Bond dissociation energy, evaluation

The bond dissociation energies in the gas phase for Me—SO2, Et—SO2 and Ph— SO2 have been evaluated to be 18, 16 and 44kcalmol respectively although Horowitz, from complex kinetic studies of the radiolysis of MeS02Cl in cyclohexane, has obtained the values of 15 and 12 kcal mol for D(Me—SOy and D(c-C6Hn—SO2), respectively these bond dissociation energies are considerably lower than the gas-phase values, and it has been suggested that the possible cause of this difference is due to the heat of vaporization of sulfur dioxide . 

Since homolytic or radical processes are largely governed by the effects of bond dissociation energies, a knowledge of BDE is required for the evaluation of chemical reactivity in such reactions. However, we have found, as we mention later, that BDE s are also an important factor influencing other types of reactions involving bond heterolyses. 

The determination of thermodynamic stability of a radical from C—H bond-dissociation energies (BDE) in suitable precursors has a long tradition. As in other schemes, stabilization has to be determined with respect to a reference system and cannot be given on an absolute basis. The reference BDE used first and still used is that in methane (Szwarc, 1948). Another more refined approach for the evaluation of substituent effects by this procedure uses more than one reference compound. The C—H BDE under study is approximated by a C—H bond in an unsubstituted molecule which resembles most closely the substituted system (Benson, 1965). Thus, distinctions are made between primary, secondary and tertiary C—H bonds. It is important to be aware of the different reference systems if stabilization energies are to be compared. 

One of the early efforts to evaluate quantitatively the bond dissociation energy of particular bonds in a compound was the work initiated by Mulliken (-3) in his so-called Magic Formula. Although this formula contains five terms, the two most important for the evaluation of a bond dissociation energy, Dq (uncorrected for zero-point vibrational energy), between two atoms i and j, are the covalent bond energy, Xjj, and the ionic resonance energy, IRE. The evaluation of Ay takes the form ... 

This value relates to the pure crystalline ester, and to discuss bond dissociation energies it is necessary to have a value for the heat of formation of gaseous phenyl benzoate. The latent heat of sublimation at 25 °C. may be derived from separate values for fusion and vaporization. We have measured the latent heats of fusion at 70 °C. as AHfus = 7.0 0.3 kcal. per mole (both electrically and from determining the cryoscopic constant). An average value of the latent heat of vaporization, AHvap = 14.2 0.2 kcal. per mole, may be evaluated from existing (17) vapor... 

The separate and quantitative evaluation of accelerating steric effects on the homolytic cleavage of C—C-bonds allowed the definition of intrinsic barriers for the bond dissociation reaction, i.e. AG or AH at Hs = 0 in Fig. 2 or 3 or in Eq. (1-11) in general. AH is equal to or slightly than the corresponding bond dissociation energy (BDE) 6a-811 as pointed out earlier. 

The differences in RSE values obtained from Equations 5.3, 5.6 and 5.8 will be illustrated here using the ethyl radical (CH3CH2, 8) and the fluoromethyl radical (FCH2, 9) as examples. Both systems are not burdened by steric effects and the RSE values can thus be interpreted as the consequences of electronic substituent effects. Also, experimentally measured C-H and C-C bond dissociation energies are available for both systems, allowing for a side-by-side evaluation of experimental and theoretical results (Table 5.1). 

Feng, Y. Liu, L. Wang, J.-T. Huang, H. Guo, Q.-X. Assessment of experimental bond dissociation energies using composite ab initio methods and evaluation of the performances of density functional methods in the calculation of bond dissociation energies, J. Chem. Inf. Comput. Set. 2003,43, 2005-2013. 

Q Fe+2x does not vary significantly in the series with the changing monovalent anion and is 40 kcal. This allowed the evaluation of Fho2+ Snoj-as 136 kcal. When this value is used in the first thermodynamic cycle, one arrives at a value of 102 kcal. for the bond dissociation energy, D(ho2.. . h>- Finally, the above value used in the third cycle (22) ... 

Shono and coworkers studied the electrochemical formation of triphenylcyclopropenyl radical and its chemical behavior in the presence of hydrogen donors and olefins (Table 10). Wasielevsky and Breslow studied the reduction of various cyclopropenyl cations by second harmonic AC voltammetry (Table 11). They detected adsorption effects for some of the cations at the Pt but not at the Au electrode. The electrochemical data were used in thermodynamic sequences to evaluate the basicity of various cycloproenyl anions as well as the C-O bond dissociation energy of cyclopropenols. 

An example of a calculation for the evaluation of the bond dissociation energy using rel. (2.2.29) for an isoprene trimer shows the preference for dissociation following a p-scission compared to a- or methyl-scission. The a-scission, p-scission, and methyl-scission are shown below ... 

Jursic, B. S. High level of ah initio and density functional theory evaluation of the C-O bond dissociation energies in the dimethyl ether anion. Int. J. Quantum Chem. 1999, 73, 299-306. 

In 1972 Wentworth, Chen, and Steelhammer set out to write a monograph entitled Negative Ions Reaction and Formation in the Gas Phase, so scientists could plan future research using the ECD. At the time few fundamental properties of thermal electron reactions had been measured. Now many molecular electron affinities and rate constants for thermal electron attachment have been measured. Currently electron affinities and bond dissociation energies can be verified using theoretical SCF calculations on desktop computers. It is especially timely to review the techniques for studying reactions of thermal electrons with molecules and to evaluate the results. 

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