Substituent effect on electron affinity, gas-phase basicity, and structure of monosubstituted propargyl 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 propargyl radicals and their corresponding anions. It is shown that when a hydrogen of the alpha-CH2 group or acetylenic CH in the propargyl system is substituted by an electron-withdrawing substituent, electron affinity increases, whereas gas-phase basicity decreases. The calculated electron affinities are 0.95 eV (CH equivalent to C-CH(2)center dot), 1.15eV (CH equivalent to C-CHF center dot), 1.38eV (CH equivalent to C-CHCl center dot), 1.48eV (CH equivalent to C-CHBr center dot) for the isomers with terminal CH and 1.66 eV (CF equivalent to C-CH(2)center dot), 1.70 eV (CCI equivalent to C-CH(2)center dot) 1.86 eV (CBr equivalent to C-CH(2)center dot) for the isomers with terminal CX at B3LYP level. The calculated gas-phase basicities for their anions are 378.4 kcal/mol (CH equivalent to C-CH(2):(-)), 371.6 kcal/ mol (CH equivalent to C-CHF:(-)), 365.1 kcal/mol (CH equivalent to C-CHCI:(-)), 363.5 kcal/mol (CH equivalent to C-CHBr:(-)) for the isomers with terminal CH and 362.6 kcal/mol (CF equivalent to C-CH(2):(-)), 360.4kcal/mol (CCI equivalent to C-CH(2):(-)), 356.3 kcal/mol (CBr equivalent to C-CH(2):(-)) for the isomers with terminal CX at B3LYP level. 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. This tendency of the electron affinities and gas-phase bacisities is greater in isomers with the terminal CX than isomers with the terminal CH. Copyright (C) 2009 John Wiley & Sons, Ltd.