Substituent effect on electron affinity, gas-phase basicity, and structure of monosubstituted propynyl radicals and their anions: A theoretical study

The substituent effect of electron-withdrawing groups on electron affinity and gas-phase basicity has been investigated for substituted propynl radicals and their corresponding anions. It is shown that when a hydrogen of the α-CH₃ group in the propynyl system is substituted by an electron-withdrawing substituent, electron affinity increases, whereas gas-phase basicity decreases. These results can be explained in terms of the natural atomic charge of the terminal acetylene carbon of the systems. The calculated electron affinities are 3.28 eV ( C==C---CH₂F), 3.59 eV ( C[triple chemical bond]C---CH₂Cl) and 3.73 eV ( C[triple chemical bond]C---CH₂Br), and the gas-phase basicities of their anions are 359.5 kcal/mol (⁻:C[triple chemical bond]C---CH₂F), 354.8 kcal/mol (:C[triple chemical bond]C---CH₂Cl) and 351.3 kcal/mol (⁻:C[triple chemical bond]C---CH₂Br). It is concluded that the larger the magnitude of electron-withdrawing, the greater is the electron affinity of radical and the smaller is the gas-phase basicity of its anion.