolytic bond dissociation energies (BDE). For the homolytic dissociation of para-substituted phenols ... 

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 ... 

In this chapter, we look closely at the performance of several ab initio techniques in the prediction of radical thermochemistry with the aim of demonstrating which procedures are best suited in representative situations. We restrict our attention to several areas in which we have had a recent active interest, namely, the determination of radical heats of formation (AHf), bond dissociation energies (BDEs), radical stabilization energies (RSEs), and selected radical reaction barriers and reaction enthalpies. We focus particularly on the results of our recent studies. 

Bond dissociation energies (BDEs) provide a measure of both the reactivity of a compound (with respect to homolytic bond rupture) and the stability of the corresponding radical. There have been many theoretical investigations of BDEs for a wide variety of species [36], In particular, the C-H BDE for a substituted methane is given by the enthalpy change for the reaction ... 

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