The prediction of gas-phase and aqueous basicities for alkyl amines

The proton affinities (PAs), gas-phase basicities (GBs), and pKas for ammonia and 18 alkyl amines were investigated by using Hartree-Fock (HF), perturbation (MP2), and density functional (B3LYP) theory. Basis set and thermodynamic effects were also examined. By using a statistical approach, the reliability of the model chemistries was assessed. Relative PAs and GBs calculated at the HF level with modest basis set led to mean errors within experimental uncertainty. The experimental PAs and GBs could be reproduced at the MP2 and B3LYP levels if large basis sets were used. The basis set dependence of thermodynamic effects was found to be significantly less than experimental error, and the importance of MP2 and B3LYP structural optimizations and thermodynamic effects are similarly negligible. Calculation of free energies of hydration via the polarizable continuum model (PCM) led to significant errors, which rendered large errors with respect to experiment in the predicted pKas.