Bond dissociation energy data values

Table II. Mean Bond Dissociation Energy Data for Some Early Transition Metal Complexesa and Estimated 5 Values for Thorium and Uranium.

For saturated hydrocarbons, exchange is too slow and reference points are so uncertain that direct determination of pK values by exchange measurements is not feasible. The most useful approach to obtain pAT data for such hydrocarbons involves making a measurement of the electrochemical potential for the one electron reduction of the hydrocarbon radical. From this value and known C—H bond dissociation energies, pAT values can be calculated. 

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

Finally, it is worth noting that all of the research described here is greatly facilitated when accurate values are available for metal-ligand bond dissociation energies. Only limited data of this type is presently available and further work along these lines is certainly warranted. 

Bond dissociation energies for several heterocyclic systems calculated by two ab initio methods, CBS-Q, G3 and G3B3, show similar values <2003JP0883>. The G3B3 data for three selected azoles are as follows ... 

The mean bond dissociation energies (E ) given in Table 12 are based on thermochemical data at 25 C19. Unless previously discussed, the heat of formation of the metal alkyl used is that given by Long60. The higher values of E and D2 for dimethyl mercury are obtained when Long s recommended value for the heat... 

RSE values can also be calculated from experimentally measured X - H bond dissociation energies or heats of formation (where available). In order to be directly comparable to the RSE values calculated at the ROMP2 or G3(MP2)-RAD level described above, this requires thermochemical data for the species in Eqs. 1-4 at 0 K. One straightforward approach is the back correction of experimentally measured heats of formation at 298.15 K to 0 K values using thermochemical corrections calculated using the rigid ro-tor/harmonic oscillator model in combination with scaled DFT or UMP2 frequencies [19,23]. 

The types of values reported in the database standard enthalpies of formation at 298.15 K and 0 K, bond dissociation energies or enthalpies (D) at any temperature, standard enthalpy of phase transition—fusion, vaporization, or sublimation—at 298.15 K, standard entropy at 298.15 K, standard heat capacity at 298.15 K, standard enthalpy differences between T and 298.15 K, proton affinity, ionization energy, appearance energy, and electron affinity. The absence of a check mark indicates that the data are not provided. However, that does not necessarily mean that they cannot be calculated from other quantities tabulated in the database. 

Next, supervised-learning pattern recognition methods were applied to the data set. The 111 bonds from these 28 molecules were classified as either breakable (36) or non-breakable (75), and a stepwise discriminant analysis showed that three variables, out of the six mentioned above, were particularly significant resonance effect, R, bond polarity, Qa, and bond dissociation energy, BDE. With these three variables 97.3% of the non-breakable bonds, and 86.1% of the breakable bonds could be correctly classified. This says that chemical reactivity as given by the ease of heterolysis of a bond is well defined in the space determined by just those three parameters. The same conclusion can be drawn from the results of a K-nearest neighbor analysis with k assuming any value between one and ten, 87 to 92% of the bonds could be correctly classified. 

The formation of molecular complexes between aluminum trihalides and pyridine or alkylpyridines has been the subject of systematic studies.35,36 Calorimetric data yield bond dissociation energies D(X3Al—py) of 323, 308 and 264 kJ mol-1 for X = C1, Br and I, respectively, and this same order is found for alkylpyridine adducts, although the A1—N bond is weakened in the case of lutidine by the effects of steric hindrance. For gallium halides the values of D(X3M—py) are smaller 248, 237 and 195 kJ mol-1 for chloride, bromide and iodide adducts, respectively. 

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