Fission dissociation energies

Figure 4.5 Fission dissociation energies, A26.P (see Eq. (44)), for the doubly cationic K26 cluster as a function of the fission channels P. Solid dots theoretical results derived from the SE-SCM method. Open squares experimental measurements from Ref. [79]. Top panel the spherical model compared to experimental data. Middle panel the spheroidal model compared to experimental data. Lower panel the ellipsoidal model compared to experimental data...

Sulfones are thermally very stable compounds, diaryl derivatives being more stable than alkyl aryl sulfones which, in turn, are more stable than dialkyl sulfones allyl and benzyl substituents facilitate the homolysis by lowering the C-—S bond dissociation energy. Arylazo aryl sulfones, on heating in neutral or weakly basic media at 100 °C, yield an aryl and arenesulfonyl radical pair via a reversible one-bond fission followed by dediazoni-ation of the aryldiazenyl radical (see Scheme 2 below) °. However, photolysis provides a relatively easy method for generating sulfonyl radicals from compounds containing the SO2 moiety. 

This stems from the weakness, i.e. ease of thermal fission, of the Pb—R bond, and radicals may be generated in solution in inert solvents, as well as in the vapour phase, through such thermolysis of weak enough bonds, e.g. those with a bond dissociation energy of < w 165 kJ (40kcal)mol 1. Such bonds very often involve elements other than carbon, and the major sources of radicals in solution are the thermolysis of suitable peroxides (O+O) and azo compounds (C+N). Relatively vigorous conditions may, however, be necessary if the substrate does not contain substituents capable of stabilising the product radical, or... 

The extremely wide range of possible dissociation energies necessitates the use of different kinds of light source to break molecular bonds. Van der Waals molecules can be fragmented with single infrared (IR) photons whereas the fission of a chemical bond requires either a single ultraviolet (UV) or many IR photons. The photofragmentation of van der Waals molecules has become a very active field in the last decade and deserves a book in itself (Beswick and Halberstadt 1993). It is a special case of UV photodissociation and can be described by the same theoretical means. In Chapter 12 we will briefly discuss some simple aspects of IR photodissociation in order to elucidate the similarities and the differences to UV photodissociation. 

Table V shows the efficient organization of this reaction chemistry into five reaction families. Bond fission, for example, is the elementary step that creates two free radicals from a parent molecule. In chain processes this will often be the initiation step. Thermochemical estimates often show that the logarithm of the Arrhenius A factor (logioA) is of the order 14-17, whereas the activation energy is essentially equivalent to the bond dissociation energy (19,42). This equality is the result of the essentially unactivated reverse reaction step, radical recombination.

Both UV and visible light have sufficient energy to initiate many processes in coordination complexes. Decarbonylation is one of the most typical photoreactions, because the dissociation energy of a common metal carbonyl oxide bond is as low as 200kJ mol 1.1048 Scheme 6.154 presents two examples the fission of (a) metal—CO1049 and (b) metal—CO—alkyl1050 bonds in some carbonyl complexes. In the latter case, irradiation of an enantiomerically pure iron complex 348 leads to decarbonylation, which is followed by alkyl migration. 

The absorption spectra of chlorine and bromine molecules have 2max at 330 nm (s = 651 mol 1 cm and 420 nm =1651 lmol 1 cm ), respectively.155 Since their dissociation energies are 243 and 192 kJ mol which correspond to photons of 492 and 623 nm wavelength, respectively, they readily undergo homolytic fission upon excitation, even when using visible (incandescent) low-power radiant sources. 

Briefly, the shock tube data give A-factors for radical fission which are about 1 order of magnitude lower than that calculated from the A-factor for the reverse reaction (which is radical recombination) and the estimated entropy of the over-all reaction. In addition, the shock tube activation energies are from 3 to 6 kcal/mole higher than the bond dissociation energies for branched radicals. 

Alkyl anions have been implicated as intermediates stabilized by a neutral molecule. Alkoxide ions when photolysed in a pulsed ICR spectrometer dissociate into alkanes and enolate anions The intermediate 19 in equation 25 can be represented by two possible extremes. In 19a the alkyl anion R is solvated by a ketone and inl9b the radical anion of the ketone is solvated by the radical R. The structure of this intermediate will then depend on the relative electron affinities of the alkyl group R and the ketone. Brauman and collaborators photolysed a series of 2-substituted-2-propoxides (18 with R = CH3, R" = H and R varied). For substituents R = CF3, H, Ph and H2C=CH, the C—R bond dissociation energies for homolytic fission are larger than the C—CH3 bond energy, i.e. if the intermediate complex has the structure 19b then methane would be expected to be produced. Conversely, since these R groups form more stable anions than CH3, decomposition via 19a should result in RH. The experimental observation that only RH is formed led to the conclusion that 19 is best described by the solvated alkyl anion structure 19a. 

Figure 4.9 Solid dots LDA-SCM results for the dissociation energies for the most favorable fission channel for doubly charged cationic parents Na v when the spherical jellium is used. The influence of triaxial deformation effects (calculated with the SE-SCM approach) is shown by the thick dashed line...

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