Transfer dissociation energies

Table XVI. Comparison of the Transfer Dissociation Energies (on Tetrahydrofuran, from Water to 50 Mass Percent Methanol,...

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. 

Figure 1.6. Quantum theory of IBr-Ar dissociation, showing a snapshot of the wavepacket states at 840 fs after excitation of the I-Br mode by a 100 fs laser pulse. The wavepacket maximum reveals predominant fragmentation of the IBr molecule along the r coordinate at short IBr-Ar distances [R coordinate), whilst a tail of amplitude stretches to longer R coordinates, indicating transfer of energy from the I-Br vibration to the IBr-Ar dimension, which propels the argon atom away from the intact IBr molecule.

Numbers used in this cycle AG° for addition of water to give a monoanionic adduct, Table 1.7 AG° for proton transfer reactions, based on pAT, values estimated by the method of Branch and Calvin.) The dissociative energy is taken from Table 1.7. 

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... 

The validity of equation (12) has been checked for several families of alkyl halides for which D and E /x- are known (Ref. 32, see particularly figure 6 therein). It was thus found that for v = 0.1 V s the constant is equal to 0.3 eV at 20°C (expressing D in eV and the potentials in V). Equation (12) was then applied to the approximate determination of unknown BDEs in several series of compounds undergoing dissociative electron transfer, namely, TV-halosultams,32 sulfonium cations,33 vicinal dihalides,34 1,3-dihaloadamantes, 1,4-dihalo-bicyclo[2.2.2]octanes, and l,3-dihalobicyclo[l.l.l]pentanes.35 In the latter case, the mutual influence of the two halogens could be rationalized thanks to the conversion of the peak potential data to bond dissociation energies. 

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