Aromatic amines:: A comparison of electron-donor strengths

The electron-donor abilities of ten aminophenyl systems and an additional aminothienyl system are compared using density functional theory calculations. The systems studied here include those with amine nitrogen atoms bearing alkyl or aryl groups and those with amine nitrogen atoms as part of a heterocycle. Their abilities to act as donors in electron-transfer processes are assessed from calculated vertical ionization potentials for the aminobenzenes, which are in good agreement with available experimental data. Their abilities to act as intramolecular pi-electron donors in conjugated systems are inferred from the bond lengths and charge densities calculated for the corresponding 4-aminobenzaldehydes and 4-aminobenzonitriles. The computed C-13 NMR chemical shifts for the 4-aminobenzaldehydes and 4-aminobenzonitriles are in good agreement with published and new experimental data. The chemical shifts correlate well with the computed charge densities and can, to some extent, be used as an experimental probe of pi-donor strength. We find that the electron-transfer-donor strengths do not correlate well with pi-donor strengths: these differences can largely be attributed to steric effects.