engleski

Hydride Affinities of Substituted Alkenes: Their Prediction by Density Functional Calculations and Rationalisation by Triadic Formula

The hydride affinities (HAs) of ethene and its derivatives, produced by substituents greatly varying in their donor/acceptor properties, are studied by the B3LYP/6-311+G(2df, p)//B3LYP/6-31G(d) method. The computed values are in very good accordance with available experimental data. A useful by-product of the calculations of thermodynamic data are the estimated adiabatic electron affinities. It turns out that derivatives involving monosubstituted CN and NO_2 groups, disubstituted CN, NO_2 and CF_3 groups and polysubstituted ethenes by these strongly electron accepting functionalities possess positive electron affinities implying that the corresponding ions are formed by the energy release thus being quite stable species. The most favourable sites of the hydride ion attack are identified and the trend of changes of hydride affinities is analyzed and interpreted by triadic formula, which discriminates the initial, intermediate and final state properties. It is found that hydride affinities are rather well correlated with the adiabatic electron affinities meaning that the initial state (neutral molecule) and intermediate step (relaxation of the nuclei and electrons upon electron capture) effects are predominant factors. The quantitative information on the H^- affinities can be obtained, however, only by including all terms appearing in triadic formula. It is shown that Pearson' s HSAB concept does not work for hydride affinities. The idea of using hydride affinities as a measure of the electrophylic propensity of molecules is put forward.

hydride affinity; nucleophilic addition; electron affinity; substituent effects; electrophilic propensity

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