Disproportionation Dissociation energy

For the most electronegative ligand, fluorine, we expect a relativistic destabilization in the Au—F bond, which was indeed determined to be —0.36eV at the coupled cluster level [182,183], Nevertheless, AuF has a sufficiently high dissociation energy of about 3.17 eV and has been identified recently in the gas phase [184]. In solution or in the solid state it would disproportionate to metallic Au and compounds of Au (AuF3 for the solid). However, a carbene-stabilized Au(I) fluoride was synthesized only very recently (see discussion in the next section) [185]. 

Why are the activation energies of the reactions of nitroxyl radicals with O—H bonds lower than those in their reactions with C—H bonds As in the case of the reaction of R02 with quinones, the difference in E values occurs as a result of the different triplet repulsions in TS [23]. When a TS of the O H O type is formed (the AmO + H02 reaction), the triplet repulsion is close to zero because the O—O bond in the labile compound AmOOH is very weak. Conversely, the triplet repulsion in the reaction of AmO with the C—H bond is fairly great, due to the high dissociation energy of the AmO—R bond. This accounts for the difference between the activation energies and between the rate constants for the reactions considered above. Thus, the possibility of the realization of a cyclic chain termination mechanism in the reactions of nitroxyl radicals with peroxyl radicals, incorporating O—H groups, is caused by the weak triplet repulsion in the TS of such disproportionation reactions... 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

Other similarities with halogens are the presence of diatomic molecules (Au2) in gold vapor, with dissociation energies comparable to those of halogens, or the spontaneous disproportionation of gold metal into Au and Au+, in a melt reaction in the presence of cesium. 

OF(g) are unstable as substances with respect to disproportionation to the respective elemental gaseous substances. The fact that NO(g) can nevertheless be isolated while OF(g) cannot is a kinetic phenomenon, no doubt related to the much greater dissociation energy of NO (630 kJ mol-1, compared with 222 kJ mol-1). It has already been noted (Section 6.4) that NO is stable to dimerisation OF dimerises to FOOF. 

M—H bond dissociation energies, 1, 287 photochemistry, 1, 251 single crystal neutron diffraction, 1, 578 stability toward disproportionation, 1, 301 Metal—hydrogen bonds bond dissociation energy in 1,2-dichloroethane, 1, 289 stable metal hydrides in acetonitrile, 1, 287 thermochemical cycle, 1, 286 in THF and dichloromethane, 1, 289 olefin insertion thermodynamics, 1, 629 in Zr(IV) bis-Cp complexes, 4, 878 Metal—hydrogen hydricity data, 1, 292... 

The characteristic of radical termination steps is that two radical centers react to produce a product that has an even number of electrons. The two most important processes are dimerization (Scheme 4.40) and disproportionation (Scheme 4.41). Dimerization is the reverse of the thermal dissociation of a o bond to produce two radicals. This process is usually quite exothermic and has small or negligible activation energy. However, significant steric hindrance can lower the dissociation energy and introduced a barrier to recombination. For example, the dissociation energy of l-(2,6-dimethylphenyl)-2-(2,6-dimethylphenyl)ethane is only 22kcal mol . ... 

PH2 Products. The pseudo-second-order overall rate constant k is defined by -d[PH2]/dt=2k[PH2]2. It is made up by the pressure-independent constant k js for the disproportionation 2PH2 PH3 + PH and by the pressure-dependent constant k ec for the recombination 2 PH2 + M P2H4 + M [13,27]. The disproportionation reaction is exothermic, since the bond dissociation energy of PH3 is greater than that of PH2 (see pp. 78, 174, and [28]). This reaction channel was proposed (and favored over recombination) to account for the PH... 

Table 2.2 Calculated (BP86/TZVPP) geometrical parameters, M-M bond dissociation energies (BDEs), and disproportionation energies for the group 2 metal(l) dimers, PhMMPh (M = Be-Ba).

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