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

Abstract

AbstractThe hydride affinities (HAs) of ethene and its derivatives produced by substituents that vary greatly 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 agreement with the 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 NO2groups, disubstituted CN, NO2and CF3groups and ethenes polysubstituted by these strongly electron‐accepting functionalities possess positive electron affinities, which implies that the corresponding ions are formed by energy release and are thus quite stable species. The most favourable sites of the hydride ion attack are identified and the trend of changes of hydride affinities is analysed and interpreted by a triadic formula, which discriminates between the initial, intermediate and final state properties. It is found that hydride affinities are rather well correlated with the adiabatic electron affinities, which means 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 the 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 electrophilic propensity of molecules is put forward. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)