| Research Abstract |
(1) Substituent effects on the stability of aromatic radical anions measured by electron affinities of benzaldehydes, acetophenones, methyl benzoates, benzophenones, and nitrobenzenes were analyzed by means of the Yukawa-Tsuno (Y-T) equation. It has been shown that resonance demand parameter (r) varies significantly with the system from 0.62 for nitrobenzene, via 1.11 for benzaldehyde, to 1.29 for methyl benzoate series. The increasing order of the r value is in agreement with decreasing stability of a parent radical anion. This trend is consistent with that observed for benzylic carbocations.
(2) Gas-phase basicities (GB) of m, p-substituted phenylacetylenes having the CF_3, t-Bu, Me, or H group at the beta-position have been determined based on proton-transfer equilibria. The analysis of the substituent effects on the GB values based on the Y-T equation showed that the resonance demand of the vinyl cation increases with the decreasing stability of the unsubstituted member of respective series. It was found that there is a good linear relationship between the r values and the stabilities of the respective parent cations, including the benzylic carbocation series, suggesting that the resonance demand is a parameter inherent in the structure of a carbocation.
(3) In order to corroborate the characterization of the resonance demands for the carbocation radical anion systems, ab initio MO calculations were performed. It was found that the changes of structural parameters such as a bond length in the carbocation system are in consistent with the resonance theory and that those are linearly correlated with the resonance demand observed for gas-phase ions. In conclusion, the empirical r value determined by the Y-T correlation should have real physical meaning which represents the degree of pi-delocalization of the positive charge.
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