Bond dissociation enthalpies compounds

Observation of the empirically linear equation 2 for the gas-phase enthalpy-of-formation data implies that AH(3) is constant for the series of compounds ML 20a b. AH°(3) can also be expressed in terms of the bond dissociation enthalpies (equation 5) by again using Scheme 1. 

TABLE 2. Bond dissociation enthalpies in organogermanium compounds (kJmol-1)... 

The main purpose of these final comments is to show a few general trends in the thermochemistry of Group 14 organometallic compounds, helped by some (hopefully) reliable values. And one of the trends is revealed by a rather usual plot1,2, in which the mean bond dissociation enthalpies of the species MR4 (i.e. one-fourth of the enthalpy required to break all the M—R bonds) are represented as a function of the enthalpy of formation of M in the gaseous state. As observed in Figure 4, for R = H and Me, D(M—H) and D(M—Me) increase with the enthalpy of formation (or sublimation) of M. It is noted, on the other hand, that the differences D(M—H) — D(M—Me) vary from 47.7 kJmol-1... 

FIGURE 4. Mean M—R bond dissociation enthalpies in MR4 compounds (M = element from Group 14 R = H or Me) versus the enthalpies of formation of gaseous M... 

Table 6. Mean bond dissociation enthalpy D (M-L) kJ mol 1 for homoleptic metal compounds ML ...

ESTIMATED BOND DISSOCIATION ENTHALPIES OF NITROGEN-FLUORINE COMPOUNDS... 

Though some more traditional thermodynamicists will be dismayed by the concept of solution phase bond dissociation enthalpy, the fact is that the database involving these quantities is growing fast. When used judiciously, they may provide important chemical insights—as is indeed the case for the stability of the O-H bond in phenolic compounds. Although solution phase bond dissociation enthalpies are not true bond dissociation enthalpies, because they include some contribution from intermolecular forces, a series of solution values like those in table 5.2 may be (and often is) taken as a good approximation of the trend in the gas-phase. 

Figure 5.4 Stepwise bond dissociation enthalpies in several first-row hydrides. For each compound, the light gray bars, starting on the left, represent DH°(A-H), DH A-H), etc. The dark gray bars represent the respective mean bond dissociation enthalpies, (DH°) (A—H). Data at T = 0 from [78],...

Apparently, there is not much advantage in using bond enthalpy contributions to discuss bonding energetics in a series of similar complexes. As already stated, we could have selected any value for Z)//,°(Cr-CO) + DH (Cr-CO) + Z)//j (Cr-CO) and then derived chromium-arene bond dissociation enthalpies in Cr(CO)3(arene) compounds, all based on the same anchor. The trend would not be affected by our choice. Nevertheless, besides emphasizing that the absolute values so obtained should not be regarded as bond dissociation enthalpies, the bond enthalpy contribution concept attempts to consider a pertinent issue in molecular energetics the transferability of bond enthalpies. 

M. M. Bizarro, B. J. Costa Cabral, R. M. Borges dos Santos, J. A. Martinho Simoes. Substituent Effects on the O-H Bond Dissociation Enthalpies in Phenolic Compounds Agreements and Controversies. Pure Appl. Chem. 1999, 71, 1249-1256. 

Mortimer and co-workers extended these studies to many other CoL2X2 compounds and, using estimated or measured enthalpies of sublimation and heat capacities of the complexes and the ligands, were able to derive the corresponding Co-L mean bond dissociation enthalpies [238]. 

In conclusion, therefore, a judicious use of CV methodology may lead to absolute thermodynamic data that are accurate to ca. 15 kJ mol-1. Relative values (i.e., differences between bond dissociation enthalpies in similar compounds) can be more reliable, but the approximations described suggest that some caution be exercised when using the results to draw conclusions that rely on small differences between bond dissociation enthalpies. This is the case, for example, for ring substituent effects on the O-H bond dissociation enthalpies in substituted phenols [346,349],... 

A compilation of bond dissociation enthalpies in organic compounds and enthalpies of formation of free radicals. 

This monograph contains enthalpies of formation, heat capacities, entropies, and metal-ligand bond dissociation enthalpies of organometallic compounds of transition and main group elements. 

In Table 2.4, we have collected background information for discussion in the following chapters. Recommended C—H bond dissociation enthalpies of selected organic compounds are reported in the first two columns, followed by a variety of heteroatom-hydrogen bond strengths including N—H, O—H, S—H, Ge—H, and Sn—H bonds. 

A. R. Dias, H. P. Diogo, D. Griller, M. E. Pinas de Piedade and J. A. Martinho Si mix s, Bonding Energetics in Organometallic Compounds, ACS Symposium Series No. 428 (Ed. T. Marks), Chap. 14 (Metal Bond Dissociation Enthalpies from Classical and Nonclassical Calorimetric Studies), 1990, p. 205. 

Ab initio molecular mechanics calculations have been used to examine the electronic states of selenoxanthene 9 and selenoxanthone 10. These results were used to explain differences in the formation of bromine adducts of these compounds (see Section 7.11.6.1.2) <1998JOC8373>. Bond dissociation enthalpies and adiabatic ionization potentials of phenolic antioxidants containing selenium and tellurium have been carried out using DFT models in an attempt to design novel vitamin E analogues such as 11 <20060BC846>. 

Enthalpies of formation have been determined for about 70 organotin compounds, principally by static bomb calorimetry, and are listed in the reviews by Pilcher and Skinner,77 Tel noi and Rabinovich,78 Harrison,79 and Simoes, Libman, and Slayden.80-81 The enthalpies of formation of the radicals of Mc, Sn Et3Sn , and BusSn" have been measured to be AH° (g) = 130 17, 99.7 17.6, and -36 kJ mol-1, respectively,80 (that for Bu3Sn by photoacoustic calorimetry) and from these values and the enthalpies of formation of the organotin compounds, bond dissociation enthalpies, D(M-L), for the reaction 2-6 can be derived from equation 2-7. 

The most common initiation or homolysis reaction is the breaking of a covalent C-C bond with the formation of two radicals. This initiation process is highly sensitive to the stability of the formed radicals. Its activation energy is equal to the bond dissociation enthalpy because the reverse, radical-radical recombination reaction is so exothermic that it does not require activation energy. C-C bonds are usually weaker than the C-H bonds. Thus, the initial formation of H radicals can be ignored. The total radical concentration in the reacting system is controlled both by these radical initiation reactions and by the termination or radical recombination reactions. In accordance with Benson (1960), the rate constant expressions of these unimolecular decompositions are calculated from the reverse reaction, the recombination of two radical species to form the stable parent compound, and microscopic reversibility (Curran et al., 1998). The reference kinetic parameters for the unimolecular decomposition reactions of K-alkanes for each single fission of a C-C bond between secondary... 

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