Silyl radical electron affinity

Thermochemical information about neutral species can also be obtained from measurements of ions. Indeed, accurate bond dissociation energies for neutral molecules have been obtained from gas-phase ion chemistry techniques. In this section, we will summarize both the negative-ion and hydride-affinity cycles involving silicon hydrides (RsSiH) which are connected to electron affinity (EA) and ionization potential (IP) of silyl radicals, respectively [22-24]. 

Table 2.5 Electron affinities of silyl radicals together with gas-phase acidities and bond dissociation enthalpies of silanes"...

Group IV substituents, especially the trimethylsilyl group, apparently enhance the electron affinity of aromatic systems. The effect is particularly noticeable in aniline derivatives. The strong electron-releasing effect of the amino group decreases the electron affinity of the aniline derivatives and hinders reduction to the radical anions. Nitroanilines may be reduced to radical anions (65). The only other aniline radical anions that have been reported bear silyl substituents either at nitrogen (62) or on the ring (83, 85, 86). 

The electron affinities of a number of a-silyl substituted silyl and carbon radicals were determined in photodetachment experiments and confirmed by data obtained from ab initio calculations. The authors conclude in this study that the stabilization a carbanion experiences through a-silyl substitution is approximately 14-20 kcalmol-1 per silyl group that of a silyl anion is approximately 6-14 kcal mol-1. The larger stabilization in the carbanionic systems is readily explained by stronger hyperconjugation of the anionic carbon center with the silyl groups as compared to that of the silyl anion with a silyl group. 

Yokoyama and coworkers investigated the discolouration of titanylphthalocyanine 60, and related porphyrins by silyl radicals derived from methylphenylpolysilane by molecular orbital methods60. Calculated values of AN, the index derived from the hard and soft acids and bases concept was determined from HF/3-21G calculated ionization potentials, electron affinities, and HOMO and LUMO orbital energies of the systems of interest. These studies predict that PhMe2Si should donate an electron readily to 60 to form 60 (equation 18). This prediction is of direct relevance to observations that irradiation of titanylphthalocyanine-coated methylphenylpolysilane films leads to discolouration of the film60. 

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